Acoustics Basics | Explanations of Those "Obscure" Acoustic Terms

Architectural acoustics contains many seemingly profound and difficult-to-understand technical terms. Today, let's unveil the mystery of these "secret words" together!

01

Decibels (dB)

The decibel, named after Alexander Graham Bell, the inventor of the telephone, is a measure of the relative intensity of sound. Its calculation follows a logarithmic ratio, rather than a simple linear addition. For example, two speakers each emitting 60dB of sound will combine to produce 63dB, not 120dB. This unique calculation method makes the decibel particularly precise in describing sound intensity.

02

Reverberation Time (RT60)

Reverberation time refers to the time required for the sound intensity to decrease to one millionth of its original intensity (i.e., a reduction of 60 dB) after the sound source stops emitting sound in an enclosed space. It is one of the important indicators for measuring the acoustic quality of a room. In venues such as concert halls and theaters, appropriate reverberation time can enhance the fullness and depth of music; while in venues such as recording studios and classrooms where clear speech is required, it is necessary to reduce reverberation to improve speech intelligibility.

03

Sound Field

A sound field is the area covered by sound propagating in space, and it is the result of the interaction between sound waves and a medium. Physical quantities in a sound field (such as sound pressure) change with spatial location and time, and this relationship is described by the acoustic wave equation.

04

Free Field

A sound field is a sound field in which the influence of boundaries can be ignored when a sound source radiates sound waves in a homogeneous, isotropic medium. That is, a sound field with only direct sound and no reflected sound or negligible reflected sound. However, an ideal free sound field is difficult to obtain. Usually, only an approximate free sound field that meets certain measurement error requirements can be obtained, such as an anechoic chamber.

05

Semi-free field

A semi-free sound field is a specific acoustic environment mainly composed of direct sound and first-reflection sound. Direct sound is the sound wave that travels directly from the sound source to the receiver, while first-reflection sound is the sound wave that reaches the receiver after being reflected by a rigid surface such as the ground. In practical applications, an open outdoor environment (with no reflective objects around and the sound source placed on the ground) and a semi-anechoic chamber in a laboratory can be approximated as a semi-free sound field.

06

Standing Wave

A standing wave is a special waveform formed by the interference of two waves with similar frequencies. A standing wave contains nodes with zero amplitude and antinodes with maximum amplitude. The standing wave phenomenon is particularly important in acoustic design because it can cause abnormal increases or decreases in sound intensity in certain areas, affecting the overall acoustic effect. Through proper acoustic design, the adverse effects of standing waves can be effectively avoided.

07

Absorption coefficient (α)

The sound absorption coefficient is an indicator of a material's ability to absorb sound energy. It represents the ratio of the sound energy absorbed by the material to the incident sound energy, and is usually expressed as a value between 0 and 1. Theoretically, if a material completely absorbs sound energy, its sound absorption coefficient is 1; if it completely reflects sound energy, its sound absorption coefficient is 0. However, in practical applications, the sound absorption coefficient of all materials lies between 0 and 1. Understanding the sound absorption coefficient of materials is of great significance for optimizing indoor acoustic environments.

08

Sound bridge

Sound bridges refer to the phenomenon where sound energy is transmitted through rigid connections when panels are directly fixed to rigid structures such as joists. The more sound bridges there are, the larger the contact area, and the stronger the rigid connections, the more severe the sound bridging phenomenon and the worse the sound insulation effect. In sound insulation design, measures need to be taken to reduce the formation of sound bridges in order to improve sound insulation performance.

Article source: Dr. Sound