At the 2007 Prosound Shootout, our noise floor was 70-75dB. This limited our distortion resolution to about 1%, depending on the SPL of the box tested. If a cabinet produced 110dB, then the lowest distortion we could measure was 1%, at 115dB, 0.5% could be reliably detected. But if only 90dB, then the lowest level distortion you could see was 10%. So whatever SPL level you see on the response curve for any given box, that sets the resolution of distortion that can be measured at that particular frequency. As a result, the resolution is greatest in the passband, less at the lower frequency limits.
All measurements were taken with the microphone 10 meters away. We could have increased resolution of our distortion measurements by moving the microphone closer. This would increase the signal-to-noise ratio, and would have allowed us to see distortion at lower power levels, like 25 watts. But this was a Prosound Sub Shootout, not a single-ended triode shootout. We weren't interested in what these speakers do at 25 watts. All the subs we measured would do pretty well at such small signal levels. There's no challenge in that.
The information we gathered at the Prosound Shootout tells us what the subs would do at power levels they will be used at, at least a hundred watts, up to several hundred watts to the maximum rating of each sub. Even the best subs generate double-digit distortion figures at low frequencies and high power levels. Most subs generate triple-digit distortion figures, with distortion higher than the fundamental at the lowest frequencies. So even with resolution dropping at the lowest frequencies because of the reduced SPL (signal-to-noise), we still could see distortion at high power levels at sub-30Hz frequencies on all boxes but one. It isn't difficult to draw a conclusion from that.
Tom Danley wrote on Mon, 05 November 2007 13:58 |
For example at 50Hz, its about 10%, at 30Hz, one has about 15 dB between them (about %17)...
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That's right. At 1600 watts, the 12Pi measurements show about 10% distortion and maybe a smidge more at 30Hz, still less than 20%.
Tom Danley wrote on Mon, 05 November 2007 13:58 |
...and at 20Hz its about 4dB or about 40%.
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No, the distortion at 20Hz is below the noise floor and so we can't reliably determine what it is. The noise floor was 70-75dB, and output of both the fundamental and harmonics is falling below 30Hz, so distortion becomes indeterminate at that frequency because it is below the noise floor. My guess is distortion at 20Hz is about 20%, maybe 30% at 1600 watts. Even if it is 40%, just under the noise floor at that frequency, it would still be impressive. Most hornsubs are well into triple-digit distortion when run that far below cutoff at full power.
Look at that squarely, Tom. We're talking about a 12Pi basshorn sub generating 30% or maybe 40% distortion at 20Hz at 1600 watts. What we've seen from the Michigan sub shootout is that a group of
four LABhorns generates 30% distortion at 20Hz with
only 25 watts input. That means even by your own characterization of the data, the 12Pi basshorn subwoofer running full tilt at 1600 watts produces no more distortion than a LABhorn sent a mere 25 watts.
At 1600 watts, distortion of a LABhorn at 20Hz will be far higher than the fundamental. Sorry, I misspoke. At 1600 watts, output from a LABhorn will be zero after about the first half of the sweep because the voice coil will have burned open. But at 800 watts where it can survive the sweep, the harmonics will be far louder than the fundamental at 20Hz.
What would be really nice is to see a LABhorn measured at 100 watts, 200 watts, 400 watts and 800 watts. Knowing what it does at 25 watts tells us very little, particularly in terms of distortion, power compression, electro-mechanical parameter shifts and the ripples in response caused by parameter shifts at high temperatures / power levels.
Tom Danley wrote on Mon, 05 November 2007 13:58 |
Actually I have never built any or measured any myself.
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