SB Acoustics Reference Speakers
We set forth to create a Reference quality speaker system for our audio club, with design plans available to the public. We had a design task force consisting of members Chuck B., Don M., Keith L. and Ron E. This group was involved in setting goals and coming up with the original design goals and concept look.
Modular design - Mid/high speaker with integral woofer stands.
Reasonably high sensitivity (90+dB at 1m/2.83V)
Large enough size combined to be full range and with enough output to work well in the club environment (optimum tweeter height ~40", and a room size of roughly 1500 square feet)
MTM upper cabinet that could be used as standalone for home theater applications or small rooms. The design is flexible and the cabinet may be either sealed or vented of various volumes depending on bass needs. In a sealed box, these woofers reach ~80 Hz, and vented they will reach ~50. Cabinet width is the only "critical" dimension, builders may vary depth (and to a lesser extent height) to meet their specific requirements. The dual woofer bass cabinet with extension to 30 Hz will make a truly full range system if desired.
We ended up specifying the drivers below from SB Acoustics, a relative newcomer to the market with designers from Denmark and manufacturing facilities in Indonesia. Links to our supplier are inline, and permanent links to the data sheets are in the files section at the bottom of this page. At current prices, the total driver cost for two speakers is approximately $730 dollars not including shipping.
On delivery, the driver build quality was perceived as excellent, especially for the price. The woofers have attractive cast metal frames designed for recessed mounting. The only negative impression noted was that the brand stickers on the back of the magnets seemed to peel off at low temperatures (i.e. when stored in a garage in Minnesota) Once warmed up, the stickers would re-adhere if pressed down. It is recommended to remove them to avoid any potential rustling noises or blocking of the pole vents.
Tweeter: SB29RDC-C000-4 - Cost ~$55 each.
This is SB Acoustics' famed dimple-dome tweeter. It combines high sensitivity (~93dB) with a very extended (beyond 40kHz) frequency response. Resonance is a low ~600Hz
Midwoofer: 2@ SB15NRX30-8 Cost ~$55 each.
5" paper cone driver with a cast aluminum frame, generous pole and under spider venting and very low inductance thanks to a copper cap on the pole piece.
Woofer: 2@ SB23NRXS45-8 - Cost: ~$100 each
8" paper cone driver with a cast aluminum frame, generous pole and under spider venting and very low inductance thanks to a copper ring on the pole piece.
Measurements (Ron E)
Initial measurements were made on a folding test baffle designed for the project, based roughly on the IEC baffle dimensions but in a more easily constructed size. The 14x14 test baffles are custom made to flush mount each driver and stored on the back of the larger baffle. Each smaller test baffle may also be screwed onto a test box for measuring the woofer parameters. A dimensional drawing of this "ASM Baffle" is in the files at the bottom of the page. These initial measurements on a test baffle were virtually identical to the published frequency response characteristics of the drivers, so in the interest of brevity they will not be included here..Enclosure design (Ron E)For the prototype, it was decided to do a vented system for each of the boxes to maximize bass extension. Builders of the full system would not need to construct the upper section as vented. Thiele - small parameters were measured for the drivers after a period of break in and were found to be a bit different from the SB Acoustics specifications, but calculations indicated that the main difference was in the parameter Cms, suspension compliance, which was lower. This causes resonance frequency and Q to increase, and Vas to decrease, and this is often observed if the measurement temperature is lower than normal, and also when parameters are measured at lower signal levels than when measured by the factory. Thankfully, even rather large variations in Cms have very little effect on the final low frequency response.Since these speakers were designed to be used in the club system, they would be powered by our Audio Research M300 monoblocks and D115 Stereo amp. These tube amps have output impedances of approximately 0.4 and 1.0 ohms, respectively. This has a direct effect on the Thiele Small parameters, reducing damping (raising Q) and requiring a larger box size. ....more details coming....Cabinet construction (Keith L)
Cabinets were designed for manufacturability and constructed on a CNC router by Keith from Larson Construction. Keith put solid wood inserts on the sides of the baffle and routed the large 1.5" round-overs by hand! Just the starting torque of that bit would be enough to wrench the router from most mere mortals' hands ;) The cabinets are heavily braced and very solid. The rear arc shaped bit on the upper cabinets proved convenient for storing the crossover boards during the tweaking stages.Finishing (Don M)Don fashioned beautifully figured solid wood top and bottom pieces from Bubinga and added rollers and a handle to simplify moving these 154lb behemoths. He also came up with a simple yet attractive paint scheme to dress up the prototype cabinets.Crossover Design (Ron E)Measurements were performed on the drivers in the finished enclosure prior to crossover design. Nearfield measurements were corrected for baffle diffraction and then spliced to the farfield data. Measurements were made using several software measurement tools. The most expedient method was to make acoustic measurements using Soundeasy (Not recommended) or HolmImpulse (free) and import these into Speaker Workshop (free, but dated) to do the crossover design and simulation. The crossover design, simulation and optimization tools in Speaker Workshop (SW) have (in my opinion) the best workflow of any of the crossover packages I have tried. Sadly, the acoustic measurement tools in SW are a bit less reliable due to a strange windowing arrangement that baffles me (I know, that was a groaner of a pun.) The impedance measurements have been repeatable and reliable (if a bit noisy at low frequencies on my hardware) as long as care is taken in setting up and calibrating a jig. SB15 woofers: this is the sum of both woofers on axis - note the diffraction bump just above 1kHz and the woofer breakup above 5kHz. Also of note is the flat impedance, which simplifies the lowpass crossover. This impedance measurement is with ports loosely stuffed with rolled-up socks.SB29 tweeters: note baffle-related dip around 3kHz and generally extended response.
The MTM speakers were designed as a two way with a passive crossover. After several design iterations, the flattest sum was attained with a classic 3rd order electro-acoustic crossover. The filter was implemented electrically with a Cauer-type filter on the midranges (a second order filter with a stop band null to tame the midrange breakup peaks) and a second order filter with L-pad on the tweeters
Best crossover frequency was determined to be ~1600Hz based on both crossover summation and directivity concerns. Since this is lower than many DIYers would cross over, some simple "enclosure" calculations were made using manufacturer specs and the tweeter is thermally rather than displacement limited with this crossover. It should still be able to meet its 100 watt manufacturer "system" rating. Listening tests at a very loud level in a large room revealed no distortion problems in use.
Measured response of MTM - actual sensitivity is approximately 91dB/2.83V/1m - the measurement is 1/12 octave smoothed. The response of the SB15 mid is exceptionally flat. The y-axis scale is actually a bit narrower than I see used in most measurements, so even the tweeter response is quite flat, and if averaged over a small window it would appear even flatter.
Measured impedance - a demanding 4 ohm load.
For best performance, the lower crossover point should be between 150 and 200Hz. It was originally planned to cross over between the upper and lower cabinets passively. Due to the ~40 ohm impedance peaks of the SB15 drivers, which have a rather high Qms, a passive crossover in the ~150Hz area would have required an expensive (>$130 per side for 7.5mH inductance and 400-600uFcapacitance if high quality parts are used) impedance compensation circuit. It was deemed less expensive, not to mention more practical and higher performance to use an extra set of amplifiers (which we already owned) and an active circuit to implement this crossover.
Details of SB23 woofer performance and sample frequency response are forthcoming.....
These units were auditioned for the club in October 2013 using other club DIY amplification (PassDIY.com BA-3 preamp and F5 amplifier built by Don M.) The woofer cabinets were driven by a small Audiosource Amp 100 and an active crossover arrangement,just to show that expensive amplification is not required.
The finished speaker system has an authority and ease of presentation, along with the dynamics and impact one would expect with a high(ish) sensitivity setup of high performance drivers. Particularly of note is the rather impressive attack on dynamic passages, and the even horizontal dispersion that allowed both speakers to be clearly heard even relatively far off axis. The bass was quite punchy and dramatic (rattling ceiling tiles when appropriate) although purists might sneer at the TDA7294 chips used in the Audiosource amps ;).