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|23rd April 2008, 09:35 PM||#1|
Join Date: Mar 2008
My First Speaker Project
After several months of fooling around with different drivers in old AR cabinets, I decided to make my own speakers, ones that are optimized to the drivers I wound up with. These drivers are: ScanSpeak 25W/8565-00 10" diameter woofer, Seas CA12RCY 4.5" Mid, and Seas 27TFFC tweeter. The woofer has a high Qts and huge Vas, so I knew I was headed for a big box. I chose an acoustic suspension approach after much modeling with WinISD. My goal was a smooth Q=0.70 roll-off and 35 Hz Fc appeared achievable.
Using WinISD, the box volume is 120 liters for the woofer and 1.2 liters for the midrange internal enclosure. I made the boxes out of 0.75" MDF (double thickness front baffle) using a table saw and a sabre saw. Boxnotes provided the cutting list and assembly instructions. The box was glued with Titebond II and sealed with latex elastomer. I used Acoustastuff for damping. The outside was painted black and finished with three coats of clear acrylic. Each one weighs 88 lbs (40 Kg). Here's some photos.
The speakers are tri-amped. Crossover frequencies are 400Hz and 2.5KHz, although I've moved the mid/tweeter xover around a bit. The sound is very neutral and pleasing, far better than I expected.
I've made some measurements and am interested in comments:
1. Stuffing - Initially, I stuffed the boxes very heavily, about 2.5 lbs of Acoustastuff in each box. I measured impedance vs. frequency and found that the impedance at resonance (30Hz) was 48 ohms, lower than the 60 ohms that WinISD predicted for a low absorption enclosure. I removed most of the stuffing (down to 0.5 lbs) and stretched the fibers out to increase the volume. Impedance went up to 63 ohms at 33Hz resonance, as WinISD predicted. But I really didn't hear much difference in the sound. I am confused about how much stuffing to use.
2. Frequency response. I constructed a microphone out of a Panasonic electret element and used a spare preamplifier to achieve a line level signal. I wrote software in LabVIEW that grabbed this signal from my external soundcard and filtered it through my homebrew LabVIEW digital 6th order filter. You've seen the FR in the attached jpegs. This measurement was made at about 8 feet away from the midpoint between the speakers (separated by 6 feet) at midrange height in the room seen in the photo with pink noise input. The elevated low frequency is from some bass boost in the preamp. I am concerned about the sharp roll-off above 10KHz. Is it the electret FR? I've used Radio Shack and Sony elements and gotten worse results. The Panasonic spec sheet is ruler flat. I have measured the soundcard, preamp, and CD player combination with white noise input and gotten flat +/-1dB response from 20Hz to 20KHz, verifying those compomnents and the software.
Can it be the tweeter? I get the same roll-off with the mic right in front of the tweeter, 6" away. I've not substituted a different driver. Can it be the mic?
I do not have very good hearing above 10KHz, but my "golden ears" friend loves the high end and says that there is plenty of sound energy out beyond 10KHz.
These conundrums notwithstanding, I love the sound of these speakers. I am truly humbled.
Thanks for reading. Please comment. I have a lot to learn and am eager for discussion.
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