Help Topics - D

Daily personal noise exposure level

The general name for the level of the amount of noise exposure that an operator is subjected to measured at the workplace. Limits vary according to various national and international Health and Safety regulations for the amount of exposure that is considered allowable for an operator without hearing protection. Typical limit values are an equivalent steady level of 85 or 90 'A' weighted dB for an 8-hour working day. This is also known as the LEP,d.

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Damping

The dissipation of noise energy with time or distance. The term is usually applied to the attenuation of sound in a structure owing to the internal sound-dissipative properties of the structure or to the addition of sound-dissipative materials.

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Data logging

The collection of a regular series of data points during a recording to enable the temporal variations of the measured values to be studied. This can be in a personal noise dosimeter, a heat stress monitor, a real time dust or particulate monitor or a weather monitoring system. All of these examples show the benefit of knowing when certain measured values occurred in order to apply corrective measures properly. Data logging can be done as rapidly as every second for fast changing acoustic signals or as slow as every hour for more slowly varying weather parameters such as atmospheric pressure.

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Day night noise level

The 24 hour average noise level of all hourly Leq measurements with a 10 dB penalty added to the levels between 2200 and 0700 hours to reflect people's extra sensitivity to noise during the night. No correction is added to the measured Leq levels taken between the hours of 0700 and 2200 in arriving at the overall single number.

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dB

The abbreviation of the decibel used to express the level of a noise or sound. It is a logarithmic quantity that represents the ratio of the actual measured sound pressure (p) to a nominal reference sound pressure (p_o). The accepted reference sound pressure is taken to be 20 µPa (0.000,020 Pa). The definition of the decibel is given by;

dB = 20 log₁₀ (p/p_o)

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decibel

The unit of sound pressure level usually abbreviated to the dB. Any noise quantity that is expressed as a “level” is measured and quoted in decibels.

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Density

A measure of how heavy a solid, liquid, or gas is for its size. Measured as mass per unit volume, e.g.. lb/cu ft or kg/m³.

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Dew point

The temperature at which the water vapor in the air first starts to condense. Typically expressed in degrees Fahrenheit or Celsius. The dew points indicate the amount of moisture in the air. The higher the dew point, the higher the moisture content of the air at a given temperature. Dew point temperature is defined as the temperature to which the air would have to cool (at constant pressure and constant water vapor content) in order to reach saturation. The warmer air is, the more water vapor it can "hold."

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Diffraction

Radiation being bent or curved from its original course, for example light as it passes through a narrow slit. Also a modification which sound or light  waves undergo in passing by the edges of solid obstacles or particles in their path from source to receiver.

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Diffusion tubes

An inert tube of plastic or stainless steel that contains an absorbing matrix capable of collecting specific target air pollutants. The tubes are used for ambient air monitoring and sent to a laboratory for analysis.

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Digital audio tape

A form of closed tape recording system using small tape cassettes with a fixed recording duration. The dynamic recording quality is approximately 75 dB from tape saturation to noise floor. This matches the output range from most current sound level meters. Up to 4 hours of continuous recording can be possible at half speed recording.

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Dry bulb temperature

The temperature reported by a thermometer shielded from the direct rays of the sun or other radiant sources that measures the temperature of the local air.

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Digital signal processing

A digital computation carried out on samples of the raw sound measured by certain sound level analyzers that can be used to derive the common units of instantaneous, maximum, minimum and average level etc. Typical DSP calculations are carried out many times a second to derive small packets of noise that can be recombined to obtain the more popular values found in more conventional sound level meters. The adoption of proprietary algorithms can yield the frequency contributions contained within the short samples. Rates as fast as sampling at 76,800 times per second are common in many analyzers generating small samples every 5 or 10 µseconds.

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Directivity

Unless a noise source is completely omni-directional there will be some of the sound energy that is radiated more in certain directions than in others.

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Directivity index

In a given direction from a sound source under consideration the directivity index is the difference in decibels between the sound pressure level produced by the source in that direction and the space averaged sound pressure level of that source measured at the same distance. This unit is often used in predicting noise levels away from a source of known output and directivity when performing noise control.

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DOD

The Department of Defense of the United States. A specific set of measurement settings in a noise monitoring instrument used to measure the exposure of certain military personnel to noise in their workplace.

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Doppler effect

The apparent upward, or downward, shift in frequency of a sound as the relative positions of the source and receiver change with respect to each other. This is most noticeable for a passenger standing on a railway platform as a speeding train approaches and recedes the station. The apparent frequency increases quickly as the source approaches the receiver and decreases as the source moves away. The faster the source is moving the more noticeable will be the change of frequency. The Doppler effect is also heard when the source is stationary and the receiver is moving such as a car passing by a steam exhaust venting to atmosphere.

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Dose

A relative measurement of the noise in a workplace usually expressed as a percentage of some allowable total daily value. Noise dose is analogous to radiation dose as a hazard in the workplace.

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Dose badge

A small lightweight personal noise dosimeter that has the microphone built in to the body of the unit so preventing problems with cables when worn by a worker in a noisy environment. A separate reader unit is required to control the noise dose badge and to access the stored results.

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Dosimeter

A measurement instrument capable of being worn by a mobile worker or operator during the day to measure the total exposure of that operator to noise in the workplace. Usually the dosimeter is supplied with a microphone on a cable that allows the measurements to be carried out in the hearing zone of the individual. The body of the dosimeter is worn on the belt or in a pocket or pouch. A noise dosimeter is essentially the same as a standard sound level meter except that it is normally designed to only measure the broadband noise levels without performing any sort of frequency analysis.

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Duration

The elapsed time from the start of a measurement run to the end of that run. Measurement durations can be from a few seconds up to several hours or even days depending upon the application. The duration of the measurement can be specified in a standard or may be calculated by knowing the process under investigation. If noise is cyclical in nature then at least one or two complete cycles should be measured to be sure of correctly classifying the noise climate.

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Dust

Solid particles, usually produced by a mechanical process, with a range of particle diameters from 0.1 to 100 microns. Dust can also include fumes that condense from the vapor state usually from the volatilization of molten metals, high boiling liquids or combustion.

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