A
Acoustic Absorption
When a sound wave strikes a hard surface, most of its energy is reflected. For a flat surface, the angle of reflection equals the angle of incidence. When the same sound wave encounters a porous material or a vibrating membrane, a significant portion of its energy is absorbed instead. The amount of absorbed energy depends on the material properties, surface mass, exposed area, and the frequency content of the sound. The remaining energy is reflected.
A material’s absorption performance is expressed using the Sabine absorption coefficient (α), on a scale from 0 to 1, where 1 represents total absorption. The Sabine coefficient is the ratio of absorbed acoustic energy to incident acoustic energy and is therefore dimensionless.
Equivalent Absorption Area
The equivalent absorption area represents the area of a perfectly absorptive surface (Sabine α = 1) that would provide the same acoustic absorption as the surface or object being considered.
It is expressed in square metres (m²).
D
Decibel [dB]
The decibel is a logarithmic measurement scale used in acoustics. It is a dimensionless unit expressing the ratio between two quantities such as sound levels or acoustic power.
Because the decibel scale is logarithmic, sound levels cannot be added arithmetically. For example, combining a 60 dB noise source with a 65 dB source does not produce 125 dB, but approximately 66.2 dB.
When the difference between two sound levels exceeds 10 dB, the quieter source has little influence on the perceived overall level. This phenomenon is known as the masking effect.
Weighted Decibel [dB(A)]
The “A” weighting indicates that the sound level has been filtered to account for the frequency sensitivity of human hearing.
Several weighting curves exist (A, B, C and D), each corresponding to different frequency-response characteristics. In building acoustics, A-weighting is the most commonly used.
Sound Decay Rate
The sound decay rate of a room, noted DL, describes the reduction in sound pressure level as the distance from a source is doubled. The reference source used is typically a pink-noise source.
DL is used to characterise reverberation behaviour in large spaces and is referenced in workplace acoustic regulations.
It is expressed in dB(A).
Acoustic Diffraction
Diffraction is a physical phenomenon that occurs when a wave encounters an obstacle or passes through an opening, causing the sound to spread in multiple directions. This explains why sound can still be heard behind an obstacle.
Diffraction occurs when the dimensions of the obstacle are smaller than the wavelength of the sound.
F
Frequency [f]
Frequency is the number of oscillations per second of a sound signal. It is expressed in hertz (Hz).
The human hearing range typically extends from 20 Hz to 20,000 Hz.
I
Statistical Noise Indices / Fractile Levels [L99…]
When noise levels fluctuate over time, they can be characterised using statistical indices derived from measured sound-level distributions. These indices represent the A-weighted sound pressure level exceeded during N% of the measurement period and are denoted LN.
For example:
• L1: level exceeded for 1% of the measurement period,
• L50: median noise level exceeded for 50% of the time.
Commonly used statistical indices include:
• L1: maximum noise level,
• L10: peak or intrusive noise level,
• L50: average or median noise level,
• L90: background or ambient noise level.
Sound Reduction Index [Rw]
To assess the sound insulation performance of a material or building element, the sound reduction index R is defined as the difference between the sound levels measured on either side of the tested element.
In general, acoustic insulation improves with increasing surface mass.
Rw is an intrinsic laboratory-measured property and is expressed in decibels (dB).
Airborne Sound Insulation [DnT,A]
Airborne sound insulation represents the amount of sound energy that is not transmitted through a separating element. It is defined as the difference between the sound level in the source room and the sound level measured in the adjacent receiving room.
It depends mainly on:
• the sound reduction index and surface area of the separating partition,
• the sound reduction indices and areas of flanking elements,
• the volume and reverberation time of the receiving room.
This parameter is measured in situ and expressed in dB.
M
Standardised Tapping Machine
To measure floor impact sound insulation, acousticians use a standardised tapping machine consisting of five 500 g hammers falling from a height of 40 mm at a rate of 10 impacts per second.
N
Ambient Noise
The total noise present in a given environment during a specified time interval. It includes all noise sources, both near and distant.
Specific Noise
The identifiable component of ambient noise that is distinguished from the overall background, typically because it is the subject of an assessment or complaint.
Residual Noise
The ambient noise level measured in the absence of the specific noise source(s) under investigation.
Pink Noise
Pink noise is a standardised random noise signal whose energy is equally distributed across octave bands. Pink-noise sources are commonly used for airborne sound insulation measurements because of their uniform energy distribution over the frequency spectrum.
Pink noise is also used as a reference signal for indoor building noise and aircraft noise assessments.
White Noise
White noise is a standardised random noise signal whose energy is equally distributed across all frequencies (similar to white light containing all colours).
It is commonly used as a reference signal for road and railway traffic noise.
Road Traffic Noise Spectrum
A road-traffic noise spectrum is typically rich in low frequencies and is used as a reference profile for road and railway noise calculations.
O
Octave Bands
An octave band is a frequency range in which the upper frequency is twice the lower frequency. The bandwidth doubles with each successive octave band. The overall sound level corresponds to the total energy (or pressure) summed across all octave bands.
Human hearing perceives pitch logarithmically rather than linearly. At higher frequencies, larger frequency variations are required to produce the same perceived change in pitch.
Standardised octave-band centre frequencies are therefore used to reflect this perception:
31, 63, 125, 250, 500, 1,000, 2,000, 4,000 and 8,000 Hz.
Octave
An octave is the interval between two sounds whose frequencies differ by a factor of two, as defined in music theory.
P
Sound Power Level [Lw]
A sound source radiates acoustic energy, referred to as sound power. Sound power is an intrinsic property of the source and is expressed in dB or dB(A).
R
Reflection / Echo
An acoustic wave may undergo reflection or diffraction.
Sound reflection corresponds to the echo phenomenon.
Diffraction, by contrast, occurs when sound bends around the edges of an obstacle and is re-radiated in multiple directions.
Refraction
Sound propagates in straight lines through a medium of uniform density. However, like light, sound undergoes refraction when entering a different medium, such as from air into concrete.
Refraction causes a change in the propagation direction of the wave. Sound waves tend to bend toward the medium in which they travel more slowly.
Reverbation Time
The reverberation time, noted RT or T60, is the time required for a sound level to decay by 60 dB after the sound source is abruptly stopped.
Reverberation time varies with frequency, and acoustic comfort in a room is closely related to the reverberation-time curve across the frequency spectrum.
It is expressed in seconds and depends on:
• the total absorption area of the room,
• and the room volume.
S
Sound Level Meter
The sound level meter is the acoustician’s primary measurement instrument. It is used to measure sound pressure levels.
A sound level meter generally includes:
• a microphone,
• an amplifier with a defined frequency weighting (A, B or C),
• and a detector/display system with specified time-response characteristics.
There are four accuracy classes for sound level meters:
• Class 0: ±0.4 dB,
• Class 1: ±0.7 dB,
• Class 2: ±1.0 dB,
• Class 3: ±1.5 dB.
Professional acoustic measurements are generally carried out using Class 1 instruments.
Sound Power Level [Lw]
A sound source radiates acoustic energy, referred to as sound power. Sound power is an intrinsic property of the source and is expressed in dB or dB(A).
Instantaneous Sound Pressure Level [Lp]
The strength of a sound is characterised by the amplitude of pressure fluctuations relative to atmospheric pressure. Human hearing responds logarithmically rather than linearly to sound intensity.
Sound pressure levels are therefore expressed in decibels [dB] or A-weighted decibels [dB(A)].
When a single value representative of human hearing sensitivity is required, all frequency components are weighted according to the ear’s response curve.
The resulting value is referred to as the A-weighted overall sound level.
Equivalent Continuous Sound Pressure Level [Leq]
To describe fluctuating noise using a single value, the equivalent continuous sound pressure level, denoted Leq,t, is calculated over a specified integration period t.
Leq represents the constant sound level that would contain the same total acoustic energy as the actual varying sound over the same time period.
When A-weighting is applied, it is denoted LAeq,t.
Daily Noise Exposure Level [LEX,d]
The daily noise exposure level, LEX,d, represents the equivalent sound exposure experienced by an individual over a working day.
It is expressed in dB(A).
Sound Reduction Index [Rw]
To assess the sound insulation performance of a material or building element, the sound reduction index R is defined as the difference between the sound levels measured on either side of the tested element.
In general, acoustic insulation improves with increasing surface mass.
Rw is an intrinsic laboratory-measured property and is expressed in decibels (dB).
Airborne Sound Insulation [DnT,A]
Airborne sound insulation represents the amount of sound energy that is not transmitted through a separating element. It is defined as the difference between the sound level in the source room and the sound level measured in the adjacent receiving room.
It depends mainly on:
• the sound reduction index and surface area of the separating partition,
• the sound reduction indices and areas of flanking elements,
• the volume and reverberation time of the receiving room.
This parameter is measured in situ and expressed in dB.
Directional Source
A directional source radiates more acoustic energy in certain directions than in others.
A loudspeaker is a typical example of a directional source.
Omnidirectional Source
An omnidirectional source radiates sound energy uniformly in all directions.
True omnidirectional sources are uncommon in everyday environments, but they are frequently used in acoustic measurements.
T
Transmission
Transmission is the portion of incident acoustic energy that passes through an obstacle, such as a wall, subjected to alternating pressure fluctuations caused by sound waves.