Help Topics - C

C weighting

One of the standard frequency correction curves (or weightings) applied to sounds in a measurement device to simulate the hearing capability of the human hearing mechanism. The C weighting is most often used for the measurement of transient or impulsive noise levels. It is specified in certain noise standards for the response of the meter to peak noise measurements since it has a defined characteristic unlike the linear (or un-weighted) frequency weighting. Other frequency weightings include the 'A', 'B' and 'Z' curves.

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Calibration

The process of checking a noise measuring instrument by applying a signal of known level and frequency to verify its operation in the field.  Any drift from the nominal reading is usually corrected prior to measurements starting by means of a change to the displayed level displayed by the meter. Some standards require that the measurement instrument be also checked at the end of a measurement to verify that no significant drift has taken place during the measurement run.

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Calibrator

A stand-alone instrument that is used to perform a field check of the sensitivity of a measurement device. These devices usually produce a single tone at a single frequency but can have multiple frequencies and levels to verify the meter.

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Carbon absorber

An add-on control device that uses activated carbon to absorb volatile organic compounds from a gas stream. (The VOC's are later recovered from the carbon.).

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Carcinogen

Any substance that can cause or aggravate cancer.

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Celsius scale

A range of temperatures over a 100-degree range expressed as 0 degrees at the freezing point and 100 at the boiling point for pure water at standard atmospheric pressure.

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Centigrade scale

A range of temperatures over a 100-degree range expressed as 0 degrees at the freezing point and 100 degrees at the boiling point for pure water at standard atmospheric pressure. Now more commonly referred to as the Celsius scale.

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Chart recorder

A hard copy device that is usually fitted with a continuous roll of preprinted paper on a roll graduated in scales that allow the level of the input signal at a particular elapsed time to be determined from a pen trace recorded onto the paper. Typical chart recorders use the dc output from a noise meter or dust monitor to drive the pen mechanism backwards and forwards across the width of the paper roll in sympathy with the changing signal levels.

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Chronic effect

An adverse effect on a human or animal in which symptoms recur frequently or develop slowly over a long period.

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Chronic exposure

Multiple exposures occurring over an extended period of time or over a significant fraction of an animal's or human's lifetime (Usually seven years to a lifetime).

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Climate change

The term 'climate change' is sometimes used to refer to all forms of climatic inconsistency, but because the Earth's climate is never static, the term is more properly used to imply a significant change from one climatic condition to another. In some cases, 'climate change' has been used synonymously with the term, 'global warming'; scientists however, tend to use the term in the wider sense to also include natural changes in climate.

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Cochlea

Part of the human hearing mechanism located within the inner ear that contains the receptor organs essential to the hearing process.  It is shaped like a small snail and contains the hair cells that detect the sound vibrations and convert a mechanical motion into nerve impulses that are sent to the brain for interpretation as sound patterns and noise identification.

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Community noise equivalent level

A twenty four hour single number equivalent noise level calculated from hourly equivalent levels where certain corrections have been added to the evening and night time noise levels to represent a penalty factor.  The evening period defined from 1900 to 2200 has a penalty of 5 dB added to the measured levels while the nigh time period of 2200 to 0700 has a 10 dB penalty added.  Thus a higher overall level is obtained for the 24 hours compared to the actual measured hourly levels obtained by normal logarithmic addition of the 24 separate Leqs.

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Compact Flash cards

Digital memory cards with storage capacity for data results from certain measurement systems such as the Casella Nomad Portable Weather Station. Capacities vary from 16Mbyte up to as much as 1 Gbyte on a single small form factor card. There are no moving parts in a Compact Flash (or CF) card so they are much more rugged and reliable in field and portable instruments designed for outdoor usage.

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Compliance monitoring

Collection and evaluation of data, including self-monitoring reports, and verification to show whether pollutant concentrations and loads contained in permitted discharges are in compliance with the limits and conditions specified in the permit.

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Compound

Chemical combination of different elements to form a substance where the compound atoms cannot be separated by physical means. Such compounds are not necessarily stable and may decay into other simpler compounds.

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Concentration

The amount of a contaminant in a unit measurement. For example, the mass in grams of particulates in a standard cubic meter of volume of air. This is referred to as mg/m³ since there is not usually enough particulate present to get beyond a few tenths of a gram in mass.

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Conical inhalable sampling head

The conical inhalable sampling head uses a 37mm GFA filter to collect samples of air in the breathing zone when used in conjunction with a personal sampling pump.

Constant bandwidth filter

A band-pass filter whose bandwidth is independent of the band center frequency. FFT analyzers are typical examples of such filters used for the analysis of vibration sources. Every filter in such an analyzer would have the same bandwidth, for example, 10 Hz wide or 100 Hz wide. The total measurement range will therefore be split up into equal parts with the same absolute resolution. For acoustic measurements across the normal range from 0 Hz to 20 kHz with a 10 Hz constant bandwidth filter there would be (20000/10) = 2000 separate results.

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Constant percentage bandwidth filter

A band-pass filter whose bandwidth is a constant percentage of the band center frequency. Whole octave or third octave filters are examples of such filters used for the analysis of acoustic sources. In this case, every filter represents a constant percentage of the center tuned frequency in an octave band analyzer. Octave band analyzers are sometimes referred to as such filters since the range of frequencies passed by the selected filter is always about 70% of the center frequency chosen by the user. For the 1 kHz octave band filter the bandwidth is from about 700 Hz to about 1,400 Hz. (1414 - 707 = 700 Hz, 700/1,000 = ~70%). For the 16 kHz octave the range is still about 70% but this represents a much larger number of frequency values that for the lower frequency bands.

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Contaminant

Any physical, chemical, biological, or radiological substance or matter that has an adverse effect on air, water, or soil.

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Contamination

Introduction into water, air, and soil of microorganisms, chemicals, toxic substances, wastes, or wastewater in a concentration that makes the medium unfit for its next intended use. Also applies to surfaces of objects, buildings, and various household and agricultural use products.

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Crest factor

This is the ratio between the peak level of an acoustic signal and its energy equivalent, or rms, level. For a continuous sine wave the peak level is 3 dB higher than the rms level when measured in logarithmic terms. When measured in linear unit terms the peak level is 1.41 (square root of 2) times the rms. value. For an impulsive signal such as a blast or an impact sound the peak level can exceed the rms level by 25 or 30 dB.

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Criterion level

The level of equivalent steady noise that lasts for exactly 8 hours that represents the allowable daily noise exposure or PEL. This level is usually taken to be 90 dB but other lower levels are also specified in certain regulations. The criterion level is the equivalent decibel level for the 100% noise dose value.

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Cumulative

The total overall sound level for the whole of a noise measurement interval from the start to the stop time including everything that occurs during the measurement run.

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Cumulative distribution

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 100% to 0% as the dynamic range increases.

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Cumulative distribution level

A distribution of samples of the instantaneous noise level taken at regular intervals during a measurement and classified according to occurrence at increments of amplitude. Typical increments of level are either 1 or 0.5 dB intervals across the measured dynamic range of the instrument plus an over range and under range counter. An instrument with a 70 dB dynamic range classifying samples at 0.5 dB intervals plus over and under range values will have 143 separate bins to store and classify the samples.  At least 1000 samples (or preferably many more) are typically needed to produce reliable values for the percentile noise levels generated from such a distribution table.

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Cutoff frequency

The frequencies that mark the end of a band or the point at which the characteristics change from a pass to a no-pass band. For example, for an octave filter in a sound level meter centered at 1 kHz the lower cutoff frequency will be 707 Hz and the upper cutoff frequency will be at 1,414 Hz. In an idealized filter all other frequencies below and above these two limits will be severely attenuated or completely removed.

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Cutoff level

The sound level at which a noise dosimeter starts to accumulate sound into the calculations used to measure a personal noise dose reading. All levels of noise below the cutoff level will be treated as if they were 0 dB and excluded form the calculation of the noise dose value according to the requirements of the OSHA workplace noise standards. Thus, if the noise levels is continuous at 79 dB all day there will be no contribution to the day's dose because the readings do not exceed the 80 dB cutoff level. The cutoff level is also known as the threshold value.

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Cycle

The complete sequence of values of a periodic quantity that occurs during one single period.

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Cycles per second

A measure of the frequency of a tone and numerically equivalent to Hertz, the preferred unit of frequency according to international standards.

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Cyclone head

A device that uses centrifugal force to remove large and heavier particles from polluted air. The smaller size particles pass up the cavity and are deposited on the filter membrane. Cyclones are designed to separate out the respirable size particles below about 4 microns such that the larger sized particles are not included in the measurement and are rejected by the sampling head.

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