When undertaking measurements of noise it is of course essential that the measurement equipment is working properly though the means available to the user to check this are limited. The regular calibration of such equipment is therefore an important requirement though the costs involved can be high furthermore it is both inconvenient and undesirable to have measurement equipment away for calibration for long periods of time. For this reason the Association of Noise Consultants developed the means by which a practical and inexpensive check on the electrical performance of sound level meters may be regularly carried out.

The test signals, stored on a compact disc, are played using a CD player, into an Interface Unit. The interface unit's output provides an electrical signal that is substituted for the measurement microphone on a sound level meter.

BS 7580 - Specification for the verification of Sound Level Meters, specifies a limited but sufficient range of tests to verify the accuracy of the measurement instrumentation at various intervals. As a general requirement the standard recommends that verification shall be performed at least every two years. The majority of the test procedures are carried out with an electrical signal substituted for the measurement microphone with a series of test signals being used to assess the performance of the sound level meter in respect of noise, linearity, frequency weightings, time weightings, peak response, RMS accuracy, time averaging, pulse range, sound exposure level and overload indication.
Many of the test signals used to assess performance are complex and are certainly not available to the majority of people involved on a day-to-day basis with the measurement of sound. The test for RMS accuracy for example requires that a reading obtained from a continuous 2kHz sinusoidal signal be compared with that obtained from a sequence of tone bursts consisting of 11 cycles of a 2kHz sine wave repeated 40 times a second and with an amplitude 6.6 dB higher than the continuous signal.

Compact discs have been around for a long time now and provide a viable means by which over 70 minutes of high quality audio signal can be stored. In the ANC system the necessary calibration test signals are stored on a compact disc as such a medium could easily accommodate the longer recordings needed for the time averaging and statistical indices tests. The compact disc medium also provides an inexpensive and convenient means by which the necessary test signals can be distributed and used by those wishing to carry out regular calibration checks on their measurement equipment.

Calibration can be carried out using a standard domestic CD player, a portable CD player or a CD-ROM player configured as a standard audio player on a personal computer. The output signal from the CD player system may be derived either from the line out socket or the headphone socket of the system. When used with a personal computer it is important to note that the PC CD-ROM drive is not being used to derive .WAV files but is simply used as an audio CD player. If used in conjunction with appropriate software capable of compiling play-lists, the personal computer option does provide a convenient means by which banks of test tracks can be played sequentially.

It is of course critical that the signal derived from the compact disc replay equipment is of an appropriate quality and level. For this reason a Calibration Interface Unit is required between the CD player and the sound level meter under test. The Interface Unit is needed to amplify the output signal from the CD player to an appropriate reference level and to ensure that the frequency response of the player system is linear. The ANC calibration system therefore comprises a Calibration Compact Disc and an associated Calibration Interface Unit, the Interface Unit being battery operated to enable tests to be carried out 'in the field' if required. The absolute accuracy and calibration of the Interface Unit is an important consideration and so this does need to be checked regularly against traceable reference standards.
Such tests that are required on the Interface Unit are however relatively simple and straightforward and may be carried out against appropriate reference standards traceable to national or international standards.

The calibration test signals included on the Calibration CD provides a fast and convenient means by which the electrical performance of sound level meters may be checked. The procedures are broadly based on British Standard 7580 with additional filtered noise test tracks included to check octave-band filter characteristics and a traffic noise track to check statistical noise level percentiles. The procedures are not intended to replace BS 7580 but rather to complement the standard by providing a viable and practical means by which the performance of a sound level meter may be regularly and easily checked by the user.

The Calibration CD comprises some 69 tracks, the total running time being in the region of 37 minutes. The adopted reference level of the CD is 12dB below maximum recording level though peak levels on some tracks do approach maximum operating level. The recorded test signals enable the performance of the sound level meter to be checked in respect of the following parameters:

1. Sensitivity
2. Self-Generated Noise
3. Linearity & Range Control Accuracy
4. Frequency Weightings
5. Time Weightings (Fast/Slow)
6. RMS Accuracy
7. Time Averaging
8. Sound Exposure Level
9. Overload Indication
10. Octave-Band Filters
11. Statistical Percentiles
12. Microphone Open Circuit Correction Factor

A sound level meter microphone reference sensitivity of 50mV/Pa has been adopted, this being one of the most commonly encountered microphone sensitivities found on currently available sound level meters. Meters with different sensitivities may however still be calibrated with minor modifications to the Calibration Interface Unit. The Interface Unit is provided with a 30dB attenuator switch to enable either a 124dB or 94 dB reference level to be adopted. Whilst the procedures are intended to check the electrical performance of the sound level meter a first order indication of the open-circuit correction factor of the associated microphone may also be obtained using a sound level calibrator in a valid state of calibration. A multi-turn level adjuster on the Interface Unit provides the means by which the reference signal from the CD player may be adjusted to the correct reference level. A calibrated VU meter is provided on the Interface Unit to adjust the sensitivity and verify that the frequency response of the CD player is satisfactory. An output on/off switch is also provided on the unit to enable the sound level meter self-generated noise to be determined without being affected by electrical noise from the CD player or Interface Unit.

The ANC Sound level meter calibration and testing system is available to members of the ANC and members of The Institute of Acoustics or their employers. If you would like to know the current price and availibility the please call, fax or e-mail the ANC.

The ANC Calibration Equipment is protected under UK Patent No. GB 2326478 B.