A Guide To Hearing Protection

In many cases the use of hearing protection is essential for protecting yourself from loud and potentially dangerous noise, but there are many different types of hearing protection available for many different scenarios. Knowing which type of protection is the most appropriate for you is important, the guide below should help you to make a more informed decision about which to be using and when.

Protection Mechanisms

In Ear

When the ear is occluded by an earplug there are 4 methods of sound transmission to the cochlea:

path8702

  1. Air gaps – A tight fit ensures a seal between the ear and the plug, which provides the maximum amount of protection. A gap can cause a reduction in sound pressure level by 5-15dB.
  2. Vibration  – The ear canal is fleshy, and can change shape through movement of the jaw, neck, and head. This movement causes the plug to act as a piston, and affects low frequency performance.
  3. Transmission through material – For disposable earplugs this is not a factor for transmission as the attenuation of the material is extremely high, but for custom moulded plugs the attenuation through the device can be controlled by the use of removable filters.
  4. Bone Conduction – In-ear methods of protection have no control over bone condution through the skull, which limits their maximum attenuation to around 50-60dB before the noise floor of the bone conduction is reached.

The introduction of a blockage to the ear canal also causes something called the occlusion effect. Heard as a boomy rumbling sound, the occlusion affect is caused by a better bone conduction path being created by the blockage, raising the SPL reaching your cochlea by around 10dB under 2kHz. The best way to avoid/reduce the affect of this is to make sure your earplugs are inserted deeply into the canal, past the external cartilage where bone conduction occurs.

Circumaural

For circumaural ear defenders the main transmission path to the ear is through air gaps around the edge of the cup, where the shape of the head makes it difficult to create a perfect seal. Depending on how they’re worn this can mean that the level of protection varies by up to 15dB.  In comparison to in-ear protection there is a large surface area exposed to incoming sound, which introduces a higher possibility that a transmission path will occur through the cup. The cushioned area around the seal of the defender is likely to be the weak point in terms of through-transmission, as the material is not likely to provide the same level of attenuation as the lining of the cups.  Bone conduction is less of a problem as the structure of the defenders is held away from the skull, due to the cushions around the ear. 

optime-3
3M Peltor Optime III Ear Defenders

Foam Earplugs & Ear Defenders

The most common type of hearing protection are foam earplugs, and over ear (circumaural) defenders. These are typically used in industrial situations, where a person is likely to be exposed loud noise for a period of time, potentially their whole working day. Although they offer a high level of attenuation (around 30dBSPL for common ear defenders) they have a frequency response that greatly affects speech intelligibility, making it much more difficult to communicate with colleagues, and making working with machinery, tools, or vehicles much more dangerous. The cause of this is in the construction of the protection, the foam used to make in-ear plugs, as well as the foam found within ear defenders, absorb high frequency sounds more effectively than low frequency sounds. The graph below shows attenuation plotted against frequency for common ear defenders and earplugs by 3M.

Ear Defender & Ear Plug Attenuation graph 1

When being able to localise sound well (i.e. know where a sound is coming from) is important, earplugs may be advantageous over ear defenders as the outer ear is still open to the incoming sound.

Musicians Earplugs

Musicians earplugs work in much the same way as regular foam earplugs, however instead of blocking as much sound as possible a cavity within the plug acts as an acoustic filter, which can be configured to almost any amount of attenuation and frequency response. This is advantageous in a number of ways as the plugs can be configured for many different scenarios, which works especially well for musicians. A drummer may require a greater level of protection than a guitarist, while a sound tech may require a greater level of attenuation than both of them.

For custom made hearing protection an impression of the ear is taken by injecting a material into the canal that hardens after a few minutes. When the impression is removed it is sent off for production of a mould, which is subsequently made into a custom moulded silicone earplug. An acoustic filter (of which there are many types) is then inserted into a cavity within the plug.

pro-10-1
ACS Custom Hearing Protection

 

The graph below shows the minimum attenuation performance of ACS custom hearing protection filters, from 125Hz to 8kHz. The graph also includes universal fit Pacato ear plugs.

ACS Hearing Protection Specifications

The suggested uses for these filters are listed below:

  • Pro 10 – Designed for classical and acoustic performers, suitable for use up to 90dB.
  • Pro 15 – Designed for brass and woodwind musicians and vocal performers, suitable for use up to 100dB.
  • Pro 17 – Designed for musicians and vocal performers, suitable for use up to 100dB. Has the flattest frequency repsonse of the range.
  • Pro 20 – Designed for drummers. Suitable for use up to 105dB.
  • Pro 26 – Designed for use by musicians in high SPL environments. Suitable for use up to 110dB.
  • Pro 27 – Designed for crew and support staff that are likely to be exposed to very high SPL. Frequently used by motorcyclists and shooters. Suitable for use up to 112dB.

In comparison to the custom fitted protection, the universal fit Pacato plugs do not have such a flat frequency response, but still provide a good level of protection for most scenarios (19dB average). These are generally well suited to people attending gigs and events, where they are not likely to be wearing them for extended periods of time.

Pacato-9
ACS Pacato Ear Plugs

Impulse Protection

In some scenarios standard hearing protection is not particularly applicable, as the user may not want to have a constant reduction in level, but only when a loud and potentially dangerous impulsive noise is heard. This type of protection is particularly applicable to the military where good communication is vital but noises such as gunfire and artillery can be very dangerous to peoples hearing. This also applies to hunting, where hearing sounds in the distance is very important for locating a target.

The principle of this type of hearing protection is that as sound pressure level increases so does the level of attenuation provide by the ear plug. The mechanism for this is a valve/diaphragm/orifice that can close when exposed to a high sound pressure level, then open again after the sound level has reduced. This is process is explained well by Berger (2010):

At low sound levels, streamlined airflow predominates and the pressure is therefore linearly related to the particle velocity. However, at sufficiently high levels turbulence occurs as vortexes are generated at the exit of the orifice, and the pressure then becomes proportional to the square of the particle velocity and the resistance increases.

ACS Pro Impulse protection (available as both custom fit and universal fit) provides around 15dB of broadband level attenuation at normal sound pressure levels, which increases when exposed to a loud peak sound. The table below shows the increase in attenuation for different peak sound pressure levels:

Peak Level (dB) 130 150 158 166
Level of attenuation (dB) 21.3 29.3 32.3 33.1

A second version is available that provides a switch to block the ear plug, increasing the broadband attenuation to 25dB, while still providing the same level of impulse protection.

ACS Pro Impulse Universal Fit Ear Plugs
ACS Pro Impulse Universal Fit Ear Plugs

References

Berger, 1980. Hearing Protector Performance: How They Work – and – What Goes Wrong in the Real World. EAR Log 5.

Berger, 2010. Empirical Evaluation using Impulse Noise of the Level-Dependency of Various Passive Earplug Designs

 

 

For more information on many subjects relating to hearing protection check out the 3M Archive.

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