Question about sound below an amps frequency specs

In order to eliminate LF "noise" such as air conditioning rumble on the recording, or TT rumble when playing a LP, many electronic units do have a sharp filter below 20 Hz. The idea is that this signal wastes amplifier power, and drives loudspeaker cones in and out, which you can't hear but which causes distortion of higher frequencies that you can hear.

In general I think this is a good idea. However, I have one CD of a Wurlitzer theatre organ that has a continuous non-musical LF rumble at and below 20 Hz (per my spectrum analyser). I have heard the actual organ, which is in a pizza restaurant in Mesa, Arizona, and this sound comes from the wind generation machinery. If you filtered it out you might like the sound better but it wouldn't be like the real thing.

Most solid state amps will "pass" frequencies below 20Hz, but most speakers will not reproduce such low notes. My guess is that the tube amp sent 2nd and or 3rd harmonics to the speakers, and that is what you heard.

Will it pass info outside of those ranges?

Yes. How much........figure that each one represents a first order filter in the amp.

IOW........those numbers are 3 dB down, and one octave away would be 6 dB down. Two octaves would be 12 dB down.

Playing low frequency tones usually results in tons of distortion, and that is very likely what you heard.

Most amps roll off gradually below 20Hz and above 20kHz, so unless the amp employs steep filters to purposely roll off the response then you should still have some output at 10Hz.

That said, I don't believe many humans can actually hear bass tones below 20Hz. Such low notes are felt more than heard. If the output from the speakers is high enough, objects in your room will start shaking, and walls, doors, and windows may vibrate loudly.

Most speakers will not produce significant output below 20Hz, and any bass you actually heard was probably an upper harmonic distortion (and not the fundamental test tone) -- especially if you heard bass while playing a 10Hz test tone and the room was not shaking...

Unless there is a reason to limit the bandwidth of the amplifer, the low frequencies will not be intentionally filtered just outside the published specs.

The frequency response of an amplifier is specified with a dB range. Your amp may be 20-20k +/- 1 db (or whatever). Any frequencies over or below simply will exceed the 1 db range.

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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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.

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.