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 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.
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 values • exposure limit values
See also the exposure action value • exposure limit value
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.
See also critical damping • sound absorption • viscous dampingDay-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
See also community noise equivalent level • evening noise level • night noise level
dB SIL under sound intensity level
dB SPL, dB(SPL), dBSPL and dBSPL, variations of sound pressure level
dB SWL under sound power level
Power in dBW = 10 lg (Power in W)
See also
AC coupling.
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.
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 a wide range of amplitude values into 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
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.
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 list • sound level calculations
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
See also stationary signals • transients may also be deterministic.
Diffraction
Diffraction Factor
Diffuse Field Microphone under random incidence microphone
Diffuse Field
Diffuse Field Distance
Diffuse Field Sensitivity
Diffusion
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 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.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 radiationSee 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 (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
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
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
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
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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