Mind your language! A lay persons’ guide to workplace noise terminology.

It used to be said that Great Britain and America are two Nations divided by a common language but ask the average person how you measure the background noise in say London or New York City, two fairly loud cities, I’d be willing to bet that most people would answer “in decibels”. 

They might even have a smartphone app at their disposal and offer to show you some values but judging by the number of people who suffer from noise induced hearing loss (NIHL) in the workplace, they are possibly unaware of the significance of the numbers dancing about on the screen, the accuracy of the actual measurement or the terminology involved. That’s frustrating given that the scientific cause and effects of too much noise exposure dates back nearly 70 years, plus there is now increasing evidence of the effects of noise on our general health including dementia. 

Sadly, it is widely reported that there are 30 million European and 22 million US workers who are regularly exposed to noise levels high enough to put their hearing at risk.  Indeed, the World Health Organisation (WHO) said ^[1] that occupational noise exposure is the second most common risk factor in the workplace behind workplace injuries and that noise exposure contributes to 22% of workplace health issues. WHO also warn ^[2] that there are millions of teenagers who are waiting in the ‘workplace wings’ already suffering from hearing loss due to loud music and gaming for long periods of time.

In nearly all cases hearing damage occurs because of this repeated exposure over time to relatively high noise from any source and for reference, the table shows some typical levels.  

Figure 1

Note the relationship between the sound pressure expressed in Pascals (Pa), which covers seven orders of magnitude and the logarithmic decibel (dB) scale, which is a much more manageable range e.g., 0-140 dB.

Distance. The distance from the source is also an important factor. Imagine a noise source hanging in free space, radiating noise in all directions. The level would reduce the further you moved away from the source as sound energy is absorbed through molecular friction and the energy dissipated as heat. Now put that same source on a hard, non-absorbing surface and the level would immediately double by 3dB (hold that doubling thought). If you now put that source, say in a factory environment with lots of hard surfaces, the noise level would build due to reverberation and decay with distance in a more random way. By contrast, soft materials help to absorb or attenuate the noise; think about how a room sounds when you are decorating or remodelling your home and why we all sound like Pavarotti in the shower!

Frequency. As well as level and (level decay with) distance, noise has a third component, namely frequency. The human ear can detect frequencies from 20 Hz to20 KHz, but this can degrade with age, disease, and hearing damage. However, even healthy hearing doesn’t respond equally to all frequencies. You will often see the letter A or C written after the dB symbol to clarify the nature of the measurement. These frequency weightings (see figure 1) are standardised and approximate to how we hear.

Figure 2

A-weighting is used most for continuous occupational noise exposure and C-weighting for instantaneous peaks or impact noise. B & D weightings are no longer used.

Understanding the frequency content of the noise source is generally useful for noise control purposes. Take hearing protection for example. C-weighting used in conjunction with A-weighting is a rudimentary measure of frequency content and indeed the ‘C-A’ value is used in the calculation of hearing protection using the H-M-L method in Europe. However, the octave band method is preferred because it is more accurate but does require a little more calculation. Here, the hearing range is broken down into a manageable span based on octaves. Starting from 1KHz simply double the center frequency i.e., 2, 4, 8, 16 kHz or halve to get 500, 250, 125, 63 and 31.5 Hz. You will see some of these octave band values written on the packaging of hearing protectors as well as the NRR (used in the US) or SNR and H-M-L values (used in Europe).

Time. The idea of hearing damage due to a combination of level and cumulative exposure time is an important relationship which leads to the concept of an acceptable noise dose (expressed as a percentage), where most of a working population would statistically not suffer permanent hearing damage. Noise levels usually vary with time (and distance), and it is the cumulative energy received over a working hour day that’s important. Expressing that energy either as a single number or as a percentage dose which can be compared with safe limits is extremely helpful. Here is where the UK (and Europe) and the US differ in their approach based on the interpretation of risk of NIHL.

The UK has always expressed a doubling of risk based on the equal energy principal i.e., a doubling of energy is equivalent to a +3dB increase, known as the exchange rate and sometimes written as Q=3. Based on this principal, the A-weighted average noise level (LAeq), is the average noise level considered as a notional steady level that has the same amount of noise energy as the actual fluctuating noise level during a specified period of time. An LAeq measured over 8 hours of 85dB is equivalent to an LAeq of 82 for 16 hours or 88dB for 4 hours; these all equate to a 100% safe daily dose based on current legislation.

In the US, occupational exposure limits are expressed as an 8-hour time weighted average value (TWA) that includes the whole of the shift exposure. Historically the US has used several different exchanges rate conventions ranging from 4, 5 or 6 dB per doubling/halving which is of course less stringent than the UK/Europe interpretation although NIOSH has more recently adopted Q=3 and a TWA limit of 85dBA. Under OSHA regulations, where Q=5 this would ‘allow’ a TWA of 85 dBA for 16 hours, meaning a 200% dose or twice the risk in UK (and NIOSH) terms!

Peak. And finally, contrary to popular belief, hearing loss arising from too much noise is rarely the result of a burst eardrum.  This seldom happens and even when it does, it is usually the result of a pressure wave typical of an explosion and having a peak amplitude of 160-180 decibels. Figure 2 shows an extreme example of 300g of plastic explosive measured at 2.8m from the detonation leading to a peak of 63KPa or 190 dBC. Remember the term peak has a special meaning in acoustics to distinguish it from the maximum sound pressure level (SPL) value and care must be taken not to mix them up. 

Figure 3

Confused? Don’t worry there is an awful lot of black art surrounding noise, but you don’t need a Degree in Acoustics to be able to undertake a risk assessment in your workplace. Increasingly there are training courses and support available on-line and modern instrumentation tends to measure all the possible combination of parameters in parallel which means you don’t have to remember to select the right instrument settings as you did in the past. A future article will look at instrumentation in more detail namely sound level meters and noise dosimeters.

1.       Addressing the prevalence of hearing loss, WHO, Feb 2018

2.        https://www.who.int/news/item/02-03-2022-who-releases-new-standard-to-tackle-rising-threat-of-hearing-loss