is the time during a single noise event where the sound level is 10 dB below the maximum level. Also known as the 10 dB-downtime during an aircraft fly-by
a measuring instrument for indicating speed or rotation.
occurred. C denotes that the C frequency-weighting was used.
can result in distant noise sources appearing significantly closer than they really are. Normally the air temperature is highest at ground level and cools as the elevation increases. However on occasions the opposite is true i.e. it is cooler at ground level and consequently the sound waves do not dissipate in all directions normally and some
back down to the ground at some distance from the source.
and is equal to a flux of 1 weber per square metre = volt second per metre-squared = one
per ampere per metre = 10,000 gauss.
sub-divide the audio spectrum typically into 33 bands with constant percentage bandwidth
filtering, three times more detailed than octave
The cut-off frequencies have a ratio of 21/6 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 standardized attenuation characteristics.
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
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 absolute threshold
and audible threshold
See also •
auditory masking •
masked threshold •
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
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.
hearing loss and
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
Note : Timbre depends primarily upon the waveform
, but also upon the sound pressure
and the temporal characteristics of the sound.
one of the seven SI base quantities.
The SI unit is the second, symbol s
time domain averaging and
, 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
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.
, a mathematical integration of a variable level or function with respect to time.
time average •
time domain averaging •
Time Mean Square
see mean square
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 level
measured over an 8 hour working day, using a 3 dB exchange rate
See also • noise dose and sound exposure
is the time-averaging characteristic used to measure oscillatory or fluctuating quantities. The most common time weightings are rectangular (perfect integration with no memory).
, on the other hand provides a running average in which recent values are more heavily weighted than those occurring earlier.
sound level time weightings and time constants
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
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.
TL : transmission loss
, the sound level difference between two locations
TNI : traffic noise index
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
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
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)
Total Energy Density Definition (IEC 801-21-41) sum of the instantaneous potential sound energy density 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
measured figures in the formula.
TNI = 4 (LA10 - LA90) + (LA90 - 30) dB
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
acoustic impedance •
transmission impedance and radiation
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
of the displacement
of the isolated system to the input displacement. Describes the response or effectiveness of a vibration isolation system.
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.
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.
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%.