The need for clear communication between diver and topside is obvious, safe practices depend on good communications as do efficient operations and effective response to an emergency. The requirements of communications systems have only recently been formalized. The rationale behind the design of communications tests are specific problems of helium speech are discussed.
Tests made on commercial diving installations show that all parts of the system are important. No matter how good the unscramble good performance may sometimes only be achieved by using it in conjunction with a particular microphone. Results of tests will be presented to demonstrate this and to demonstrate that the commonly held belief that speech is more intelligible without the unscrambler may sometimes be true. The acoustic characteristics of the space from which the sound is being transmitted influences the intelligibility and two identical systems, used in different chambers, give very different performances.
These results suggest that investing large sums of money to design new unscramblers may be wasteful and much smaller investments, which produce intergrated systems to suit the particular space in which each will be used, will give much greater improvements in intelligibility.
It has become accepted practice to evaluate diver communications systems using a Modified Rhyme Test (MRT). The MRT is a modification of the Rhyme Test devised by Fairbanks (1958) and was suggested and evaluated by House et al (1965) for use where it is not possible to devote a great deal of time to training the readers and listeners. A further modification, that of Griffiths, is the format used by the industry. The MRT of house et al is widely accepted as a means of evaluating speech communication and was accepted by the American Standards Committee for Acoustics (1989)
The usual procedure is to play pre-recorded test tapes through each communications link and to use a team of listeners to score the tests "live", "on location" in the control room. This is carried out in the presence of an independent observer who marks the score sheets and calculates the average intelligibility for each link.
The presence of helium in the breathing gas distorts speech and unscramblers are used to restore this distorted speech to a form which can be recognized by the listener. Over the years a great deal of time, money and ingenuity has been expended to improve the design of unscramblers the experience gained from testing communications systems in recent years suggests that the main problems do not now arise from the performance of the unscrambler in that those available make a good job of unscrambling helium speech. The balance of the evidence seems to indicate that distortions generated by the rest of the communications link, which the unscramblers were not designed to correct, are the main cause of loss of intelligibility.
The earliest suggestion for intelligibility testing was made by Campbell in 1910 for testing the new telephone system of communication.