One of our acoustic consultants, Stephen Jay has recently completed his MSc in Environmental and Architectural Acoustics at London South Bank University and was awarded a distinction.
As part of his studies he had to undertake a final project which he decided to undertake on his second passion after acoustics, the RNLI. Stephen based his project on the attenuation and speech intelligibility properties of the Gecko helmet, worn by inshore lifeboat volunteer crews for protection while at sea.
The Gecko helmet is designed to provide impact protection within the marine environment, but with its use and the inclusion of an intercom system the direct transmission path of sound to the user’s ear is obstructed. Anecdotal evidence from volunteers showed that when wearing the helmet, users were having trouble to understand verbal commands and communications. The study looked at objective measurement of attenuation and speech intelligibility, taking advantage of London South Bank University’s anechoic chamber.
Measurements of the attenuation of the Gecko helmet was undertaken within an anechoic chamber using a directional sound source, with measurements undertaken using replicated engine noise and pink noise sources. Measurements showed the helmet provided slight attenuation of sound levels from 1 kHz to 16 kHz measured with the highest level of attenuation noted at 4 kHz, while between 31.5 Hz and 1 kHz no significant attenuation was observed. When looking at the overall level of attenuation across 8 Hz to 16 kHz frequency spectrum, it can be considered that the Gecko helmet provides no attenuation.
With the measurement of speech intelligibility objective and subjective test methods were used. Objective measurement was undertaken within the anechoic chamber at London Southbank University, using a STIPA signal at varying levels in relation to a masking noise source and the speech transmission index score calculated. This method was repeated, but with a change to the angle of incidence of the sound source. Subjective measurements of speech intelligibility were also undertaken with the help of 22 volunteers over two locations. Volunteers listened to 4 sets of phonetically balanced word score recordings and asked to write down the words they heard. Initial measurements were undertaken with no helmet and masking noise, this created the baseline results for the study; this was followed by introducing the Gecko helmet and masking noise source to the test. Both objective and subjective measurement showed a decrease in speech intelligibility with the introduction of Gecko helmet. When the Gecko helmet was tested without a masking noise, objective measurements showed only a slight decrease in STI score which under the common intelligibility scale was scored as excellent. When the same test was undertaken subjectively the result achieved a fair rating. With the introduction of a masking noise to the helmet performance in tests reduced significantly for both objective and subjective measurements.
With the measurement of the level of attenuation of the Gecko helmet led to an assessment of the likely noise exposure for volunteers. When an Atlantic 85 craft is traveling at 35 knots, the noise levels measured at the fourth man position was measured at LAeq,T 99 dB with an LAmax measurement of 100 dB. These levels were considered in relation to exposure levels cited within the The Control of Noise at Work Regulations (2005), although it should be note that RNLI volunteers would be exempt from the regulations under the following conditions:
- “(A) Under regulation 9 (Health surveillance) shall not extend to persons who are not employees.
- These regulations shall not apply to the master or crew of a ship or to the employer of such persons in respect of normal shipboard activities.
Although exempt from The Control of Noise at Work Regulation (2005) the exposure levels cited are a good indicator of levels that can lead to a permanent shift in the threshold of hearing. If a volunteer were to be exposed to the noise levels measured for just 1 hour their daily exposure would of LEP,d 87 dB(A), if calculated as weekly exposure this would reduce to LEP,w 83 dB(A). These levels are considered to be higher than the lower exposure action level stated within The Control of Noise at Work Regulations (2005). It should also be noted that it is not typical of inshore lifeboats (including the Atlantic 85) to continuously run at a speed of 35 knots for over an hour on exercise or operational duty. But this level could be exceeded due to cumulative exercises and operations during busy periods.