Acoustic Glossary

Third Octaves sub-divide the audio spectrum typically into 33 bands with constant percentage bandwidth filtering, three times more detailed than octave band filters.

The cut-off frequencies have a ratio of 2^1/6 for example a 1 kHz third-octave band filter has a centre frequency of 1000 Hz with lower and upper frequencies of 891 Hz and 1122 Hz respectively

Third Octave Band Filters centred at one of the preferred frequencies, should meet the IEC and ANSI standardised attenuation characteristics.

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Threshold Level most regulations specify that for the measurement of noise at work, the sound levels below a certain threshold, should be disregarded. A noise dose meter, therefore only sums up the contributions from the levels above the threshold and uses these values to calculate the noise dose parameters.

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Threshold of Hearing is the lowest level of sound (pure tone) that can be perceived by people and is close to the standard reference level of sound pressure, 0.00002 pascals at 1 kHz. Defined as the minimum level of a sound that is just audible in given conditions on a specified fraction of trials (conventionally 50%), in quiet conditions. Also known as the audible threshold.

See also audible range • audible sound definition • auditory masking • masked threshold • threshold shift

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Threshold of Pain is subjective and therefore varies in the literature from 120 dBA to 140 dBA. However 120 dBA is most common.

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Threshold of Speech Intelligibility

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Threshold Shift a change in the threshold of audibility at a specified frequency from a threshold previously established. The amount of threshold shift is expressed in decibels.

It is usually assumed that the following components are additive, at least for small values of the components.

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See also loudness and related terms

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See also time domain averaging and time series.

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Time-average, when used in the mathematical sense, it refers to the mean average, the sum of the values divided by the number of values

Time-average Sound Energy Density
Time-average Sound Level
Time-average Sound Pressure Level

See also time domain averaging • time series • time weighting

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Time Constant

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Time Domain is a term used to describe the analysis of mathematical functions, or physical signals, with respect to time. An oscilloscope is a tool commonly used to visualize real-world signals in the time domain.

A time domain graph shows how a signal changes over time, whereas a frequency domain graph shows how much of the signal lies within each given frequency band over a range of frequencies.

Time Domain Averaging also known as time synchronous averaging is often used in machinery monitoring. It requires a tachometer connected to the trigger input of the analyser to synchronise each snapshot of the signal to the running speed of the machine.

Time Domain Averaging is very useful in reducing the random noise components in a spectrum, or in reducing the effect of other interfering signals such as components from a nearby machine.

See also other types of averaging

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Time Frequency Analysis when analysing non-stationary signals, time frequency analysis gives optimum resolution in both the time and frequency domains. Data is presented in a map with time shown on the x-axis, frequency on the y-axis and the amplitude indicated by various colours or grey-scales in the contour map.

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Time Integral, a mathematical integration of a variable level or function with respect to time.

See also time average • time domain averaging • time series

Time Mean Square see mean square

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Time Series a sequence of measurements where a single measurement is not considered appropriate to establish the mean, maximum and/or other statistical information. In this case mean is used to differentiate the results from the wide range of averaging methods widely used in acoustical measurements.

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Time Synchronous Averaging (TSA) under time domain averaging.

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Time Weighted Average (TWA) is the workplace noise exposure level measured over an 8 hour working day, using a 3 dB exchange rate in Europe.

See also noise dose and sound exposure

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Time Weighting is the time-averaging characteristic used to measure oscillatory or fluctuating quantities. The most common time weightings are rectangular (perfect integration with no memory).
Exponential weighting, on the other hand provides a running average in which recent values are more heavily weighted than those occurring earlier.

See also sound level time weightings and time constants

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Time Window, fourier analysis tells us that time and frequency are simply two alternative ways of observing a signal. By changing the nature of a signal in the time domain, we implicitly change the nature of the spectrum in the frequency domain.

This is exactly what we do when we apply a weighting function or time window to a specified period of time record. Examples are rectangular, hanning, etc.

See also windowing

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Tinnitus ringing in the ear or noise sensed in the head. Onset may be due to an acoustic trauma and persist in the absence of acoustical stimulation (in which case it may indicate a lesion of the auditory system). Not directly due to external acoustic stimulation. It can be associated with exposure to high levels of noise.

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TL : transmission loss, the sound level difference between two locations

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TNI : traffic noise index

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Tone a sound or noise recognisable by it's regularity. A simple or pure tone has one frequency. Complex tones have two or more simple tones, the lowest tone frequency is called the fundamental, the others are overtones.

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Tone Assessment sound containing discrete frequencies or 'tones' has for many years (BS 4142, BS 7445 and ISO 1996) attracted a 5 dB penalty when assessing industrial and environmental noise.

BS 7445 suggests that if the level in a 1/3-octave band is 5 dB or more higher then the adjacent two frequency bands then it's likely to constitute a nuisance and should be rated accordingly.

Tone Assessment Measurements, some meters include software to make measurements directly.

Tone Assessment Parameters include :
Kt : the value added to the LAeq to give the tone-corrected rating level
Lta : the audibility of all tones found in the same critical band as the selected tone
Lpn : the total level of the masking noise in the band containing the selected tone
Lpti : the level of the selected tone
Lpt : the total level of all tones in the critical band containing the prominent tone

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Torque also called the moment of force is the tendency of a force to rotate an object about some axis. It is the product of the force and the length (radius) to the point where the force is applied.

The unit is the N·m (newton metre)

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Total Energy Density

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Traffic Noise Index (TNI) a method developed in the 1970's based on the LA10 and LA90 measured figures in the formula.

TNI = 4 (LA10 - LA90) + (LA90 - 30) dB

Transfer Impedance Definition (IEC 801-25-18) quotient of a dynamic field quantity at one point in a system by the corresponding kinematic field quantity at another point in the same system

See also acoustic impedance and related terms

Transient Definition (IEC 161-02-01) pertaining to or designating a phenomenon or a quantity which varies between two consecutive steady states during a time interval short compared with the timescale of interest. Applies to an adjective and noun

See also maximum transient vibration value

Transient Oscillation Definition (IEC 801-24-03) oscillation that results from a change in external excitation

See also other oscillation terms

Transmissibility the ratio of the displacement of the isolated system to the input displacement. Describes the response or effectiveness of a vibration isolation system.

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Transmission Impedance and Radiation when acoustic waves pass from one medium to another, part is reflected back. The transmission and reflection factors are determined by the relative impedances of the two materials.

See also acoustic impedance and related terms

Transmission Loss Definition (IEC 801-23-39) reduction in sound pressure level between two designated locations in a sound transmission system, one location often being at a reference distance from the source.

Also known as propagation loss, but should not to be confused with sound insulation transmission loss IEC 801-31-39.