Loudness

The subjective impression of the intensity or magnitude of a sound. It is dependent on frequency, waveform and duration, as well as sound intensity or sound pressure. It is expressed quantitatively in units of sones and phons for sine tones or narrow band noise, and in terms of Perceived Noise Level (PNdB) for broad band environmental sounds.

Even for fairly simple sounds composed of sinusoidal components (called complex tones), the measurement of loudness is very difficult; however, the Stevens method of loudness summation is perhaps the most useful. See Appendix F.

The masking effect that one tone has on another, particularly one of higher frequency, making it harder to hear, complicates most cases, although for sinusoids, this is less serious when heard in rooms rather than with headphones. The presence of beats between closely spaced components of two sounds tends to enhance their loudness. However, when the components lie within a critical bandwidth, the total loudness is the same as a single sine tone with equal SPL. For tones further apart than a critical band, the loudness contributions of each component (in sones) will add together.

The loudness of short sounds will tend to fall off below a duration of half a second. That is, to sound equally loud as longer sounds, short ones must be more intense. See: Click.

Exposure to constant tones results in a decrease in their apparent loudness, described as adaptation of the ear. Constant exposure to moderate or intense noise levels leads to a temporary threshold shift, which is experienced as a loss of sensitivity after the stimulus is removed.

The brain cannot fully react to extremely short, intense impact sounds or those which contain impulses of shorter duration than the brain's averaging time (35 ms). Therefore, although these sounds are not heard as loudly as their energy content would suggest, the full impact of the sound reaches the inner ear with consequent danger of hearing loss. See: Damage-Risk Criteria.