P : Sound and Vibration Terms and Definitions
, the derived SI unit
It is a measure of force per unit area i.e. equivalent to the N/m2 (newton per metre-squared)
1 Pa in SI base units is kg·m-1·s-2
= 1 N/m2
= 1 J/m3
= 10-5 bar
, the reference sound pressure
= 20 μPa ≡ 0 db the threshold of hearing
at 1000 Hz
See also sound pressure • sound energy density
Pa·m3 : pascal metre-cubed
is a derived SI unit for energy
, equivalent to the Joule
Pa·s/m : pascal second per metre
, the SI unit of characteristic acoustic impedance
• specific 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
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.
: A-weighted sound pressure
PAC : sound power
, an attribute with a value - for example, weighting.
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
to accelerate an air particle is to change its velocity over a period of time, the units are m/sec2
is the rate of change of
per unit time, and is a
quantity. Also known as sound particle acceleration
See also instantaneous 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 and displacement amplitude.
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 Relationships
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 acoustic particle velocity and the 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 × sound pressure
= sound intensity
Particle velocity = sound pressure
÷ acoustic impedance
Particle velocity = sound intensity
÷ sound pressure
Particle velocity = √
÷ acoustic impedance
See also peak particle velocity
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)
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
PCM : pulse code modulation
PE : piezoelectric
a peak detector responds in less than 100µs (microseconds), according to the sound level meter standards. A typical response time is 40µs.
See also time weightings
Peak Frequency-weighted Sound Pressure Level
Peak Hold : peak detection process
, to retain 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 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 velocity • PPV measurements.
Peak Sound Level
Peak Sound Pressure
Peak Speech Power
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
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.
aircraft noise •
effective perceived noise level •
judged perceived noise level •
maximum perceived noise level •
noise and number index •
noise exposure forecast •
tone-corrected 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.
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
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
describes where in its cycle a periodic waveform
is at any given time. the phase of a wave is given in
, or fractions of a
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
the relationship in time of two or more waveforms with the same or harmonically related periods gives us a measurement of their phase difference.
is usually computed with coherence
function, shows phase difference as a function of frequency between two sets of time series data.
Phase Index : pressure intensity index
the delay between two tones of the same frequency measured in angular units of degrees or radian.
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. Unit, m/s
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 •
loudness and related terms
PI : privacy index : speech
PI Index : pressure intensity 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
, 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.
trade name for IEPE (integrated electronics piezoelectric)
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
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
= 40 + 10 log2
See also noise exposure forecast
PNL : perceived noise level
PNLmax : maximum perceived noise level
PNR : predicted noise level reduction
, 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.
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.
, 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
, 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
See also our full page on sound power and related parameters.
are directly proportional to the total power; sound power
, sound energy
and sound intensity
are power quantities
A root-power quantity, also known as a field 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 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 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.
, 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 see power spectral density
Power Spectrum Level the level of the power in a band one hertz wide referred to a given reference power.
, 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 standardised octave and third-octave centre frequencies defined by BS EN ISO 266 : ISO 266, also known as nominal frequencies.
Preferred Noise Criterion (PNC)
Preferred Speech Interference Level (PSIL)
Presbycusis impairment of hearing with age.
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)
atmospheric pressure •
static pressure •
sound pressure and related parameters
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
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
free-field microphones •
random incidence microphones
Pressure Residual Intensity Index
Pressure Intensity Index : pressure intensity index
PRN : pseudo random noise
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.
• under amplitude probability
when analysing signals, the probability density is the probability that the signal level at some point in time lies within a defined area.
ProjDose : projected noise dose
, two variables are proportional if there is always a constant ratio
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.
inverse distance law
inverse square law
PSEM : personal sound exposure meter
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.
Other noise descriptors,
ambient noise •
background noise •
broadband noise •
gaussian noise •
narrowband noise •
pink noise •
random noise •
residual sound •
specific sound •
white noise •
PSIL : preferred speech interference level
, the interaction of the human auditory system and acoustics.
PTS : permanent threshold shift
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.
PWL : sound power level
Certified Sound and Vibration Instrumentation for Hire.