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PSYCHOACOUSTICS

This field of research looks at the relationship between physical acoustic stimuli and the sensations perceived by a listener. A simple example is our ability to distinguish variations in sound pressure and intensity as being loud or soft sounds.

 

There are limitations to human hearing, which shapes the way we interpret the propagation of vibrating air. These limitations are caused by both the ear and the brain, and are a result of evolutionary development. Similarly, other animals interpret sound differently to humans.


 

Sound pressure and loudness

 

Like all of our senses, human hearing is a subjective phenomenon, and our anatomomical construction and cerebral processes shape the way sound is heard. Loudness is this perception of sound pressure and the auditory sensation that permits us to be able to discern loud from soft.

Expressing sound pressure level; LSPL is the SPL of a stimulus, p is the pressure of stimulus in Pa, p0 is the reference pressure of a tone with a frequency of about 2 kHz, I is sound intensity of the stimulus, and I0 is the reference’s intensity.

Not all frequencies are equal to our ears; some frequencies require more sound energy in order for those sounds to be perceived. This non-linear perception of sound has been studied and mapped into equal-loudness contours.

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Fig. 1  The Fletcher-Munson, or Equal Loudness Contours, illustrate that we are most sensitive to sounds around 4kHz; for example, a 73 dB SPL sine wave at 4kHz needs to physically be 80 dB SPL at 400Hz to be perceived to be of equal loudness.

It is important to differentiate between the physical properties of sound like sound pressure (measured in decibels) and subjective quantifications like loudness. Weighting functions like the commonly used “A-weighting” help account for these differences and compensate the measurements to correspond more accurately to how someone would typically perceive loudness.
 

Auditory Masking

 

Perception of a sound isn’t isolated according to its source, but rather in conjunction with the frequency and intensity of other co-occurring sounds (both simultaneous and nonsimultaneous).  Auditory masking is the psychoacoustic phenomenon that occurs when a sound is made inaudible by the presence of another louder sound. Consider when you’re more likely to hear your phone ringing: in bed while reading a book, or while mowing the lawn?

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Fig. 2  Visualization of simultaneous auditory masking, where the dotted line represents an equal loudness curve, ie. the hearing threshold in silence.

Auditory masking raises the threshold of perceivable loudness of a sound according to its sound pressure level and the span of frequencies. In Fig. 2, four hypothetical sounds with varying fundamental frequencies are produced. S0 (at 0.5kHz) now raises the hearing threshold above the equal loudness contour around its bandwidth, creating a “mask” which obfuscates the quieter sounds that fall under.

The critical bandwidth of a sound typically spans from around its fundamental frequency through to its harmonic frequencies higher in the spectrum, tapering off. Because S1 lies in the critical bandwidth of the masker S0, it is more probable that it won’t be perceived by a listener than S3, although it physically possesses a greater sound pressure level value.

 

Select research and further reading:

 

Buss, E. & Hall, J.W. III. (2011). Effects of non-simultaneous masking on the binaural masking level difference. Journal of the Acoustic Society of America 29(2): 907–919

Rosen, S., Faulkner, A. & Smith, D.A.J. (1990) The Psychoacoustics of Profound Hearing Impairment. Acta Otolaryngol 499: 16-92

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