Mixing and matching instrument components
By William Egan
29 Nov 2010
Living up to sound level meter standards is harder than you might think … William Egan sifts through the details
|
Technology
|
|
|
 |
|
Sound level meters and noise analyzers all have three main components: a microphone, a measuring system including a display, and a sound level calibrator. Modern noise measuring instruments are usually designed to meet an international standard [BS EN 61672 Part 1 (2003)] defining their performance so that measurements from different instruments are comparable.
Most noise measurement standards stipulate the grade of sound level meter required to carry out the measurements. For example, BS ISO 20906:2009 ‘Acoustics – unattended monitoring of aircraft sound in the vicinity of airports’, specifies that each measurement channel must conform to Class 1 of BS EN 61672-1.
Although the instruments from different manufacturers are designed to carry out a similar range of noise measurements, the user should be aware of the dangers of swapping components from different manufacturers. In addition, mistakes can be made by assuming that a particular instrument conforms to the standard when used for outdoor noise measurements.
Sound level meter standards
The older standards for sound level meters originate in the 1970s and 1980s, and because of the longevity of instruments, many from this period are still in regular use. These standards refer to Type 1 or Type 2 instruments for field use, but there were also Type 0 (laboratory use) and Type 3 instruments.
These older standards were replaced in 2003 with the current BS EN 61672 Part 1 (specifications) and Part 2 (pattern evaluation), after which Part 3 (periodic tests) followed in 2006. This standard replaced the original four grades of instrument with just two — Class 1 and Class 2. It is noticeable that some updated versions of older standards still mistakenly refer to the older Type classification — see for example BS5228 Part 1 2009!
Part 1
Part 1 of the standard for sound level meters is a very detailed document on the specifications for Class 1 or Class 2 instruments, and is mainly of interest to the manufacturer of such products. However, the operator of such instruments should note a few points:
1 - Microphones
“The instruction manual shall state the models of microphones with which the complete sound level meter conforms to the specifications …” (Section 5.1.6)
It is not uncommon for the unwary user to assume that they can use any similar microphone on their sound level meter. In all cases known to the author, there is just one microphone specified for use with each particular model because the implications of costly ‘Pattern Evaluation’ (see below: Part 2 of the standard) means that it is impractical to provide a range of suitable models of microphone. The BS EN 61672 Standard (Part 1: Specifications) specifies the performance of the sound level meter as a complete instrument that includes its microphone.
Consequently, if you wish to ensure your measurement results comply with the Class 1 or Class 2 standards, please ensure your instrument is fitted with the correct microphone - either the one originally supplied by the manufacturer, or a replacement of exactly the same type.
2 - Calibrators
“At least one model of sound calibrator shall be stated in the instruction manual for checking and maintaining the correct indication on the display of the sound level meter.” (Section 5.2.1)
It is very likely that a manufacturer will specify one of their own range of calibrators for use with the instrument, for obvious reasons! Do not use a different sound level calibrator from the one stated by the manufacturer in the handbook, as there are many pitfalls in trying to use a different one.
Richard Tyler’s article ‘Coupling an acoustic calibrator to a sound level meter’ in Acoustics Bulletin from March/April 2009 details the problems, and his advice should be followed: “The mix-and-match of manufacturer A’s calibrator with manufacturer B’s adaptor for calibrating manufacturer C’s microphone/sound level meter is a recipe for error on a grand scale and should never be employed!”
3 - Windscreens
Windscreens: “The instruction manual shall contain … a description of the average effects … of enclosing the microphone with a recommended windscreen, rain protection device or other accessory provided or recommended. A statement of the performance class to which the sound level meter conforms when such accessories are installed …” (Section 9.2.6 [a])
Evidently, it is important that the correct type and size of windscreen is used for the sound level meter.
Part 2: Checking on Part 1
Part 2 of the Standard describes ‘Pattern Evaluation tests’ to verify conformance to all the specifications in Part 1 has been met. In these tests, at least three sound level meters must be submitted for testing in an independent acoustical test laboratory (often PTB in Germany).
This procedure means the buyer of any instrument can be sure that it fully complies with all of the many requirements laid down in Part 1 (Specifications).
The alternative is for the user to assume that the manufacturer’s claims are true, but this is not always the case, as shown in Liz Brueck’s (HSE) paper in Acoustics Bulletin March/April 2010, which states that “responses can be more than 15dB outside standard tolerances even after a field calibration!”
 |
|
Although most of the main manufacturers of sound level meters submit their new or modified instruments for pattern evaluation, it is worth checking that the complete measurement system you intend to use has been approved. This particularly applies for outdoor noise measurements using a weather-protected microphone system linked to the sound level meter.
The meter and its normal-use windscreen may well have gone through the pattern-evaluation process to confirm specifications, but often the configuration including a weather protection system for the microphone has not undergone such approval. You can check on this with the manufacturer or find many of the certificates on the PTB website.
In summary, if you want to ensure that your measurement results fully meet the relevant standards and cannot be challenged later, make sure all of the instrumentation you use including your outdoor microphone protection complies with the latest standards (Class 1 or Class 2), and that the full system has been pattern-evaluated to confirm its compliance.
 |
| Pattern evaluation certificate for a sound level meter and its outdoor microphone system to ensure it meets the requirements of a Class 1 instrument (both for the sound level meter and its outdoor microphone), and provides appropriate results in that configuration. |
|
Read the latest industry news
