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Not sure I'm in the right StackExchange group for this question, but it seems linked enough. Let me know if it's not the case.

I heard about an acoustic and/or signal processing technique that allows small speakers to kind of fake low bass sound waves, and that would allegedly be used by most good quality bluetooth speakers nowadays.

I wonder:

  • what is this technique?
  • do the sound waves it produces have the same quality and nature as the ones that could natively be produced by huge speakers?
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    $\begingroup$ If there were such a thing as an acoustics stackexchange, this would be a good question to ask there. There are some tricks that can be played acoustically to make a speaker act bigger than it is (Bose was active in this area back in the 1990's, I think, and they still seem to be the best ones at it). Certainly the bass response of teeny speakers got a lot better after Bose showed that it could be done -- but that's about all I know about it. $\endgroup$
    – TimWescott
    Jul 20, 2022 at 15:43

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The acoustic efficiency of a small speaker drops drastically with frequency. The standard way of dealing with this is to simply boost the bass through a linear EQ. However that's limited by the physical constraints of the speaker: power, driver excursion, port noise, etc.

There used to be techniques to create "fake" bass. That done by band-splitting the signal and running the low frequency band through a non-linear distortion, high-passing the result and adding it back the high frequency band.

Example: let's say you have a speaker that goes down to 90 Hz and you feed it a 50 Hz sine wave. If you don't do anything, the 50 Hz sine wave is just gone. The algorithm would run the 50Hz through a non-linearity and crete harmonics at 100Hz, 150Hz, 200Hz, etc and the harmonics would be played back. So at least something comes out and it could be identified as a 50Hz note since the spacing between the harmonics is still 50 Hz

do the sound waves it produces have the same quality and nature as the ones that could natively be produced by huge speakers?

Not at all. It sounds pretty bad.

would allegedly be used by most good quality bluetooth speakers nowadays.

Nope. Good quality and "fake bass" do not go together well.

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Perhaps it was an usage of the missing fundamental phenomenon? The basic idea is that if you pick some frequency $f$, even if your signal contains just some of its overtones $(f_1, f_2, f_3, ...)$, but not $f$ itself, your brain will still "listen" your signal as if $f$ were a part of it.

I'm not sure how common it's to exploit it in order to enhance bass frequencies for playback in small speakers, but the same wikipedia article I linked points out there's been at least one patent in the late 80s which proposed synthesizing such overtones in order to induce the perception of low frequencies, as well as an audio plugin implementation.

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