Keep the noise down
23 January 2013
Growing awareness of the dangers of exposure to excessive noise is driving the requirement for better performing hearing protectors at work. Andy Todd reports on the testing undertaken at SATRA's recently installed state
Hearing protection comes in many different forms: standard ear-muff or ear-plug designs, known as passive devices, or more complex models incorporating electronic systems which react differently in varied noise environments, known collectively as active devices.
The protection provided by any device will be compromised if it is poorly fitted or incorrectly worn - even the slightest break in a seal around an ear-plug or the cushion of an ear-muff will reduce the attenuation or noise reduction provided by the hearing protection. It is, therefore, important that health and safety managers know which type of protection is suitable for particular tasks and that wearers understand how to use their hearing protection correctly.
The performance of models in their passive state is dependent on several factors. The depth of the cups which enclose the ears, the force of the headband, and acoustic absorption characteristics of liners are just a few of the variables contributing to the effectiveness of ear-muffs, while depth, fit and construction material generally govern the performance of ear-plugs.
Selecting the correct protection It is vital that the choice of hearing protector is suited to the noise environment in which it will be used, providing adequate protection while avoiding over-protection (this is when the hearing protection provides too much attenuation, making it difficult to hear speech, alarms and warning signals).
Health and safety managers can ensure they are selecting the right device by referring to the SNR and HML values provided by the manufacturer.
Simplified Noise level Reduction (SNR), also referred to as Single Number Rating, provides a single attenuation value based on the subjective attenuation tests.
Theoretically, this value can be subtracted from measured external noise levels to estimate the noise level at the ear beneath the hearing protection. It should be noted, however, that this method does not provide any information as to how much protection is provided in different frequency ranges - this is why the HML rating system is also required.
High - Medium - Low (HML) provides further detail, allowing the attenuation provided to be assessed across high, medium and low frequency ranges.
This is particularly useful if a staff member is subjected to narrow-band noise, which covers a small range of frequencies such as continuous hissing from compressed air, as it allows a more accurate assessment of the noise level at the ear.
Ensuring quality At SATRA our acoustic test programme consists of two parts: subjective attenuation and insertion loss. Subjective attenuation testing is undertaken using 16 human test subjects. During this test, we measure the threshold of hearing (the quietest sound a person can hear) with and without the hearing protection under test worn; from these two measurements we can calculate the acoustic performance of the device. This test procedure requires extremely stringent test conditions, which is why SATRA has recently invested in a high performance hemi-anechoic chamber. The new chamber is isolated from external noise and provides the environment required to test the subjective attenuation provided by hearing protection devices.
The second acoustic test is the insertion loss assessment which is conducted in our acoustic tunnel. This test is designed to calculate the deviation in performance between ten samples of the same device.
The physical test programme is designed to replicate the day-to-day wear and tear which the device will be subjected to. During this test schedule we assess resistance to damage when dropped, ignitability, headband flexing and cushion pressure to name a few, as well as a sizing assessment to ensure that the size of the device corresponds to the size designated by the manufacturer.
Within SATRA's chemical and analytical testing laboratories, we are also able to conduct innocuousness assessments, to confirm that the materials used in the manufacturing process are not harmful to the wearer.
When purchasing a hearing protection device, it is important to check that it is marked 'EN 352' as this shows it has been tested to and meets the required European Standards. Without this marking, the quality of the device cannot be assured and may be putting employees at risk.
Andy Todd is acoustic engineer at SATRA