The Assumed Protection Value of a hearing protector – what does it mean, how is it derived, and how does it relate to ‘real’ world conditions?
You may have seen the Assumed Protection Value (APV) printed on the packaging of hearing protectors and probably thought “what does this actually mean?”
The APV data is typically represented as shown in the table below:
|Mean attenuation (dB)||14.1||11.6||18.7||27.5||32.9||33.6||36.1||35.8|
|Standard deviation (dB)||4.0||4.3||3.6||2.5||2.7||3.4||3.0||3.8|
The APV values are determined from tests run in a laboratory environment and assumes the minimum attenuation protection that is provided by a hearing protector, in each of the octave frequency bands, for a percentage of wearers. It is these values that are used to calculate the resultant noise levels at the ear with the hearing protector in place, i.e:
Noise level external to hearing protector (dB) – APV (dB) = Resultant noise level (dB) at the ear [for each of the octave frequency bands].
Laboratory testing and derivation of the APV
The APV is derived from the mean attenuation obtained of a hearing protector tested under controlled conditions in a certified laboratory. The test, in accordance with an EN352 standard, follows a subjective based method where the hearing threshold of subjects are measured with and without the hearing protection in place. The noise attenuation is simply the difference between these two measurements.
Test conditions include trained and motivated wearers (receiving supervision, if required) fitting the hearing protection in accordance with the manufacturer’s instructions. This test method provides the best assurance that attenuation data obtained will be repeatable.
The test data is collected across a combination of a specified number of wearers and hearing protectors, and by processing this data, the mean value of the attenuation and the standard deviation (effectively, the spread of the data) may be calculated at each of the frequency bands in the above table for the hearing protector type under test. By definition, we have:
APV (dB) = mean attenuation (dB) – p x standard deviation (dB)
where p is a whole number.
For p = 1 (as it is the case in the above table), this implies statistically that 84% of the population (if tested under laboratory conditions) will be assumed to achieve a minimum attenuation which is given by the published APV. By implication, 16% of wearers will be predicted to under achieve the APV. An important point of note is that 100% of the population will NOT achieve the APV.
The statistical spread of test results in the laboratory are down to, amongst other things, anthropometric differences in head shapes between the test subjects and any manufacturer’s tolerances across the build of the hearing protectors tested.
APV and the ‘real’ world
In the ‘real’ world, however, user conditions are highly uncontrolled compared to a laboratory type environment, due to factors such as incorrect fitting, poor maintenance, the wearing of spectacles and jewellery, plus interference by other personal protective equipment. For these reasons it’s highly likely that the manufacturers’ specified APV performance will suffer some degree of degradation. The HSE recommends that a ‘derating’ factor of 4dB be applied to your calculations (i.e. add 4dB) when predicting the noise level at the ear with the hearing protector in place.
Although it is highly unlikely that the APV performance of a hearing protector can be replicated in the ‘real’ world, with the provision of instruction and training (which is a legal requirement in high risk noisy areas), staff can be taught how to properly wear, maintain and look after hearing protection. The outcome would more than likely maximise the effectiveness of the hearing protection and therefore minimise the chances of noise induced hearing damage.
Confused? Then leave it to us. Essel Acoustics has the skills, experience and qualifications to correctly prescribe appropriate hearing protection and provide training in noise awareness and the correct use of hearing protection at operator and supervisor/manager level.