D : Sound and Vibration Terms, Definitions, Units, Measurements ...
D • Level Difference
under sound insulation
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 values • exposure limit values
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
dissipation of energy of an oscillating system with time or distance
ratio of the actual damping to the critical damping.
See also •
critical damping •
sound absorption •
, a digital audio tape recorder.
Data Logging of on-going measurements, stored at regular intervals for subsequent downloading to a PC etc..
, 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 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
. 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)
the connection of a signal from one circuit to another in a manner that passes both AC and DC components.
See also • AC coupling.
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 pressure, voltage or
sound intensity, etc.
Decibels compress a wide range of amplitude values to a more manageable set of numbers.
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 formulae for example 20 log10 p/po) dB for sound pressure levels.
The parameters below use the 10 log formulae for example 10 log10
) dB for
sound power levels
See also the acoustic reference quantities list
• sound level calculations
included in this Glossary are IEC Definitions
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
, 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.
trade name for IEPE (integrated electronics piezoelectric)
under Lden (day-evening-night noise level)
, the density of a material is it's
, p = m/V
where p is the density, m is the mass and V is the volume.
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
, 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 signals •
transients may also be deterministic.
DFT • discrete fourier transform
DI • directivity 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
Diffuse Field Microphone under random incidence microphone
Diffuse Field Distance
Diffuse Field Sensitivity
recording or storing information as series of the numbers 1 and 0, to show that a signal is present
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.
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
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
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 displacement •
particle acceleration •
particle displacement used in acoustic wave theory •
particle velocity •
standard reference levels table
Distance, change in position of an object in metres - see also displacement.
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.
Dn • normalized level difference
under Ldn (day-night noise level)
DnT • standardized level difference
DnT,w • weighted standardized field level difference
DnT,w + Ctr • weighted standardized level difference with spectrum adaption term Ctr
DOD : Department of Defence (USA)
a general dodecahedron is a polyhedron having 12 faces. Acoustic examples are dodecahedron and hemi-dodecahedron loudspeakers arrange to provide isotropic
Dose related subjects
DOHR (noise dose per hour)
Dose - Noise
Dose per Hour
Dose - Vibration
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
Home • Glossary Search •
Certified Instrumentation for Hire