Thales is investigating three-dimensional (3D) sound and hearing protection for helicopter crews as another way to deliver information and make missions easier. Most promising seem to be communication ear plugs, with tailored spatialized sound to distinguish between the various radio channels and alarms. Applications will be mainly military, according to a presentation made at the European Rotorcraft Forum 2010 in Paris in September. The symbology on head-up and helmet-mounted displays give a limited amount of information, Thales researchers say. Hence the need for exploration of other man-machine interfaces. Sound, as it can be associated with hearing protection in noisy helicopter environments, is a promising one.
Active ear plugs (that contain earphones) appear as a good combination of protection and enhancement of information delivery. Wireless plugs have been evaluated but have a major shortcoming—the pilot’s head needs to be positioned very precisely in the helmet because of the transmission coils. Therefore, wireless communication ear plugs are prone to signal loss. So Thales tends to favor wired ear plugs.
Morphology—including that of the pinnas, head and torso—impacts 3D sound perception. Moreover, left/right sound segregation is easy, while up/down or front/rear segregation is not. The generation of virtual 3D sound requires specific algorithms. This is done by a computer through a head-related transfer function (HRTF). In other words, for each individual, an algorithm can be determined to link a sound to where the brain “sees” its source.
The conventional HRTF determination method is well-known but takes too long. One transfer function has to be defined for each pilot and Thales thus set a time limit at 25 minutes per pilot. The faster method assumes that, for most users, an HRTF can be found in a database. It does not always select the best function but a sufficient one, Thales researchers explain.
Trials so far have focused on enhancing intelligibility of multiple concurrent audio communications. Thales used its TopOwl helmet as supporting hardware in a NH90 military transport. The company conducted tests in the context of a helicopter mission—because stress influences perception, researchers note.
Testing has proved that the HRTF determination procedure is accurate enough. One early application could be helping the pilot discriminate between radio channels. Also, the system could be used to warn about hostile fire. In that case, the system has been designed so that the simulated source of the sound uses the earth as a referential. This is to keep the threat’s origin consistent, even when the helicopter is in a turn. (From January 2011 Rotorcraft Report)