Help Topics - T

Tape recorder

A mechanical recording device utilizing a medium such as audiotape to store the original ac-varying signal from the microphone output of a sound level meter. Tape recorders can use reel-to-reel open tapes of various sizes or can be of the closed types such as a DAT (Digital Audio Tape) cassette of a certain size. The audiotape can be used to verify the source of a nuisance noise or for archiving purposes to show before and after sound quality.

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Telemetry

The transmission of data sets from a remote monitoring device over large distances to a central readout location. Can be done by radio signals or by telephone modems.

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Temporary threshold shift

A temporary impairment of hearing acuity as indicated by a change in the threshold of audibility.

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Third octave band

A frequency band whose cutoff frequencies have a ratio of 2 to the third power (about 1.25). the third octave band centered at 1000 Hz has a lower cutoff frequency of 891 Hz and a higher cutoff frequency of 1112 Hz. These are the frequencies at which the response of the filter has dropped to half, sometimes called the 3dB down points.

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Third octave band analysis

The process of performing a frequency analysis of a noise by breaking up the sound into a series of contiguous bands whose bandwidth is a third of an octave. 31 third octave frequency bands cover the typical audio range from 20 Hz to 20 kHz.

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Thoracic fraction

The mass fraction of inhalable particles penetrating beyond the larynx. Typically this will represent particles with a mean aerodynamic diameter of less than 10 microns.

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Threshold level

The sound level in dB below which an instrument will ignore any quieter noise levels for the purpose of measuring a noise dose in the workplace. Typical threshold levels are 80 and 90 dB used in personal noise dosimeters to comply with workplace noise measurement techniques specified by the OSHA regulations. Sometimes it is also called the cutoff level.

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Threshold of audibility

The minimum sound pressure level at which a person can hear a specified frequency of sound over a specified number of tests carried out in acoustically controlled conditions.

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Threshold of pain

The minimum sound pressure level of sound outside the ear that will produce a change from discomfort to definite pain. Typically taken to be at about a level of 130 A weighted dB.

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Threshold shift

A change in the threshold of audibility at a specified frequency from a threshold previously established for that individual. The amount of the threshold shift is normally expressed as a dB value.

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Timbre

An attribute of auditory sensation allowing a subject to judge that two sounds similarly presented and having the same loudness and pitch are dissimilar. Effectively, the ability of an individual to be able to tell the difference between the two sounds even though they both appear to be equally as loud and of the same pitch.

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Time

One of the three fundamental variables of noise measurement along with amplitude and frequency.

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Time history

The resultant set of values produced by an instrument to record (or data log) the changing noise levels during a run. The measured changes indicate the time history variations of the noise.

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Time history profile

The regular short values of certain levels of a parameter (such as noise or temperature) that are recorded during a measurement run to be able to visualize the temporal changes in that (noise or temperature) climate at a chosen time resolution.

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Time weighting

Another name for the time response in the rms section of a sound level meter. The standard time weightings generally found in an instrument include the Slow, Fast and Impulse weightings.

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Time weighted average level

Occupational exposure limits for a material or hazard expressed as an 8 hour time weighted average value (TWA) that includes the whole of the shift exposure. For noise it is a single number equivalent value in decibels that represents the equivalent average level of the actual changing noise levels. When the exchange rate is chosen as 3 dB the average noise level is called the Leq. When the exchange rate is chosen as 4 dB the average noise level is called the LDoD. When the exchange rate is chosen as 5 dB the average noise level is called the LOSHA. In the example above the blue line shows the minute by minute changing noise levels from 65 to 92 dB while the horizontal black line shows the equivalent TWA value at 80.9 dB.

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Tinnitus

A ringing in the ears or noise sensed in the head. The onset may be due to an acoustic trauma and still persist in the absence of the stimulus. A person suffering from tinnitus will complain of being able to hear noises even in very quiet surroundings. This condition is very difficult to prove or to treat.

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Tone

A sound with a definite pitch. A pure tone from a device like an acoustic calibrator will produce a sinusoidal waveform when viewed on a suitable display.

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Total inhalable fraction

The mass fraction of total airborne particles that are inhaled through the mouth and nose.

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Transducer

A device capable of being stimulated by waves from one or more transmission systems or media and supplying related waves to one or more other transmission systems or media. Typical examples are microphones that convert (or transduce) sound waves into an equivalent electrical signal or accelerometers that convert vibrations into useful electrical signals.

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Transmission loss (dB)

The decibel reduction in noise level across a partition as measured in third octave bands. Usually measured with a real time frequency analyzer from 100 Hz to 3k15 Hz.

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Type 1

A grade of overall accuracy taking into account the physical response of the complete measurement system that has tolerances in terms of frequency, direction, temperature, humidity, ambient air pressure etc. A sound measurement device having a claimed accuracy of type 1 according to ANSI (or other equivalent international standards) is sometimes called a precision instrument. Type 1 instruments are usually specified where the best accuracy is mandated.

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Type 2

A grade of overall accuracy taking into account the physical response of the complete measurement system that has tolerances in terms of frequency, direction, temperature, humidity, ambient air pressure etc. A sound measurement device having a claimed accuracy of type 2 according to ANSI (or other equivalent international standards) is sometimes called a general-purpose instrument. Type 2 instruments are usually specified where the absolute accuracy is not critical or where cost is a compelling factor.

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