Infinite Baffle Subwoofer

Introduction:

This page describes my basement home theater infinite baffle (IB) subwoofer. The performance of this beast is SO far superior to my decade-old sonotube subwoofer (which is no slouch!) that its doesn't come anywhere close to being a fair comparison. It features four 15" Tempest-X drivers powered by a professional-grade 1000 watt amplifier. There are two primary benefits to building an IB subwoofer over a more traditional box-style sub: 1) significantly higher output level and 2) vastly superior sound quality. Let's start with the easy part first. A properly installed and configured IB sub can easily generate sound pressure levels in excess of 125dB while maintaining low levels of audio distortion. By comparison, a high-quality entry-level "box-style" sub (say, one costing a minimum of $750+ on the retail market) will probably max out somewhere near 105-110dB. Make no mistake, both are significantly loud, but the additional output capability of the IB sub (significantly higher SPL at much lower frequency) makes a tremendous difference in the physical impact of an action movie. As traditional "box-style" subs approach their maximum output levels, audio distortion quickly increases to noticeably high levels. This happens because at its maximum output level, the driver, its box, and its amplifier are all working at their individual design limits. The key to the higher output level with an IB sub is the use of multiple (either four or eight) large (usually 15" to 18") drivers that are capable of moving significantly more air than can be moved by a single 10-12" driver when mounted in a box. 

With a subwoofer, output level is a direct function of the volume of air it can move with a single stroke (referred to as "swept volume" or "displacement"). In my setup, each driver has a radiating surface area of approximately 800 sq cm (converted from a 15" diameter) and a linear one-way travel (Xmax) of 2.7cm. Doubling Xmax (for peak-to-peak driver movement) and multiplying by the cone area provides the maximum swept volume for one driver - just about 4 liters in this case. Multiplying by four (for four drivers) provides a total swept volume of about 16 liters! Compare this with the swept volume of my 12" subwoofer of just under 2 liters and the difference between this new IB sub and my current sonotube sub becomes very clear: my IB sub has 8 times the physical air displacement capability of my very capable sonotube sub! If you have a typical 10" commercial subwoofer, the comparison is even more dramatic: my IB sub has approximately 20-25X more displacement! Using multiple drivers in an IB configuration allows the drivers to work together to produce high output levels while simultaneously keeping each individual driver well within its physical design limitations. Think of comparing the performance of a family sedan (say, a Ford, Toyota, Honda, or whatever) and a capable racing car (say, an Indy car that would race in the Indianapolis 500 on Memorial Day Weekend) where both are travelling at 100 mph. While the speed of each vehicle is the same, one is starting to suffer from reduced stability, lack of additional acceleration, and difficulty steering and braking. The vehicle that is still just comfortably "cruising along" even at speeds that exceed 100mph continues to exhibit excellent control over steering, braking, and acceleration (I'll let you figure out which is which). In addition to greater sound pressure output and lower overall distortion, the sound produced by an IB subwoofer is of much higher quality than that from a subwoofer in a box. To explain why, I have copied the basic description of an IB sub from ThomasW's web page (linked above):

"For ease of explanation it's convenient to think of an IB sub as a massively large sealed box alignment. This is overly simplistic, but it serves the purpose of an introduction until such time as greater technical explanation benefits the conversation. Think of the baffle as an infinitely large boundary that provides isolation/separation between the front-wave and the rear-wave coming off the driver/drivers. For practical purposes this means the drivers are mounted so they share a common boundary between the listening room and an adjacent space. This adjacent space can be the attic, basement, crawl space, garage, or any room that shares a common wall with the listening area.  However the IB is created, there must be adequate isolation between the front wave and rear waves. Sealed and ported subs [the two primary types of "box" subs] are enclosed systems where the enclosure and rear-wave coming off the cone impacts its operation. In the case of a ported alignment; the rear-wave coming off the cone interacts with the air-mass inside the enclosure to create a Helmholtz resonator (like blowing over the top of a bottle). The vibrating mass of air colors the sound. In the case of smaller sealed box systems; the box interaction of the rear-wave and the compressing air-mass inside the box changes the 'Q' of the system. This too colors the sound. In poorly designed or inadequately built ported or sealed box subs, the walls of the enclosure itself can be excited by the rear-wave coming off the driver. If the walls of the box are vibrating there's a detrimental effect on the sound quality.   

IB subs feature no Helmholtz radiator effect, no compression of the air-mass and in most cases no box walls to negatively impact the output of the driver." 

Thus, the tag line for IB subs: Hear The Bass, Not The Box

I know by now some of you are thinking about hearing loss and other associated damage to your ears. Since the bass effects in movies tend to be brief in duration and intermittent throughout the movie (not sustained for long periods of time), the risk of hearing damage is rather small - much smaller than you are thinking. Now, continuous music playback at levels of 105+ dB is a COMPLETELY different story. Check out the following link for additional information about duration of exposure to loud noises and hearing loss and see the charts at the bottom of this page.

The Drivers:

I purchased a set of four 15" Tempest-X drivers from the great folks at DIYCable (sadly, now also defunct). What you are looking for is a large driver (15 or 18 inch diameter) with a low resonant frequency (lower is better, preferably in the < 25Hz range) and a fairly stiff suspension (Qts near 0.5). The Tempest-X driver is quite a capable driver! It is based on the Adire Audio Brahma motor with a highly optimized, fourth generation XBL^2 motor. They are standard 12-spoke cast baskets with very robust construction. Each driver has a dual 8-ohm voice coil. Parameters for the driver are: Re: 3.95 ohm, Qts: 0.578, Qes: 0.673, Qms: 4.095, Fs: 24.5hz, mms: 130g, cms: .300mm/N, BL: 11, Vas: 284L, and an Xmax (linear one-way travel) of about 27mm (over one full inch!). I picked these up a few years ago at pricing that was too good to pass up (a deal that just can't be matched today!) knowing that one of these projects was in my future. With fewer and fewer manufacturers operating in this arena, many of the IB sub builders choose the 18" drivers from FiCarAudio - they have a model (the IB3) that is specific to Infinite Baffle installations. There is also a great 18" driver available from StereoIntegrity: the HT18.

The driver is quite large and heavy. I thought the 12" TCSounds driver that I used for the Avro speaker project was large, but these 15" drivers clearly dwarf those little 12" guys! I'll let you figure out which is which below... You can see the weather stripping that I added to the sub to cover the mounting holes in the basket to prevent air leaks when its mounted in the manifold.

Which/How Many Drivers Do I Need? 

Enough drivers that are sufficiently large should displace a minimum of 12 liters of air with a single stroke is generally considered to be a good starting point. My four Tempest-X drivers have a maximum displacement of about 16 liters. Four of the FiCar IB318 drivers will have about 32 liters of displacement!

Chrisbee from "The Cult of the Infinitely Baffled" makes the following recommendations about driver size and quantity for an IB sub:

To calculate how many drivers you need, of any particular kind or size, you only need to know the displacement of one of them:

Volume of displacement= Vd = (2 x Xmax) x Sd. 

Ideally an IB ought to have at least 12 litres of total displacement for reasonable safety in IB use for action films. 

Once we have our displacement for one driver we can find how many we need to achieve our 12 litres of total displacement.

Example= Xmax for the Peerless XLS12 = 12.5mm. Sd = 466 sq.cm

Displacement Vd = 2.5 cm x 466 = 1165/1000 = 1.65 litres per driver.

Notice how we convert the 2 x Xmax(mm) to cm to keep the terms for both Xmax and Sd equal. We divide our answer (in cc) by 1000 to convert to litres. 

So 12 litres of required total displacement = 12 /1.65 litres = 7 drivers. 

But! We can't use 7 drivers because they are so difficult to drive without using seven amplifier channels. There are no sensible series-parallel wiring options with odd numbers of drivers. More amps = wasted money. 

So (being sensible) we increase the number of drivers required to 8. 

8 x 1.65 = 13.2 total displacement = an extra margin of safety. 

8 drivers can be wired series-parallel to run 4 per channel. Or all 8 on a mono, bridged amp.

The tragedy now is that 12" peerless drivers cost about as much as Fi IB318 18" drivers. 

Only two 18" drivers will provide you with 14 litres of displacement! 

So after all our calculations we have discovered that using 12" drivers is not at all sensible unless you already own an awful lot of good ones. Or can buy them for a song. Both options are very unlikely. 

Buy cheaper 12" drivers and this will probably reduce the Xmax. So you'll need twice as many! Which makes the economics even more ridiculous! 

This is why we normally choose two 18" drivers. Or four 15" drivers for IB use. Minimum! They offer sensible displacement in smaller numbers at prices we can afford. 

Given their pricing and performance, people with "smaller" rooms typically choose two 18" drivers, while those with larger rooms (think 5000 cubic feet or more) often go with four 18" drivers. As a quick rule of thumb for driver quantity and size considerations, the air displacement capability of two 18" drivers = four 15" drivers = eight 12" drivers = sixteen 10" drivers (assuming each has an equally long X-Max - which is almost never the case as drivers get smaller). 

Below is what one person did with his old box-style subwoofer after installing his IB sub. Anyone else with a 10" (or smaller) box subwoofer that was purchased for $100 or less should go and sit in the corner....

Driver Mounting: The Manifold

OK, so now you've got a bunch of large cardboard boxes taking up room in your garage or basement, so what do you do with them? You have two primary choices, you can either build a line array of drivers in your theater (essentially flush mounting each driver in a line so it fires directly into the listening space) or you can build a manifold system. To learn more about the differences between these two options and other general information about IB subs, I recommend reading the IB Subwoofer FAQ pages before you start your IB sub project. You may need to create an account for yourself before you can read these pages - it's free and they don't send out any mass e-mail messages, to its a no-brainer to register and learn! 

Due to the large size of these drivers and their high output capabilities, building a line array requires a very, very, very stiff wall to hold them (think concrete and steel). If you wall is not absolutely stiff (meaning it is made from wood), it will flex in the opposite direction that the drivers move (darn that Newton and his third law of motion...), thus most of the output will be cancelled before it has a chance to fill your theater with bass. Since my wall is made from sticks, I opted for a dual manifold system. The manifold has the added advantage of having any potential physical resonances induced into the structure by the drivers canceling one another out (it won't vibrate on its own like an improperly built line array will). It also has the strange effect of keeping thing on the "back side" of your IB install relatively quiet as the output from the drivers cancels out due to the out-of-phase mounting on the back side. Having two manifolds helps to provide more even in-room bass response but does so by trading off the absolute output level achievable by creating a single, larger manifold (no free lunch here, either...). 

The image at the top of this page was my starting place for the manifold. I started with some left over Baltic birch plywood and MDF from the Avro project. Each manifold is essentially a 5-sided rectangular box. The sides where the drivers are mounted are cut from Baltic birch and have been laminated with 3/4" MDF to add some mass and rigidity to them. Thus, each side wall is 1.5" thick. For extra strength and vibration control, I installed a 2x2 crossbar toward the opening of the manifold. 

Since exact size is not critical here, I cut all of panels freehand on my table saw. Precision is not necessary, only consistency so you can actually get tight-fitting seams for your box. The outer dimensions of the manifold are 19" tall, 9.5" wide, and 28" deep. The resulting opening that fires into the theater room measures 8" wide by 17.5" high - more than enough breathing room for two 15" drivers. 

I cut all of the circles with my Jasper Circle Jig before I assembled the boxes. The Baltic birch was cut to allow the driver cone and surround to fit into the hole with a little bit of breathing room when flush mounted. I used my round-over router bit when I was finished with the hole as an extra measure of making sure the driver surround didn't contact the wooden box at all while its operating. Next, I routed a larger hole in the MDF to allow the driver to sit completely inside the hole. The recesses in the MDF that you see above are to allow the driver mounting clips to be installed. I used the speaker grill mounting clips available from PartsExpress (see image below on the right). Don't worry about what anyone says about these not being heavy-duty. The 1/8" thick steel is plenty strong enough to hold your driver firmly in place! The supplied screws are not very long, though, which is why I cut the recesses into the MDF. I discarded the rubber mounting ring that came with my drivers and instead just used some weather stripping to make sure the driver seals to the manifold reasonably well. You can see where I drilled holes and mounted the T-nuts on the inside of the box for the drivers. This mounting method takes a bit of work ahead of time, but allows you to easily remove a driver from the outside of the box in the future should the need arise (perhaps to replace a blown driver because you over-drove it?).

The space on the manifold to the right in the above image is where I will mount it to the wall stud, thus, the MDF didn't need to go all of the way to the end of the manifold. Below is a shot down the throat of the manifold. In both images (above and below) you can see the 2x2 cross brace that I added to the manifold before I installed the drivers. This was glued and screwed in place and was added to help keep the manifold from vibrating on its own since this is the least supported part of the structure.

The picture below shows the completed manifold twins. Before I mounted the drivers, I used silicon caulk to make sure the seams of the cabinet were sealed and painted the insides black. I might have been better off painting before I caulked, or using a latex rather than silicone caulk as paint doesn't stick well (or at all) to silicone.... Make sure the caulk is fully cured before adding the drivers and the outgassing from the caulk can dissolve the glue that holds the speaker surrounds in place.

And finally, you can see one completed manifold with both drivers installed below. This little sucker is quite heavy, over 100 lbs. worth of wood and drivers together! I had a hard time dragging the completed manifolds around the corner for installation (shown on the right below). Behind the front wall of the theater is shelving system. One side of each manifold is lagged to a stud for the front wall, the bottom of the manifold is lagged to the footer of that same wall, the inside bottom of the manifold screwed (into the supporting stud, of course) to the shelf that it rests on in the other room, and finally, the face of the manifold is screwed to the drywall from the theater side. After getting things up and running, I'm happy to see that the manifolds are rock-steady. Even at very high output levels, the manifolds themselves are vibration and resonance free. The rest of the room, though, is not so lucky ;-)

From the theater room side of things, you can see where the IB subs are mounted on the front wall between the main speakers. You can also see where I drove screws through the drywall to anchor it to the perimeter of each manifold. In the final iteration, I used some spare 1x2 that I had laying around and made two quick frames - one to cover each hole in the wall - with some glue and clamps. The frame measures approximately 20" square. I rounded over the outside edges of the frame with my router and then cut away some of the frame on the edge where it presses against the wall so I could run some wires behind the grille and still keep it flush-mounted to the wall. Then I stapled some black grille cloth to the frame and applied some Velcro to the frames and to the wall to hold the frames in place. Basically any "open weave" cloth will do. To tell if the weave is sufficiently "open," just hold it up to a the light and you should easily be able to see through it. Alternatively, hold it across your nose and mouth - you should be able to breath easily and normally right through the fabric. If the fabric fails either of these tests, look for something else.

The result works quite well to hide the holes from the manifolds on the front wall. I made the frames a bit oversized in terms of their width. Due to the placement of the studs in the wall, the holes for the manifolds are somewhat non-symmetric in their placement relative to the edges of the screen. Having oversized grilles allows me to place the grilles on the wall so they are evenly spaced from each edge of the screen and completely cover both manifold holes. 

Using the flash on my camera, you can see the cover panel and the frame behind the grille cloth. Using only the room lights, the cover disappears and blends into the front wall quite nicely. When the lights are off and a movie is playing, it all just disappears. The final result looks quite nice. 

Amplification:

Total amplification for my new sub is just about 1,000 watts continuous into 8ohms. Each manifold contains two drivers and is powered by ~450w. This presents an amplifier-friendly load of 8-ohms (voice coils for each driver are wired in parallel, drivers are wired in series) for each of two channels - a very nice arrangement. Thus, each driver sees about ~225 watts or about 110w for each voice coil. Indicated maximum input power (Pmax) for each driver is about 600w, so things are in good shape.

For power, I opted for some overkill to allow flexibility across a number of domains. First, the best amplification option is to use a professional grade amplifier - something that bands would purchase to power their speaker while they are on the road performing. Its no secret in this industry that Behringer offers just about the best bang for the buck on the planet. I opted for their larger amp (actual measurement indicates the EP-4000 capable of delivering 2 x 450w into 8ohms, 2 x 635w Watts into 4 Ohms; 2 x 815 Watts into 2 Ohms; or 2,000 Watts into 4 Ohms in bridge mode). This is one hell of an amp! I picked the larger one for three primary reasons: 1) some pro-audio gear requires a much higher input level signal than does comparable consumer-grade gear to produce the same level of output, thus having extra output power reduces my sensitivity to this issue, 2) many IB subs need some low-frequency added equalization to hit the really low notes (15-20Hz). Since equalization places huge demands on amplifiers (adding a mere 3dB of output at 20Hz requires exactly double the output power from the amp), I figured it would be good to have some headroom with the amp, and 3) should circumstances dictate new drivers down the road (which I hope to avoid), this amp has enough power to drive just about any configuration of drivers. As added bonuses, this amp is build like a tank and holds its resale value quite well. 

The downside to this amp (as with many pro-amps) is that its construction features banks of output mosfets that are mounted internally to a heatsink tunnel that is kept cool by rather noisy fan. If you use this amp in another room, it's really not a problem. If the fan makes too much noise for use in your listening room, though, the solution is simple enough: open the amp and replace the fan. Can you tell I'm not too concerned about voiding warranties? Below is an image of the original fan inside the chassis. A drop-in replacement for the original fan is available from Digi-Key, part P9739-ND fan 24VDC 1.30W 80mm x 80mm x 25mm, 25dB for $8.25 + shipping. You need to remove the screws around the perimeter of the top of the amp and the four screws on top that hold the lid to the heatsinks. Cut the cord on the old fan, leaving yourself enough room to solder the leads to the new fan and you're back up and running in just a few minutes (really!). Please make sure that the new fan blows air in the same direction as the old fan before closing the chassis. There have been a few reports of overheating amps because the airflow was both reduced and reversed. The new fan spins a bit more slowly, so it is inaudible from just a few feet away. The airflow is reduced, but since a subwoofer amp sees somewhat intermittent use (as opposed to running it full tilt as your main amplifier while on stage) the air flow is still more than plenty to keep the amp reasonably cool. For more details on replacing the fan just do a web search on "behringer amp fan mod" and you'll find all kinds of information.

Performance Tweaking (Bass Traps, Custom High Pass Filter, REW, Parametric Equalization, & Phase):

Let's face facts, building a box and cutting a few holes in it is just about as simple as things get. Here is where the REAL work (and sometimes frustration) begins - this is the stuff that makes or breaks (literally) your subwoofer! 

It may be tempting to want to watch movies and listen to your new mega-subwoofer as soon as it is up and running, but there is still a GREAT DEAL of important work to do once your IB sub is powered up (because it doesn't sound anywhere near as good as it can just yet). There are a TON of online resources for setting up your subwoofer, the best one that I've found is the Bass Integration Guide at Red Spade Audio. It is a three-part article with lots of great links for explanations. It's a long read, but read it anyway. To boil things down a little, the "uncorrected" performance of your new subwoofer is likely to be pretty awful. This means that you likely have large peaks and troughs in your frequency response, your room exhibits significant "ringing" at room modes, and spectral decay is so long that bass lines just blur together making things a mess. It will, however, have lots of bass, but it needs significant work to clean it up so that it can live up to its full potential.

What do you need to do? Glad you asked! Start with Bass Traps to get the room under control and begin to remove some of the room modes (bass echo, for lack of a better description). Then, you need something to help protect your drivers from massive over excursion. This can lead to distortion (audible nasties) or even damage to your drivers (physical nasties) and can be addressed with a high pass filter. Finally, a good parametic equalizer will work wonders for the response of your system, though it will take some patience to really learn how to use it well.

Bass Traps:

Bass traps are an essential element of a dedicated theater room. Every room will exhibit modal ringing just like an organ pipe (or blowing across the top of an empty soda bottle). Ringing adds coloration to the sound since it exaggerates the amplitude of certain frequencies. The specific frequencies are a direct function of the room's physical length, width, and height dimensions and these negative effects are most pronounced for bass in the 20Hz to 100Hz range. Bass traps are devices intended to "trap" this unwanted resonance and help reduce its impact on your listening experience. Do yourself a favor and build a nice set of bass traps (ideally, one for each corner in the room) before you begin any other form of analysis, equalization, or tweaking for your subwoofer.

Creating a Custom First-Order High-Pass Filter:

Do yourself a favor and install your high pass filter BEFORE you begin tweaking things with REW and your equalizer. The primary goal of a high pass filter is to keep your drivers from exceeding their X-Max specification (one-way maximum linear travel) by reducing the magnitude of the very low frequency content (single digit stuff) in movie soundtracks. Begin by downloading the latest copy of WinISD Pro and creating a new driver profile for your drivers by entering all of the technical specs as published by the driver manufacturer. Next, model your IB sub by specifying a closed box with 999 cubic meters of volume (in the Box tab), the number of drivers you are using (in the Driver tab), and the maximum output power of your amplifier (in the Signal tab). Then, click around until you find the "Cone Excursion" graph. Somewhere in the 20Hz region, it will likely show your driver greatly exceeding the driver's X-Max specification (graph going above the horizontal red line). When the excursion travels above this line, you are introducing huge amounts of audio distortion (which sounds awful) and putting your drivers at risk for physical damage since they are being pushed beyond their design capabilities. Every driver can be pushed beyond its design limits - it's just a matter of how the drivers are arranged (their ability to share power) and just how much power (and EQ) they receive.

If your drivers exhibit over-excursion behavior in the WinISD model, it is time to begin the trial and error part of establishing a high pass filter. Find the EQ/Filter tab and create a first-order high-pass Butterworth filter. Then, plug in various cutoff frequencies until your cone excursion curve stays under the red X-Max line. The resulting cutoff frequency will likely be somewhere in the 10-15Hz range. In my case, the high pass filter seems best set at 11Hz (see image below). Setting the filter frequency too low won't protect your drivers from over excursion and setting it too high will unnecessarily limit the output of your drivers by restricting their excursion (and corresponding bass output!). Of course, you'll need to take other things into consideration to protect your drivers from over excursion. These include raising the output level of the LFE channel on your preamp and boosting output in the <30Hz region with your EQ. 

So, now you have determined the high pass crossover frequency that should keep the driver safely within its design limits for the number of drivers and output power of the amp you are using. Since the point is to preserve headroom in your system, you want to filter the subwoofer output signal before it gets to your amp. If you are using parametric equalization (such as the BFD), you'll want to place the filter before your EQ to help keep it from clipping the signal that it passes to your amp. Creating the actual high pass filter is a very simple exercise - you just need to insert a capacitor and a resistor into the signal cable. The good news is that the resistor is already there for you - it is the imput impedance of your equalizer or amplifier (whichever is first in your signal path for your sub) and it is indicated by the input impedance (check the spec sheet) of your equalizer or amp. The input impedance of my Behringer DPS1124P is given as 30kohm for an unbalanced (RCA) cable. Then use the first order crossover calculator on the ApICS web page to enter the input impedance of your EQ as the High Pass Impedance (30,000 ohms), plug some bogus number into the Low Pass Impedance field (since we have no interest in a low pass filter), type in your desired high pass frequency as determined with your WinISD model (11 Hertz), and click the Calculate button. The C1 value will provide the proper sized capacitor to achieve your desired high pass crossover point. For me, this works out to about 0.48uF. If you are using an unbalanced signal cable (standard RCA plug), place this cap in series with the hot lead of your signal cable as shown in the schematic for the first order high pass filter and you're all set! If you are using a balanced signal cable (standard XLR plug), place this value of capacitance in series with BOTH the positive lead AND the negative leads. It's really just that simple and now you are ready to move on to using REW to set your equalizer.

Room Equalization Wizard and Parametric Equalization with the BFD:

In order to use REW, you need a calibrated microphone (such as a Behringer ECM8000 or a Dayton EMM-6) or a sound pressure meter with an output jack (the old-style Radio Shack analog meters are great for this), a tripod to hold the mic, a computer with a decent sound card, and a few cables. In REW, you will need to import the calibration settings for your microphone as well as create a custom calibration setting for your PC's sound card. The help document is pretty well written and describes how to do this. For your REW measurements, be sure to set your measuring/graphing limits to 45dB to 105dB and measure from 5Hz to 200Hz. Using these limits will make analysis and interpretation consistent with measurements that others have made when you post over at the cult of the infinitely baffled. 

Since the BFD is notoriously cumbersome to program by hand (its doable, just very time consuming, see Thomas' instructions on his web site), using a MIDI cable greatly speeds the calibration process along! The cable connects your PC (via USB cable) to the BFD (via MIDI connectors) and saves a TON of time, especially when you are working in a "trial-and-error" mode (measure, calculate, adjust, repeat...). You'll also need a Sound Pressure Level (SPL) meter. Tons of these are readily available - the most common one is the "old" analog one from Radio Shack (shown below). Since many of these meters have some inherent inaccuracies, you'll also need the calibration file for which ever meter you end up using.

The automated/predicted calculations provided by the Room EQ Wizard are typically unable to account fully for peaks (nodes)/troughs (anti-nodes) in the frequency response behavior that are a result of your room dimensions, hence the trial-and-error approach. Thomas suggests a general protocol to get things going:

1. The fewer filters used the better.

2. Narrow band filters (less than 1/3 octave or 20/60 in REW/BFD) decrease sound quality so use as few of those as possible.

3. The idea is to start off with wide-band cut filters to get the large/wide peaks under control. 

4. Then apply wide-band boost where needed - but only boost a little bit (< 3-4dB). A 10dB boost requires 100 times more amplifier output power!

5. Fine tune with narrow band if needed. Sometimes these look better on the measurements but don't really do that much when auditioned. 

6. Finally don't get all hung up in having a perfectly 'flat' plot, most of the time those are boring to the ear.

Lastly, it is important to keep the BFD from clipping the audio signal. To prevent the BFD from clipping, you need to keep an eye on its signal level meters - these actually show the magnitude of the output signal and clipping really kills the quality of the output signal. On the left side of the BFD front panel are two vertical columns of LED's that light up indicating signal level. Always keep these operating in the green and yellow indicators. When the red LEDs are on or flickering, it means the digital circuits are clipping which will result in audible distortion being sent to your sub amp. Two things cause the BFD to clip: a signal level from your preamp's LFE (sub) channel that is too high, and/or you've added too much boost to the signal in your BFD. You will need to balance these two items with one another so that only the green (and occasionally, the yellow) LEDs light up with the loudest movie scenes. Think of it as a fun excuse to demo each of your favorite action scenes again...

For more on setting up your IB sub with REW and the BFD, check out ThomasW's excellent advice.

Measurement Results:

OK, enough with the theory and all of the background - it's time to actually get some work done! Below are before and after graphs of frequency response, waterfall plots, and impulse response plots that I made with the Room EQ Wizard software. The "before" plots (on the left) were recorded at the primary listening position (about 17 feet away from the manifold opening) without any added equalization or my rear-corner bass traps. Just the preamp running directly to the subwoofer amp. The initial starting place is pretty ugly. The "corrected" plots (on the right) were recorded at the same listening position after several days of tweaking using the Room EQ Wizard (referred to as REW) to adjust the filters on my Behringer DSP1124P Parametric Equalizer. Because some of my measurements were made at different points in time, the scales differ a little from plot to plot which makes direct comparison a bit more difficult. The important points are the differences in the shapes of the curves.

In the original frequency response plot above (left in purple), note the extreme peaks (10Hz, 45Hz, and around 90Hz) and troughs (22Hz, 75Hz, and 95Hz). This means that sounds at 45Hz are nearly 15dB louder than sounds at 25Hz and 70Hz! In an ideal world, the frequency response plot for a subwoofer should not deviate more than plus/minus 5dB from the "average" response level in the frequency range of 20-80Hz or so. This makes sounds at all frequencies sound equally loud with one another. Ignoring the peaks out past 70Hz in the "before" graph (because the LFE crossover in my preamp is set to 60Hz), my original response plot varies by more than plus/minus 8dB (a 16dB swing that sounds awful). After several days of experimenting with equalizer settings in the BFD, recently completed corner bass traps, and an outboard first order high pass filter (a 0.48uF cap inserted in the +sig and -sig XLR wires before the BFD), my frequency response is displayed in the graph above on the right (in blue-green). I achieved this with onlt three cuts at 20Hz, 45Hz, and 97Hz. Note that all of the peaks and troughs (with the exception of the room mode at about 105Hz that I just can't seem to make go away) have been smoothed out to within plus/minus <3dB from 25Hz to 125Hz! The 10dB response hump at 10Hz is the result of a room mode that I can't address because it is outside of the adjustment range of the BFD. On the plus side, it's not really so bad at all to have a natual response hump at 10Hz... When is the last time you saw a commercial subwoofer offered for sale with specifications that include significant response down to 5Hz?!? At 5Hz, sound pressure is equal to the average of the rest of the response curve (the Behringer EP4000 amp filters frequencies below 5Hz and my preamp filters below 8Hz)! This is incredible response and a remarkable transformation from the original response curve. Hold on to your seats during movie time - bass this deep and loud shakes the entire house!

The waterfall plots are 3-dimensional plots that show frequency response over time. The above image on the left (purple) shows the same peaks and valleys as the earlier frequency response graph, but shows how sounds at various frequencies "stick around" and "echo" around the room - sounds these frequencies decay more slowly than do sounds at other frequencies making them sound exaggerated and boomy. The measurements start with the "back layer" in the graph and as time progresses (millisecond scale is shown on the z-axis on the top right) new layers are added in front of the original measurement. Thus, you can see that after 300ms, there is still significant reverberations across most of the frequency range - especially at about 12Hz, 23Hz, 45Hz, and 90Hz (these are primarily room modes due to the physical dimensions). The waterfall plot after corrective equalization and corner bass traps appears on the right (turquoise color). The overall level is shifted down a bit due to the filtering of the in-line capacitors, but the difference is pretty clear. There is still a peak at about 105Hz that I am relatively unable to address, but it has been tamed quite a bit from its starting point. Remarkably, the frequency response across the entire spectrum has dropped by more than 20dB within the first 150ms, with the exception of the <40Hz range. In the 40-90Hz region, the output exhibits a decrease of closer to 30dB within 150ms. This is a MAJOR improvement!

The two graphs above show the impulse response for the before and after conditions for the room. Impulse response graphs show the time it takes (the x-axis) for a single impulse of sound to drop in decibel level (the y-axis). Note that in the "before" graph (green on the left) the sound pressure level is barely down by 10dB after the first 200ms and takes 600ms before the average sound pressure level is down by 20dB. At 1000ms (one full second) the sound pressure level is down by only 25-27dB. In the corrected graph (turquoise on the right), the impulse level is down by nearly 40dB after the first 200ms and within 500ms is down by more than 60dB - this is very significant improvement again! As indicated on the Red Spade web site, impulse response should ideally be down by 20dB within the first 150ms - my response curve is down by almost 24dB within this timeframe! Those bass traps in the rear corners of the room really make a difference! Also, notice the larger time delay in the corrected response plot - there is a greater space between the 0ms mark and the onset of the largest peak directly to the right. This means we have some phase issues to address.

Phase Adjustment:

Phase is an important element of getting your subwoofer properly dialed in. Remember back to all of that "node" and "anti-node" stuff from high school physics class? Here is where it really makes a difference! Getting your subwoofer and main speakers phase alligned means that the timing of the signals sent to your speakers arrive at your ear in unison and the drivers are working together in constructive rather than destructive ways. Exploring the phase setting for your subwoofer is important to do at the same time that you are using REW to try to smooth out your frequency response curve as they interact with one another. This is very important as each element in your playback chain introduces various levels of delay. The bass management signal processing in your receiver or preamp creates some delay, your parametric equalizer adds its own delay, your amplifiers add a little more. The distance from the speakers to your listening position adds even more delay. If you subwoofer is not placed at the same distance as your main speakers, this further complicates the picture. While there are formulas and calculations for quantifying each of these delays (see the very nice example provided by Rythmik Audio), there is no replacement for actual measurement of the entire system to see what is going on in your room.

The graph below shows the result of altering the phase of the subwoofer with respect to that of the main speakers. Initially, my frequency response graph was left with a rather large "hole" in the 60-100Hz region (dark blue line at the bottom in the graph below). I was unhappy with this for quite a while - it made voices from the center channel sound strange and unnatural. I couldn't figure this out for the longest time and then I decided to play around with integrating the phase of the subwoofer with respect to the phase of the main speakers. In many cases, your preamp with Audyssey is capable of taking care of this automatically with the "distance" settings for the individual speakers, though it is certainly a worthwhile effort to verify that everything is set correctly. In all of my previous work, I was feeding the REW test tones directly into the BFD and then to the sub amp, so I was adjusting the subwoofer only. This time, I fed the REW test signals from the computer into my preamp while also running the front array of speakers (I ran the analog output from my PC sound card directly into the L&R inputs for an analog input on the back of my preamp and used the bass management features of the preamp for adjustment). The initial result using the actual measured speaker distances from the main listening position are displayed with the blue line in the graph below. My main speakers are about 13 feet from the listening position and the subwoofer is about 18 feet away. I changed the preamp distance setting for the subwoofer to 17 feet. The result is the pink curve below. Then I changed it to 16 feet (orange trace), then 15 feet (purple trace), and finally to 14 feet (turquoise trace at the top of the graph below). I was VERY surprised by this result - with the subwoofer distance set to 14 feet, the hole in the response graph from 60-100Hz completely disappeared! In fact, it raised the response from 25Hz all of the way to 130Hz! This provided nearly 15dB more output in the 70-80Hz range, and nearly 5dB of increase from 25-100Hz! Wow! What a difference this made! I was able to decrease the LFE output level, which kept the BFD from clipping the signal as much and now I get WAY more bass output! No more signal clipping in the BFD, no more amp clipping, and no more overloading the output of the sub! 

Finally, my preamp runs the main speakers full range. I experimented with the sound by switching back and forth between different low-pass crossover settings for the subwoofer (40Hz, 60Hz, 80Hz, 100Hz) - I greatly prefer the 60Hz setting. This adds plenty of oomph to the soundtrack without making it sound too overdone (this keeps voices out of the sub - except the ones that belong there!). Overall, I'm looking at strong response down into the single digits Hz range and is flat (plus or minus about 3dB) from 25Hz through about 125Hz. Below 25Hz, there is a 10dB room-induced bump at 10Hz. While the strong single digit response originally caused some concern for me, I don't worry about this since implementing the custom high-pass filter. The high amplitude signals in the 5-10Hz range (fairly common on BluRay action movies) provide plenty of room shake without endangering the safety of the drivers. 

One Last Tweak:

Take your sound pressure meter, set it to "C" weighting, "slow" response, and adjust your preamp until the output level of your subwoofer is 10-12dB higher than the output level of your main speakers (main speakers should be adjusted to about 75dB using the white noise generator in your preamp). Finally, adjust the subwoofer output level up or down to taste by listening to some favorite movies and music. You now have "properly calibrated" audio setup that will give the bass some kick to it! 

Overall, the graphs above confirm what my ears are already telling me - this is just one VERY impressive subwoofer! When the bass hits, it hits HARD, it hits FAST, and then its GONE! No "bloated" subwoofer boom here! Below is the individual frequency reponse plots for the IB sub (with a 60Hz HP filter in my preamp) and my Avro mains.

Three little letters: OMG!  With my sound pressure meter (which reads artificially low at lower frequencies), I can measure over 130dB (response mode set to slow/average, not fast) at the opening of each manifold and over 120dB at the primary listening position that is 17 feet away! Talk about bang-for-the-buck! For just about a grand, you can pick up four drivers, a hefty amp, and a decent parametric equalizer. If you are into building projects, the manifold can probably be constructed out of scraps of wood that are already stacked up in your garage... The only power tool you need is a router or a jig saw to cut the driver holes.

The sound is VERY clearly different from my (very capable) Sonotube sub. Like they say, you hear the bass, not the box! Its quite a difference! Bass is very clearly present, but doesn't have the "bloated" in-your-face sound sometimes associated with box-style subs (some of the cheap ones are known as "one-note-wonders") that are built around a set of compromises. It's challenging to adequately describe the performance of this remarkable subwoofer, but a quick audition of the final fight scene in Iron Man 2 on Blu-Ray clears things up right away! Visitors literally jump up off the couch when things really get going! 

Look Out! Duck! Run For Cover! Sorry, THERE IS NO PLACE TO HIDE! 

"But Isn't 120+dB Just Plain Overkill ?!" you ask...

Well, yes and no. Let's examine the situation more carefully - maybe we'll learn something along the way... There are many excellent sources of information on this topic. One that I found interesting is Art Ludwig's writeup on Music and the Human Ear.

Let's start with the "Yes" answer because we are talking about DIY audio and NO commercially available product could ever HOPE to even begin to approach this level of power output at such low frequencies (single digit Hz range). One notable exception to this rule is the rotary subwoofer, but at nearly $25,000 it is out of reach for most home theater enthusiasts... Overkill is one of the primary and most dearly held tenants of the DIY community - this is the point! We do it because we can!! Need more dB? Just add more drivers and a bigger amp! Finally, we do this for movie soundtracks, NOT FOR MUSIC! Sustaining this level of playback for music can cause permanent hearing damage inside of 10-15 minutes! Talk about a buzz kill...

More realistically - NO, this is NOT overkill. Let's start by recognizing that loud bass effects in movies tend to be brief in duration (transient peaks) - not sustained (long duration averages) like they are with music that is driven by bass guitars and drums. Thus, the associated risk of hearing damage from bass effects with movie playback is much smaller than with music. 

First, you need to understand a few things about the physiology of human hearing. As frequencies reach lower and lower (100Hz and below, in this case because we are talking about bass), our hearing becomes increasingly insensitive. Think about it this way: there just isn't much in our natural world that makes sounds this low that needs our attention on an ongoing basis. Notable exceptions to this include events such as earthquakes, volcanoes, lightning, waterfalls, and communication among whales, elephants, alligators. Sounds at 20Hz require greater sound pressure levels in order to reach our threshold of hearing than do higher frequency sounds. Follow the green (threshold of hearing) line in the image below on the left. While humans are capable of detecting a 1-2dB sound (provided there is no ambient noise) in the 4kHz-5kHz range (typically, the same frequency range as a baby's cry), we are essentially unaware of frequencies at 20Hz until they reach closer to 75dB of sound pressure. At 15Hz, over 92dB of sound pressure are required in order to be detected by the human ear. At 7-8Hz, more than 104dB are required for perception. That is a TREMENDOUS difference in our ability to hear low frequencies, especially when you consider that the dB scale used to measure sound pressure (like our hearing) is Logarithmic in nature - not linear! That 70dB difference really means that a just barely perceptable sound at 20Hz needs to bring approximately 3,150 times greater pressure to your ear drum than does a sound at 5kHz in order for us to just barely recognize its presence! Now, go back and read those last three sentences again so this concept sinks in - I'll wait...

Although the above picture looks bleak, there is a bit of benefit to be achieved depending on the placement of your sub in your listening room. When you locate your sub within close proximity of a room boundary (another wall, the floor, or in the corner) you typically pick up an additional 3dB per boundary. This means placement on/near the floor gives you an additional 3dB "for free" while placement in the corner can potentially gives you between 6-9dB of additional output (corner interaction effects can sometimes yield even greater levels of increased output). This may also lead to unusually large peaks in your frequency response, so you really need to experiment with placement of the sub in the room and measure the results to make comparisons. Overall, this effect is called "room gain" and is a function of the physical dimensions of your room, sub placement, and listening position choices.

Finally (as if things weren't bad enough already), each time you go one octave lower with bass frequencies (subwoofers generally produce sounds up to about 80Hz; thus, 80Hz to 40Hz is one octave, 40Hz to 20Hz is a second octave, 20Hz to 10Hz is a third octave), you need to DOUBLE the amplifier output power to produce the same sound pressure level (SPL) from your speakers. The main reason for this is the difficulty of achieving proper acoustic "coupling" of the drive unit and the air it radiates into. Typical loudspeakers have an efficiency of about 1% - that is, only 1% of their electrical input power is converted to acoustical output power. As frequency decreases, the "coupling" or efficiency of the driver decreases as well. This results in an increasingly poor patch between the air and the driver. For the sake of demonstration, let's say that you need 1w of audio output power in order to produce a 93dB tone at 80Hz. In order to produce that same 93dB level of sound pressure at 40Hz, you need 2w of amplifier power. At 20Hz, you need 4w. At 10Hz, you're looking at 8w of output power just to equal the sound pressure that 1w of power got you at 80Hz! This represents an 8-fold increase in amplifier power that is necessary to be able to span the frequency range from 80Hz down to 10Hz due to acoustic coupling losses!! It's a pretty sad reality in terms of power needs...

Now, let's put these three behaviors together:

1. Our hearing is exponentially insensitive to lower frequency sounds. 

2. Amplifiers need geometrically increasing amounts of output power in order to produce each additional 3dB of sound pressure. 

3. Subwoofers need geometrically increasing amounts of input power in order to re-produce each successively lower octave. 

These three physical phenomena combine (and not in a good way for our amplifiers) to require VERY SIGNIFICANT levels of output power in order to produce low bass frequencies (20Hz) that we even are capable of hearing, let alone being perceived of equal "loudness" to the other frequencies that we are hearing at the same time with the rest of the movie soundtrack. Now do all of this without simultaneously generating massive levels of audible distortion because your drivers are being driven beyond their design parameters. This is no small feat and is the reason why IB subs employ multiple large drivers driven by massive amplifiers.

This is also why commercial movie theaters (including THX and IMAX) have POSITIVELY NO HOPE OF EVER CREATING ANYTHING EVEN REMOTELY CLOSE to realistic levels of bass output... 

So, is having 1000+ watts to generate 120+dB of bass output overkill when watching action movies? 

NO WAY! Though, this much audio power is completely inappropriate for people that live in structures with attached neighbors (condos, apartments, townhomes, etc). You'll have the police at your door before you get to the second scene of your movie...

But we really haven't discussed the most fun part of having an IB sub yet: Things REALLY get fun when you start getting into single digits and low teens of Hz because at this point you can no longer HEAR the sound effects (provided there are no corresponding higher frequency harmonics or rattles in the room). You FEEL them instead! Sure, a movie is fun when you both hear and feel the effects, but how many of you have only felt the impact that had no associated audible component? Infrasonic bass (below 20Hz) is well known to produce human reactions of awe, fear, anxiety, uneasiness, and chills down the spine (yes, the 16Hz low note on the pipe organ in church is supposed to inspire awe among the parishioners). Low bass helps to set the tone for a movie, create tension for the plot, and increase the realism of the overall experience. A number of BluRay movies have great tactile sensations that you cannot hear: Thor, The Hulk, Transformers, the list goes on and on beyond movie titles that start with the letter "T." In the scene from Thor where Odin throws the hammer into the portal and it lands in the desert on Earth there is an intense pressure wave effect that literally sweeps from the front to the back of the room and is accompanied by only physical shaking. WHAT A THRILL! How to Train Your Dragon also has a superb soundtrack on Blu-Ray, especially the LFE channel! The next thing you'll need to do is get yourself some Fun Tack and walk around the house securing fragile, light-weight items before they crash to the floor and break...

Finally, do watch out for continuous exposure time, especially with music playback (though we've been talking movies here). The following chart is based on the outdated 1998 standard developed by the US Dept of Health and Human Services. The continuous dB levels in this chart were subsequently revised by United Kingdom regulators in the standard known as "The Control of Noise at Work Regulations 2005." Essentially, the UK revision reduces the dB levels by 5dB for each associated time period. It is important to note that this data refers to "A-weighted" noise measurements. Since the F10 point of the A-weighted scale occurs at about 250Hz, however, this scale does not account for bass content at all. By comparison, the F10 point for the C-weighted measurement scale is about 15Hz, thus even the UK-based 2005 revised time durations with reduced dB levels are somewhat optimistic (too long) where subwoofer exposure is concerned. This is likely because the standards apply to industrial noise levels that typically don't contain low bass content.

Conclusion:

THE OUTPUT LEVEL OF THIS SUBWOOFER IS SIMPLY FRIGHTENING!!! It lays waste to anything that comes its way, including visitors! It creates feelings of awe and sends chills down your spine... 

After getting things dialed in, I cued up the train arrival scene from the opening to The Polar Express. The preamp's volume control was set to -5dB (below reference level, subwoofer output is set at about 10dB above mains level) and my SPL meter was indicating 115dB+ when the train first arrived! (side note: at these low frequencies, my analog Radio Shack SPL meter actually reads LOW by 5-7dB and the amp still had headroom left...) I've never before experienced a movie soundtrack that made my HAIR actually MOVE from the bass . I didn't detect any signs of driver distress, though my wife told me that full pots were dancing across the stovetop upstairs in the kitchen. 

I have NEVER experienced bass like this before... Neither have ANY of the visitors to my home theater...

One of the steps I took when building my theater room was to run an additional wire for bass shakers to be mounted to the bottom of the couch. Its pretty clear that these are completely unnecessary - I don't think I would actually notice bass shakers at all with the IB sub running! There are a number of dominant players in the truly high-end subwoofer market (SVS, HSU, Epik, Rythmik, Seaton, JTR, JL-Audio, etc) where flagship subwoofer configurations can produce output levels of 120+dB and typically cost anywhere from $2,000 to $5,000 (and more!). Make no mistake, this is extremely impressive performance, but the price tag is equally extreme. For under $1,000, I achieved deeper and stronger bass by building my own Infinite Baffle subwoofer and didn't have to give up any additional floor space in my theater for the box! That's a Win-Win-Win-Win in the DIY column for those who are keeping track.

If you are looking to give your new subwoofer a workout, have a look at the list of Movies with Bass over on AVS Forums. Other great resources include Setting Up your Subwoofer 101, and Setting Up Your Home Theater Audio 101, also on AVS Forums. 

Epilogue:

The bass produced by virtually all commercial subwoofers and even the best movie theaters is COMPLETELY LAME!! Now you know why! There's not really much else to say except go build one if you have a room layout that allows IB (almost all rooms do, see example layouts here and here). Your listening experience will NEVER the be the same again...