Why You Hear What You Hear


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Supplements for Chapter 22


Sound files:

From Auditory Demonstrations CD (see discussion here):

Does pitch depend on loudness?:
Loudness scaling
The decibel scale
Dependence Of Pitch On Intensity
Asymmetry of masking by pulsed tones
Backward and forward masking

This PDF file from the University of the West Indies Department of Computer and Electrical Engineering gives a very nice summary of masking, a subject we only touch on in Why You Hear What You Hear, in connection with critical bands. Much of the phenomenon of masking can be guessed by asking whether sounds have overlapping critical bands, but there is much more worth knowing.

Smartphone decibel meters

iPhone and Android and other smartphones have free or very cheap decibel meter applications. Most use the A-weighting; some, like the iPhone app SoundMeter by Faber Acoustical, are switchable. Many users testify in the case of some apps that they compare very closely to expensive dedicated decibel meters. In any case you have your phone with you most of the time, and violations of noise levels occur sometimes when you don't expect it.

Noise levels from industirial side effects to deliberate roadway or backroad noise makers have consistently increased, in violation of local and state ordinances and usually with no enforcement consequence. For instance, the Massachusetts constitution was amended some years ago to read "people shall have the right to clean air and water, freedom from excessive and unnecessary noise, and the natural, scenic, historic, and esthetic qualities of their environment; and...". The law states that no new noise source shall stand more than 10 dB above the background noise level for the area. Neither the consitiutional amendment nor the 10 dB rule seem to be enforced. Get out your dB meter and hold your local officials accountable! Noise contributes to stress, reduced sleep, raises blood pressure in some people, etc. etc.

decibel level meter for iPhone

A dB meter "Sound Level Meter" from Cateater for the iPhone


The Wikipedia article on this topic is unusually comprehensive and informative.


Project: Pitch-Loudness Illusion

Perceived pitch can depend on loudness. This effect differs from person to person, and can switch whether louder means a higher or lower perceived pitch depending on the frequency of the note. The effect is not large, but it is noticable. To get you started, here is a pair of notes of the same mathematical pitch - do they sound exactly the same pitch to you? Using any of several software packages that you are familiar with, you can make your own trials - for you and perhaps some friends. How does perceived pitch depend on loudness AND on (1) whether the "note" is high or low frequency, and (2) the number of partials in the notes (e.g. a pure sine tone vs. a sawtooth of the same period).

Here is an idea that may or may not ever have been investigated: What about residue pitches, as in "missing fundamentals" or the autocorrelation pitch of a chime tone - are these more sensitive to loudness, less sensitive, or the same?