Acoustic Glossary


D : Sound and Vibration Terms and Definitions ...

D-weighting under frequency-weighting networks

D : level difference under under sound insulation

Daily Acceleration Exposure (Aeq8), the vibration a worker is exposed to during a working day, normalised to an 8-hour reference period, taking account of the magnitude and duration of the vibration - more details.

See also vibration at work regulationsexposure action valueexposure limit values

Daily Noise Exposure (LEX,8h) is the time-average, A-weighted noise level for a nominal 8-hour working day, also known as LEP,d.

Daily Personal Noise Exposure (LEP,d) is the time-averaged, A-weighted noise level for a nominal 8-hour working day, also known as LEX,8h, used for assessing the noise exposure of a worker during a working day. If the noise exposure of workers varies markedly from day to day, the personal noise exposure may also be assessed over a week rather than a day and is noted as the LEP,w.

The UK Noise Regulations require action if the following Noise Exposure Action Levels are exceeded

The Lower Exposure Action Levels = 80 dBA and a peak sound pressure of 135 dBC ≈ LCpeak

Measurements should be made with a precision Leq sound level meter equipped with a Cpeak network. Alternatively noise dose meters may be used to check the levels bearing in mind these are usually Class 2 devices

If the Leq (equivalent continuous sound level) is measured over 8 hours, then the LAeq, the LEP,d and the LEX,8h will all be the same.

See also exposure action valuesexposure limit values

Daily Vibration Exposurehand-arm vibration A (8)whole-body vibration A (8), normalised to an 8-hour period.

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

See also the exposure action valueexposure limit value

Damped Natural Frequency Definition (IEC 801-24-10) frequency of free oscillation of a damped linear system

Damping refers to the attenuation of sound in a structure owing to the internal sound-dissipating 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.

The removal of echoes and reverberation by the use of sound absorbing material is under reverberation time

Damping Definition (IEC 801-24-19) dissipation of energy of an oscillating system with time or distance

Damping Ratio Definition (IEC 801-24-11) ratio of the actual damping to the critical damping.

See also critical dampingsound absorptionviscous damping

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

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

Day-average Sound Level Definition (ASA 3.17) in decibels is the time-average frequency weighted sound level between 0700 and 2200 hours. Units decibel (dB), abbreviation DL, symbol Ld.

Day Evening Night Noise Level (Lden) the A-weighted, Leq (equivalent noise level) over a whole day, but with a penalty of 10 dB(A) for night-time noise (23:00-07:00) and 5 dB(A) for evening noise (19:00-23:00), also known as the day evening night indicator.

See also community noise equivalent levelevening noise levelnight noise level

Day Night Average Sound Level Definition (ASA 3.19) on a given day of the week, twenty-four-hour average frequency-weighted sound level after addition of 10 decibels to levels from midnight to 0700 hours and from 2200 hours to midnight. Unit, decibel (dB); abbreviation, DNL; symbol, Ldn.

Day Night Noise Level (Ldn) is the LAeq (equivalent noise level) over a 24 hour period with a penalty of 10 dB(A) for noise during the hours of 23:00-07:00, also known as the day night indicator.

Day Noise Level (Lday), the A-weighted, Leq (equivalent noise level), over the 12-hour day period (07:00-19:00), also known as the day noise indicator.

dB : decibels

dBA • dBB • dBC • dBD • dB Flat • dB Linear and dBZ see sound level frequency-weightings

dB calculations

dB Level the logarithm of the ratio of a given acoustic quantity to a reference quantity of the same kind. The base of the logarithm, the reference quantity, and the kind of level must be indicated.
Note 1 : the kind of level is indicated by use of a compound term such as sound power level or sound pressure level.
Note 2 : the value of the reference quantity remains unchanged, whether the chosen quantity is Peak, the RMS, or otherwise.
Note 3 : the base of the logarithm is indicated by use of a unit of level associated with that base.

dBm and dB(mW), power relative to 1 milliwatt. No reference impedance is assumed, though 600 ohms is common in audio equipment.

dB SIL under sound intensity level
dB SPL, dB(SPL), dBSPL and dBSPL, variations of sound pressure level
dB SWL under sound power level

dBV and dB (1 V RMS), voltage ratio with a reference voltage of Vo = 1.00 volt = 0 dBV, regardless of impedance.

dBW or decibel watt, is a unit for the measurement of the strength of a signal expressed in decibels relative to one watt. It is used because of its capability to express both very large and very small values of power in a short range of number, e.g. 1 watt = 0 dBW, 10 watts = 10 dBW, 100 watts = 20 dBW and 1,000,000 W = 60 dBW.

Power in dBW = 10 lg (Power in W)

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) and is related to the reverberation time by the formula T = 60 dB/d.

Decay Rate Definition (IEC 801-31-08) at a given frequency, rate at which the sound pressure level decreases with time, for example in a reverberant room
Note : the unit of decay rate is the decibel per second.

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

The decibel (dB) is a relative unit of measurement, widely used in acoustics. The dB is a logarithmic ratio between the measured level and a reference (threshold) level of 0 dB. The ratio may be
sound power in watts
sound pressure in pascals
sound intensity in watts per metre-squared, etc..

Effectively the decibel compresses the wide range of sounds we hear into more manageable numbers.

Decibel Definition (IEC 801-22-03) one-tenth of the bel
Note 1 : the decibel is more often used than the bel as a unit of level.
Note 2 : the decibel can be defined as a unit of level of a power-like quantity when the base of the logarithm is the tenth root of ten. Also, the decibel is the unit of level of a field quantity when the base of the logarithm is the 20th root of ten.

See also root power quantity and our decibel calculations page

Decibel Reference Levels
  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 formula for example 20 log10 p/po) dB for sound pressure levels.

The parameters below use the 10 log formula for example 10 log10 (W/Wo) dB for sound power levels.
Sound   Power Level, LW     Sound Intensity     Sound Energy Density  
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

See also the acoustic reference quantities listsound level calculations

Definitions included in this Glossary are ASA and IEC Definitions

DEFRA : Department for Environment, Food & Rural Affairs (UK)

Degree ° a measure of angles. There are 360 degrees in a full rotation or circle and 90 degrees (90°) is a right angle. The symbol for degree is °

Although in common use, the degree is not part of the International System of Units (SI) the derived unit is the radian

See also angles

Delay 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.

Deltatron ® trade name for IEPE (integrated electronics piezoelectric).

DENL under Lden (day-evening-night noise level)

Density (p) the density of a material is it's mass per unit volume, p = m/V
where p is the density, m is the mass and V is the volume.
Density is a scalar quantity and the SI units are kilogram per metre-cubed, kg/m3

Density of air, air density decreases with increasing altitude, as does air pressure.
At sea level and at 20 °C, air has a density of approximately 1.2 kg/m3
At sea level and at   0 °C, air has a density of approximately 1.3 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 energy in the waveform.

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 signalstransients may also be deterministic.

DFT : Discrete Fourier Transform

DI : Directivity Index - radiation pattern

Differentiation in acoustics, a mathematical operation that converts a displacement signal to a velocity signal, or a velocity signature to an acceleration signature. The opposite process is called Integration

Diffraction Factor

Diffuse Field Microphone under random incidence microphone

Diffuse Field
Diffuse Field Distance
Diffuse Sound Field

Digital recording or storing information as series of the numbers 1 and 0, to show that a signal is present

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 Analyser 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 unlike an analogue signal, which is continuous and contains time-varying quantities, a digital signal has a discrete value at each sampling point.

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.

See also sampling.

DIRAC : Dual Input Room Acoustics Calculator software developed by Acoustic Engineering to measure a wide range of room acoustical parameters, according to the ISO 3382 (room acoustics) and IEC 60268-16 (speech intelligibility) standards. Based on the measurement and analysis of impulse responses, DIRAC supports a variety of measurement configurations and is distributed by Bruel & Kjaer.

Directivity is a measure of the directional characteristic (radiation pattern) of a sound source.

Directivity Factor (Q), if a noise source radiates uniformly in all directions, it has a Q = 1. Placing the source on the ground so the energy can only radiate hemi-spherically, then the directivity factor Q=2.

Placing the source on the ground next to a wall so the radiation is concentrated into a 1/4 of the sphere, then Q=4.

If the source is placed on the floor in a corner, the sound energy is further concentrated into 1/8th of a sphere so Q = 8.

Directivity Index (DI) is 10 times the logarithm to the base 10 of the directivity factor Q. Say for example we buy a piece of equipment with a sound power rating of 80 dB and we install it in the corner of an empty building. The resultant sound power level radiated into the building would be 80 dB + 10·log (8) = 89 dB.

Directivity Pattern, the 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.

See also radiation

Direct Sound

Directly Proportional

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

See also tonal assessment

Discrete Fourier Transform (DFT) mathematical calculation that converts or transforms a sampled and digitised waveform into a sampled spectrum. They reveal periodicities in input data as well as the relative strengths of any periodic components. The fast fourier transform, is an algorithm that allows a computer to calculate the discrete fourier transform very quickly.


Displacement is the change in position of an object in metres and is a vector quantity.

Displacement (ξ) = v/ω = a/ω², where v = velocity, a = acceleration and ω = 2·π·f = angular frequency.

If Displacement (s) = v/ω and Velocity v = a/ω
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 angular displacementparticle accelerationparticle displacement used in acoustic wave theory • particle velocitystandard reference levels table

Dissipation Definition (IEC 801-31-29) conversion of sound energy into heat

Dissipation Factor Definition (IEC 801-31-30) ratio of sound energy dissipated as heat to the energy of the incident sound wave.

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

Distortion Definition (IEC 801-21-48) undesired change of waveform
Note : Distortion may result from:
a) non-linear relation between input and output;
b) non-uniform transmission at different frequencies;
c) phase shift not proportional to frequency.

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

Divergence Loss Definition (IEC 801-23-41) that part of the transmission loss due to the divergence, i.e. spreading of sound waves in accordance with the configuration of the system.
Note : divergence loss exists, for example, for spherical waves emitted by a point source and is also known as spreading loss

Dn : normalised level difference - under sound insulation

DNL see Ldn : day-night noise level

DnT : standardised level difference - under sound insulation
DnT,w : weighted standardised level difference - under sound insulation
DnT,w + Ctr : weighted standardised level difference with spectrum adaption term Ctr

DOD : Department of Defence (USA)

Dodecahedron a general dodecahedron is a polyhedron having 12 faces. Acoustic examples are dodecahedron and hemi-dodecahedron loudspeakers arrange to provide isotropic sound sources.

Dose related subjects
DOHR : noise dose per hour
Dose - noise dose
Dose Badge
Dose per Hour
Dose - Vibration

Driving Point Impedance Definition (IEC 801-25-17) quotient of a dynamic field quantity at one point in a system by the resulting kinematic field quantity at the same point.

See also acoustic impedance and related terms

DSP : digital signal processing

Dual Input Room Acoustics Calculator

Duration of Shock Pulse

Dw : weighted level difference - under sound insulation

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 decibels

Dyne the force that will accelerate a 1 gram mass at the rate of 1 cm/s.
1 dyne = 1 g·cm/s² = 10-5 kg·m/s² = 10 μN

The dyne was the 'old' reference level for sound pressure (0.0002 dyne/cm²), these days the SI reference level of 20 micro pascals (20 μPa) is used

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