LXC

The LXC (The "C" is for Cardioid) is a low cost prototype of Siegfried Linkwitz's LXmini (About $96 for all the drivers and $105 for an MiniDSP Active Crossover). I created this speaker using only the publicly available information from linkwitzlab.com that he generously shares to advance the state of the art in sound reproduction. I'm not publishing any propriety information that Siegfried Linkwitz includes with his plans. As such, this is not meant to be an exact duplicate of his design but only a lower cost speaker to evaluate the concept and experiment.

If you like the sound, I would recommend purchasing the LXmini plans as I'm guessing the LXmini sounds even better.

Drivers

• Tweeter - 3.5” Vifa Paper Cone Full Range Driver Vifa TC9FD18-08 ($9)

• Woofer - 6" Dayton RS-150-4 - Aluminum Cone ($39)

You can find the measurement for these drivers and for the drivers used in the LXmini at zaphaudio.com. The woofers are on the 5.5 in driver comparison page and both the full-range drivers are on his blog. All the drivers for LXC and LXmini are excellent performers.

Parts List

Here is a parts list for a pair of speakers (not including 4 channels of amplification needed) along with the source (PE is parts-express.com) and the approximate price:

Note: You can purchase the MiniDSP from Madisound.com as well as the Vifa Full Range driver.

Construction

I choose the height of my speakers so the center of the full-range driver would be at my ear level when I 'm seated. After listening to the speakers, it doesn't make a big difference in the height (most likely due to the low crossover point and large sweet spot). If you like, you can change the height by a couple inches without making much of a difference. I cut the 4" woofer pipe 30 3/4" long. The total height of the speaker is 39 1/2" tall.

The base is 8" at the front and 6" at the back and is 11" deep. I drilled a 1 1/4" hole where the wires come out so they would not be crimped). The center of the pipe is 4" back from the front. I rounded off the corners with a belt sander after cutting out the base.

To attach the pipe to the base, I cut a 4" MDF disc and then sanded it to fit snugly inside the pipe. I used 100% silicone to attach it flush with the bottom of the 4" PVC pipe. I waited a day to allow the the silicone to cure.

I then drilled holes in the disc for the wires and put silicone around them to create an airtight seal. I attached the pipe to the base with 3 drywall screws driven through pilot holes from the bottom.

Woofer Mount

The woofer mount is a teardrop shaped piece of MDF (5/8" thick). The outside diameter is 6 3/8" and the woofer cutout is 4 7/8". The rear is 2" diameter with the total length being 9 1/2". Holes for the full-range mount are at 1" and 2 1/2" from the back.

Full-Range Driver Pipe

The full-range driver pipe/absorber is made of 3" Schedule 40 PVC pipe. The rear of the tube is cut at a 10 degree angle to make it more stream-lined but you could leave it straight like the LXmini if you like.

The full-range PVC pipe is connected to the woofer base with 1 3/8" long pieces of 1/4" aluminum tubing with pan head machine screws inside. I put the nuts inside the 3" pipe since they will be hidden by the acoustic-stuff. If desired, you can hide the speaker wires to the full-range drivers by running them through the machine screws using crimp on connectors at each end of the machine screws.

I hand sanded the PVC tubes and just left it unpainted. You could also paint the PVC special spray paint that is intended for plastic. I painted all the wood parts with flat black spray paint.

Painting Tip: To get a smooth finish when painting MDF, smear a thin coat of wood-filler (I use Elmers) over all the cut edges. Let it dry and sand smooth. This will prevent the spray paint from being absorbed into the wood. Otherwise, you will need many coats of paint. You can also use a 50/50 mix of wood glue/water but I find that wood filler is easier to sand.

Assembly

I heavily stuffed 4" woofer pipe with Acousta-stuff. I packed it more dense at the bottom and a little looser at the top (I used approximately 1/4 pound of stuffing). If you don't have a kitchen scale, just take the bag of 1 lb. bag Acousta-stuff and divide it into fourths. The pipe creates a infinite transmission line, absorbing most of back wave of the woofer. Measuring with an the impulse response at 1/2 in from the woofer cone, I measured about 30 db of attenuation from the source impulse to the reflected impulse (The LXmini measured even better at about 40 db. This may be due to the fact that the RS-150 has a phase plug that may leak more sound, different stuffing, or difference in measurement technique). At 30 db down, it's much better than a typical box speaker.

I drilled the pilot holes in the woofer mount (make sure you use a backer board so the MDF does not split) and I painted it with 2 to 3 coats of black spray paint and let it dry. I then used 100% silicone to attach the rubber coupler to the woofer mount, using my finger and some rubbing alcohol to get a smoother finish.

I used a pipe clamp to secure the woofer so the bottom of the coupler was 1 1/2" down form the top of the tube (mark 3 spots 1 1/2" down with a pencil to get it level). I placed a piece of black electrical tape over the chrome pipe clamp to give it a cleaner look. I used speaker gasket tape and #6 screws to mount the woofer.

The full-range driver is mounted with three black #6 x 3/4" course threaded screws that have their points filed flat (driver mounting screws). To make it easy to file the screws, I drilled them into a piece of 1/2" thick scrap wood until the head stuck out and filled them down by about 1/16 of an inch. The screw holes are 60 degree from each other and 3/16" back from the front (hint: You can use a 1/2" strip of paper to mark off the 1/3 the circumference to evenly space the screws). I put a 1/2" in piece of speaker gasket tape on the magnet of the driver where each screw touches. You could also use a small dab of silicone at each point.

I heavily stuffed the full-range pipe with Acousta-stuff. I also added a thin scrap Sonic Barrier (1/2 in thick but cut down to 1/4 in thick) so you don't see the Acousta-stuff from the rear giving it a more finished look. The Sonic Barrier I used had an adhesive back so I attached it to a scrap board and used a serrated knife cut off the adhesive. I then traced the size of the opening and used utility scissors to trim it to snugly fit the back of the full-range pipe. I don't think it makes a noticeable difference in the sound but it's easy to try it with and without.

Crossover

Here are settings for the "2 way + Sub (2.1) crossover" plug-in. Make sure you have the latest plug-in version 1.08 (you can sign into the MiniDSP.com site and download the latest) since with other versions of the software that I had there was bug with the way low-shelf filters were handled.

You can download the MiniDSP Configuration file with these settings at the bottom of the page.

Woofer Crossover

I used a LR12 at 700 Hz for the woofer. This is the same as used in the LXmini. I also added a high pass filter to prevent over excursion of the woofer since it's really pushing the limits of a 6" driver - you can bypass if you like but the 6" driver is going to be past it's limit on bass heavy music (Just try Holly Cole's "Train Song" or Lorde's "Royal" for an example of low bass). Also if you are using this with a subwoofer you can change the high pass filter to where you want to cross over to the subwoofer (e.g. LR 12 at 100Hz).

Low Pass Filter: Cut Off Frequency: 700 Hz, Type: LR 12dB/oct

High Pass Filter: Cut Off Frequency: 37 Hz, Type: BW 48db/oct

Full-range Crossover

Same as LXMini that uses LR12 at 700Hz for a wider overlap (Woofer driver is inverted in the Output Channel since a LR12 is 180 degrees out of phase) and more gradual transition from omni-polar to cardioid.

Low Pass Filter: Bypass

High Pass Filter: Cut Off Frequency: 700 Hz, Type: LR 12dB/oct

Woofer Parametric EQ

The woofer needs a little extra boost in the low end since it roll off around 100 Hz. I'm using a peak filter like in the LXmini instead of a Linkwitz Transform you would normally use on a sealed woofer. It doesn't even atempt to get output from the woofer below around 40 Hz. The F3 is around 45Hz with this EQ.

EQ1: Filter Type: Peak, Frequency: 50 Hz, Gain: 6.2 db, Q: 0.9

This cuts a rise in the response that is relatively close to the 700 Hz crossover frequency and was showing up in my measurements. I suspect this is either a small bump in the response of the woofer or baffle step gain from teardrop woofer mount and also the 4" pipe that reinforcing the wave. This bump does not show up as prominent when I measure 1/2" from the woofer cone. I don't think LXmini includes this EQ and you can bypass it and see what you like better.

EQ2: Filter Type: Peak, Frequency: 1054 Hz, Gain: -4 db, Q: 1.4

Full-range Parametric EQ

Compensates for the dipole roll off of 6 db per octave. I EQ'ed the driver so it was flat down to about 500 Hz.

EQ1: Filter Type: Low-Shelf, Frequency: 1288 Hz, Gain: 15 db, Q: 0.7

Cuts out for the dipole hump as a result of the back wave reinforces the front wave.

EQ2: Filter Type: Peak, Frequency: 2800 Hz, Gain: -5.7 db, Q: 2.9

Remove a peak at 8900 Hz in the full range driver. I seems like this peak might be causing some sibilance on vocals so I cut it a little bit. This is optional and can be bypassed if you like.

EQ3: Filter Type: Peak, Frequency: 8900 Hz, Gain: -2.3 db, Q: 5.8

Woofer Output Channel

Polarity of the woofer is inverted since with a LR12 crossover the woofer is 180 degrees out of phase.

Gain: 0 db, Delay: 0 ms, Polarity: Inverse

Full-range Output Channel

Full range driver is dropped 5 db to match the level of the woofer. Full range is also delayed by 0.10 ms (1 1/3 inch) to align the acoustic centers of the drivers. This delay achieved the deepest reverse null when measured outdoors.

Gain: -5 db, Delay: 0.10 ms, Polarity: Normal

Parametric EQ

The HRTF (Head Related Transfer Function) filter compensates for the difference of the phantom image that appears to be directly in front of you but is actually being reproduced by stereo speakers at approximately 30 degrees angle from your ears and therefore is too bright as a result of your head is not blocking the higher frequencies. You can read more about in this Frequency Response Requirements (PDF) . I have tried it with and without this filter and I think it sounds more natural with the filter on. Try it both ways and see what you like. This is the setting that I used that I think closely match the curve on page 12 version 3.2 of the PDF.

Filter Type: High-Shelf, Frequency: 2242 Hz, Gain: -3.2 db, Q: 0.7

Note: All drivers are wired with normal polarity (Polarity is inverted in the crossover)

Here is the complete crossover for the woofer and full-range measured from the outputs of the MiniDSP:

Measurements

Frequency Response (On Axis - Indoor Combined)

Combined woofer response measured at 1/2 inch up to 500 Hz and then the total response from 12 inch away with 1/6th octave smoothing. The HRTF eq was bypassed for this measurement to achieve a flat response.

Polar Response

Indoor, 1 meter, 0 - 180 degrees at 15 deg intervals, 1/6 octave smoothing. These measurements were done before I fixed the dip in the 1.2 to 2.2 KHz area . Also, Ignore frequencies below 1 KHz since this was done indoors.

Orange - 0 Degrees

Purple - 15 Degrees

Green - 30 Degrees

Yellow (higher one) - 180 degrees (directly behind speaker)

Response with HRTF Eq

Indoor, 12in, 1/6 octave smoothing. Orange without HRTF. Purple and Green are left and right speakers with HRTF function enabled.

Frequency Response (On Axis - Outdoor)

Outdoor measurement, 1 meter on axis , gated measurement (no smoothing). HRTF is on. Ignore response below the gating frequency of 480 Hz. Full ranger driver was about 6 ft off the ground. The small rising response from 10 - 18 Khz is from the Sure 4x100 digital amp. It does not show up on my NAD amp that I used for indoor measurements.

SPL

The maximum SPL at 40 Hz is limited by the excursion of the 6" Dayton driver. The LXC has a maximum SPL of 85 db at 40Hz. The Seas driver used in the LXmini should be about 5db louder (90 db) since it has a larger surface area and Xmax. Here is a comparison chart calculated with SpeakerBoxDesignerV1 spreadsheet:

When used with a subwoofer the Seas and Dayton (crossed at around 100-140 Hz with a LR12) have the same maximum SPL since they are limited by power input and not cone excursion:

The Seas driver can handle more power (80 Watts continuous) but is less efficient. The Dayton driver is more efficient but can only handle about 40 Watts of continuous power. The maximum continuous SPL for both is about 103 db at 1 meter when you get above 150 Hz.

Speaker Placement

The placement of these speakers is much more flexible than many of the other speakers I have built. They sound good placed next to the TV and just 1.5 feet from the front wall but they really excel when placed in an equilateral triangle at least 3 ft from the walls. They are very light and easy to move around so I typically move them out to the optimal position when I'm going to listen to music and leave them by the TV for home theater, gaming, and casual music.

Listening Impressions

Imaging is excellent and the speakers disappear in the room with the very wide and deep auditory scene. The sweet spot is huge and seems like it about 1 1/2 foot square (more than twice of most my other designs). The perceived seating position is closer than with a dipole speaker (more like in the front few rows at a concert) and is more similar to a box speaker. It also sounds much better than a omni-polar or dipole speaker when placed close to a wall and as a result is much more flexible in room placement. You can also turn your head almost 90 degrees to each side and the phantom image stays in the same location. It also seems to be much less dependent on listening height since the drivers are so close together and integrate so well. With well-recorded music these speaker can create a plausible acoustic scene that if you close your eyes, you can imagine you are at the live performance whether it be outdoors, in a large concert hall or a small room.

This may be my favorite speaker so far as it sounds excellent with both acoustic music and all other varieties of music (Acoustic, Jazz, Classical, Alt. Rock, Rock, Folk, Choir, Country, Pop, etc.). It also makes an excellent home theater speaker (left/right channel) since is can be place relatively close to the wall/TV and still sound good. It sounds very natural with voices depth is place well into the scene.

Why do they sound so good?

• All the off axis frequency responses (> 30 degrees) are very similar (good polar response)

• Floor and ceiling reflections are minimized as a result of radiation pattern

• Less front wall reflections so you hear more direct sound than with a dipole

• Very little internal box reflections (woofer use an infinite transmission line, full-range has a rear absorber instead of a box)

• Very Low diffraction

  • Full-range has almost no diffraction since front/back wave cancel each other at midrange frequencies and beams at higher frequencies.

  • Woofer has a very small upward facing baffle and low crossover frequency so sound wraps around instead of diffracting.

• 2-way design has one low crossover point with the acoustic centers of the drivers very close to one another (and time aligned) allowing for the drivers to integrate fully

• Shallow crossover for low group delay

• Very low structural resonances since the woofer is isolated with a rubber mount. The woofer pipe is very rigid and has minimal vibration.

• Low Distortion, High Quality, Linear Drivers

• DSP Active Crossover

  • This crossover is not practical to create with passive components

    • Accurate and consistent results due to digital signal processing

    • Each driver is connected directly to amp increasing the damping factor

    • Amplifiers are isolated so drivers do not interact with each other

  • No crossover changes due to power compression (drivers getting hot)

Cons

• Omni-polar bass is not quite as tight as dipole bass in my room. Dipole speakers excites less room modes (I have a smaller 4 way dipole design that I have not posted yet that does have a little better base from 40 - 160 Hz). But, the bass is still excellent.

• No Ultra-Low Bass - Unless used with a subwoofer. The LXC sounds surprisingly large and for most acoustic music there is no need for a subwoofer but I do miss the visceral feel of a good sealed subwoofer with a Linkwitz transform for some types of music.

  • Ultra-High Treble Drops Off - Frequency response drops off at 18 KHz. This is outside of my hearing range as it is for most people past their 20's.

  • Loudness - They do get plenty loud for reasonable listening levels (9-12 ft away in an equilateral triangle) but they are not intended to fill an auditorium.

Amplifier

Most of the time I use a NAD T753 AV Receiver that has direct inputs to the amplifier and has about 90 watts per channel into 8 ohms. You don't really need more than about 40 watts into 4 ohms since this will max out the 6" Dayton woofer but having some headroom is always nice. I'm experimenting with other less expensive digital amps and they do sound ok but are not the caliber of the NAD.