This report examines the findings of three recent studies by the Life Sciences Department, DRA Alverstoke, all of which are connected with diver noise exposures The first examines the self noise and protection to waterborne noise of a bandmask hood type breathing system [I], the second the protection to external noise provided by a lightweight diving helmet (Diving Systems International, Superlite 17) , and the final study is of the self noise of a continuous flow air helmet.
The noise to which a diver is exposed comprises of both the self none generated by the diver's breathing apparatus, and that which passes into the helmet from external sources such as compressed air tools, sonar transmissions, boat engines and propellers etc. The diver's life support equipment may produce noise as a result of gas flow, as high pressure gas is expanded to the ambient pressure at which it must be supplied to the diver, and that which is generated as exhaust bubbles form. These factors will all contribute to the noise exposure to the diver, with very high overall Sound Pressure Levels (SPL) of the order of 140 dB re 20 µPa, being typical.
The noise exposure to a diver using a bandmask hood type diving system (DSI KMB28) was measured at the Exmouth Diver Training School. Two calibrated hydrophones (Bruel and Kjaer 8105's) were fitted to the diver, one positioned inside the bandmask hood at the diver's left ear position and the other outside on the top front of the faceplate. These Instruments enabled the ambient noise to be recorded whilst the diver breathed normally, and then in turn operated a small compressed air rock drill (CP9) and an air lifting device. The depth of the diver's workstation was 6 metres of seawater (msw)
A diver in a bandmask hood effectively has a water filled external ear canal or "wet" ear and so "weightings" have not been applied to the data in this instance, hence, the breathing noise time history shown in figure 1 indicates the raw hydrophone data from inside the bandmask. This type of breathing system supplies gas on demand and hence, the noise level time history vanes in synchronisation with the diver's breathing cycle, from a peak level of 135 dB during exhale to 105 dB during Inhale This would suggest that external exhaust bubble noise is able to transfer efficiently from the water medium into the hood.
The breathing noise frequency spectra of these time histories (obtained via the FFT technique) indicates that the sound is predominantly low frequency, in the region below 1 KHz (Figure 2). The extent to which the human ear is able to respond to waterborne sound at these low frequencies is not yet fully understood, although the results of A1 Masri  suggest that they may be of more significance than in air. At typical speech frequencies the sound is at a level of approximately 70 dB re. 20 µPa.