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A measure of the sound absorption of a surface. It is equivalent to a unit area of perfectly absorbing material. It is related to the reverberation time of the room.

**Sequential frequency analysis**

The process of carrying out a frequency analysis of a sound in which the octave or third octave bands are determined using a single filter and scanning across the range of frequencies of interest. At the lower frequency bands more time will be required to allow the energy to stabilize. As the analysis proceeds through the frequency bands the noise level stabilizes much quicker since a wider range of higher frequencies are being included. Some sound analyzers perform the scan under automatic control governed by the instrument in order to minimize the time needed to obtain a statistically reliable answer in every band. Carrying out third octave band sequential analysis will take longer than an octave band analysis since more bands are involved. The noise source must be stationary during the sequential analysis so this type of measurement technique is best suited to noise sources that are continuous such as fan or pump noise.

**Shielding**

The attenuation of a sound that is achieved by placing barriers in between the source and the receiver.

**Short term exposure limit STEL**

A 15 minute short term exposure limit rather like a Time Weighted Average (TWA). If no STEL is quoted for an environment, then a level 3 times the 8 hour TWA may be assumed but the total duration of the excursions should not exceed one hour in any 24 hour period.

**Slow Time weighting**

The slow weighting is the same as the slow response in a sound level meter and is part of the rms. circuit controlling the response of the meter to the variability of the instantaneous levels of the noise. It has a value of 1000 milliseconds (1 second) and is a continuous function that is calculated all the time by the instrument.

**Sociocusis**

The loss of hearing caused by noise exposures that are part of everyday social life. Not related to any occupational noise exposure, physiological changes with age or disease.

**Sound**

An oscillation in air pressure in an elastic medium. It is also an auditory sensation evoked by these oscillations. Not all sound waves will evoke an auditory response, for example, ultrasonic waves.

**Sound exposure level (SEL)**

The total noise energy in an event expressed as if it had only lasted for a single 1 second duration at the notional level in dB that contains the same amount of noise energy as the actual noise event. Measurement of the sound exposure levels of different noise sources allows their contribution to the overall noise climate to be easily compared since all results are referenced to the same time interval of 1 second. Sound Exposure Level is displayed as LAE, LCE or LZE.

**Sound level**

The frequency and time weighted sound pressure obtained by the use of a sound level meter (or other equivalent device) as specified by the relevant

__ANSI standards__or

__IEC standards__for sound level meters. The level is expressed in decibels relative to the reference pressure level and unless specified to the contrary is normally taken to be the A weighted level.

**Sound level meter**

A measuring instrument comprising a transducer (the microphone), a frequency weighting circuit (the A, B or C responses), an rms circuit (the slow, fast or impulse weighting) , data processing (with a microprocessor, if fitted) and an output device (digital LCD) for the measurement of noise and sound. Sound level meters can be battery operated for field use or can be powered from external power sources depending upon their power consumption and the length of time required for operation.

**Sound power**

The quantity of sound from a source irrespective of its local position in the environment. It is the total sound energy radiated by a noise source in unit time. The unit of sound power is the watt.

**Sound power level**

Ten times the logarithm to the base 10 of the actual weighted sound power to the reference sound power. The reference sound power is taken to be 1 µW (or 0.000,001 Watt). This result is a logarithmic quantity called the decibel or dB.

**Sound pressure**

The instantaneous difference between the actual pressure produced by a sound wave and the current atmospheric barometric air pressure at a given point in space.

**Sound pressure level**

Twenty times the logarithm to the base 10 of the weighted sound pressure to the reference sound pressure. The reference sound pressure is 20 µPa (or 0.000,020 Pa). This result is a logarithmic quantity called the decibel or dB.

**Sound quality**

The measurement of sounds perceived by a listener that involve more than just the simple overall level of noise. Frequency content is important and is often measured with a third octave band analyzer.

**Sound transmission**

The passage of sound through structures. Measurements may be made to determine the effectiveness of the insulation of different materials used for the construction of walls, floors ceilings etc.

**Sound transmission class**

The preferred single figure rating system which gives an estimate of the sound insulation properties of a structure or a rank ordering of a series of structures.

**Sound transmission loss**

A measure of the sound insulation provided by a structure of specific design and construction. Expressed in dB, it is 10 times the logarithm to the base 10 of the reciprocal of the sound transmission coefficient for the configuration.

**Span or Dynamic Range**

The range of measurement capability in a monitoring instrument from the smallest to the largest value.

**Spectrum**

The transformation of a sound wave’s resolution into its components of frequency and amplitude.

**Spectrum analysis**

The measurement and determination of the contribution of the frequencies that make up the overall sound level. For sound pressure it is measured with a sound level meter.

**Speech interference level**

A calculated overall quantity providing a guide to the interference of a noise with the reception of speech. The speech interference level is the average of the octave band levels of the interfering noise in critical bands of the spectrum. The levels in the 500, 1000 and 2000 Hz bands are averaged together to give the speech interference level in dB.

**Speed of sound**

The speed that sound wave travel through air under given conditions. Sound waves travel through different media at different speeds depending upon the composition of the medium. For sound waves traveling in air at normal temperature (21ºC, 70ºF) and pressure (1013 mB) the speed of sound is 344 m/s (1128 f/s). sound waves traveling through steel have a faster speed of transmission.

**Spherical wave**

A sound wave in which the surfaces of constant phase are concentric spheres. A small point source radiating into an open space produces a free sound field of spherical waves.

**Standing waves**

The condition within enclosed spaces, such as rooms, in which sound reflects off opposing surfaces, such as the floor and ceiling, and produces noticeable 'hot spots” of higher energy sound and "dead spots" with low energy. The effect is frequency and room size dependant. Highs and lows of sound will occur at integer multiples of half wavelengths as the waves add or cancel out in the space between the reflective surfaces(walls).

**Statistical analysis**

A calculation performed by a Sound Level Meter on the noise levels measured during the measurement period to describe the statistical spread of the noise. The classification of the magnitudes of a measured parameter into a cumulative frequency distribution from which various statistical levels can be derived.

**Statistical noise levels**

The calculated noise level based on sampling the varying noise climate and expressing the result as a percentage of time above the chosen statistical level. For example, the LN10% is the noise level exceeded for 10% of the measured time interval. The samples of the noise are sorted into a distribution table at fixed dB intervals and the statistics are calculated from the cumulative distribution curve that is produced. In a 10 minute measurement the LN10% value represents the level of noise exceeded for 1 minute in total. The noise does not have to be continuously above the calculated statistical level but the sum of the times when it is will give the statistical value required.

**Steady noise**

Noise that remains constant within a typical limit of ± 2 dB and without large fluctuations over extended periods of time.

**Steady state noise**

Sounds whose average characteristics remain relatively constant over time. Typical examples of steady state noises are fans, air conditioning units and compressors or pumps.