Acoustic Glossary


D : Sound and Vibration Definitions, Terms, Units, Measurements etc., ..


DLevel Difference under sound insulation

Daily Acceleration Exposure (Aeq8), the vibration a worker is exposed to during a working day, normalized 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 Exposure Level

(LEX,8h) is the sound exposure averaged over 8 hours.

The Noise at Work Directive 2003/10/EC defines the following limit and action values:

Exposure Limit Values (LEX,8h) = 87 dB(A) and LCpeak = 200 Pa respectively;

Upper Exposure Action Values (LEX,8h) = 85 dB(A) and LCpeak = 140 Pa respectively;

Lower Exposure Action Values (LEX,8h) = 80 dB(A) and LCpeak = 112 Pa respectively.

LCpeak is the maximum value of the C-weighted, instantaneous sound pressure.

LEX,8h is the Leq (equivalent continuous sound level) corrected for the length of the working shift, in this case 8 hours

See also the Daily Personal Noise Exposure (LEP,d) below

Daily Personal Noise Exposure (LEP,d)

, the average, A-weighted noise exposure 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.

LEP,d is calculated from the measured sound exposure the exposure time and a reference time of 8 h.

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,8h is adopted from 2006 onward.

See also • Leq (equivalent continuous sound level)

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

See also • the exposure action valueexposure limit value

Damping Definition

IEC 801-24-19, dissipation of energy of an oscillating system with time or distance

The term often 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.

Removal of echoes and reverberation by the use of sound absorbing material. See reverberation time

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

Damping Ratio Definition

IEC 801-24-11, ratio of the actual damping to the critical damping.

Viscous Damping Definition

IEC 801-24-22, damping that occurs when a particle in an oscillating system is resisted by a force that has a magnitude proportional to the magnitude of the velocity of the particle and direction opposite to the direction of the particle

DAT Recorder, a 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-evening-night noise level

Day-evening noise level

Day-night noise level


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

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 T by the formulae 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.

Decibel (dB), a relative unit of measurement widely used in acoustics. The dB is a logarithmic unit used to describe a ratio between the measured level and a reference or threshold level of 0dB. The ratio may be sound power, sound pressure, voltage or sound intensity, etc.

Decibels compress a wide range of amplitude values to a more manageable set of 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.

The decibel is used extensively in acoustics - so we have a more detailed description.

  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 Lg formulae for example 20 Log10 (P/Po) dB for sound pressure levels.

The parameters below use the 10 Lg formulae for example 10 Log10 (I/Io) dB for sound intensity 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 Reference Quantities List

Definitions included in this Glossary are IEC Definitions

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


, 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).

Density (p)

, the density of a material is it's mass per unit volume.
ρ = m/V where ρ is the density, m is the mass and V is the volume.
SI units • kilogram per cubic metre, 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


, 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.


, 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 bull; stationary signalstransients may also be deterministic.

DFTDiscrete Fourier Transform

DIDirectivity Index


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 Field Sensitivity

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.

Direct Sound

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

Direct Sound Field

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.



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


IEC 801-31-29, conversion of sound energy into heat

Dissipation Factor

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, also known as spreading loss.

Dnnormalized level difference

DnTstandardized level difference

DnT,wweighted standardized field level difference

DnT,w + Ctrweighted standardized 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 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 topics

DSPDigital Signal Processing

Dual Input Room Acoustics Calculator

Duration of Shock Pulse

Dwweighted level difference

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

The Dyne was the old standard reference level for sound pressure (0.0002 dyne/cm²), the same reference level today is 20 micro Pascals, or 20 μPa


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