There may be an increasing emphasis on speech intelligibility in aircraft, including military aircraft, from a safety and operational standpoint. An aircraft operator's ability to communicate during an emergency may be important. Speech intelligibility in a flight environment may not only include a crew's ability to hear, but may also include the accuracy with which the crew can understand spoken words in the aircraft's noise environment. Generally there are two forms of speech intelligibility measurement: (1) human-based or direct testing and (2) machine-based or indirect testing. With direct testing expert listeners monitor specially constructed speech samples directly or broadcast over a sound system and the listeners may mark the words or sentences they hear on a prepared test sheet. In indirect testing either speech or a special test signal may be broadcast over a sound system, and a received signal may be picked up by a microphone and analyzed to produce a result signal. Degradation components may also be produced. A ratio of useful signal to detrimental signal may be computed.
A human-based testing procedure such as the Modified Rhyme Test (MRT) method may be used to evaluate speech intelligibility in the cabin, flight deck and communication systems in a flight environment. MRT may be regarded as a subjective speech intelligibility metric that not only requires substantial resources in terms of jury training and flight testing costs, but may also require a dedicated test airplane with a capability to accommodate a jury of 10-15 people, comprising of speakers and listeners, for an extended period of time. A machine-based testing procedure that may yield an intelligibility metric that may correlate with speech intelligibility and MRT may be obtained using a machine-based Speech Transmission Index (STI) signal.
STI is a measurement technique for determining speech intelligibility that employs a principle that speech intelligibility is based upon slow modulation of the strength of a carrier sound pressure signal associated with speech. STI considers background noise level, reverberation time and space size using special test signals having a fundamental wave form modulated by low-frequency signals. Depth of modulation of a received signal (after traversing a test space) is compared with the original test signal (broadcast into the test space) in each of a number of frequency bands. Reductions in modulation depth may be associated with loss of intelligibility. Using STI according to prior art techniques may require recording special signals at each measurement location or test space in an aircraft during expensive flight tests requiring a dedicated test airplane operating aloft.
There is a need for a system and method for effecting a machine-based determination of speech intelligibility in an aircraft during flight operations that minimizes or eliminates a need for airborne testing procedures.