Natural Hearing Protection: Hands?

Have you ever covered your ears with your hands to protect yourself from loud noise? That’s the closest to natural hearing protection that we’ve got, but just how much does it reduce the sound pressure level reaching your ear? And what’s the best method? This experiment aims to find out.

In order to investigate this, a Brüel & Kjær head and torso simulator with in-ear microphones and realistic pinna has been used to replicate the response of a real ear and the effects of a hand placed over them.

A sine sweep from 50Hz-20kHz has been generated and output from our NTi FX100 Audio Analyser and RT-MicFX software via a loudspeaker, facing the HATS. The level received from the loudspeaker has then been measured using the microphone in the artificial ear.

A frequency response has first been taken without any attenuation involved to give a reference level. This reference has been compared to 3 different methods for reducing the level reaching the ear:

Cupping a hand over the entire ear:

Pushing on the tragus to block the ear canal:

And placing the tip of a finger into the canal:

Results

Five frequency responses have been taken for each of these attenuation methods, removing and replacing the hand each time. A logarithmic average has then been used to average the sound pressure levels measured in decibels.

The results of this experiment are shown below, using a logarithmic X-axis showing the test frequency range of 20Hz-20kHz, and a Y-axis showing sound pressure level in dB. Please click on the image for an interactive graph:

Analysis

When comparing the responses to one another a number of things can be noted. Firstly, both holding in the tragus and covering the entire ear give very similar results, most likely due to small air gaps being present through which sound is still able to transmit into the ear canal.

The level reaching the microphone when using these two methods is hardly affected, and although some of the sound above 2kHz is being absorbed by the hand, the cavity created by cupping the ear or pushing on the tragus is causing an amplification of around 10dB below 2kHz.

Pressing on the tragus appears to provide slightly better high frequency attenuation (above 8kHz) than cupping the entire ear, which may be related to the size of the air gap that is allowing sound to propagate into the canal.

Placing a finger directly over the canal and blocking it provides the most amount of attenuation of the 3 methods (seen in red). Above 100Hz this method provides 20-30dB’s worth of attenuation across the spectrum, a considerable reduction in level. Below 100Hz there is an increasing amplification down to 20Hz due to the occlusion effect. This is caused by bone conduction of low frequencies caused by the body (movement of the jaw etc.) reflecting off of the object blocking the canal and being transmitted to the ear drum. This sound would normally escape the canal into the air when not occluded, which is why we don’t normally notice it.

Conclusions

Based on these results if you ever find yourself in a loud environment without hearing protection available to you, the safest method of reducing the level reaching your ear is to fully occlude your ear canal with the tip of your finger, as this gives a substantial reduction in level (minus the occlusion effect) across the whole audible frequency spectrum, potentially saving you from hearing damage!

If you’re interested in knowing how our hands can be used to amplify sounds reaching our ears, click here!