# Acoustic Glossary

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

Pa (pascal), the derived SI unit for pressure and sound pressure
It is a measure of force per unit area i.e. equivalent to the N/m2 (newton per metre-squared)
1 Pa = 1 N/m2 = 1 J/m3 = 10-5 bar
1 μPa = 1 micropascal = 10-6 Pa

Pa·m3 (pascal metre-cubed) is a derived SI unit for energy, work, equivalent to the Joule

Pa·s/m (pascal second per metre) : the SI unit of characteristic acoustic impedancespecific acoustic impedance

Pa·s/m3 (pascal second per metre-cubed) : the SI unit of acoustic impedance also known as the acoustic ohm

Pa·s2/m3 (pascal second-squared per metre-cubed) : the SI units of acoustic mass and inertance.

Pa2·h (pascal-squared hour) : measurements of an employees sound exposure, (noise dose) are recorded hourly during the working day.

Pa2·s (pascal-squared second) : the sound exposure of single or short term noise event, recorded second by second during the event.

Pa2·s (pascal-squared kilosecond) : the sound exposure of noise events, recorded for a thousand seconds.

SI unit is the pascal-second per metre (Pa·s/m).

PAC : sound power

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

Partial one of a group of frequencies, not necessarily harmonically related to the fundamental-frequency, 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

Particles • particle acceleration, particle displacement and particle velocity are terms used when discussing sound waves, where the particles 'assist' the transmission of the wave through the medium 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. Also known as sound particle acceleration

Particle Displacement, is the movement of the medium, about it's equilibrium as it transmits an acoustic wave. In most cases this is a longitudinal sound wave, but it can also be a transverse vibration wave. Also known as sound particle displacement
Particle Displacement Definition IEC 801-21-26, RMS of the instantaneous particle displacement, over a given time interval, unless otherwise specified

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. Also known as sound particle velocity

Particle Velocity (v) is the speed of a particle and should not to be confused with the speed of sound (c).

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

Particle Velocity Relationships :
Particle velocity × sound pressure = sound intensity.
Particle velocity = sound pressure ÷ acoustic impedance
Particle velocity = sound intensity ÷ sound pressure
Particle velocity = (sound intensity ÷ acoustic impedance)

Particle Velocity Level (Lv) is also known as acoustic velocity level and sound velocity level

Particle Velocity Level (Lv) = 20 lg (v/vo) dB, where
v is the effective particle velocity and
vo is the reference particle velocity = 5 x 10-8 m/s ≡ 0 dB

The following notations : dB SVL, dB(SVL), dBSVL or dBSVL are often seen but are not strictly correct

Pascal (symbol Pa)

Passband is 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

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 a 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

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.

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.
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.

Percentile Noise Level (LAN,T) is the A weighted, Fast time-weighted, sound levels exceeded for n% of the specified time, where 'n' is between 0.1 and 99.9% and calculated by statistical analysis

For example LA90,1h is the A-weighted level exceeded for 90% of 1 hour.

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 Hearing Protectors

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

Personal Sound Exposure Meter

Phase describes where in its cycle a periodic waveform is at any given time. the phase of a wave is given in radian, 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 Coefficient under acoustic phase coefficient

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 radian.

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"

PI Index under sound intensity pressure 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

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

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.

Pistonphone Definition IEC 801-28-11, apparatus having a rigid piston which can be given a reciprocating motion of known frequency and amplitude so permitting the establishment of a known sound pressure in a closed cavity of small dimensions

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.

PNdB = 40 + 10 log2 (noy)

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

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

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

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 (W), a scalar quantity 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

Power Density is the amount of power per unit volume. The SI Unit is W/m3 (watt per metre-cubed).

Power Level

Power Quantity is a quantity directly proportional to power. In acoustics sound power, sound energy and sound intensity are power quantities.

A root-power quantity is a quantity, like sound pressure, which when squared is proportional to the sound power.

See also our sound level calculation article and the IEC decibel definition.

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 watt per Hertz (W/Hz) and is also known as power spectrum density.

Power Spectral Density Definition IEC 801-21-44, limit as the bandwidth approaches zero, of sound power divided by Bandwidth

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.

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.

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

Preferred frequency is also known as nominal frequency.

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 a scalar quantity is defined as the force exerted per unit area. The SI unit of pressure is the pascal (Pa) or newton per square metre (N/m2).

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 Microphone designed to measure the pressure that exists in front of the microphone diaphragm as opposed to the more common free-field microphone that compensate for the effect of the microphone on the sound field . Used to measure the pressure in cavities or flush mounted in aircraft wings, etc., i.e. 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

Pressure Residual Sound Intensity Index

Privacy Index

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 • under 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 noise dose

Propagation see sound propagationsound propagation coefficientsound transmission

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 propagation transmission loss, but should not to be confused with sound insulation transmission loss IEC 801-31-39.

Proportionality, two variables are proportional if there is always a constant ratio between them

Direct Proportion, as one value increases, another value increases at the same rate.

Inverse Proportionality, when one value decreases at the same rate that the other value increases.

Pseudo Random Noise electronically generated noise which may appear to lack any pattern, but does consist of pulses that repeat themselves periodically. The period is determined by the generator span and the number of generator lines.

Psychoacoustics, the interaction of the human auditory system and acoustics.

Pulse Code Modulation and Adaptive Delta Pulse Code Modulation are 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 Definition IEC 801-21-05, sinusoidal acoustic oscillation, also known as pure sound