Acoustic Glossary
Reproduced courtesy of Adrian James Acoustics Ltd
Absorption Coefficient
Frequency dependent coefficients commonly used to describe the fraction of acoustic energy absorbed at a boundary.
A value of 1.0 indicates all energy is absorbed and there will be no resultant reflection from this surface. A value of 0 indicates no energy is absorbed. These limiting values are theoretical, and in practice some energy will always be absorbed / reflected.
NOTE: Measured a values are often quoted as being greater than 1. This is a result of the measurement technique and not actually achievable. 1.0 is actually the practical limit.
Acoustician
An expert in the branch of physics concerned with the properties of sound [definition taken from OxfordDictionaries.com].
Airborne Sound
Term used to describe sound sources that originate from within free space or a room, which may then propagate into the structure. Used to differentiate from structureborne sound.
Assessment of non-steady sound
Most sounds are not steady, so that the sound pressure level fluctuates with time. A measurement is therefore meaningless unless we know whether it represents a minimum, maximum or some kind of time-averaged level. Various parameters have been derived to measure sounds of differing characters, and the most relevant to this report are as follows:
- Leq,T: The equivalent continuous noise level is used widely to measure noise that varies with time. It is defined as the notional steady noise level that would contain the same acoustic energy as the varying noise. Because the averaging process used is logarithmic, the Leq,T level tends to be dominated by the higher noise levels measured.
- L90,T: This is the sound pressure level exceeded for 90% of the measurement period T. It is an indication of noise levels during the quieter periods of measurement, and is widely used to measure background noise.
- Lmax: This is the maximum level measured, and is used to assess sleep disturbance from intermittent sources such as aircraft and train noise. Lmax is normally defined as the maximum reading given by a sound level meter set to "Fast" response (with a 0.125 second time constant).
Attenuation
General term for the reduction in amplitude, magnitude or intensity of a physical quantity, in this case, acoustic energy. This is generally a result of absorption, scattering or dispersion within a medium, although this latter parameter is often not considered a form of attenuation.
A-Weighting
An A-weighting network is used to reflect the differential sensitivity of human hearing to sounds of different frequency. I.e. it is a set of correction terms applied to sounds of different frequency measured using absolute 'linear' sound measurements (dB(lin)). The A-weighting sound pressure level, LpA, is measured on a scale defined by the dB(A).
A number of frequency weightings have been developed to imitate the ear's varying sensitivity to sound of different frequencies. The most commonly used is the 'A' weighting. The 'A' weighted SPL can be measured directly or derived from octave or one-third octave band SPLs. The result is a single figure index which gives some idea of the subjective loudness of the sound, but which contains no information as to its frequency content. The addition of the subscript "A" to any of the indices described above indicates that these have been measured using the 'A' weighting (e.g. LAeq,T or LAmax).
Decibel (dB)
This is the unit used to measure sound. The human ear has an approximately logarithmic response to sound over a very large dynamic range - typically from 0.00002 to 200 Newtons per square metre (N/m2). Decibels provide a logarithmic scale to describe sound pressure and sound power levels. The threshold of hearing for most people corresponds to a sound pressure level of 0 dB and the threshold of aural pain to a sound pressure level of 140 dB.
Environmental Acoustics / Noise
Noise sources from road, rail, aeroplanes, industry and entertainment facilities all count as sources of environmental noise. Rated according to their own international (ISO) and national (BS) standards, it is an area of acoustics that is taken very seriously. To monitor and assess environmental noise, complex systems of monitoring and prediction are used to interpret existing scenarios as well as the likely change / increase due to development and expansion, and their likely environmental impact on local communities.
Commonly used descriptors for defining environmental noise are the LAeq,T,L90,T, and L10,T. The LAeq,T is defined as "the value of the A-weighted sound pressure level of a continuous, steady sound that, within a specified time interval T, has the same mean square sound pressure as a sound under consideration whose level varies with time." The L90,T is a measure of the level of sound exceeded over 90 percent of a period of time, T. Similarly with L10,T this is the level exceeded 10 percent of a period of time.
Recent projects include:
Guru Nanak Sikh VA Secondary School
Langley Grammer School, Slough
Stamford Endowed Schools, Lincs
Kendrick School, Reading
Loudness and addition of decibels
Because of the logarithmic scale, decibels do not add in a linear fashion. When two identical sounds occur simultaneously, the resulting level is only 3 dB higher than for a single source. By contrast, an increase of 10 dB normally represents a doubling of "loudness" of the sound. Hence doubling the amount of sound energy results in very much less than a doubling in subjective loudness.
Noise Rating (NR) and other curves
'A' weighted levels can not be used to define a spectrum or to compare sounds of different frequencies. NR curves convey frequency information in a single-figure index by defining the highest measured or specified level at each frequency. To measure the noise rating of a given environment, the SPL is measured in octave or one-third octave bands and the noise rating is then the highest NR curve touched by the measured levels. The graph below shows curves NR20 to NR40.
NC curves are similar to NR curves and are more commonly used outside Europe. They are not defined at 31.5 Hz. NR and NC curves were derived theoretically from the response of the human ear under laboratory conditions. More recent research has found that at low noise levels, noise spectra matching these curves allow too much noise at very high and very low frequencies compared with the levels at mid-frequencies. PNC curves have begun to be used more widely in the USA and UK for noise control in concert halls and theatres.
Octave and One-Third Octave Bands
The human ear is sensitive to sound over a frequency range of approximately 20 Hz to 20,000 Hz (1 Hz = 1 cycle per second), and is generally more sensitive to medium and high frequencies than to low frequencies. To define the frequency content of a noise, the spectrum is divided into frequency bands, the most common of which are octave bands, in which the mid frequency of each band is twice that of the band below it. For some applications, each octave band may be split into three one-third octave bands, and for finer analysis narrow band filters may be used.
Sound Power Level (Lw or PWL)
This is a function of the noise source alone, and is independent of its surroundings. It is a measure in decibels of the amount of sound power emitted by the source.
Sound Pressure Level (Lp or SPL)
This is a function of the source and its surroundings and is a measure in decibels of the total instantaneous sound pressure at a point in space. The SPL can vary both in time and in frequency. Different measurement parameters are therefore required to describe the time variation and frequency content of a given sound. These are described below.
Transmission, Absorption and Reflection
Three terms used for describing the resultant distribution of acoustic energy impinging on a boundary such as a wall partition.
Transmission describes the acoustic energy that passes through the partition. Absorption is the mechanical conversion of energy into heat as the sound wave does 'work' on the partition. This is the energy that is lost by the sound wave upon interaction with the interface. With the case of absorption panels situated on a surface, the deeper the absorption material, the more it will attenuate low frequencies - this is evident in comparison of the absorption coefficients for our absorption panels - the depth of the panel becomes more and more comparable to the wavelength of the low frequency sound waves leading to greater mechanical interaction.
Reflection describes the acoustic energy that is deflected by the object.
In summary: Incident sound energy = Transmitted + Absorbed + Reflected energy