What a noise meter is
A noise meter converts a variation in air pressure into a sound level in decibels, in a way that has been standardised so that measurements from different instruments and different laboratories can be compared. The instrument itself is straightforward; what decides whether a measurement is valid is the set of choices made around it - the frequency weighting, the time weighting, the accuracy class, and the period over which it is averaged.
Two quite different duties dominate its use. A workplace noise assessment asks how much sound a person is exposed to over a working day, and the answer feeds a hearing-conservation decision. An environmental or boundary assessment asks what level is being emitted at a defined point over a defined period, and the answer feeds a regulatory or contractual obligation. The quantities and averaging periods differ between the two, so the instrument and the method have to be chosen for the duty rather than in the abstract.
The accuracy class of the instrument sets how much its own uncertainty can be relied on. Frequency analysis, where it is available, is what turns a reading that exceeds a limit into a decision about which control measure would reduce it. This page covers those choices, and the boundary against the other environmental measurements taken on the same site.
How a noise meter measures
Sound level is expressed in decibels on a logarithmic scale, which means a change of a few units represents a substantial change in acoustic energy rather than a small one, and that two sources of equal level together raise the total by a fixed small increment rather than doubling the figure. This is why noise control is rarely a matter of a small adjustment and usually a matter of dealing with the dominant source.
Frequency weighting applies a correction across the spectrum to approximate how the ear responds, so that the single figure reflects perceived loudness rather than raw energy. A-weighted and C-weighted readings of the same event differ, and a method states which is required - which is why a reading quoted without its weighting cannot be interpreted. Time weighting determines how quickly the instrument responds: a fast setting follows the level closely, a slow setting smooths it, and an impulse setting captures very short events such as a hammer blow.
For any assessment covering a period rather than an instant, the instrument has to average. An equivalent continuous level expresses a varying sound as the steady level that would contain the same energy over the same period, and it is the quantity most occupational and environmental criteria are written around. Taking instantaneous readings at intervals and averaging those numbers is not the same calculation and does not give the same answer.
Configuration choices
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
| Instrument class | The two standard classes, differing in how tightly the accuracy is specified | What the measurement can be used to demonstrate. A stricter assessment or a regulatory purpose usually names the more accurate class. |
| Frequency weighting | A-weighting, C-weighting, and a flat or unweighted setting | Whether the reading reflects perceived loudness, the low-frequency energy present, or the raw level. The method states which is required. |
| Time weighting | Fast, slow and impulse settings | How closely the instrument follows a rapidly changing level, and whether a short transient event is captured at its true magnitude. |
| Averaging over a period | The equivalent continuous level over a set period, and the period itself | Whether the result matches the criterion being assessed, which is normally written as an average over a working day or over a defined interval. |
| Personal exposure measurement | A body-worn instrument that accumulates dose over a shift | Whether the exposure of a person who moves around a site can be measured at all, which a static instrument cannot do. |
| Frequency analysis | Octave band and third-octave band analysis | Whether a reading above a limit can be attributed to a frequency range, which is what a control decision needs. |
| Data logging and reporting | Internal logging at an interval, and whether the record can be exported | Whether the evidence for an assessment can be produced, which a single reading cannot supply. |
| Acoustic calibration | The calibrator used, and whether the instrument can be checked before and after a session | Whether the reading is traceable. A measurement session with no check at either end has no demonstrable validity. |
How to choose a noise meter
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|
| The purpose of the measurement | Workplace exposure and boundary emission need different quantities and different averaging | State the duty - occupational, environmental or a contractual limit - rather than the level expected. |
| Criterion, and the class it demands | A regulatory limit or a contractual term often names the instrument class | Quote the requirement, including any class or method it references. |
| Weighting and averaging in the criterion | A figure without its weighting and averaging period cannot be compared with a limit | State the weighting and the period, and whether the criterion is a level or a dose. |
| Whether the source, the person, or both move | A moving source or a mobile worker needs a different measurement from a fixed point | Describe the situation, and whether the instrument has to be worn. |
| Need for frequency information | Choosing a control measure usually requires knowing which frequency band dominates | Say whether the measurement has to support a decision about reducing the level. |
| Duration and repetition | A full working day, a night period, or a single short event | Give the period to be assessed, and whether it repeats. |
| Calibration requirement | Traceability has to be demonstrated for the record to be of use | State who holds the acoustic calibrator, and whether certificates are required. |
If any of these are still open, send what is known. A quotation can be issued against a duty description and refined once the gaps are filled.
The nearest alternatives
These are the types an enquiry for a noise meter usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a noise meter |
| Air quality monitors | Measure the composition of the air rather than its acoustic effect, and are concerned with gases and vapours. The two often share a monitoring station because both describe how a site affects its surroundings, but nothing about one reading informs the other. |
| Particle counters | Report particulate concentration, usually within a defined size fraction. Like noise, particulate exposure carries both occupational limits and boundary obligations, so the two are assessed in parallel rather than substituted for each other. |
| VOC detectors | Respond to volatile organic compounds, either as a total figure or against specific substances. They are a chemical measurement, and the instrument, the unit and the sampling method have nothing in common with an acoustic one. |
| Light meters | Measure illuminance, which is the other environmental factor with both a workplace adequacy and a comfort dimension. The instrument and the unit are unrelated to a sound level measurement. |
| Weather stations | Record wind, temperature and rainfall alongside other parameters, and are relevant to a noise measurement because wind on a microphone or rain on a surface is itself a source of error. An outdoor measurement normally needs weather data in order to be interpreted. |
| Environmental monitoring systems | An integrated station that may combine several of these measurements on a common data platform. Where one parameter has to be measured to a defined method, a dedicated instrument is used; the system is for continuous long-term observation. |
The mistake that costs most here is treating a sound level reading as self-describing. Without its frequency weighting, its time weighting and the period it represents, a decibel figure cannot be compared with a limit or with another measurement.
What to include in the enquiry
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
- The purpose of the measurement: occupational exposure, boundary emission, or a contractual limit
- Criterion, including any instrument class or method it names
- Frequency weighting and averaging period required
- Whether the source, the person, or both move during the measurement
- Whether the measurement has to support a decision about noise control
- Duration and repetition of the period to be assessed
- Whether the instrument must be worn by a person
- Whether results must be logged and exported, and whether a calibrator is held
- Quantity, destination port and required delivery date
