FINALLY got my BoseBuster sequel done and written up!! I've shortened the name to BBV2 so I don't get any hassle from Bose. My Soma Sonus
site is now hosting my projects, and will have the most up-to-date version. Here is copy of that page:
A small format sub/satellite system
The goal for this project was a small, high quality speaker that was shallow enough to be mounted on wall next to the thin LCD TV’s that are now common. The Fountek FR88EX was perfect for this project, with its low distortion and ability to fit in a truly tiny box (you can get the cones really dancing with little change in character). After several tweeters, I finally settled on the SB Acoustics SB26STCN-C000-4. This tweeter not only measures well, but is subjectively a very clear and open sounding tweeter. Crossover point is 2.8khz @ LR4. Here is the on-axis frequency response:
With reverse polarity to check phase integration:
I did considerable testing of both the crossover point and notching out the breakup around 12khz of the mids. I found that I didn’t really need to do anything too special for the breakup of the mids, a nominal LR4 network was good enough. The tweeter sounded most well behaved at 2.8khz versus pushing much lower.
The frequency response measurements consist of a far field measurement with a 6.5ms gate (no smoothing!!), merged to a near field measurement at 300hz, resulting in a quasi-anechoic response. The low frequency ripple is the noise floor of my shop.
Here is the crossover schematic:
Inductors are 18g on the woofer and 20g on the tweeter. The small resistance value shown with each inductor is the resistance of the inductor itself, not a separate resistor.
I built them with a depth of 3 ½” (external dimension) for that low profile mounting. However, if I were to put these on stands, or even wall mount next to something other than a super thin TV, I would probably go with a 4 ½” depth at least. Not for the bass (these are sealed), but having the rear baffle too close to the woofer can interfere with the sound quality, and if possible it would be better to get some breathing space for the woofers. Speaking of breathing space, the backside of the front baffle must be chamfered slightly (I used a Dremel):
I prefer to line the inside of the box with the denim insulation from Home Depot if possible, or the .75lb/cf sheets from Meniscus Audio. Obviously with a speaker this small I went with an external crossover, with all three front channels in a small box with connections on the outside for speaker wire. Home Depot or Lowes has some nice 16g in-wall 4 conductor wire that is perfect for connecting the crossover box to the speakers. I used Techflex over the wire for a clean look.
There are many very good woofers to use for the subs. In my case I had bought some nice 100w plate amps with boost at 30hz when a speaker manufacturer was getting rid of old stock (the Dayton Audio SA100 would probably be another good candidate), so I wanted to use a reasonably efficient woofer since I didn’t have a huge amount of power, and one that would be the best combination of small box size and extension. Since these would cross around 175-250hz I didn’t really want something with a terrible cone breakup either that could show up as non-linear distortion in the passband. The Silverflute W20RC38-08 was a perfect match for my requirements. Using dual Dayton Audio 8” passive radiators enabled a truly tiny box of only 12”x12”x12 ¾”! Using eleven 3/8” washers on each passive radiator (for an effective 30hz box tuning) and the included 6db boost @ 35hz of the plate amp, enabled really good extension considering the size of the woofer and box. Of course any sub could be used instead of the Silverflutes, as long as they can play cleanly given the higher than normal crossover point. My sub integration walk through makes it easy to integrate other subs.
The following is how I initially integrated the sub and satellites. This is intended to be a quick way to get the system up and running. Obviously there is always a lot more one can do to get the smoothest bass and mid-bass response, and this is only one simple method.
First, I use two subs, one roughly at the foot of each main channel. Multiple subs in different locations is one of the best ways to get smooth bass response. In this case I tried REW as the measurement software since it’s free and easy to use. I use a mic from Cross Spectrum Labs. I didn’t bother with all the calibration of the measurement setup since we just want to see real-time in-room response for integration. Given that, it is pretty straightforward to setup: just open the RTA window, change the scale to go up to 2khz, 1/6 octave smoothing, select FFT Length 32768, Blackman-Harris 7 window, 8 averages, and that’s it. You might have to open the preferences and set the soundcard output if you get no sound, or the input if the mic doesn’t read anything. I aimed for a 175hz crossover, I’d like to experiment with 200-250hz in the future.
1) First set the levels of each channel (without subs on), I use a Radio Shack meter on A weighted Fast
2) in the AVR set the sub to “LFE+main” (this is on a Marantz, other brands should be similar)
3) set the sub crossover to 250hz
4) set the mains to small and crossover of 80hz
5) starting with just the left channel outputting pink noise (sub off), adjust the crossover until your -6dB point is at roughly 175hz:
My bass doesn’t appear to drop off quick, but that is due to the noise floor of my apartment being fairly high. Raising the signal will correct this, but I didn’t want to get my neighbors bent out shape!
6) Now running only the left sub, adjust its level to match the main. I like a little boost in my home theater bass though
7) Adjust the sub crossover until its -6dB is also 175hz:
8) Now turn on both the main and sub, and check integration:
9) Looks good so I moved on to the right channel
I was lucky and integration was very good right away. After integration I turned on both subs and the dip around 50hz filled in nicely and all I was left with was the hump at 40hz. A simple notch EQ here took care of that. Anyone who measures their setup knows how badly the front wall (often in combination with the floor and side wall) can really mess up the response in the 100-500hz area. One of the nice things about a sub/sat system crossed around 200hz is that you now have drivers with different crossover slopes/points, and in two different positions in the room, to smooth out the response. At this point I switched 1/24 octave smoothing to make sure I wasn’t smoothing over the ugly parts.
If one did not have good integration yet, you can adjust the phase on the sub, or adjust the delay setting in the AVR, or move the sub around, to get the proper phase match. Sometimes I find the response improvement to be ambiguous as I change settings. When that happens I like to run the main in reverse polarity, switch to 1/24 octave smoothing, and adjust delay/phase and sub placement to get the deepest null at the crossover. When you find it, you then switch the polarity back to normal and you should have excellent phase integration.