Introduction to Computer Music: Volume One

14. How do we perceive pitch? | page 2

Further psychoacoustic phenomena related to pitch perception

In the range of 20 to 2000 Hz, the ear has the ability to fuse harmonically-related frequencies into a single entity (i.e., a single note with a particular timbre) with a fundamental frequency, even if the fundamental is missing.

When we listen to a violin section, we are hopefully hearing numerous instruments playing approximately the same pitch, with some very slight differences in frequency (called chorusing) that give the sound a depth and richness beyond what a single instrument would produce. As mentioned above, a slight difference in frequency will lead to the phenomenon of beating. We perceive the mistuned notes as a single chorused pitch up to the limit of discrimination, a difference of approximately 10 to 15 Hz, beyond which we hear two separate tones. At the very point of such a perceptual separation lies an area of tonal roughness.

While a single pitch may maximally stimulate a specific spot on the basilar membrane, it also stimulates some adjacent hair cells as well. These lie within what is called the critical band. Other pitches that are close in frequency may also share some hair cells in common, which is theorized to cause intervallically close tones to sound more complex than more widely separated tones. The intervallic width of the critical band varies with register, being a large percentage of the frequencies of two low tones, and a smaller percentage of the frequencies in a higher register. (It is about a minor third above A440.) This may account for our orchestrational penchant for using wider intervals in lower registers. Another applicable phenomenon is that when two sounds of equal loudness are close in pitch, thereby in the critical band, their combined loudness will be only slightly greater than one of them alone.

Our perception of pitch is also affected by the duration and intensity of a sound. Sustained sounds above 2 kHz may be perceived as rising in pitch as their intensity increases, whereas sounds below 2 kHz may be perceived as dropping in pitch. I might theorize that this phenomenon contributes to the intonation difficulties all high school band or choral conductors experience with changes in dynamics.

In preparing this article it became clear from the existing research that the complete and exact mechanism for our extraordinary pitch resolution and discrimination is not completely understood and is still being investigated. The placement theory coupled with the critical band does not completely explain our ability to detect the minute frequency changes we are capable of.


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For further study, see Hyperphysics->Hearing

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