Gracey & Associates
Noise and Vibration Glossary - T
GLOSSARY INDEX: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
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 : the SI unit of magnetic flux and is equal to one weber per square metre = volt second per square meter = one newton per ampere per metre = 10,000 gauss.
THD : Total Harmonic Distortion .
Third Octave Band : a frequency band whose cut-off frequencies have a ratio of 21/3, which is approximately 1.26. The cut-off frequencies of 891 Hz and 1112 Hz define the 1000 Hz third-octave band in common use.
Third Octave Filter : a filter whose upper-to-lower passband limits bear a ratio of 21/3, which corresponds to 23% of the centre frequency. 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, duration : t : one of the SI base quantities. The SI base unit of time is the second, symbol s
Time Averaged Sound 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 - Leq. - the equivalent continuous noise level
Time Averaging : there are two ways to create an average or sample mean. Take one component and average the numbers at ten different times, which produces the Time Average. Alternatively take the numbers for ten components at any one time (i.e. a vertical column) add them and divide by ten. This is known as the Ensemble Average. Conventionally the Ensemble Average is written E(x) and the Time Average is indicated by putting a bar over the x.
Time Constant : standardized time constant used in exponential time weighting for acoustical analysis. The standard time constants for sound level meters are Slow = 1000 ms, Fast = 125 ms, and Impulse = 35 ms while the signal level is increasing or 1,500 ms while the signal level is decreasing. See also fast, impulse and slow .
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 or just Synchronous Averaging. Here the waveform is averaged in a time buffer before the spectrum analysis (FFT). 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.
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.
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 Time Constant.
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.
TL : Transmission Loss .
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.
Total Sound Exposure : see sound exposure
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.
TTS : Temporary Threshold Shift
TWA : Time Weighted Average
Type 1 : precision grade meters for laboratory and field use - renamed Class 1
Type 2 : general grade meters for field use - renamed Class 2
GLOSSARY INDEX: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
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