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Bass extension
and power handling - how much ?
In an average listening room,
with normal music, we typically listen to music at about 87 dB average
levels. This is fairly loud, but not uncomfortably so.
The average loudspeaker sensitivity
is 87dB/W, therefore the amplifier is typically producing about
1 watt.
Peaks however can be up to
20dB higher than the average level, so then the amplifier is producing
100W for the speaker to make those107dB peaks.
Woofer excursion increases
4 fold for every halving of frequency. Studies show that for music,
the peak spectral content at 40Hz is approximately 10dB below the
overall peak levels. Therefore, the bass power handling needs to
be 107-10 = 97dB at 40Hz at which point the amp will be at 10W.
Ideally we'd want the bass driver to be operating within +/- 1/2
Xmax excursion at this level to keep harmonic distortion and compression
in check.
The 10" Scanspeak 26w8861,
which has an Xmax of 9mm, is able to produce 96dB at 40Hz within
1/2 Xmax with the amp at 16 watts. When at Xmax at 40Hz, it will
be producing 6dB more, or 102dB, with the amp at 60W.
So it appears that a good long
throw 10" woofer with a 100W amplifier is nicely adequate for
normal music listening. Greater low frequency and extension requires
much larger cabinets or much more more amplifier power, both of
which go against my goal for a compact and simple system. For movies,
which have greater low frequency content below 40Hz, the driver
will exceed Xmax/2 and therefore distortion will increase.However,
I think a bit more distortion during sound effects is acceptable.
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