Volume

Professional Definition

Loudness is a characteristic of auditory perception that allows for the ordering of sounds on a scale from quiet to loud.

It is important to clearly distinguish loudness (a psychoacoustic quantity) from sound pressure level (a physical quantity expressed in decibels). Loudness depends not only on pressure but significantly on the frequency of the sound, as well as its duration and spectral character.

This relationship is described by equal loudness curves, which show that the human ear is much less sensitive to very low and very high tones than to mid-range tones (the range of human speech, around 3–4 kHz). The unit of loudness level is phon, while loudness as perception is measured in son.

Acoustics in Simple Words

The best analogy is the difference between air temperature and feels-like temperature.

A thermometer might read -5 degrees Celsius – that is a physical, objective measurement, equivalent to decibels. However, if there is a strong wind, your body feels as if it is -15 degrees. This is your subjective feeling, which is equivalent to loudness.

In acoustics, the "wind" that changes our perceptions is the frequency of the sound:

• Deep bass at a level of 50 decibels may be barely audible to you and seem very quiet.

• Baby crying (mid and high tones) at the same 50 decibels will seem very loud, piercing, and irritating.

Loudness is simply your brain's response to the question: "How strong does this sound seem to me?", rather than a dry result that a microphone of a meter shows.

Summary

• Impression, not measurement: Loudness is a subjective feeling, not pure physics.

• Influence of frequency: Our hearing is programmed to perceive the human voice best, while ignoring part of the energy from low bass.

• Units: By using phons and sons, acousticians try to "measure human impression", which allows for the design of better hearing aids, sound systems, or quieter household appliances.