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Question about sound below an amps frequency specs

if an amps frequency response are spec'd out to be between 20hz to 20khz, will it pass information above and below those numbers?For instance will it pass a 15hz tone or will any information below 20 hz be cut off by some internal filters. If an A/v receiver has the same frequency specs in their manual, will it also cut off info above or below its stated specs? Could info going through its LFE be cut off below 20hz as well. The reason I ask is when I was playing a Realtraps bass test CD I downloaded, my speakers had no info below 20hz, though the disc starts with 10hz tones. Just for fun I played the same disc using my tube amp and had significant bass through my mains well below 20hz. Which made me wonder.
shiva
jeffreybehr's avatar
jeffreybehr
805 posts
 
I believe that few amplifiers intentionally limit frequency response anywhere near the audible range. A rule of thumb I heard years ago was that any high-pass filter had to be at least 2 octaves below the lowest point one wants to be audible. If one uses 20Hz for that lowest point, than the HIGHEST-frequency filter should be no higher than 5Hz. I think that most filter points, and these filters, generally, are 'single-order' ( = 6dB per octave) filters composed of a capacitor in series with the signal, loaded with a resistor to ground, are far below 5 Hz. I've calculated the filter points of the 2 original coupling caps in my Audio Refinements Pre 5 preamp at 0.07Hz and 0.3Hz.

Gs5556 is correct in that the FR specs of an amp have a tolerance of so many deciBels. That's usually quite small at the extremes of the audio band...say one-half dB or less. The -3dB point (the accepted definition of 'filter point') would be WAY below that. So virtually any amp will pass frequencies WAY below its published specs, and the lower the frequency goes, the less energy it has.

Ar_t: "...those numbers are 3 dB down, and one octave away would be 6 dB down. Two octaves would be 12 dB down." Not quite. A single-order filter reduces energy by 6dB per octave. So the energy at the filter point is -3dB or half power. One octave further down, the energy is NINE dB down (nine equaling 3 plus 6), and one more octave down, the energy is 15dB down, and so on.
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eldartford
4,262 posts
 
Jeffreybehr...The typical "rumble filter" (every one I ever had) was third order, 18 db/octive with 18 HZ as the -3 dB point. Such a filter would have only a couple of dB effect at 20 Hz, and most loudspeakers require more than a few dB boost to get to 20 Hz.
ar_t
165 posts
 
NO...........

Filters are spec'ed for "-3 dB point". Using a straightline approximation, the response at the "-3 dB point" is really 0 dB.

We do things like that to confuse the non-technical crowd. Devious nerds, us engineer types are.

Back to the topic at hand......

FR specs that don't give a +/- X dB number, are assumed to be -3 dB points. Amp FR spec's usually give a value of the 20 Hz and 20 kHz points.

There is no way a typical amp has a -3 dB point of 20 kHz on the top end. Typical amp has -3 dB point on the top of between 100 and 200 kHz. At a point 5 times lower than the "-3 dB point", the response is down only 0.2 dB. Which would be around 20-40 kHz in this example.
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