Psycho-acoustics – A Very Human Perspective of Sound

Your ear is an amazing piece of engineering. It converts vibrations in the air into meaningful perception. As sound travels along the auditory pathways it is converted into three different forms of energy (mechanical, hydraulic and neuro-chemical) before it is actually heard.

By physical definition, sound is a repeating molecular disturbance that propagates in all directions. Faster pulse rates are interpreted as high pitch, while slower pulse rates are interpreted as low pitch. Sound is the seed of hearing and signifies movement in our environment.  The relationships between the physical properties of sound in our environment, and the perceptual experience of sound in the mind, are complex and difficult to describe.

The science of psycho-acoustics attempts to study and define the interactions between physical sound and our perception of sound. By very definition, psycho-acoustic relationships rely on subjective judgements, and two people  can easily perceive the same physical phenomenon in different ways. While the language of physics may be exact, the language of perception is subjective. Melding the two is a challenging task and often requires complex formulas.

A bridge between the physical properties and human perception of sound is needed to conduct even the most routine of hearing tests. The audiogram plots an individual’s thresholds of hearing along a frequency (pitch) axis. The audiogram is calibrated in dBHL (Decibels Hearing Level) to display results in a way which is more easily grasped in terms of human experience. 0 dBHL describes the average lower limit of volume perception at a given pitch for young adults with good hearing. 0 dBHL should not be mistaken for signifying no sound in the environment, but simply describes that the sound pressure level is at the threshold of human detection. Sounds well below 0 dBHL do exist, and can be described with another unit of measure called dBSPL (Decibels Sound Pressure Level). dBSPL is not referenced to human perception, rather it uses a physical unit of measure called the pascal to quantify the amplitude of sound in terms of disturbances in atmospheric pressure.

It would be cumbersome to present test results in dBSPL, as the ear’s sensitivity to amplitude varies according to frequency or “pitch”. Equal loudness contours map out the human response to changes in amplitude  across the frequency spectrum audible to human ears (20-20,000Hz). Sounds in the 2-5kHz range are detectable to human ears at much lower dBSPL than adjacent frequencies, partly due to the resonances of the ear canal and the amplifying effects of the middle ear bones. Conversely, sounds below 10Hz and above 10kHz require substantially more dBSPL to be heard by humans.

dBSPL is therefore re-scaled into dBHL for simplicity of presentation and to allow an individual’s hearing thresholds to be quickly and easily compared to normative data.