P-wave: a longitudinal compression wave in an elastic medium.

Particle velocity: see under acoustic particle velocity.

Pascal: a unit of pressure equal to 1 N/m².

Pass band: a band of frequencies, which are transmitted by a band pass filter.

Passive: a device, which does not require a source of power for its operation, e.g. a passive filter or a passive noise control (cf. active).

Peak: the maximum deviation of a signal from its mean value within a specified time interval.

Peak-to-peak: the algebraic difference between the extreme values of a signal occurring within a specified time interval.

Perceived noise level: of a sound; the sound pressure level of a reference sound which is assessed by normal observers as being equally noisy; the reference sound consists of a band of random noise centred on 1000 Hz.

Percentile level, LAN,T: the sound level, in dBA which is exceeded for N% of the time interval T, for example in LA10 and LA90. The time weighting (F or S) should always be specified, e.g. LAF10 or LAS10. Note that percentile levels determined over a certain time interval cannot generally be extrapolated to other time intervals.

Period: a repetitive signal; the time for one cycle.

Periodic signal: one which, repeats itself exactly.

Permanent threshold shift: the component of threshold shift, which shows no progressive reduction with the passage of time when the apparent cause has been removed.

Personal daily noise exposure level, LEP,d: that steady or constant level which, over eight hours, contains the same amount of A-weighted sound energy as is received by the subject during the working day.

Phase: of a sinusoidal signal; an angle whose value determines the point in the cycle, i.e. the magnitude of the signal, at some reference time.

Phase difference: the difference between the phase angles (of two sinusoidal signals of the same frequency).

Phon: the unit of loudness level; the loudness level of a sound, in Phons, is the sound pressure level of a 1000 Hz pure tone judged by the average listener to be equally loud.

Piezoelectric: the behavior of certain crystalline materials whereby a deformation of the material (caused by force or stress) results in the production of electric charge on the stressed faces, and a voltage difference between them.

Pink noise: a random broadband signal which has equal power per percentage bandwidth and therefore has a flat, i.e. horizontal, frequency spectrum when plotted on a logarithmic frequency scale (cf. white noise).

Pinna: the external part of the ear leading to the ear canal.

Pitch: that attribute of auditory sensation in terms of which sound may be ordered on a scale related primarily to frequency; the unit of pitch is the mel.

Plane wave: a wave in which the wavefronts are plane and parallel everywhere, so the sound energy does not diverge with increasing distance from the source.

Plenum: a chamber or space used to collect air prior to its distribution via a duct system.

PNdB: the unit of perceived noise level.

Point source: an idealised concept of an acoustic source, which radiates spherical waves.

Polarisation: a property of transverse waves but not longitudinal waves; it relates to the direction of the particle displacement in the plane normal to the direction of propagation.

Power spectral density: the amount of power per unit of frequency in a signal. It is measured in W/Hz, or more generally for a voltage signal in V²/Hz (since power is proportional to V²).

Ppv, peak particle velocity: a measure of vibration, usually measured in mm/s, used for assessing likelihood of damage to buildings from vibration.

Preamplifier: a circuit which acts as an electrical impedance matching device between a transducer with a high output impedance, such as a microphone or accelerometer, and the signal processing circuits of the sound level or vibration meter.

Precedence effect: see under Haas effect.

Pre-completion testing (PCT): sound insulation tests required of newly built (and some converted) dwellings, in order to test compliance with the performance requirements of the 2002 Building Regulations (unless built in accordance with Robust Standard details).

Preferred speech interference level: the arithmetic average of the sound pressure levels in the three octave bands 500 Hz, 1000 Hz and 2000 Hz.

Prepolarised microphone: see under electret microphone.

Presbycusis: hearing loss, mainly of high frequencies, that occurs with advancing age.

Progressive wave: a wave that travels outwards, from its source, and is not being reflected.

Psycho-acoustics: the study of the relationship between the physical parameters of a sound and its human perception.

Pure tone: a sound for which the waveform is a sine wave, i.e. for which the sound pressure varies sinusoidally with time.

Pure tone audiometer: an instrument for measuring hearing acuity to pure tones by determination of hearing levels.