Gracey & Associates
Noise and Vibration
Glossary - P
GLOSSARY INDEX:
A B C
D E F
G H I
J K L
M N O
P Q R
S T U
V W X
Y Z
p-I index : : the p-I index is equivalent to the mean pressure level minus the sound intensity level.
Pa : under Pascal
Pa².h : E : Sound Exposure .
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, which appear in a complex tone. Bells and other percussion instruments have rich partials in their spectra.
Particle Acceleration : a : to accelerate an object (air particle) is to change its velocity over a period of time. Acceleration is defined as the rate of change of velocity of an object with respect to time and is given by the equation a = (v - u)/t : where v = final velocity, u is the initial velocity and t is the time interval in seconds.
The units are m/sec2
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.
The units are m
Particle Velocity : v : the instantaneous value of the distance travelled by a particle per unit time in a medium that is displaced from its equilibrium state by the passage of a sound or vibration wave.
The units are m/sec
When applied to a sound wave through the medium of air, particle velocity would be the physical speed of an air molecule as it moves back and forth in the direction the sound wave is travelling as it passes.
Particle Velocity v = I/p = ξ·ω = ξ(2·π·f) = a/ω
where
I is the instantaneous acoustic intensity
p is the instantaneous sound pressure
ξ is the particle displacement
ω is the angular frequency
π is about 3.142
f is the frequency
a is the particle acceleration
In acoustics particle velocity is sometimes denoted by u
Particle Velocity Level : Lv = 20 log (v/vo) dB re 5 x 10-8 m/s - also known as dB SVL, sound velocity level
Particle velocity 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
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 : pico-coulomb : 1 pC = 10-12 coulombs
PE : Piezo-electric .
Peak : the highest sound pressure level at any instant during a measurement period - measured with a very short time constant meter - not to be confused with Lmax which is usually measured with a Fast or Slow weighting. Peak can only be accurately measured with a C or Flat [Z] frequency weighting, and even C-weighted Peak should be treated with discretion. The A curve and other filter introduce a time constant which makes the measurement of True Peak impossible.
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 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 or PPS is the vector sum = the square root of the summed squares of the maximum velocities, regardless of when in the time history those occur.
Peak-to-Peak : The amplitude difference between the most positive and most negative value in a time waveform, that is, the total amplitude.
Perceived Noise Level : PNL : Jet engines are perceived to be noisier than propeller aircraft and led to the development 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 Effective Perceived Noise Level - EPNL
Perceived Maximum Noise Level : PNLmax : during aircraft flyover, used in Effective Perceived Noise Level calculations.
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 Sound Exposure Meter : PSEM : a small instrument designed to be worn by an individual to provide a measure of the accumulated sound exposure received by the wearer while moving about during the workday. The instrument is calibrated in Pa².h. If the meter is worn for only a representative part of the working day, the reading must be corrected appropriately. Noise dosemeter (or dosimeter) is a more general term for instruments having a similar purpose but which may be calibrated to suit differing standards around the world - see also sound exposure.
Phase : the relative position of a sound wave to some reference point, the phase of a wave is given in radians, degrees, or fractions of a wavelength.
Phase Cancellation : phase describes where in its cycle a periodic waveform is at any given time. 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 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 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
Pico-coulomb : pC : 1 pC = 10-12 coulombs
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.
Pink Noise : 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.
Pistonphone : microphone calibrator generating a known sound pressure level, typically at a certain reference frequency.
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.
PNC : Preferred Noise Criterion. .
PNdB : Perceived Noise Level
PNL : Perceived Noise Level
PNLmax : Perceived Noise Level
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.
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 meter per second : base unit m2 kg s-3
Power Density : volume based power density - watt/m3
Power Level Sound Power Level
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 Level : the level of the power in a band one hertz wide referred to a given reference power.
PPV : Peak Particle Velocity. .
Presbycusis : impairment of hearing with age.
Preferred Frequencies : a set of standardized octave and third-octave centre frequencies defined by ISO R266. See also 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 : 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.
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 Intensity Index : LK : = Lp - LI = sound pressure level minus sound intensity level, an index of the accuracy of a sound intensity measurement, the lower the Lk the better the quality.
Pressure Residual Intensity Index : LK,0 - the residual intensity index for a given measurement system is defined as the difference between the indicated sound intensity level and the measured sound pressure level when exactly the same signal is fed into the two channels of an intensity analysing system. This index will normally be negative.
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 Density : when analysing signals, the probability density is the probability that the signal level at some point in time lies within a defined area.
Projected Dose : Proj D : uses the current dose value, logged during the present measurement, to be projected forward to give the predicted 8 hour period, assuming the the sound levels for the remaining time stays the same.
PSD : Power Spectral Density
PSEM : Personal Sound Exposure Meter .
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.
PSIL : Preferred Speech Interference Level. .
Psychoacoustics : the interaction of the human auditory system and acoustics.
PTS : : Permanent Threshold Shift. .
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.
PWL : Sound Power.
GLOSSARY INDEX:
A B C
D E F
G H I
J K L
M N O
P Q R
S T U
V W X
Y Z
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