D : Sound and Vibration Terms ...
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, normalized to an 8-hour reference period, taking account of the magnitude and duration of the vibration - more details.
vibration at work regulations •
exposure action value •
exposure 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
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 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.
exposure action values •
exposure limit values
Daily Vibration Exposure
hand-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 value •
exposure limit value
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
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.
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-average Sound Level ASA 3.17, in decibels is the time-average frequency weighted sound level between 0700 and 2200 hours. Unit 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
community noise equivalent level •
evening noise level •
night noise level
Day Night Average Sound Level 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)
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
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.
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.
the time taken for the sound pressure level
to fall by 60 dB - a million to one.
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 0dB. 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 sound levels 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
) dB for
sound power levels
See also the acoustic reference quantities list • sound level calculations
included in this Glossary are 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
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
stationary signals •
transients may also be deterministic.
DFT : discrete fourier transform
DI : directivity index
- radiation pattern
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 Sound Field
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.
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
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.
angular displacement •
particle acceleration •
particle displacement used in acoustic wave theory •
particle velocity •
standard 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 : normalized level difference
- under sound insulation
DNL see Ldn : day-night noise level
DnT : standardized level difference - under sound insulation
DnT,w : weighted standardized level difference - under sound insulation
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 sound sources
Dose related subjects
DOHR (noise dose per hour)
Dose - Noise
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
DSP : digital signal processing
Dual Input Room Acoustics Calculator
Duration of Shock Pulse
Dw : weighted level difference
- under sound insulation
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 (20 μPa) is used
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