Acoustic Glossary


 

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


D-weighting

D • Level 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 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. This is written as LEP,w.
 
The UK Noise Regulations require action if the following 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), normalized to an 8-hour period.

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

See also • the exposure action valueexposure limit value


Acoustic 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.

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

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


DAT Recorder, a digital audio tape recorder.

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-evening-night noise level
Day-evening noise level
Day-night noise level
Day noise indicator


dB • decibel-acoustics

dBA • dBB • dBC • dBD • dB Flat • dB Linear and dBZ - 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 T 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.

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.

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 IEC Definitions

DEFRA • Department for Environment (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 bull; stationary signalstransients may also be deterministic.


DFT • discrete fourier transform

DI • directivity index


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
Diffraction Factor


Diffuse Field Microphone under random incidence microphone


Diffuse Field
Diffuse Field Distance
Diffuse Field Sensitivity
Diffusion


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 Distance
Direct Sound Field


Directly Proportional under proportionality

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.

Dispersion


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 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 and is also known as spreading loss

Dn normalized level difference


DNL under Ldn (day-night noise level)

DnT standardized level difference

DnT,w weighted standardized level difference

DnT,w + Ctr • weighted 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%
Dose Badge
Dose per Hour
Dose - Vibration
Dosimeter


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


DSP • digital signal processing

Dual Input Room Acoustics Calculator

Duration of Shock Pulse

Dw weighted 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' reference level for sound pressure (0.0002 dyne/cm²), these days the SI reference level of 20 micro Pascals, or 20 μPa is used

 

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