Acoustic Glossary


 

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


Pa : Pascal


Pac : Sound Power.


Pa²h : Sound Exposure.


Parameter an attribute with a value - for example, weighting.


Partial

one of a group of frequencies, not necessarily harmonically related to the fundamental, which appear in a complex tone. Bells and other percussion instruments have rich partials in their spectra.
 

Partial Definition.

IEC 801-30-02, sinusoidal component of a complex sound wave

Particle Acceleration, Particle Displacement and Particle Velocity

are terms used when discussing sound waves, where the particles 'assist' the transmission of the wave but then return to their 'original' state, i.e. no net movement.

On the other hand an object, like a cricket ball experiences acceleration, velocity and displacement literally so the general acceleration, displacement and velocity terms are more apt.

 

Particle Definition.

IEC 801-21-24, portion of a medium whose volume has dimensions which are small compared to the wavelength of the sound.

Particle Acceleration

to accelerate an air particle is to change its velocity over a period of time, the units are m/sec2 : where acceleration is the rate of change of velocity per unit time, and is a vector quantity.

See also • instantaneous particle acceleration


Particle Displacement

is a measurement of distance (in metres) of the movement of a particle in a medium as it transmits a wave. In most cases this is a longitudinal wave of pressure (such as sound), but it can also be a transverse wave, such as the vibration.
 

Particle Displacement Definition.

IEC 801-21-26, RMS of the instantaneous particle displacement, over a given time interval, unless otherwise specified

See also • peak particle displacement


Particle Velocity the particles of a medium are displaced from their random motion in the presence of a sound wave. The velocity of a particle during this displacement is called the particle velocity, units m/s.

Particle Velocity is the speed of the particle and should not to be confused with the speed of sound.

Particle Velocity is related to the particle displacement and particle acceleration - as described above.

Particle Velocity multiplied by the sound pressure = sound intensity.

Particle Velocity Definition.

IEC 801-21-29 is the RMS of the instantaneous particle velocity, unless otherwise specified

See also • peak particle velocity


Particle Velocity Level (Lv) = 20 lg (v/vo) dB re 5 x 10-8 m/s and is also known as Acoustic Velocity Level and Sound Velocity Level

Particle Velocity Level Definition.

IEC 801-22-08, logarithm of the ratio of a given velocity to the reference velocity. Velocity level in decibels is 20 times the logarithm to the base ten of the ratio
Note 1 : unless otherwise specified, the reference velocity is 1 nm/s.
Note 2 : unless otherwise specified, the velocities are understood to be expressed in root-mean-square values.

Reference Particle Velocity Level (vo) = 5·10-8 m/s ≡ 0 dB

See also • the IEC Definition of Level


Pascal (Pa) is the SI derived unit of pressure. It is a measure of force per unit area i.e. equivalent to one newton per square metre or one joule per cubic metre or 0.00001 Bar

1 Pa = 1 N/m2 = 1 J/m3 = 10-5 bar : kg·m-1·s-2

1 μPa = 1 micropascal = 10-6 Pa

Sound pressure is measured in pascals.


Passband the range of frequencies between filter cut-off frequencies defining the frequency band that is not attenuated.


Passive Sound Absorber

pC : picocoulomb : 1 pC = 10-12 Coulomb

PCM : Pulse Code Modulation


PE : PiezoElectric


PeakPeak Level


Peak Detector a peak detector responds in less than 100µs (microseconds), according to the sound level meter standards. A typical response time is 40µs.


Peak Frequency-weighted Sound Pressure Level


Peak Hold peak detection process retaining the 'true' maximum value of a signal.


Peak Particle Displacement Definition.

IEC 801-21-27, greatest instantaneous particle displacement during a given time interval

See also • particle displacement


Peak Particle Velocity (PPV) Definition.

IEC 801-21-30, greatest instantaneous particle velocity during a given time interval

If measurements are made in 3-axis then the resultant PPV (peak particle velocity) is the vector sum i.e. the square root of the summed squares of the maximum velocities, regardless of when in the time history those occur.

See also • particle velocityPPV measurements.


Perceived Maximum Noise Level (PNLmax) during aircraft flyover, see the Effective Perceived Noise Level definition below.


Peak Sound Level
Peak Sound Pressure
Peak Speech Power
Peak-to-Peak


Perceived Noise Level (PNL) jet engines are perceived to be noisier than propeller aircraft and led to the development in the 1960's of a scale based on equal loudness contours called Noys.

PNdB = 40 + 10 log2 (noy). As a rule of thumb, 100 dBA ~ 112 PNdB i.e. add 12 and an increase of 10 PNdb is equal to doubling it's Noy value.

See also • the Noy

Perceived Noise Level Definition.

IEC 801-29-11, the frequency-weighted sound pressure level in decibels, obtained by a stated procedure that combines the sound pressure levels in the 24 one-third octave bands centred on 50 Hz to 10 kHz
Note 1 : the procedure is stated in ISO 3891-1978: Procedures for describing aircraft noise on the ground.
Note 2 : Perceived noise level is intended to approximate judged perceived noise level.

See also • effective perceived noise leveljudged perceived noise leveltone-corrected perceived noise level


Percentile Levels (Ln) percentage exceeded levels where 'n' is between 0.1 and 99.9% calculated by statistical analysis - may also include other descriptors i.e. A, C, L or Z weightings. Most common Ln values are A-weighted L10 and L90 levels


Period (P) a signal that repeats the same pattern over time is called periodic and the period is defined as the time it takes to complete one cycle, or repetition. The period of a periodic waveform is the inverse of its fundamental frequency = 1/f.


Permanent Threshold Shift (PTS)


Personal Noise Exposure see daily personal noise exposure and/or weekly personal noise exposure.


Personal Sound Exposure Meter (PSEM)


Phase describes where in its cycle a periodic waveform is at any given time. the phase of a wave is given in radians, degrees, or fractions of a wavelength.


Phase Cancellation occurs when two signals of the same frequency are out of phase with each other resulting in a net reduction in the overall level of the combined signal.


Phase Difference the relationship in time of two or more waveforms with the same or harmonically related periods gives us a measurement of their phase difference.


Phase Function is usually computed with coherence function, shows phase difference as a function of frequency between two sets of time series data.


Phase Lag the delay between two tones of the same frequency measured in angular units of degrees or radians.

Phase Shift the angular difference between two signals, which reflects the time difference.

Phase Velocity Definition.

IEC 801-23-20, velocity in the direction of propagation of a surface of constant phase

Phon

two sounds may have the same sound intensity but may not sound equally loud because the human hearing sensitivity varies with frequency. equal loudness contours which show the variation for the average human ear have been plotted. If 1000 Hz is chosen as the reference frequency, then each equal loudness curve can be referenced to the decibel level at 1000 Hz.

This is the basis for the measurement of loudness in phons. If a given sound is perceived to be as loud as a 40 dB sound at 1000 Hz, then it is said to have a loudness of 40 phons.

Phon Definition.

IEC 801-29-07, unit of loudness level, judged or calculated as specified in definition of "loudness level" or definition of "calculated loudness level"

See also • sones


PI Index under Sound Intensity Pressure Index

PI : Privacy Index

Picket Fence Effect information between samples in FFT spectrum analysis may be missing. hanning windows may help


pico (p) a SI prefix = 10-12 • see other SI units

Picocoulomb (pC) 1 pC = 10-12 coulomb

Picofarad (pF) a million millionth of a farad, 10-12 farad

Picowatt (pW) a million millionth of a watt; 10-12

Piezoelectric (PE)

any material which provides a conversion between mechanical and electrical energy. Piezo is a Greek term which means 'to squeeze'. If mechanical stresses are applied to a piezoelectric crystal then an electrical charge results. Conversely, when an electrical voltage is applied across a piezoelectric material, the material deforms.

The piezoelectric property of materials is used in transducers that convert acceleration (or force) into electrical signals, and vice versa.


Piezotron ® trade name for IEPE - Integrated Electronics PiezoElectric.

Pink Noise

unlike white noise which is uniform and characterless, the pink noise spectrum falls at 3 dB per octave; so the energy content is inversely proportional to frequency i.e. -3 dB per octave or -10 dB per decade.

This is useful when using sound analysers with constant percentage bandwidth octave or third-octave filters, the net result is a flat spectrum

 

Pink Noise Definition.

IEC 801-21-11, noise whose power spectral density is inversely proportional to frequency

Other noise descriptors • ambient noisebackground noisebroadband noisenarrowband noiseperiodicpink noisepseudo random noiserandom noisespecific noisewhite noisewideband noise

See also • constant bandwidthconstant percentage bandwidthsoctave bandsFFT - fast fourier transform.


Pistonphone

a microphone calibrator generating a known sound pressure level, at a reference frequency. They are highly accurate, typically 0.1 dB and as the name suggests the sound level is generated by pistons moving air in a fixed coupler formed by the Pistonphone and the microphone under test.

They are single frequency devices, usually 250 Hz and include a calibrated barometer to correct for local changes in atmospheric pressure.


Pitch

is a subjective auditory sensation and depends on the frequency, the harmonic content and to a lesser extent on the loudness of a sound, see also tone
 

Pitch Definition.

IEC 801-29-01 that attribute of auditory sensation in terms of which sounds may be ordered on a scale extending from low to high
Note 1 : the pitch of a complex wave depends primarily upon the frequency content of the stimulus, but it also depends upon the sound pressure and the waveform.
Note 2 : the pitch of a sound may be described by the frequency of that pure tone having a specified sound pressure level that is judged by subjects to produce the same pitch.

Plane Wave or Planewave

Planning and Noise the Planning Policy Guidance PPG24 sets out the UK Government's policies on different aspects of planning. Local authorities must take their content into account in preparing their development plans.

Some UK planning policy guidance is available for downloading


PNC : Preferred Noise Criterion


PNL : Perceived Noise Level

PNLmax : Perceived Maximum Noise Level

PNR : Predicted Noise Level Reduction

Point Sound Source Definition.

IEC 801-29-10, source that radiates sound as if from a single point.

Point Source

a noise source whose dimensions are small compared to the propagation distances involved.

We know from the inverse square law that the sound level pressure level decreases by 6 dB every time the distance between the measurement point and the source is doubled.

See also • line source


Polar Pattern or Polar Response microphones respond to sound coming from different directions with varying degrees of sensitivity. A plot or graph of this response is called a polar pattern (sometimes polar response curve). Similarly loudspeakers and other sound sources have Polar Responses.

Polar Patterns are frequency dependent, the low frequency response may be almost omnidirectional but the polar pattern will be come more directional as the frequency rises up the audio range.


Post Processing

the application of a mathematical function to a signal after measurement to further improve the information that can be obtained from the analysis.

Potential Sound Energy Density.

Power (P)

is the rate at which work is performed or energy is transmitted. The unit of power is the watt

W = J/s joule per second = N m/s newton metre per second : base unit m2 kg s-3

See also • our full page on sound power and related parameters.


Power Density
Power Level


Power Quantity

is a power or a quantity directly proportional to power, for example sound power. Energy quantities, for example sound energy may also be labelled as power quantities. A root-power quantity is a quantity such as sound pressure, when squared is proportional to the sound power.

Power Reference Levels

Power Spectral Density

(PSD) the spectral density of the wave, when multiplied by an appropriate factor, will give the power carried by the wave, per unit frequency, known as the power spectral density (PSD) of the signal. PSD is commonly expressed in watts per Hertz (W/Hz)
 

Power Spectral Density Definition.

IEC 801-21-44, limit as the bandwidth approaches zero, of sound power divided by Bandwidth, also known as power spectrum density.

Power Spectrum

the average squared magnitude of multiple frequency spectra.

Power Spectrum Averaging also called RMS Averaging, calculates the weighted average of the sum of the squared levels. The weighting is either linear or exponential. Power Spectrum Averaging reduces random fluctuations in the levels but does not reduce the noise floor.

See also other types of averaging

Power Spectrum Density Definition under Power Spectral Density Definition

Power Spectrum Level the level of the power in a band one hertz wide referred to a given reference power.


P-P the amplitude difference between the most positive and most negative value in a time waveform, that is, the total amplitude.


PPG : Planning Policy Guidance 24

PPV : Peak Particle Velocity

Predicted Noise Level Reduction (PNR)

is the calculated noise level reduction at the ear when using ear protectors, based on the manufacturers' HML figures and the measured noise levels at the operators position.

Preferred Frequencies a set of standardized octave and third-octave centre frequencies defined by BS EN ISO 266 : ISO 266 - also known as Nominal Frequency.

Frequency Octave 1/3 octave   Frequency Octave 1/3 octave   Frequency Octave 1/3 octave
16 Hz   x x                
20 Hz     x   200 Hz     x   2000 Hz   x x
25 Hz     x   250 Hz   x x   2500 Hz     x
3.15 Hz   x x   315 Hz     x   3150 Hz     x
40 Hz     x   400 Hz     x   4000 Hz   x x
50 Hz     x   500 Hz   x x   5000 Hz     x
63 Hz   x x   630 Hz     x   6300 Hz     x
80 Hz     x   800 Hz     x   8000 Hz   x x
100 Hz     x   1000 Hz   x x   10000 Hz     x
125 Hz   x x   1250 Hz     x   12500 Hz     x
160 Hz     x   1600 Hz     x   16000 Hz   x x


Preferred Noise Criterion (PNC) a noise measurement system for continuous or ambient noise in indoor environments proposed by Leo Beranek in 1971.


Preferred Speech Interference Level (PSIL)

Presbycusis impairment of hearing with age.

Pressure is defined as the force exerted per unit area. The SI unit of pressure is the Pa pascal or N/m2 newtons per square metre.

See also • atmospheric pressurestatic pressuresound pressure and related parameters


Pressure Gradient

the change in pressure with distance, from lower to higher pressure, or vice versa. Used in the determination of sound intensity, the pressure gradient enables particle velocity to be measured.

Pressure Index
Pressure Intensity Index


Pressure Microphone

designed to measure the pressure that exists in front of the microphone diaphragm as opposed to the common free-field microphone. Used to measure the pressure in cavities or flush mounted in aircraft wings, etc.

The presence of the microphone should not to effect the measurement.

Because of their importance in acoustics we have a full page on measurement microphones

See also • free-field microphonesrandom incidence microphones


Pressure Residual Intensity Index

Privacy Index (PI)

PRN : Pseudo Random Noise

Probability

a number between 0 and 1 which represents how likely an event is to occur. Events with probability equal to 0 never occur. Events with probability equal to 1 always occur.

Probability Amplitude used to investigate the amplitude distribution of signals. Also 'known' as amplitude probability.

Probability Density when analysing signals, the probability density is the probability that the signal level at some point in time lies within a defined area.


Progressive Waves

ProjDose : projected dose


Propagation Loss Definition.

IEC 801-23-39, reduction in sound pressure level between two designated locations in a sound transmission system, one location often being at a reference distance from the source. Also known as Transmission Loss

See also • sound propagation


PSD : Power Spectral Density


PSEM : Personal Sound Exposure Meter


Pseudo Random Noise (PRN)


PSIL : Preferred Speech Interference Level


Psychoacoustics the interaction of the human auditory system and acoustics.


PTS : Permanent Threshold Shift


Pulse Code Modulation (PCM) and Adaptive Delta Pulse Code Modulation (ADPCM) subclasses of the WAV : waveform audio file format

Pulse Code Modulation works by taking discrete samples at even intervals (called the sampling rate). Common intervals are 11 kHz, 22 kHz, and 44 kHz. The higher the sampling rate, the better the representation of the original analogue wave and the better the sound quality.

Adaptive Delta Pulse Code Modulation (ADPCM) is a form of compression, is a more efficient way of storing waveforms than 16-bit or 8-bit PCM


Pulse Rise Time Definition.

IEC 801-24-29, interval of time required for the leading edge of a pulse to rise from some specified small fraction to some specified larger fraction of the maximum value.

Pure Tone a tone with a single frequency, no harmonics, for example a sine wave.

Pure Tone.

IEC 801-21-05, sinusoidal acoustic oscillation, also defined by the IEC as Pure Sound

PWL : Sound Power Level

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