Human Ear

Not a Hearing Test

Since our receiver does not provide an unambiguous indication we can use to perform the aural search, we have to use our ears. Since peoples lives may depend on us performing the technique effectively we would do well to take some time to understand how human hearing works. We might be tempted to believe that since we have been using our ears since the day we were born that we know everything about them. However, unless you have made a study of aural perception there are many properties of hearing that will surprise you. The first is that aural search techniques are not a hearing acuity test. One does not need superior, or even average hearing acuity to perform these techniques well, the receiver does all the heavy lifting for us.

Monaural vs Binaural Hearing

Our hearing has been optimized for the kind of tasks that we do every day: communicating with others, listening for danger, locating food, etc. This optimization has taken advantage of the fact that we have two ears and binaural hearing. What we believe we hear is the result of a lot of signal processing performed by the brain. This signal processing helps us perform the tasks listed above. One of the important and impressive things this processing can do is separate the signal (the sound we want to hear) from the noise (those sounds we don't want to hear). The brain is able to do this processing because it receives information from two physically separate ears. Because the ears are physically separate signals from physically separate sources arrive at each ear with different phase and amplitude characteristics. The brain can use those characteristics to separate out signals from different sources. For example, at a party with many people all talking around you, you are still able to hold a conversation with the person next to you. If you replace your natural hearing, with a microphone and headphones that present exactly the same audio to each ear this ability will be substantially limited. This is exactly the situation a search crew is in when performing an aural search. The receiver provides one audio channel which is provided to each ear. Nor will it help to vary the amplitude sent to one ear with respect to that sent to the other ear since these changes will affect the signal we are listening for and the noise that may be masking it in the same way.

To make the best of this situation there are a number of things we can do. Make sure the volume of the receiver, intercom and headphones (if equipped) are all set to provide the highest comfortable volume. Many descriptions of aural searches tell us to set the volume and leave it at one level. This is not required at all. The important thing to remember is that if you reduce the volume as the output of the receiver becomes uncomfortable, that it is readjusted later to keep the volume at the highest comfortable level. There is no need to endure long periods of overly loud output in the hope that keeping the volume knob fixed will improve the accuracy of the procedure. In fact the opposite will happen. Any period of listening to very loud signals will temporarily (or maybe even permanently) reduce hearing acuity. 

The second thing we can do is limit the amount of competing audio from other sources during critical phases of the procedure. Critical phases are those periods when the search aircraft is outside the area where the emergency transmitter can be received, but proceeding into that area. During these phases the crew is trying to identify earliest point when the signal is detected. The crew talking, traffic from other aircraft on another radio or identification signals from navigation receivers may all reduce hearing acuity for short periods which may coincide with signal detection. Of course we can not expect the crew to eliminate all these competing signals for the entire flight, that would reduce both operational effectiveness and safety. It is possible to do so for short periods, especially when the likely duration is known to be short. This is one of the important reasons that we recommend that the aircraft fly outside of the signal area then turn around and fly back in. For the short period returning to the signal area unnecessary communications may be turned down or off.

Next we will look at the emergency transmitter.
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