Help Topics - P

Particulate

Can be an aerosol, mist, fog, dust, fumes or smoke depending upon the source.

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Passive sampling

Air sampling without the aid of a sampling pump typically employing diffusion tubes.

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Peak

The absolute highest positive or negative pressure of a sound wave at a given point in time and space.

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Peak response

This is one of the standard responses in a sound level meter (or dosimeter).

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Peak noise level

The noise level measured with a sound level meter set to the peak response in dB usually with no frequency weighting, i.e. a linear peak, or with the C weighting, i.e. a C weighted peak level.

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Peak sound pressure level

The maximum absolute value of the instantaneous sound pressure in a specific interval of time. In the case of a sine wave produced by a steady noise source such as an acoustic calibrator the peak sound pressure level is 1.4 times the rms pressure level. This is equivalent to the peak level being 3 dB higher than that rms steady level.

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PEL

The Permitted Exposure Level of a hazardous substance is the amount that a worker can be exposed to for a standard 8 hour workday. For noise measurements the PEL is a time weighted average level of 90 dB according to the OSHA workplace noise regulations.

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Percentile noise level

The notional noise level in dB exceeded for a given percentage of the measurement duration. Usually expressed as LN% where n represents the percentile value from 0 to 100 %. The L0% is equivalent to the maximum noise level since it is the level exceeded for 0% of the time, i.e. no readings are higher than the L0%. The L100% is equivalent to the minimum noise level since it is the level exceeded for 100% of the time and so is exceeded all of the measurement time. The L10% is often taken as a measure of the noise produced by highway traffic while the L90% is often taken as the background noise level. The L50% is the mean sound pressure level but it is not the same as the time average level, which is the unit Leq.

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Period

The duration in time that it takes an oscillation to repeat itself. For example a frequency of 1 kHz repeats itself with a period of 1/1000 = 0.001 seconds. A period can also mean the shorter measurement interval during a long run such as a 10-minute period in an 8-hour day measurement at a specific monitoring site.

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Permanent threshold shift

A permanent decrease of the hearing acuity of the ear at a specific frequency as compared to a previously established reference level. The amount of permanent threshold shift is commonly expressed in decibels and is unrecoverable by the individual who will not regain his or her hearing even by halting any exposure to excessive noise levels. It is an irreversible condition of the inner ear caused by damage to the individual hair cells in the cochlea.

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Personal noise dosimeter

A small portable noise meter especially developed to be worn by an individual during the working day. Its purpose is to accumulate all the different noise exposure suffered by the individual whether it comes as steady noise exposure or whether it is in the form of sudden impulses of noise. The microphone is usually clipped to the collar close to the hearing zone and the body of the dosimeter is worn on the belt or in a pocket for safety. Some newer noise dosimeters have the microphones built-in to the body of the unit so there are no cable to get tangled or caught. Traditionally noise dosimeters readout the results in dose based on a percentage of the allowed daily maximum. More modern noise dosimeters also produce the results in dB form for the average level plus maximum and minimum levels.

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Personal noise exposure

Noise exposure in the workplace that can affect a worker's hearing detrimentally over extended periods of exposure at high noise levels. Regulations exist in most industrialized countries to limit the danger to worker's hearing from such high levels of noise.

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Personal protection

Protection worn by an individual against an anticipated hazard at work. Possible examples include hard hats, boots with steel tips, ear plugs, ear muffs etc.

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Personal sampling

Occupational sampling of airborne contaminants in particulate form with sampling equipment worn by the operator. Measurements are normally taken in the breathing zone of the operator.

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Personal sampling pump

A small battery operated device worn by a worker that draws air at a constant rate across a filter mounted in a cassette. By keeping the flow rate constant a known volume of air is sampled in a given time interval. For example, at 2 l/m flow rate a sampling pump will pull through 120 liters of air in 1 hour. The particulates in the air will be collected on the filter in the cassette and can be weighed at the end of the measurement to give a concentration in mg/m³.

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Phon

The logarithmic unit of measurement of the loudness level of a sound.

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Pink noise

Noise with a constant energy per octave (or third octave) bandwidth. A spectrum analysis of a pink noise source with a real time octave (or third octave) band analyzer would reveal a flat response across the frequencies of interest. Pink noise is often used as a stimulus signal in measurements of the characteristics of buildings and rooms since it produces equal energy across the whole audio bandwidth.

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Pitch

The attribute of auditory sensation that orders sounds on a scale extending from low to high. Pitch depends primarily on the frequency of the sound stimulus, but it also depends on the sound pressure and waveform of the stimulus.

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Plane wave

A sound wave whose wave fronts are parallel and perpendicular to the direction of propagation that the wave is traveling in.

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PM2.5

Particulate matter having a mean aerodynamic diameter of 2.5 microns usually related to ambient air monitoring.

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PM10

Particulate matter having a mean aerodynamic diameter of 10 microns usually related to ambient air monitoring.

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Preamplifier

An electrical device that acts as the interface between the microphone (or accelerometer) on a sound level meter (or vibration meter) that is used to match the high impedance of the transducer to the following electrical circuits of the measuring instrument. The preamplifier comes before any extension cable, where the microphone is remotely mounted from the meter, to prevent any significant signal loss down the extension cable caused by capacitive effects. Some preamplifiers are equipped with heater elements to minimize the harmful effects of moisture around the microphone capsule.

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Precision

An indication of the uniformity or reproducibility of a result of a physical measurement. Precision relates to the quality of an operation by which a measurement result is obtained, and is distinguished from accuracy, which relates to the quality of the result itself. Thus a measurement can have good precision in that repeated results will all be close together but still be inaccurate if they differ from the "true" result by some offset value or bias.

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Presbycusis

The reduction in human hearing acuity that is attributed simply to the normal aging process. This will occur irrespective of any exposure to high noise levels in the workplace.

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Pressure drop

The differential pressure drop across a restriction, such as a sample filter in a cassette, normally measured in inches (or cm) of water.

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Probability

A distribution of samples of the instantaneous noise level taken at regular intervals during a measurement where the samples are arranged according to their percentage of occurrence. The distribution ranges from 0% to a maximum % as the dynamic range increases. Each sample is allocated a place in the distribution according to its sampled level.

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Probability distribution level

The distribution of noise samples across the dynamic range of an instrument arranged at regular class widths such as every 0.5 dB. For an instrument with a 70 dB dynamic range there will be 143 class widths if the sampling is carried out at 0.5 dB steps. This will include an under and over range bin. The total number of samples to produce the percentage probability level divides the number of samples in each bin. For a random noise the distribution will have a Gaussian distribution.

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Projected noise dose (Proj%)

This is the projection of the actual measured noise dose to what it would be if the measurement were to continue for a full 8 hours. This value is usually used to predict what the eventual outcome would be at the end of a standard working day when a representative period of time has been used to carry out the measurement. If the actual measurement lasted for 1 hour 30 minutes. and a measured noise dose was found to be 27% then the 8 hour projected value would be calculated as 27(8/1.5) = 144%.

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PUF filters (PolyUrethane Foam)

Small cylindrical foam filters that can be inserted into the air stream when performing air sampling to limit the size of the average particles reaching the cassette or passing through an optical system. Different sized foams are available to provide different cut values depending on the purpose required for the measurement. Typical pore sizes for the PUF filter foams are 10, 4 and 2.5 microns. A size selective adaptor will be required to hold the PUF filter in place.

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Pure tone

A sound for which the sound pressure is a simple sinusoidal function of time, and characterized by a singleness of pitch (or frequency). Such examples are acoustic calibrators that generate a pure tone as the reference level.

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Purge

To keep an air sampling system clean and monitoring correctly by providing a supply of clean air across the optical lenses to prevent contamination. If the lenses become contaminated the system will under read the true concentration levels encountered during the run.

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