Audio and Acoustics Reference Data

Acoustics is the science of sound, and audio engineering applies it to the recording, measurement and reproduction of sound. Sound is a pressure wave travelling through air, and its intensity is most often expressed using sound pressure level (SPL), a logarithmic measure stated in decibels relative to a standard reference pressure. The logarithmic decibel scale is convenient because human hearing spans an enormous range of intensities, and because perceived loudness changes roughly in proportion to the logarithm of the pressure.

The range of human hearing is conventionally taken to span roughly 20 hertz to 20 kilohertz, though sensitivity falls off at the extremes and varies between individuals. The ear is not equally sensitive at all frequencies: it is most responsive in the midrange around speech frequencies and less so at very low and very high pitches. This frequency-dependent sensitivity is captured by equal-loudness contours, historically known as the Fletcher–Munson curves, which trace the sound pressure levels that listeners judge to be equally loud across the audio spectrum. Loudness level is expressed in phons, defined with reference to these curves.

Reference tables collect the practical figures used in audio work, including typical sound pressure levels of common sources, the audio frequency ranges of instruments and voices, and weighting curves used in measurement. Another familiar chart is the DTMF (dual-tone multi-frequency) table, which lists the pairs of tones assigned to telephone keypad buttons, each key sounding one tone from a low-frequency group and one from a high-frequency group so that any pressed key produces a unique, decodable combination.

Frequently asked questions

What is sound pressure level measured in?

It is measured in decibels relative to a standard reference pressure. The decibel is a logarithmic ratio, which compresses the very wide range of audible intensities into manageable numbers.

What are the Fletcher-Munson curves?

They are equal-loudness contours showing the sound pressure levels at different frequencies that are perceived as equally loud. They illustrate that the ear is less sensitive to very low and very high frequencies.

How does DTMF dialing work?

Each telephone keypad button emits two simultaneous tones, one from a low group and one from a high group. The unique pair identifies which key was pressed.