Definitions, Terms, Units and Parameters
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Pa : Pascal: 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
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.
Particle Acceleration, Particle Displacement and Particle Velocityterms are used when discussing sound waves, where the particles 'assist' the transmission of the wave but then return to their 'original' state, 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 Acceleration : a: to accelerate an air particle is to change its velocity over a period of time, the units are m/sec2.
Acceleration is defined as the rate of change of velocity and is a vector quantity.
Particle Displacement : ξ: 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.
Displacement ξ = v/ω = a/ω2, where v = velocity, a = acceleration and ω = 2·π·f = angular frequency.
Displacement is the change in position of an object in metres and is a vector quantity.
Particle Velocity : v: the particles of a medium are displaced from their random motion in the presence of a sound wave. The speed or velocity of a particle during this displacement is called the particle velocity, units m/s.
Particle Velocity v is the RMS - root mean square of the instantaneous velocity over a time interval at the given position and value depends on the sound pressure level, the density of the medium and the speed of sound.
Particle Velocity is related to the Particle Displacement and Particle Acceleration - see above
Velocity is the rate of change of position and is a vector quantity as both speed and direction are required to define it.
In acoustics particle velocity is sometimes denoted by u
Sound Velocity x Sound Pressure = Sound Intensity.
See also Peak Particle Velocity.
Particle Velocity Level : Lv= 20 lg (v/vo) dB re 5 x 10-8 m/s - also known as
Sound Velocity Level : SVL
Particle velocity is the speed of the particle and should not to be confused with the Speed of Sound.
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
Passband: the range of frequencies between filter cut-off frequencies defining the frequency band that is not attenuated.
Passive Absorber: a sound absorber that dissipates sound energy as heat.
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 Hold: peak detection process retaining the 'true' maximum value of a signal.
Peak Particle Velocity : PPV: the maximum or peak velocity value during a given measurement period. If measurements are made in 3-axis then the resultant PPV is the vector sum = the square root of the summed squares of the maximum velocities, regardless of when in the time history those occur.
See also Particle Velocity
Peak-to-Peak: the amplitude difference between the most positive and most negative value in a time waveform, that is, the total amplitude.
Perceived Maximum Noise Level : PNLmax: during aircraft flyover, used in Effective Perceived Noise Level calculations.
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 lg2 (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 later Effective Perceived Noise Level - EPNL
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: a signal that repeats the same pattern over time is called
periodic, and the period is defined as the length of time encompassed by one cycle, or repetition. The period of a periodic waveform is the inverse of its fundamental frequency.
Permanent Threshold Shift : PTS: the component of threshold shift that shows no progressive recovery with the passage of time when the apparent cause has been removed. Noise-induced permanent threshold shift (NIPTS) is the component of PTS associated with a noise exposure. Age-related threshold shift (ARTS) is the component of PTS related to age. It is usually assumed that these components are additive, at least for small values of the components. A permanent decrease of the acuity of the ear at a specified frequency as compared to a previously established reference level. The amount of permanent threshold shift is customarily expressed in decibels. See also Threshold Shift : Temporary Threshold Shift
Personal Noise Exposure: Daily Personal Noise Exposure or Weekly Personal Noise Exposure.
Phase: 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: 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.
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. See also sones
pico : p: 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
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, pink noise has equal power in constant percentage bandwidths, for example, octave bands, over a specified frequency range. The energy content is inversely proportional to frequency i.e. -3 dB per octave or -10 dB per decade. Other Noise Terms.
Pistonphone: microphone calibrator generating a known sound pressure level, typically at a certain 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: depends primarily upon the frequency of the sound stimulus, but it also depends upon the sound pressure and waveform of the stimulus.
Plain Wave: a wave whose wave fronts are parallel planes perpendicular to the direction in which the wave is travelling.
Planning and Noise: Planning policy guidance notes set 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 Planning Policy information is available for downloading
Point Source: a noise source whose dimensions are small compared to the propagation distances described in reference to it.
Polar Pattern or 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.
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
Power Density: is the amount of power per unit volume - watt/m3
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. Power spectral density is commonly expressed in watts per Hertz (W/Hz)
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. RMS averaging reduces random fluctuations in the levels but does not reduce the noise floor.
Power Spectrum Level: the level of the power in a band one hertz wide referred to a given reference power.
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||1/1 octave||1/3 octave||Frequency||1/1 octave||1/3 octave||Frequency||1/1 octave||1/3 octave|
|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: is the arithmetic average of the 500 Hz, 1 kHz and 2 kHz octave bands.
Note: although the PSIL, SIL and SIL3 are defined for octave band levels they are also calculated from the 1/3-octave bands within each octave before doing the average.
Presbycusis: impairment of hearing with age.
Pressure : p: Pressure is defined as the force exerted per unit area. The SI unit of pressure is the Pa pascal or N/m2 newton per square metre - see also sound pressure.
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 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 we have a full page on Measurement Microphones
See also Free-field Microphones, Pressure Microphones, Random Incidence Microphones
Privacy Index : PI: Privacy Index is a measurement of speech privacy. PI can be derived from the AI, articulation index, measurement using the following formula: PI = 100(1-AI)
PI = 0 indicates a complete lack of privacy
PI = 100 means complete privacy
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.
Pseudo-random Noise: a periodic signal where one period is a segment of a random signal. The period is determined by the generator span and the number of generator lines. Other Noise Terms.
Psychoacoustics: the interaction of the human auditory system and acoustics.
Pure Tone: a tone with a single frequency - no harmonics a sine wave. A sine wave is characterized by its frequency - the number of cycles per second, or its wavelength - the distance the waveform travels through its medium within a period, and the amplitude - the size of each cycle.
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