Definitions, Terms, Units and Parameters
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T60: see reverberation time.
Tachometer: a measuring instrument for indicating speed or rotation.
TCpeak: the time when the peak sound level occurred. C denotes that the C frequency weighting was used.
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 Threshold Shift : Permanent Threshold Shift .
tera : T: SI prefix = 1012 see other SI Units
Tesla : T: the 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.
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, sound levels below a certain limit (the threshold) should be disregarded. The noise dose meter, therefore, sums up the contributions from the levels above the threshold only, 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 the human auditory system. This is close to the standard reference level of sound pressure, 20 μPa at 1 kHz. The minimum level of a sound that is just audible in given conditions on a specified fraction of trials (conventionally 50%). In quiet conditions this is referred to as the absolute threshold. In the presence of masking sound or noise it is called masked threshold.
Threshold of Pain: the threshold of pain, being subjective, varies in the literature from 120 dBA to 140 dBA. However 120 dBA is most common.
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.
See also Temporary Threshold Shift : Permanent Threshold Shift.
Time : t: one of the SI base quantities. The SI base unit of time is the second, symbol s
See also Measurement Time : Reverberation Time.
Time Averaged Level : LAT: the equivalent steady level over a given period of time that contains the same amount of noise energy as the actual fluctuating level.
Also known as Leq. - the equivalent continuous noise level
Time Averaging: see Time Domain Averaging
Time Constants: standardised Time Weightings used in exponential time weighting for acoustical analysis.
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: a type of averaging that is important in machinery monitoring is time domain averaging, or Time Synchronous Averaging, and 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.
Other averaging methods : Exponential Linear RMS Spatial Spectrum
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 Synchronous Averaging : TSA: see Time Domain Averaging above.
Time Weighted Average : TWA: using a 5 dB Exchange Rate, the total amount of workplace noise exposure expressed as an equivalent standard 8 hour working day. American OSHA specification.
Time Weighting: 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 Meter Time Weighting.
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.
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.
Tone Burst: A short signal used in acoustical measurements to make possible differentiating desired signals from spurious reflections.
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 Harmonic Distortion : THD: the RMS sum of all the harmonics relative to the amplitude of the fundamental signal.
Traffic Noise Index : TNI: 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.
Transient: a sudden and very short duration signal. These signals start and stop within the analysis time (or measurement window). In most cases averaging is neither possible nor needed.
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 : TL: the difference between the noise levels across a partition between two or more rooms. taking into account the area of the common partition and the sound absorbing characteristic of the receiving room. See also Sound Transmission Loss and our Sound Insulation. page
Type 1: precision grade meters for laboratory and field use - renamed Class 1
Type 2: general grade meters for field use - renamed Class 2
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