I thought 87 db was on the higher end these days,especially for bookshelves. Seems like the majority are 85 or even less. Would they really require that much power?
I was surprised also. My first thought was that I had misinterpreted the oscilloscope scale and the voltage was off by a factor of ten, but that would make no sense because then I would be content with a 5W amplifier. I should have emphasized that these transients are very short, typically 10 to 50 mS and much shorter than the 1000 mS of an SPL meter which would smooth them out substantially. Except for classical music and jazz, recordings are typically mixed with significant volume compression (see
https://musicproductionnerds.com/compression-settings-vocals for example) and you wouldn't see as high a dynamic range in pop music as I measured with classical.
I have another test in mind. REW will let me generate a sinewave with a level shown as dB full scale and I can measure the voltage on the speaker terminal when I play it with my normal volume setting. Obviously a properly mastered track will come close to digital full scale at some point so the amplifier needs to be able to handle that level. This approach would be less subjective.
I finally did the test with the REW generator. I set the system gain to a comfortable listening level (80 dB most of the time with peaks visible at 90 dB), just using the Radio Shack SPL meter to give a rough idea of how loud I generally play music. I measured pk-pk voltage at the speaker. At 1 kHz, 40 dB below full scale in REW gave 1 V pk-pk. 20 dB gave 10 V (exactly as you would expect). So full scale would need 100 V pk-pk, very close to the 126 V limit of the 500W amplifier. This is not unreasonable, looking at the chart in
https://en.wikipedia.org/wiki/Headroom_(audio_signal_processing). At this point, I'm convinced that I really do need this much brute power capability.
This won't be true for most people. It's mostly because the Magnepan speakers are not very efficient. I think that a couple of hundred watts per channel is usually adequate.
It bugged me that the Mackie sounded better to me than other amplifiers I tried - I wouldn't have expected it. A borrowed oscilloscope really helped figure out why.
I think I'll start looking for a cheap oscilloscope!
