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Noise and Vibration

Glossary - D

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D-weighting :   not in common use, developed for 'non-bypass' jet engines like Concorde and military aircraft.


Daily Acoustic Immission : DAI :   Sound Exposure Level.


Daily Personal Noise Exposure : Lep,d :   is the average A-weighted noise exposure level for a nominal 8-hour working day, also known as LEX,8h. Lep,d is calculated from the measured LAE the setting of Exposure Time and a Reference Time of 8 h. Used for assessing the noise exposure of a worker during a working day.

If the Leq is measured over 8 hours then Leq and Lep,d and LEX,8h would all be the same.

The European Directive recommends LEX is used from 2006 onwards.


Damage Risk Criteria :   recommended maximum noise levels for given exposure periods.


Damping :   any means of dissipating vibration energy within a vibrating system.

The dissipation of energy with time or distance. The term is generally 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.

The action of frictional or dissipative forces on a dynamic system causing the system to lose energy and reduce the amplitude of movement.

Removal of echoes and reverberation by the use of sound absorbing material, see also reverberation time.


Dat Recorder :   Digital audio tape recorder.


Data Logging :   on-going measurements, stored at regular intervals for downloading to a PC.


Data Source :   the identity and location of the data that are used in an analysis.


Day Noise Indicator : Lday :   EU defined for describing the annoyance due to environmental noise, based on the LAeq measured over the day-time period 07.00 to 19.00.


Day-evening-night Noise Indicator: Lden :   EU defined for describing the annoyance due to environmental noise, based on the LAeq measured over the whole day with a penalty of 10 dBA for the night period.


Day-night Level : Ldn :   a US noise indicator for describing the annoyance due to environmental noise. Based on the LAeq measured over the whole day with a penalty of 10 dBA for night time noise from 22.00 to 07.00.


dB :   under decibel


dBA :   most commonly used standard weighting of the audible frequencies designed to reflect the response of the human ear to noise - also written as A-weighting or dB(A).


dBB :   a weighting curve between A and C. Not used very much these days, was popular in the motor industry.


dBC :   a standard weighting of the audible frequencies - also written as dB(C) or dBC. Commonly used for the measurement of Peak Sound Pressure level


dBD :   frequency weighting developed for measuring high level aircraft noise especially non-bypass military engines. Later ISO standards recommend A-weighting for commercial aircraft noise.


dBZ :   Z weighting is a flat frequency response between 10 Hz and 20 kHz ±1.5 dB excluding microphone response. Replaces the old Flat and Linear curves. Alternatives dB(Z) and dBZ


DC Coupling :   The connection of a signal from one circuit to another in a manner that passes both AC and DC components - see also AC Coupling.


Decay Rate : d :   is the time taken for the sound pressure level in a room to decay - measured in decibels per second (dB/s). Decay rate d in a reverberant room is related to the reverberation time T by the formulae T = 60 dB / d.


Decay Time :   The time taken for the sound pressure level to fall by 60 dB - a million to one.


Decibel : dB :   a relative unit of measurement widely used in acoustics, electronics and communications. The dB is a logarithmic unit used to describe a ratio between the measured level and a reference or threshold level. The ratio may be power, sound pressure, voltage or intensity etc. The term is used extensively in acoustics - so we include a more detailed description.

Decibel scale :   A linear numbering scale used to define a logarithmic amplitude scale, thereby compressing a wide range of amplitude values to a small set of numbers.

  Vibration Levels Sound Levels
    Acceleration   La     Velocity   Lv     Particle Velocity   Lv     Pressure   Lp  
dB m/s2 m/s m/s Pa : N/m2
0 dB 1 x 10-5 1 x 10-9 5 x 10-8 2 x 10-5
20 dB 1 x 10-4 1 x 10-8 5 x 10-7 2 x 10-4
40 dB 1 x 10-3 1 x 10-7 5 x 10-6 2 x 10-3
60 dB 1 x 10-2 1 x 10-6 5 x 10-5 2 x 10-2
80 dB 0.1 1 x 10-5 5 x 10-4 0.2
100 dB 1.0 1 x 10-4 5 x 10-3 2.0

Table Notes:
0 dB is the reference level for each parameter.
The parameters above use the 20 Log formulae for example : 20 Log (P/Po) dB for sound pressure levels.
The parameters below use the 10 Log formulae for example : 10 Log (I/Io) dB for sound intensity levels.

Sound   Power Level : LW     Intensity Level : LI     Energy Density Level : LE  
dB Watts Watts/m2 Joules/m3
0 dB 1 x 10-12 1 x 10-12 1 x 10-12
20 dB 1 x 10-10 1 x 10-10 1 x 10-10
40 dB 1 x 10-8 1 x 10-8 1 x 10-8


Delay :   a time difference that has to elapse after (or before) triggering of a measurement and sampling of the time history data begins. There are situations where entering a trigger delay is useful.


Density of air : ρ :   is the mass per unit volume of the atmosphere. Air density decreases with increasing altitude, as does air pressure, units kilogram per cubic metre.

At sea level and at 20 °C, air has a density of approximately 1.2 kg/m3


Detector :   an electronic circuit that determines the amplitude level of a signal in accordance with certain rules. The simplest type of detector consists of a resistor and a capacitor, which measures the rectified average value of a fluctuating DC signal. A more complex and much more useful type of detector is an RMS detector whose output is proportional to the sound power or sound energy present in the signal or a vibration.


Deterministic :   a type of signal whose spectrum consists of a collection of discrete components, as opposed to a random signal, whose spectrum is spread out or smeared in frequency. Some deterministic signals are periodic, and their spectra consist of harmonic series. Vibration signatures of machines are in general deterministic, containing one or more harmonic series, but they always have non-deterministic components, such as background noise. See also stationary signals. Transients can also be deterministic.


Diffraction :   the distortion of a wave front caused by the presence of an obstacle in the sound field. The scattering of radiation at an object smaller than one wavelength and the subsequent interference of the scattered wave fronts.


Diffuse Field :   the region in a room where the reflected sound dominates, as opposed to the region close to the noise source where the direct sound dominates. The same as reverberant field - see also far field : near field.


Diffuse Sound :   sound that is completely random in phase; sound that appears to have no single source.


Diffusion :   the scattering or random reflection of a sound wave from a surface. The directions of reflected sound is changed so that listeners may perceive the sound coming from all directions.


Digital Filter :   a digital processor that receives a sequence of input data values, executes an operation on them, and outputs a corresponding sequence of values that have been filtered with respect to the input.


Digital Filter Analyzer :   constant percentage (or relative) bandwidth resolution. This is often preferred for acoustic measurements because it best simulates the way in which the human ear perceives sound.


Digital Signal Processing : DSP :   is the analysis of digital signal data. The original analogue signal is sampled at regular time intervals, and an Analogue to Digital converter converts the sampled amplitudes into a number series.


DIRAC : Dual Input Room Acoustics Calculator :   software to measure impulse responses and calculates various room acoustical parameters, according to the ISO 3382 and IEC 60268-16 standards. Dirac supports a wide variety of measurement configurations.


Direct Field :   the region in which the sound measured can be attributed to the source alone without the effects of obstructions, walls, etc.


Directivity Factor :   The ratio of the mean-square pressure (or intensity) on the axis of a transducer at a certain distance to the mean-square pressure (or intensity) that a spherical source radiating the same power would produce at that point.


Directivity Index :   in a given direction from a sound source, the difference in decibels between (a) the sound pressure level produced by the source in that direction, and (b) the space-average sound pressure level of that source, measured at the same distance.


Directivity Pattern :   a graphical description, usually in polar co-ordinates, of the response of the transducer as a function of the direction of the transmitted or incident sound waves in a specified plane and at a specified frequency.


Discrete :   with reference to a spectrum, discrete means consisting of separate distinct points, rather than continuous.


Discrete Fourier Transform :   mathematical calculation that converts or transforms a sampled and digitised waveform into a sampled spectrum. The Fast Fourier Transform, or FFT, is an algorithm that allows a computer to calculate the discrete Fourier transform very quickly. See also Fast Fourier Transform.


Displacement : s :   change in position of an object in metres - vector quantity.

v = u + at   where v = velocity, u = start velocity, a = acceleration in m/s2 and t = time.

In the field of vibration acceleration a, velocity v, displacement s and angular frequency ω are related. - for example v = a/ω, s = v/ω, where ω = 2·π·f

It follows that 10 m/s2 = 0.01 m/s = 10 μm at 159 Hz

This works for all frequencies, we just chose 159 Hz to keep the numbers simple. We also have a vibration nomogram for downloading.

See also particle displacement


Distance :   change in position of an object in metres - see also displacement.


Distortion :   signal components not in the original signal due to non-linearities in the system or transmission path.


Dn : Normalized Level Difference :   for airborne sound transmission. A sound insulation index measured under field conditions, between 'real' rooms and deliberately includes effects due to flanking routes and differences in the relative size of the rooms. It attempts however to normalize the measured difference level to the level which would be present when the rooms are furnished by measuring the quantity of acoustic absorption in the receiving room and correcting the difference level to the level which would be expected if there was 10 m² sabine absorption in the receiving room. Accurate knowledge of the dimensions of the receiving room are required.


DnT : Standardized Level Difference :   for airborne sound transmission. Similar to the Dn, but this index corrects the measured difference to a standardized reverberation time of 0.5 seconds. This RT value is often cited as approximately average for a medium sized, carpeted and furnished living room. It does not require detailed and accurate knowledge of the dimensions of the test rooms.


DnT,w : Weighted Standardized Level Difference :   for airborne sound transmission. A single number quantity which characterises the airborne sound insulation between rooms, calculated from the DnT.

Building Regulations : Approved Document E - preferred parameter.


DnT,w + Ctr :   a single quantity which characterises the airborne sound insulation between rooms using spectrum no 2 as defined in BS EN ISO 717-1   - see also C - Spectrum Adaption Term and Ctr.

Building Regulations : Approved Document E - preferred parameter .


Dw : Weighted Difference Level :   D the Difference Level = (S - R) where S is the Source room levels and R is the Receiver room levels). Then Dw is a single integer number found from comparing the D-spectrum with the 'standard' curves for airborne and impact insulation and the unfavourable deviation is 32 dB.


DOHR :   Dose per hour - shows the dose profile for each hour.


Dose : D :   the noise exposure expressed as a percentage (%) of a fixed level for the working day.

If the noise limit is 85 dB and a person is exposed to a constant or equivalent sound pressure level of 85 dB for eight hours, then the result is a 100% noise dose.

ISO use an energy-based average for noise dose calculation, based on the 3 dB exchange rate. Basically a noise level of 88 dB has twice as much energy as a level of 85 dB, so a constant level of 88 dB = a 200% dose.

When using the 3 dB exchange rate, nearly always in the UK, then the 8 hour average level is known as LEP,d Daily Personal Noise Exposure or LEX,8h.

The sound exposure (in physical units) for the whole measurement duration or Dose is known as E (Pa².h)

Leq = 10 log(Dose/100 x 8/T) + 85 : where T = measurement time in hours.


Dose % :   noise exposure expressed as a percentage (%) of a fixed level for 8 hours


Dose Badge :   small device worn by operator to store fluctuating noise levels during working day. Similar to a Dosimeter.


Dose per Hour :   DOHR - shows the dose exposure over each hour.


Dosemeter :   a pocket sized instrument designed to be worn during all or part of the day as a personal monitor in order to check an operator's personal exposure to workplace noise - see sound exposure meter.


DSP :   Digital Signal Processing.


Dual Input Room Acoustics Calculator : DIRAC


Dynamic Range :   all audio systems are limited by inherent noise at low levels and by overload distortion at high levels. The usable region between these two extremes is the dynamic range of the system. Expressed in dB.


Dyne :   The force that will accelerate a 1 gram mass at the rate of 1 cm/s. The old standard reference level for sound pressure was 0.0002 dyne/cm². The same level today is expressed as 20 micropascals, or 20 μPa

1 dyne = 1 g·cm/s2 = 10-5 kg·m/s2 = 10 μN


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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