Hi all, saw something posted and thought I might be able to clarify a few concerns:
Quote:
I wish he would have done more testing on it, but his methods and my methods are so different, no one should expect our data to absolutely parallel anyway. Different PCs, different mics, different measuring distances, different input signals(for FR for example) and probably a lot of different setting in the Soundtechnology software itself.
|
If a test system is calibrated, then from a frequency response standpoint the type of mic, type of PC/soundcard/digitizer, software settings, and even test signal should not make a difference. I've done extensive sets of measurements in the past for FDA-approval of new Ultrasound systems, and have been "through the wringer" in terms of measurement types.
MLS (MLSSA, LAUD, many others), FM Slide/CHIRP (predominantly TEF), stepped sine (LMS), pink/white noise (RTAs), all will produce the same frequency response given the following:
- Data is properly truncated/windowed to eliminate effects of reflections
- System is properly calibrated.
I've spoken at length with Dr. Johnson in the past, and believe he is properly windowing the data, to eliminate reflections. A good examination of the impulse response shown at his site will show that the data is windowed correctly; there isn't any extra reflections in the data. As is shown in his review of the B4+, it's clean to 60 msec, which would indicate good data acquisition down to 16 Hz.
If there is a discrepancy between measurements, then both measurements should be called into question, until one or the other can be verified as accurate. Given that the only measurements posted have been from Dr. Johnson, and personally knowing how he captures and analyzes his data, I'd say his results are most likely quite accurate.
As far as MLS accuracy showing 12-13 dB of drop at 20 Hz, this is not an artifact of the measurements; rather, this is showing the true anechoic measurement of the system. In-room will of course have room gain, flattening the curve. Properly windowed, MLS is as accurate as any other test signal, from a frequency response standpoint. The difference is the environment where the measurements and the claimed performance are taken.
On another note:
Quote:
Did anyone ever figure out what was up with the frequency response curve which is posted? The maximum output numbers cited are also more in line with a 90-92dB/1W/1m sensitivity. Were you able to duplicate these measurements? I am somewhat curious if there were any proximity issues in measuring at 1m off the side.
|
My guess is that the environment being discussed is different. I suspect the B4+ may have the claimed sensitivity when in-room, where 8-12 dB of gain is expected down below 50 Hz. I know Mark is typically working in "largish" rooms (like 4,000 seat theaters), so that free-air sensitivity is truly anechoic. An anechoic efficiency of 90 dB @ 1W, 1m would fall in line with Dr. Johnson's findings of 122 dB SPL @ 1500W (~32 dB of power gain, meaning a 1W output of ~90 dB SPL).
I personally would be VERY surprised to see an efficiency of 100 dB SPL @ 1W, 1m anechoic for the B4 at any frequency, anechoic. If it does perform that way, then SVS has truly shattered the limitations of Hoffman's Iron Law, and should immediately patent when they are doing. An efficiency of 100 dB @ 1W, 1m is 6%, which is extremely high, especially in the bass range. Even large bass horns have a difficult time reaching that kind of sensitivity down in the sub 50 Hz range, free-air. For example, the LAB Subwoofer, a large (32 cubic foot - nearly 3X the size of the B4+) horn subwoofer drops below 100 dB efficiency at 100 Hz. At 30 Hz, still within the cutoff of the horn (and thus benefitting from the loading and directivity gains of the horn), the LAB Subwoofer is down to 95 dB @ 1W, 1m anechoic.
My guess is the difference in claimed efficiency is in-room versus anechoic. I know we've seen upwards of 95 dB SPL @ 1W, 1m efficiency for the Sadhara in our home theater room (2300 cubic feet), but anechoic measurements in our smaller 77,000 cubic foot warehouse (a bit more than 2100 cubic meters) show a true efficiency around 84 dB @ 1W, 1m.
Dan Wiggins
Adire Audio