Anders Hägglin - Hearing Protection Devices

Measuring Damping Effects of Hearing Protection Devices for Divers

A Hägglin, T Lindell Jernström, M Björk
Department of Naval Architecture and Ocean Engineering,

Chalmers University of Technology

ABSTRACT

A study concerning passive acoustic damping from hearing protection devices on divers, has been performed. The objective was to measure the acoustic damping effect from different hearing protection devices, which directly or after modification, can be used by divers.

An objective method of measuring acoustic damping was used. The measurings were done on a dummy head that is acousticly buildt as a human head. During the trials the dummy head was placed in an in-door pool were it was exposed to noise from an underwater speaker and the damping effects from the protective devices were measured.

Parallel with these measurments, subjective views on the damping effect was collected from divers exposed to noice in the same way as the dummy head.

The result show great difficulty obtaining good acoustic damping in the lower octava. However, in the area of frequency between 500-3000 Hz, important to the understanding of speach, damping of up to 40 dB was achived.

INTRODUCTION

During the past years, a lot has been done to improve diving safety. However, compared to the ordinary construction worker and the conditions at his worksite, much R/D remain before acceptable levels of dangerous exposure are reached for the diver, at the underwater worksite.

One of the larger problems among professional divers is the exposure to unacceptable levels of sound, without adequate hearing protection devices. Such exposure can lead to permenant loss of hearing. Our objective was to measure the acoustic damping effect from different hearing protection devices, which directly or after modification, can be used by divers. Earlier investigations have concentrated on measuring levels of sound from water jets, pneumatic drills, free flow streams in diver helmets, etc when using wet hoods. All relevant projects are accounted for in "Underater Noise and the Conservation of Divers Hearing : A Review"( ). This report show that the differens in auditory threshold between air and water is smaller than expected. This means that it is more relevant to provide good hearing protection for divers than anticipated.

The wet hood on a diving suit give a certain protection. Investigations show that down to 30 meters depth, the hood is a valuable part of the protection from frequencies over 1000 Hz and give no or small protection from frequencies under 250 Hz( ). The damping of the frequencies in between is not well defined and depends on ea. the thickness of the hood and how well it fits. The thickness of the hood should be at least 5 mm.

HEARING PROTECTION DEVICES

Measurements have been performed on three different kinds of hearing protection devices:
Hoods (Viking and Nokia dry suit hoods, Swedac damping hood) Earmuffs (Bilsom standard earcmuff Chalmers modified earmuff) Earplug (EAR standard earplug)

Viking dry suit hood: This standard hood is mainly intended for keeping the diver warm. It is made of a light and porous material which dampens higher frequencies.

Nokia dry suit hood: This standard hood is made of latex and provided with rubber spacers at the ears to prevent squeeze.

Swedac: This hood is specially made as a hearing protection device for divers, intended to absorb both high and low frequencies. It is made of a rubber material containing lead and it has a porous textile on the inside and a nylon textile on the outside.

Bilsom standard earmuffs: These earmuffs are worn by hunters, construction workers etc. To fit into a dry suit hood the earmuffs were removed from their standard frame and fitted into an eleastic hood.

Modified earmuffs: Modified earmuffs were made at Chalerms . They have a shell of lead and a porous foam material as filling. They are supposed to dampen lower frequencies as well as higher.

EAR-plug: This standard earplug is made of a foam material. To make it possible to use this plug when diving, it was ventilated through a 0.5 mm plastic tube.

OBJECTIVE ACOUSTIC MEASUREMENTS

Equipment
The tests were performed in the in-door tank at Chalmers University of Technology. The tank is 4 by 6 meters and 5 meters deep. An overhead crane make heavy lifts possible.

The source of sound was an underwater speaker, Argotec model 214 Projector, that was placed on the bottom of the tank. It was oriented so that the levels of sound was equivalent in every chosen point of measurement. The speaker was connected to a sound generator, B&K 1405, giving a so called pink noise, useful when measuring in all octavas.

The sound levels were measured with two microphones, B&K 4134, connected to an amplifier, B&K 2619 and a sound level measuring device B&K 2209.

The measurings were done on a dummy head, of the brand KEMAR, that is acousticly buildt as a human head. The microphones were placed in each ear. The neck of the head was slightly modified to contain the amplifier. A Nokia dry suit hood with its facial opening sealed, was placed over the head, giving a dry environment resembling that of a diver.

The head was placed on a test rig built of aluminium tubing.

Measurings
The level of referens was chosen as the Nokia dry suit hood, since it was used under all measurings. After calibration the backgound sound levels were measured and the dummy head with respective hearing protection device, was fasten to the rig.

Three positions were chosen (fig X) and for each position measurings were made in the octavas 31.5, 63, 125, 500, 1000, 2000, 4000, 800, 16000 Hz. Measurings were made with the head facing the speaker and with the head turned 180 degrees. This makes it a total of six measuring positions for each ear and device, giving a better accuracy in the results.

During the measurings the sound effect level of the speaker was kept constant at 85 dB, resembling 20 uPa.

Treatment of measuring results
When testing hearing protection devices according to ISO standard, both average sound level and standard deviation for each measuring position should be presented.

The average sound level was calculated according the formula (ref xx)

The standard deviation was calculated according to the formula (ref xx)

The following graphs show the damping effect of the respective protection device.

Results
The results show great difficulty obtaining good acoustic damping in the lower octava. However, in the area of frequency between 500-3000 Hz, important to the understanding of speach, damping of up to 40 dB was achived, when using two protection devices in combination.

SUBJECTIVE TESTS

The subjective tests were performed to give an opinion of the effect from the hearing protection devices when worn by human divers. Six divers performed these tests. The divers estimated the damping effect, but more important, gave opinions on how the devices fitted and if any trouble occurd when the diver moved his head.
The subjective test were performed with the Swedac hood, the modified lead earmuffs and the EAR-plug.
The source of sound was the underwater speaker, Argotec model 214 Projector, and the dives took place in the same in-door tank.

Scale of estimation
To support the estimation of damping effect and comfort, a scale from 1-6 was used. Every diver was exposed to seven different levels of sound from 0-110 dB. On the scale, 1 was corresponding to "very pleasent" and 6 was corresponding to "intolerable".

Performance of tests
Before the diver entered the water he was asked to estimate the effect from the hearing protection device on his comfort. When the diver was on the bottom he took the speaker in his hand and pointed it to his face. He then estimated the damping effect and comfort at different sound levels. Each test was concluded with the diver performing the test without hearing protection device, as a reference.

Results
All divers established an obvious damping effect from the different devices, but had great difficulty ranking the devices from this effect.
In comfort the earplugs was ranked as the best but also the earmuffs was regarded as comfortable. The hood was regarded as stiff when turning the head, but otherwise comfortable. It should be mentioned that only one size of the hood was available.

SUBJECTIVE TEST OF COMMUNICATION DEVICE

Rastronic Communication Earphone is not intended for the diving market but it has some advanteges when diving. It is an earphone which is both microphone and speaker. It amplifies the sound from the diver, conducted through the bone, dampens outer noise and provide the diver with communication to the surface. The obvious advanteges with this system is that disturbing sound from the breathing gas and the surroundings are eliminated.

A subjective test on this earphone was performed in a pressure chamber and in the in-door tank. In the pressure chamber, no extra source of sound was needed since the level of sound, when pressurizing, is 105 dB. Two divers were placed in the pressure chamber, one wearing the earphone and the other a standard headset. The divers spoke to two assistants on the outside during the pressurizing. When the divers had reached 20 meters, the divers and the assistants changed equipment and the test was repeted during ascent.

No physiological problems occurd during the dive. Both divers had great difficulty hearing the assistants and both assistants noted a lower level of background noise and a better understanding of speach when the diver wore the earphone.

A test was also performed in the in-door tank. The diver wore the earphone in his right ear and a Dyfo bone conductor on the left side. The diver communicated with two assistants at the same time, one using the Rastronic equipment and one using Dyfo diving communication set.

The diver was alternately exposed to noise and the assistants changed equipment after half the test time.
The differencies were small, but it was noted that the diving communication set affected the intelligibility of the speach since it was disturbed by sounds of breathing and the outer noise. The Rastronic earphone worked very well during the trials and good communication was established in both directions.

CONCLUSIONS

There are no acoustic obstacles that prevents the development of a comfortable och well functioning hearing protection device for divers. It is therefore important to continue the tests of materials and concepts.

Summerizing our measurements and experiences, we establish that it is hard to compare the results from the different concepts and materials. Finally a list of profits and consequences for the different concepts is presented.

Earmuffs
Earmuffs give a stable and reliable damping of sound and they can be used when there is a risk of ear infections.
Earmuffs are uncomfortable to wear for a longer period of time since a hard preasure on the skin can affect the blood circulation and a low preasure can give a poor damping effect. They are also hard to fit under dry suit hoods and in helmets.

Earplugs
Earplugs must be ventilated if used by divers.
Earplugs are cheap and comfortable to wear and they can be used with all hoods and helmets.
Earplugs can not be used when there is a risk of ear infection and they are easily contaminated with ear wax.

Hoods
Hoods are used with dry suits and helmets to prevent the diver from freezing. They have a certain damping effect, which can be increased if material and design is considered.
Hoods are easy to use, dampens bone conducted sound and keeps the head warm.
Hoods must fit very well to the head to give a good damping effect on sound.