A7S-450 Measurements and Analysis
Measurements were performed using the Sample Champion program developed by Paolo Guidorzi of Purebits. The program is a Maximum Length Sequence (MLS) program that generates an impulse response in the time domain from this pseudorandom signal. Fast Fourier Transformation is applied to the impulse response to generate the system frequency response. The operating principle is the same as that of the well known MLSSA program.
Test samples were taken using 16 bit resolution with a 48 kHz sampling frequency. Fast Fourier Transformations were performed at 64K allowing for a frequency resolution of .73 Hz, which is more refined than even 1/24th
octave resolution at the lowest octave; the A0 key on the piano has a fundamental frequency of 27.5 Hz.
All measurements of the driver were taken nearfield at the approximate acoustic center of the driver and referenced to a simultaneous, independent SPL measurement taken at 1 meter from the subwoofer. For these measurements, the low pass filter and the 25Hz bass boost were defeated.
Figure 1: Nearfield Impulse Response
Figure 1 shows the impulse response of the A7S-450 subwoofer measured nearfield at an SPL of 80 dB at 1 meter. This fairly typical response curve for a bass driver shows the A7S-450 damping out the impulse input quickly within two cycles of free vibration.
Figure 2: Nearfield Frequency Response
Figure 2 shows the nearfield frequency response of the A7S-450 at an SPL of 80 dB at 1 meter. The frequency response curve shows peak output at approximately 60 Hz with tapering of output below 50 Hz at approximately 12 dB/octave. No inflection point indicating significant roll off to the bass response occurs in the subwoofer’s response, which has a smooth roll off to 10 Hz. The specified 22 Hz appears to be dependent on the presence of either room boundary effects or utilization of the 25 Hz bass boost and perhaps a combination of both.
Figure 3: Nearfield Frequency Response at Various SPL
The curves in Figure 3 are overlays of the nearfield frequency response of the A7S-450 measured at various SPL. The green curve was measured at a peak SPL of 80 dB at 1 meter with the red curve at 90 dB, the blue curve at 100 dB SPL, and the purple curve at 104 dB, all measured at 1 meter from the subwoofer.
From the identical shape to the plotted curves at various SPL, it is clear that the sub had more to give before suffering the ill effects of compression even if my computer soundcard did not. Based on my experience with the sub, I would anticipate at least 3-5 dB more output is possible from the A7S-450 without any significant sonic troubles.
