The following pages present guidance intended to help you identify your calibration requirements and to identify an appropriate calibration strategy for your acoustical measurement needs.

• Measurement Objectives
• Risk Assessment
• Calibration and Measurement Traceability
• Measurement Instruments

What this process WILL do:

• help you to understand the relative importance of your measurements
• help you to understand the risks that could be incurred if measurements are taken with instruments which are inadequately calibrated
• help you to identify areas where you need a higher level of calibration, or could tolerate a lower level of calibration provided that instruments are only used to take certain types of measurement.

What this process WILL NOT do:

• Define a calibration strategy for you. As every user's needs are different, a suitable calibration strategy can only be devised by the user.

The process is illustrated by examples. It is recommended that the user follows through the process and adapts the methods used to aid the definition of a calibration strategy appropriate to their needs.

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Measurement Objectives

To define an appropriate calibration strategy, we need to start at the beginning and establish what measurements are taken, and why.

Why are measurements being taken? What is the end purpose?

Examples might include:

• Noise survey to regulatory or legislative requirements
• Routine monitoring tasks
• Production conformance testing
• Scoping study or preliminary feasibility study
• Medical/Audiometric Testing

It is quite likely that you may carry out more than one activity - in which case, list them all.

These responses will help to define the relative importance and necessary level of confidence that is required in your measurements.

How often are measurements taken?

For each activity which you have identified above, consider how often you typically carry out this activity, for example, weekly, monthly, yearly, or less often.

These responses will help to define which instruments are needed regularly and therefore need to be maintained in a suitably calibrated state. Calibration intervals for instruments that are used infrequently could be extended, as long as the instruments are appropriately calibrated for the occasions when they are needed.

What measurements are actually taken?

Examples might include

• Sound Pressure Levels
• Frequency Content
• Acoustic Intensity
• Noise Dose

You must consider these quantities later on when choosing which types of calibration are required. For example, if you are interested in the frequency content of a noise then it is important that that the frequency response and potentially any frequency analysis functionality of an instrument is checked during calibration. Alternatively, if you are only interested in whether a noise may be audible to a test subject, then the instrumentation and calibration requirements may be less stringent.

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Risk Assessment

For each measurement activity, try to assess the levels of risk associated with each activity. To do this, you may find it helpful to consider the consequences that would result in each case if inaccurate or invalid measurements were made.

In the example table shown, risks are divided into "Technical" and "Commercial" risks. Other classifications may be more suitable for your organisation. For example, for audiometric testing there may be no direct or obvious "Commercial" risk, but it may be appropriate to include "Medical" risks.

Commercial Risk

Commercial risk is considered as anything which might impact upon the operation of your business. Examples might include

• Potential impacts upon sales or profitability
• Impacts upon company reputation

For example, a noise survey to legislative requirements might represent a high commercial risk. "Questionable" measurements may be closely scrutinised and subsequently challenged in court, leading to additional costs and damage to the reputation of a company.

Technical Risk

Technical risk is considered as anything which impacts upon the technical operations of your business. Examples might include

• Impacts upon product quality
• Impacts upon technical decisions

For example, measurements made during a preliminary feasibility study represent low technical risk, if these measurements are used for indication only.

Scoring

In order to quantify the risks involved in each activity, it is suggested that the levels of risk associated with the activity be assessed, for example by using a numerical scoring system from 0 to 5.

A score of 0 would imply no risk at all, while a score of 5 would represent a severe impact upon the technical or commercial operations of your organisation, for example, severe impacts upon profitability, loss of sales, or severe impact upon product quality.

The risk levels associated with each category can then be added together to signify the combined risk level.

Level of Importance of Measurements

The level of importance of your measurement can be assessed using the combined risk level, which here represents the sum of the technical and commercial risk levels.

• For example a level of 10 represents measurements with a high level of risk, for which the highest level of confidence in the instrument readings is required.
• A level of 0 would represent a measurement which carried no risks of any kind and had little or no importance.
• The higher the result, the higher the risk, and thus the higher the importance of your measurements.

The following table contains some examples of this process:

Suggested Example	Technical Risk (0-5)	Commercial Risk (0-5)	Combined Risk Level (Technical Risk + Commercial Risk)	Importance
Noise survey to regulatory or legislative requirements	5	5	10	Very High
Medical / Audiometric Testing	5	3	8	High
Routine Monitoring Tasks	3	3	6	Medium
Scoping Study	1	2	3	Low

Note that this table is for demonstration purposes only.

Probability of Risks Occurring

A risk assessment is generally formed of a combination of severity of the identified risk, and the probability of its occurrence.

The probability of a risk occurring, for example, the risk of calibration drift, may be linked to the following factors:

• Frequency of instrument use, assuming that an instrument which is used often is more likely to be subject to impacts, vibration and general damaging factors.
• Environmental factors, for example, exposure to moisture, temperature extremes, dirt or dust.

It can be seen that the frequency of use of an instrument can affect the probability that an individual risk will occur (i.e. the probability of a specific fault occurring per hour of use, or the likely severity of the fault) as well as increasing the hours of use. Thus the aggregate probability that a risk will cause an impact (i.e. the probability of the fault occurring in any one hour multiplied by the number of hours used per year) is doubly impacted.

There are various methods which can be used to assess specific risk, for example, BS EN ISO 14971:2000 may help. In this guide and associated example we have addressed the frequency of use by considering that either an instrument is calibrated for any occurrence of use for defined activities or else it is barred from use for such activities. The user will need to define their own appropriate measures for assessing the impact of frequency of use.

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Calibration and Measurement Traceability

What is Traceability?

A measurement can be said to be "traceable" if the calibration of the individual acoustical instrument(s) used to make the measurement can be traced to the primary standard (for example the National Standards maintained at NPL) by a chain of calibrations. The traceability level is distinct from the extent or scope of the calibration, which may cover only a range of performance parameters.

The calibration of acoustic systems is further described in the NPL guide, Performance Verification of Measurement Systems.

UKAS document LAB 23 defines three general levels of calibration traceability:

• Level 1: a calibration carried out by a laboratory accredited by UKAS for the measurements concerned for which a UKAS certificate is issued.
• Level 2: a calibration which may be carried out in-house, performed to a documented method by competent staff using appropriate reference standards and equipment. The reference equipment used will need to be calibrated by a laboratory that can demonstrate competence, measurement capability and traceability, eg, a UKAS accredited calibration laboratory.
• Level 3: a certificate of conformance issued by the manufacturer of the equipment or an appropriate service organisation.

These traceability levels can be correlated with the levels of Measurement Importance defined in the Risk Assessment. The following table gives a suggested scheme, but this can be adapted according to the needs of the user.

Measurement Importance	Suggested UKAS Level
Very High	Level 1
High	Level 1 / Level 2
Medium	Level 2
Low	Level 3

Note that there may be overriding requirements. For example, if your instruments are ever to be used to carry out noise surveys to assess compliance to any regulatory or legislative requirements, then your instruments should have a Level 1 calibration or equivalent.

The level of calibration traceability will have implications upon the calibration options for your instruments.

Whatever the level of calibration traceability, it is still very important to ensure that the correct type of calibration is carried out, for example, that the frequency response of an instrument is checked if required. It is important to obtain advice from your calibration testing laboratory to ensure that a suitable type of calibration is undertaken.

It is also important to remember that the above levels of calibration traceability are not exhaustive and do not define the scope and extent of the tests to be carried out during calibration. For example, a "certificate of conformance" provides a general assurance that an instrument performance is within specification. However, the checks carried out may not cover the full range of functionality and capability of the instrument. Similar considerations will apply for all levels of calibration traceability.

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Measurement Instruments

The next stage of the process considers the instruments and equipment used to make your various acoustical measurements.

Instruments and equipment in your inventory may include:

• Microphone
• Filters
• Cables
• Sound Level Meter
• Reference Sound Sources
• Signal Generators
• Analysers
• Computer
• Sound Intensity Meters
• Acoustical Calibrators
• Audiometers
• Ear Simulators
• Mechanical Couplers
• Storage Oscilloscopes
• Frequency Counters
• Distortion Meters

It is likely that each item will require some form of checking or calibration, but the level of traceability of the calibration, and the frequency of the calibration, will depend upon the level of importance of the measurements made using this instrument, and the environment in which the equipment is stored and used.

Level of Use

Instruments may be used for a range of different measurement activities. These measurement activities might have varying levels of importance. It is useful to understand, for each instrument, the types of activities for which it is used. This information can then be used to assess the level and type of calibration needed for each item.

The following table gives some examples of how this information might be recorded, using a simple checking system.

	Level of Importance - (see Risk Assessment)
Item	Very High	High	Medium	Low
Microphone #1
Microphone #2
Cables
Sound Level Meter
PC

From the above example, the following points might be noted:

• Microphone # 1 is used for measurements of Very High and High importance. Even if these measurements take place rarely, it is still important that this microphone has a sufficient level of calibration for very high importance measurements.
• Microphone # 2 is used for measurements of medium or low importance. The level of calibration for this instrument can therefore be lower than Microphone # 1, but note that this instrument can not be used to replace Microphone # 1 for any more important measurements unless it is then calibrated to a higher level.
• Some items cannot be formally calibrated, but appropriate checks should be carried out regularly. For example, cables are an important part of the measurement chain, and cannot be calibrated separately, but the continuity of such cables should be checked. They should also be carefully inspected for damage.

Wherever possible, it is useful to have some 'end-to-end' check or calibration of equipment that is habitually used together as a system. For example, if a particular microphone, cable, filter and sound level meter is used as a system, some check that the system is giving its usual response is a valuable way of quickly checking the integrity of all the system elements.

Environment and Care of Instruments

It should be noted that the environment in which acoustic equipment is stored and used is very important. For example, a microphone is a delicate piece of equipment and easily damaged, and the damage may not be exposed by the application of a single frequency acoustic calibrator.

Instrument accuracy can be adversely affected by:

• Dampness or humidity
• Temperature extremes or rapid changes in temperature prior to use, for example, storage overnight in the boot of a car followed by measurements in a warm environment
• Dirt, dust
• Shocks, impacts or vibration
• Rapid or significant pressure changes, for example, travel in lifts or aircraft

If equipment is exposed to any of these additional risk factors, then calibration may be necessary on a more regular basis than if the equipment is generally used carefully in a laboratory environment.

Calibration may also be required following any component replacement, service or repair which may affect the calibration of the equipment.

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