Acoustic Glossary


 

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


Early Decay Time (EDT)

is derived from the reverberation time decay curve; between 0 dB and 10 dB below the initial level.

A short EDT is a good indicator of speech clarity, as early reflections that reach the listener within 50 milliseconds integrate with the direct sound and can improve speech clarity - see the Haas effect.

See also • Schroeder

Ear Simulator Definition IEC 801-28-05,

device for the calibration of earphones, incorporating a calibrated microphone for the measurement of sound pressure and an acoustic coupler such that the overall acoustic impedance is similar to that of the normal human ear in a given frequency band, also known as an artificial ear

EAV • exposure action value

Echo

, sound waves are reflected by walls and other hard surfaces and if the arrival of the reflected wave is delayed by more than 25 milliseconds, see the Haas effect, then the delayed sound is noticeable and called an echo. When a sound wave experiences multiple reflections this is known as reverberation
 

Echo Definition IEC 801-31-21,

sound wave that has been reflected and arrives with such a magnitude and time interval after the direct sound as to be distinguishable as a repetition of it.

See also • flutter echomultiple echo


Effective Level

is the root mean square of the instantaneous level over a given period of time. The shortened suffix eff is also used in formulae, for example Peff for effective sound pressure

The effective level is also known as the average level and the effective value.

See also • RMS (root mean square)

Effective Perceived Noise Level (EPNdB)

, is a rating used to certify aircraft types for flyover noise, based on the perceived noise level, corrected for tone and duration, also known as LEPN and EPNL.
 

Effective Perceived Noise Level Definition IEC 801-29-15,

level in decibels of the time integral of the antilogarithm of one-tenth of the tone-corrected perceived noise level, over the duration of an aircraft flyover; the reference duration is 10 s.
Note 1 : the integral is usually approximated by one-half the summation, over the top ten decibels of an aircraft flyover, of the antilogarithms of one-tenth of the tone-corrected perceived noise level in successive 0,5 s intervals.
Note 2 : Effective perceived noise level purports to represent subjective noisiness.
Note 3 : Effective perceived noise level of an aircraft flyover tends to be 2 dB or 3 dB greater than A-weighted sound exposure level.

See also • judged perceived noise levelmaximum perceived noise levelnoise exposure forecastnoise and number indexperceived noise leveltone-corrected perceived noise level


Effective Sound Intensity
Effective Sound Pressure


Effective Value Definition IEC 103-02-03,

for a time-dependent quantity, positive square root of the mean value of the square of the quantity taken over a given time interval

See also • effective levelRMS (root-mean-square) value


Elastic Medium

, a term used when discussing sound waves, where the particles in the medium 'assist' the transmission of the wave but then return to their 'original' state, i.e. no net movement, also known as elastic wave

Electret Microphones

are widely used in noise measurements because they offer the good linearity, wide frequency range and high stability.

Because of their importance in acoustics, we include more details, see measurement microphones


Electrostatic Actuator Definition IEC 801-28-10,

device comprising an auxiliary electrode that permits the application of an electrostatic force to the metallic or metallized diaphragm of a microphone in order to obtain a calibration.

Elementary Attenuation of Propagation Definition IEC 801-23-36,

real part of the elementary exponent of sound propagation.

See also • propagation losssound propagation


Elementary Dephasing of Sound Propagation Definition IEC 801-23-38,

imaginary part of the elementary exponent of sound propagation.

Elementary Exponent of Sound Propagation Definition IEC 801-23-34,

with respect to a system having a periodic structure, the natural logarithm of the complex ratio of particle velocities (or pressures) measured at two successive corresponding points of the structure, when this structure is assumed to be of infinite length

ELV • exposure limit value

Energy (J)

= force x distance = work, a scalar quantity equivalent to the capacity of a physical system to do work; the SI unit of energy is the Joule (J).
 

Energy Level Definition IEC 113-06-01,

for a system of particles and fields, set of states associated with a specified energy
Note 1 : this concept is particularly useful when energy changes by quanta.
Note 2 : the term energy level is also used for the associated energy.

See also • sound energy


Energy Density

is the amount of energy stored in a given system or region of space per unit volume, the SI Units are J/m3

See also • sound energy density


Energy Flux

is the rate of transfer of energy.
In the case of energy transfer per unit area, also known as energy flux density, the SI units are W/m2 and sound energy flux is in this category.

In the case of total rate of energy transfer, the SI unit is the Watt (W).

See also • sound energy flux

Energy Spectral Density (ESD) describes how the energy of a signal or a time series is distributed with frequency.

Engineering Units (EU)

, the units in which a measurement is made; for instance velocity may be expressed in millimetres per second, feet per second, miles per hour, etc., depending on the use to which the data will be put. Some instruments enable you to specify what the engineering units are and to apply conversion factors if needed.

Ensemble Averaging there are two ways to create an average or mean sample. Firstly take the values for ten components at any one time (i.e. a vertical column) add them and divide by ten. This is known as the ensemble average. Or take one component and average the levels at ten different times, to produces the time average. Conventionally the ensemble average is written E(x) and the time average is indicated by putting a bar over the x.

See also other types of averaging


Environmental Noise (UK Regulations)>


Aircraft Noise IndicatorsLAeq,16hLdayLevening.
Industrial Noise and Port Noise IndicatorsLAeq,16hLdayLevening.
Railway Noise IndicatorsLAeq,6hLAeq,16hLAeq,18hLdayLevening.
Road Traffic Noise IndicatorsLA10,18hLAeq,16hLdayLevening.


EPA - Environmental Protection Agency (USA)

EPNdB • effective perceived noise level
EPNL • effective perceived noise level


Equal Loudness Contours

have been arrived at by group consensus. 0 dB is the threshold of hearing at 1 kHz. However we are less sensitive at low and high frequencies, so the sound pressure level at 10 Hz, for example would have to be increased to 70 dB for the sound to be just detectable, or of equal-loudness - the unit is the Phon.
 

Equal Loudness Contour Definition IEC 801-29-08,

curve that shows, as a function of frequency, the sound pressure level required to cause a given loudness for a listener having normal hearing, when listening to a specified kind of sound in a specified manner.

See also • BS ISO 226 Equal-loudness contours


Equilibrium the state of a body or physical system at rest or in un-accelerated motion in which the resultant of all forces acting on it is zero and the sum of all torques about any axis is zero.

A stable situation in which all the forces cancel on another.


Equivalent Acceleration (Aeq, Aeq4, Aeq8)

Equivalent Continuous Sound Level Definition IEC 801-22-16, logarithm of the ratio of a given time-mean-square, standard-frequency-weighted sound pressure for a stated time period, to the square of the reference sound pressure of 20 μPa. Time-average sound level in decibels is 10 times the logarithm to the base ten of that ratio, also known as the time-averaged sound level and the LAeq.

Note 1 : if a frequency weighting is not specified, the A-frequency weighting is understood.
Note 2 : in principle, exponential time weighting is not involved.

Equivalent Continuous Sound Pressure Level Definition IEC 801-22-11, logarithm of the ratio of a given root-mean-square sound pressure, during a stated time interval, to the reference sound pressure. Average sound pressure level in decibels is 20 times the logarithm to the base ten of that ratio and is also known as the time-average sound pressure level

Note : unless otherwise specified, the reference sound pressure for airborne sound is 20 μPa.

Note the fundamental differences between these two definitions


Equivalent Sound Absorption Area

Erg the amount of work done by a force of one Dyne exerted for a distance of one centimetre.

In the CGS base units. 1 erg = 1 dyne cm = 10-7 Joule


ESD • energy spectral density

Estimated Noise Dose.

Estimated Vibration Dose Value
EU • engineering units

eVDV • estimated vibration dose value

Evening Noise Indicator

Exchange Rate (Q)

is the increase in noise level that corresponds to a doubling of the noise level.

Every time the sound energy doubles the measured dB level increases by 3 dB, i.e. the exchange rate Q = 3. For example if a machine produces 80 dBA then adding another identical machine increases the measured level to 83 dBA. So four identical machines, 4 times the sound energy, would measure 86 dBA, we are adding decibels not numbers. This is explained in detail with examples in our sound level calculations article.

Sound Exposure and Noise Dose follow the same rule - a doubling of the level results in a 3 dB increase. However they are also both time dependent. So if the exposure time is doubled the total sound exposure increases by 3dB. Similarly if the noise dose was 100% of the acceptable daily maximum, then doubling the exposure time results in a total noise dose of 200%, twice the daily limit.

Leq in the UK, Europe and many other places is also based on the exchange rate = 3, and the 8 hour working day level is known as the LEP,d or LEX,8h.

In America, the exchange rate defined in the OSHA standard is 5 dB and their 8 hour workplace average level is known as time weighted average. For other US exchange rates and threshold levels the average level for the measurement duration is known as Lavg.


Excitation Definition IEC 801-21-46,

external force, or other input, applied to a system, also known as stimulus.

Exponential

a decaying exponential weighting function, specified by a starting point (shift) and a constant time (length). Use exponential weighting for exponentially decaying transients longer than one time record, to avoid leakage caused by truncation.

See also • time weightings


Exponential Averaging

generates a continuous running average where the most recently sampled levels have more influence on the average than older samples. This provides a convenient form to examine rapidly changing data with the benefit of some averaging to smooth the spectra.

See also other types of averaging


Exposuredaily-noise-exposurenoise dosesound exposure,
Exposurevibrationvibration dose


Exposure Action Value (EAV)

is an 8-hour daily exposure to noise, hand arm vibration or whole body vibration exposure above which employers are required to take action to control exposure. For noise there are two action levels,
The Lower EAV is 80 dBA and a peak sound pressure of 135 dBC ≈ LCpeak

The Upper EAV is 85 dBA and a peak sound pressure of 137 dBC ≈ LCpeak

 
For hand arm vibration the EAV is 2.5 m/s2 A(8)
For whole body vibration the EAV is 0.5 m/s2 A(8)

See also • noise dosesound exposureacceleration equivalent levelvibration dose value


Exposure Limit Value (ELV)

is the maximum amount of noise and vibration an employee may be exposed to on any single 8-hour day.

For noise the daily or weekly exposure is 87 dBA and a peak sound pressure of 140 dBC.
For hand arm vibration the ELV is 5 m/s2 A(8)
For whole body vibration the ELV is 1.15 m/s2 A(8)

See also • noise dosesound exposureacceleration equivalent levelvibration dose value


Exposure Time

is the time a person is exposed to noise during a working day, and used for calculating the LEP,d (daily personal noise exposure)

See also • noise dose


External Sampling, where a level vs time record is triggered by an external signal.
 

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