Acoustic Glossary


 

T : Sound and Vibration Definitions, Terms, Units, Measurements etc., ..


T10 the time within a single noise event where the level is below 10 dB of the maximum level. Also known as the 10 dB-downtime during an aircraft fly-by

See also perceived noise level

T60Reverberation Time

Tachometer a measuring instrument for indicating speed or rotation.

TCpeak the time when the peak sound pressure occurred. C denotes that the C frequency weighting was used.

Temporary Threshold Shift (TTS)

Tera (T) ▷ SI units prefix = 1012 - see other SI units

Tesla (T), SI unit of magnetic flux density and is equal to a flux of 1 weber per square metre = volt second per square metre = one newton per ampere per metre = 10,000 gauss.

THDTotal Harmonic Distortion

The Control of Noise at Work Regulations under Noise at Work Regulations

The Control of Vibration at Work Regulations under Vibration at Work Regulations.

Third Octave Band

octave bands sub-divided into three parts, equal to 23% of the centre frequency. Used when octave analysis is not discrete enough. Divides the audio spectrum into 33 or more equal parts with constant percentage bandwidth filter.

The cut-off frequencies have a ratio of 21/3, which is approximately 1.26. 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 1112 Hz respectively

Third Octave Filter

alternative name for Third Octave Band. Which is preferably centred at one of the preferred frequencies in ISO R266. Should meet the attenuation characteristics of IEC R266, IEC R255 and ANSI S1.11-1966 Class III.

Threshold Level most regulations specify that for the measurement of noise at work, then sound levels below a certain limit the 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.


Threshold of Hearing

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 absolute threshold and threshold of audibility.

Masked Threshold threshold of audibility for a specified sound in the presence of another (masking) sound

See also • auditory masking


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


Threshold of Speech Intelligibility


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.

Noise Induced Permanent Threshold Shift (NIPTS) is the component of permanent threshold shift associated with a noise exposure.

Permanent Threshold Shift (PTS) the component of threshold shift that shows no progressive recovery with the passage of time when the apparent cause has been removed.

Temporary Threshold Shift (TTS) an upward shift in the threshold of human hearing. Usually caused by being subjected to a loud sound. When the noise abates the hearing usually returns to normal. However continual exposure will cause the shift to be permanent.

See also • noise induced hearing loss


Timbre Definition

IEC 801-29-09, that attribute of auditory sensation which enables a listener to judge that two non-identical sounds having the same loudness and pitch are dissimilar.
Note : Timbre depends primarily upon the waveform, but also upon the sound pressure and the temporal characteristics of the sound.

Time (t), one of the seven SI base quantities. The SI unit is the second, symbol s

See also time domain averaging and time series.


Time-average Sound Level Definition. IEC 801-22-16, logarithm of the ratio of a given time-mean-square standard-frequency-weighted sound pressure for a stated time period, to the square of the reference sound pressure of 20 ╬╝Pa. Time-average sound level in decibels is ten times the logarithm to the base ten of that ratio. Also known as the equivalent continuous sound level (Leq).

Note 1 : If a frequency weighting is not specified, the A-frequency weighting is understood.
Note 2 : In principle, exponential time weighting is not involved.

Time-average Sound Pressure Level Definition. IEC 801-22-11, logarithm of the ratio of a given root-mean-square sound pressure, during a stated time interval, to the reference sound pressure. Average sound pressure level in decibels is 20 times the logarithm to the base ten of that ratio and also known as the equivalent continuous sound pressure level.

Note the important and fundamental difference between these two definitions

Time Averaging see also time domain averaging and time series

Time Constant

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


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.

Time Integral an integral of a variable or function with respect to time.

See also • time averagetime domain averagingtime series


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.

Time Synchronous Averaging (TSA) under Time Domain Averaging.

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

See also • noise dose and sound exposure

Time Weighting is the time-averaging characteristic used to measure oscillatory or fluctuating quantities. Common time weightings are rectangular (perfect integration with no memory) and exponential. Exponential weighting 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


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


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.

TLTransmission Loss

TNITraffic Noise Index

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.
 
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

Tone-corrected Perceived Noise Level Definition IEC 801-29-14, a sound pressure level in decibels, obtained by adding to perceived noise level an adjustment that is related to the degree of irregularity that may occur among contiguous one-third-octave band sound pressure levels of an aircraft noise
Note 1 : the adjustment is described in ISO 3891-1978 ; it may vary from 0 dB to 6,7 dB.
Note 2 : the adjustment purports to account for the extra subjective noisiness caused by pronounced audible tones such as may be generated by propellers, compressors, turbines or fans.

See also • perceived noise level


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 newton metre, N·m = kg m2 s-2


Total Energy Density Definition

IEC 801-21-41, sum of the instantaneous potential energy and kinetic energy density and is also known as the sound energy density

Total Harmonic Distortion (THD)

, the RMS sum of all the harmonics relative to the amplitude of the fundamental signal.

Traffic Noise Index (TNI)

, a method developed in the 1970's based on the LA10 and LA90 measured figures in the formulae.

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

Transfer Function

acoustic signals are modified by any structure or medium they encounter. If for example you can measure the input to a vibration isolation mount and simultaneously measure the output, then the transfer function is the difference or ratio of the system output to the system input.

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


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.


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. Transmission Loss is also known as propagation loss

See also • sound insulation definition


Transverse at right angles to the main direction of energy flow

Transverse Amplitude

accelerometers, for example, are normally designed to measure the acceleration amplitude in the mounted direction. The transverse amplitude is usually stated as a percentage of the measured signal in the transverse (90°) direction; a typical value is 5%.

Transverse Wave

True Peak

TTSTemporary Threshold Shift

TWATime Weighted Average

Type 1 precision grade sound level meters for laboratory and field use - renamed Class 1

Type 2 general grade sound level meters for field use - renamed Class 2

 

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