A Guide to Sound Level Measuring

Introduction

The industrial environment has changed drastically in recent decades, with an increased level of automation within the workplace. This has given rise to many changes in employee work patterns. It used to be the case on the majority of production lines an employee would stay in one place during his or her shift. With the increase in completely automated production lines, employees now may supervise several machines. This means they move around from workstation to the next, varying their exposure to noise in a much more dramatic way than previously. Monitoring the worker's noise exposure plays an important role in these types of work conditions.

Making precise measurements of noise used to be quite a tricky business, but now there are automated electronic instruments that do the job for you. The best way to monitor precisely an individual's exposure to noise is by using either a sound level meter or a noise dosimeter. Your first step then is choosing the right type of instrument to measure noise. What are the differences?

Let's take a look at sound basics first for a clearer understanding of the essence of both tools...

Sound Loudness Perception and Precision

How loud a sound seems to be, depends on who's listening. A young person playing rock up in the bedroom might not think the music is loud, but his/her parents in the room down below might have other ideas. In other words, how loud things seem is a subjective thing and not something we can precisely measure. However, what makes one sound seem louder than another is the amount of energy that the source of the sound is pumping towards the listener in the form of pressure variations in the air. Sound travels through the air as pressure waves - the stronger the pressure waves, the louder the sound.

Meters that measure sound levels work by calculating the pressure of the sound waves traveling through the air from the source of the noise. That's why you'll sometimes see them referred to as sound pressure level (SPL) meters. Devices like this give a measurement of sound intensity in units called decibels (dB), a scale first devised by telephone pioneer Alexander G. Bell.

Decibel Scale

Decibels are not a linear but logarithmic measurement. Every increase in 10 decibels represents 10 times the strength of sound waves, so an increase from 40 dB (the sound level of a quiet conversation) to 60 decibels (quiet traffic sounds) represents a 100-fold increase in sound energy, however, it won't sound 100 times louder to you (human hearing perception is logarithmic too). Loud enough sounds contain so much energy that they can physically damage the eardrums. Decibel meters help protect the hearing of people who work around loud noises.

Levels	Typical example	Times louder than 10dB
10dB	Rustling of fallen leaves.	1
20dB	Watch ticking.	10
30dB	Birds flying by.	100
40dB	Quiet conversation.	1000
50dB	Louder conversation.	10,000
60dB	Quiet traffic noise.	100,000
70dB+	Louder traffic	1,000,000
80dB+	Loud highway noise at close range	10,000,000
85dB	Hearing damage after approximately 8 hours.
100dB	Jackhammer (pneumatic drill) at close range	1,000,000,000
100dB	Hearing damage after about 15 minutes.
110dB+	Jet engine at about 100m	10,000,000,000
120dB	Threshold of pain. Hearing damage after very brief exposure.

Sound Level Meters

A Sound Level Meter (SLM), also known as a Decibel meter, is the basic hand-held instrument used for measuring general area noise levels. You can use them to spot check noises such as hums, noises from industrial machinery or equipment (i.e. large trucks, jackhammers), or noise across boundaries, such as noise from a construction site or your loud neighbors whether they’re a truck stop or a train station that lets its trains idle during the wee morning hours.

Different kinds of sound level meters are available. The most basic ones give a reading of the maximum instantaneous sound pressure level (SPL), in decibels, at the moment when you switch them on.

Roughly speaking, that corresponds to how loud noise sounds at the moment you are taking the measurement are. That isn't always tremendously useful if you want to measure the average sound levels over a period of time in somewhere like a noisy production plant. For that, you need a slightly more sophisticated (and expensive) device called an integrating sound level meter. Integrating, in this case, means summing over a period of time. For measuring sound levels over longer periods, you can use data logging meters that take measurements every so often. Upload these measurements to a computer to draw a chart showing how the sound level varies during a day, week, or longer. This kind of longer-term analysis is a fairer and more accurate way of sensing whether the sound is a nuisance or a health issue than simply relying on one or two instantaneous measurements.

Noise dosimeter

Where it is difficult to get close to employees with a sound-level meter, as in the case of forklift truck drivers, or where workers are exposed to many different noise levels, they should wear a noise dosimeter. This is the case more often than not in the modern workplace, where if you are using a standard meter you would have to measure the noise levels at each location, find out how long the worker stays at that location, and then calculate an overall exposure. This can take hours of calculations to perform and will not always result in precise measurements.

The Use of Noise Dosimeters

If mobile work patterns exist in your workplace and a noise dosimeter fits the bill, it is important to realize precisely how these instruments must be used and understand their limitations. Given the logarithmic nature of the decibel scale, a variance of only 1 or 2dB can often mean serious misinterpretation of noise levels. This margin of error should be accounted for, and the worst-case scenario measurement should be taken as the reading, particularly when close to an action level.

A noise dosimeter consists of a microphone on a cable, which can be clipped to a collar. The microphone cable is then passed under the clothing to the unit itself, which is small enough to be located in a pocket or clipped to a belt. The dosimeter can then be started at the beginning of the shift. If it runs until the end of the working day, the noise dose can be directly read from the instrument or downloaded without the need for calculations. Another useful feature of noise dosimeters is that they will log the noise data so that when downloaded to a PC, the time history of the noise can be viewed. This gives the ability to analyze when and where high noise exposures occur.

This can be even more useful when the dosimeter is placed on an employee who is prepared to make a list of the times and jobs he or she was performing throughout the day. This will give the employer the ability to see which operations most need noise control to reduce exposure.

A traditional noise dosimeter is fixed to the worker's belt, and then a microphone on a cable is attached to the collar near the ear. You should make allowances for human nature. Employees fitted with dosimeters and their colleagues will often shout into the microphones, distorting the readings, so it's best to ignore the first few days' results until the novelty wears off.

One advantage of dosimeters is that if employees wear them for complete working shifts, the noise dose is measured in full. However, if you need to make several measurements of different employees in the same day, a dosimeter can be moved to different employees, as long as the measurements taken for each employee are representative of their working day. Most modern dosimeters can also project the noise dose forward to the standard eight hours, so no calculation is needed.

Calibration

Whether you use an SLM or a dosimeter, you must ensure that your instrumentation is factory calibrated each year to current NIST standards. If you are in a litigious situation, data provided by calibrated instrumentation is an absolute must.

Conclusion

Modern sound level meters and noise analyzers or dosimeters have facilitated the accurate measurement of the noise levels provided that the instruments are properly used, calibrated, and regularly re-certified. These measurements provide the basic data upon which the degree of hazard may be decided and the correct remedial measures prescribed.