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. 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 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. Also known as Threshold of Audibility.
Masked Threshold : threshold of audibility for a specified sound in the presence of another (masking) sound
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.
Age Related Threshold Shift : ARTS : is the component of PTS related to age.
Noise Induced Permanent Threshold Shift : NIPTS : is the component of PTS 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.
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.
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 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 : see Time Domain Averaging above.
Time Weighted Average : TWA : is the workplace noise exposure measured over an 8 hour working day, using a 5 dB Exchange Rate. 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.
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
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 : 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
Tone Assessment : Noise 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.
Parameters associated with Tone Assessment 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
Total Harmonic Distortion : THD : the RMS sum of all the Harmonics relative to the amplitude of the fundamental signal.
Total Sound Energy Density : sum of instantaneous potential and kinetic energy densities.
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.
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%.