
Illusions of Sound Perception - DavidSJ
http://sethares.engr.wisc.edu/htmlRT/4%20sound_illusions.html
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Stenzel
There is no illusion of pitch, and it is a common misconception that the
fundamental frequency must be present in a tone. Pitch is the perceived
periodicity of a tone, which is roughly the greatest common divisor of the
harmonics. If perceived pitch without fundamental is considered an auditory
illusion with, common pitch detection techniques should fail if the
fundamental is not present, but they work quite well in the absence of the
fundamental. So either there is no illusion of pitch or algorithms have
illusions too.

~~~
DoctorOetker
the answer is: algorithms have illusions too.

consider 2 ideal harmonic notes with a frequency ratio of 3:2, say 3kHz and
2kHz ... The brain / algorithm must doubt between interpreting the collection
of frequency peaks at m * 3kHz, and n * 2kHz as either (occasionally
overlapping) harmonics of 2 notes at 2kHz and 3kHz, OR it could interpret this
as harmonics of a single note at 1kHz (as you say the GCD of the frequencies).

There is inherent ambiguity between interpreting as 2 notes of each a timbre,
vs interpreting as 1 note with another timbre...

One could physically construct 3 bowed strings with modekilling on the 1kHz
string, such that these could make perceptually identical sounds whether the
2kHz and 3kHz strings are played simultaneously vs the 1kHz string.

at that point from the sound alone one can not discern in an ABX test which is
the case, neither a human brain nor any algorithm. The doubt forces to guess
(deterministically or not).

The sound is a projection of properties occuring in reality, and loses
information.

~~~
Stenzel
True, but ambiguity does not imply that one possible interpretation must
necessarily be an illusion.

~~~
AstralStorm
Tones and harmonics get clustered into pitches, e.g. mistuned harmonics as
seen in bass guitar or piano still get decoded into pitches via some sort of
best match if the mistuning does not exceed certain percentage. And it works
even if some harmonics disappear and reappear.

The pitch is higher level than purely perceptual.

~~~
DoctorOetker
this is correct, and the reason we are tolerant is because of dispersion: even
though the different harmonics are present on the same string of the same
length, the resonant frequencies don't need to be integer multiples of the
fundamantal since waves of different frequency have different propagation
speeds on the string.

in the case of bowed strings mode-locking ensures the phases of all the
harmmonics are reset each cycle (the bow sticks and slips), so that bowed
instruments can be played harmonically to parts per billion.

since a lot of sounds are plucking we must be tolerant for frequency dependent
propagations speeds in regular strings / media

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dr_dshiv
The Shepard tone illusion, of an ever rising pitch, is used in the movie
Dunkirk

[https://www.businessinsider.com/dunkirk-music-christopher-
no...](https://www.businessinsider.com/dunkirk-music-christopher-nolan-hans-
zimmer-2017-7)

~~~
rrss
And the dark knight films. Apparently Nolan is fond of it.

~~~
dawnerd
And rightly so. It's incredibly effective at building suspense. Hans Zimmer's
incredible scoring also helps.

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sp332
Here is a video I enjoyed that explains the basics of how our sense of sound
works. It makes it easier to understand why some of the illusions happen.
[https://vimeo.com/147902575](https://vimeo.com/147902575)

~~~
holy_city
Typo in the video (I think, not an anatomist), it's "basilar" membrane, not
"vasilar." Awesome video though, I wish my speech processing professor had
used that instead of teaching hearing like a filterbank, even if that is how
we needed to understand it.

Another weird thing about hearing: the hairs that vibrate aren't just tuned to
particular frequencies, they actually vibrate over a range, and the response
isn't symmetric (although iirc, part of that is from the fact the hairs are
mechanically coupled). That's why low frequency noise masks high frequency
noise more than vice versa, which is exploited in lossy codecs (if there's low
frequency energy, you don't need high frequency energy that it masks).

~~~
sp332
That correction is in the video description, so yeah.

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tony
For more:
[https://en.wikipedia.org/wiki/Auditory_scene_analysis](https://en.wikipedia.org/wiki/Auditory_scene_analysis)

This book _Auditory Scene Analysis: The Perceptual Organization of Sound_ by
Albert S. Bregman has more.

More foundational info on how we "fill in" information we see / hear
[https://en.wikipedia.org/wiki/Gestalt_psychology#Properties](https://en.wikipedia.org/wiki/Gestalt_psychology#Properties)

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carapace
See also "sine-wave speech".

[http://www.scholarpedia.org/article/Sine-
wave_speech](http://www.scholarpedia.org/article/Sine-wave_speech)

Oooo! Don't miss the _acoustic chimera_ on that page...

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anotheryou
That was just one :/

I give you another one related one for rhythm:
[https://youtu.be/oQf_tS5WAP4](https://youtu.be/oQf_tS5WAP4)

