P : Sound and Vibration Terms, Definitions, Units, Measurements ...
, 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 N/m2
= 1 J/m3
= 10-5 bar
1 μPa = 1 micropascal = 10-6
Pa·m3 (pascal metre-cubed)
is a derived SI unit for energy
, equivalent to the Joule
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).
, an attribute with a value - for example, weighting.
, 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.
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
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
See also peak particle displacement
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)
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)
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
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 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 velocity • PPV measurements.
Peak Sound Level
Peak Sound Pressure
Peak Speech Power
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 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
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.
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.
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
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.
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.
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.
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 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 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.
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 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.
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.
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 Sound Intensity 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.
• 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.
sound propagation •
sound propagation coefficient •
, 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.
inverse distance law
inverse square law
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.
a tone with a single frequency, no harmonics, for example a sine wave
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