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Old 16th October 2012, 09:02 AM   #1
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Question White Noise and Amplification Wattage (newbie question)

I tried to ask a question about how much power is required to create an equal amount of dB over the entire 20-20,000Hz spectrum. In other words, when only one watt is needed for 1kHz, the math says 500 watts are needed for 20Hz.

But, I realize my mistake in asking. It all depends on the program material and crossover/drivers.

However, to simplify the problem, would White Noise reveal how many watts at any spectral frequency would be required for equal acoustic production (assuming perfect drivers and human Fletcher/Munson hearing).

I know this is asking a lot but there must be an equation for the amount of wattage required to acoustically produce one frequency versus another. I am thinking of a reverse exponential scale or something like that with respect to frequency from 20,000 down to 20Hz.

Perhaps I am completely misunderstanding this. But, if I put 10 watts into a pure tone of 1kHz and get xxdB at one meter, how many watts would it take to get xxdB (the same) at 100Hz at one meter given perfect drivers?

The gist of it is that I fully expect that when the frequency reaches 1.0Hz (only 1Hz) and then even lower, the wattage should be off the scale.

I just cannot believe that no amplifier engineer has not figured this out and just said "well, its power consumption seems OK to me from 160Hz down to 20Hz." There must have been more curiosity than that!

BTW, this whole question came into mind when I started to send to my 150Wrms/ch amp ONLY frequencies above 160Hz. Therefore, I was curious to know how much wattage I was saving that would be available for "peaks" in the frequency ranges that I did tell it to amplify.

Thanks.
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Old 16th October 2012, 05:02 PM   #2
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Default Hmm....

You seem to have confused a lot of issues there. Start with the concept of SPL (sound pressure level). Wiki it, Google or read it in a book . Note that it is proportional to the total audio amplifier power when using an ideal loudspeaker, right across the audio bandwidth, whether you use single tones, voice, music, wideband noise or whatever.

The matter of human perception and Fletcher Munson sensitivity curves is not part of this consideration since all measurements are recorded with instruments scaled in SPL units (dB). It is actually much simpler to think of amplifier power as directly proportional to acoustic power, though of course, our hearing is anything but linear, narrower band, varies widely between individuals and can be more than 10 times more sensitive at upper midband frequencies than with bass or treble. We don't try to counter nature by equalizing frequencies so as to give a theoretical flat perception for our ears though, as we are kind of used to compensating for our weaknesses from a very young age and really don't like people messing with that.
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Old 16th October 2012, 05:41 PM   #3
sreten is offline sreten  United Kingdom
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Location: Brighton UK
Hi,

You need pink noise (equal power per octave) rather than white noise
(equal power per Hz). You can use F&M curves to see that 90dB of
1/3 octave pink noise at 100Hz takes a lot more power than it
does at 1KHz.

However for Hifi's 1KHz can peak much higher than 100Hz.

Regarding your question, depending on the programme material
a high passed amplifier @ 160Hz probably has about 3dB to 10dB
extra available for peaks, its very spectrum and dynamics related.

Around 6dB is good average approximation for full range material,
the higher the bass content (you have removed) the higher the
headroom above 160Hz you create compared to no high pass.

However to use that you have to crank up the bass power which
isn't really practically on. 160W to below 160Hz and say 80W
to above 160Hz is more useable for most, most of the time.

rgds, sreten.
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