The precedence effect
How a binaural psychoacoustic effect tricks some audiologists to believe something which is not entirely true.
When sound travels in a room, it reaches your ears through line of sight and also through reflections of the walls, ceiling and floor. The direct path from the source to you is the shortest and therefore the fastest. The first wavefront tells your brain where the sound source is located, based on differences between the left and right ear in time of arrival, phase and signal level. The reflected sound arrives later, coming from all directions. As long as the reflections arrive within ~40 ms of the arrival time of the first wavefront, the different signals will merge into one perceived auditory image in your auditory cortex.
At a large distance to the talker, the amount of acoustic energy of the first wavefront is much less than that of the reflections. However, your auditory system is not fooled. If you have normal hearing, you can locate sound sources inside a room fairly accurately, even at a larger distance.
Reflections of sound are useful for localization as the different wavefronts arriving via different signal paths are integrated to enhance the overall signal level. These reflections are also very useful for hearing and for communication. A teacher facing the blackboard can only be heard and understood because of reflections. Reverberation is good because it increases the desired signal level. Only too much reverberation is not good, just like too little reverberation is not good either.
There is a bit more to this. Inside a room, the direct sound is dominant over reflections at a short distance, but at a larger distance, most of the acoustic energy is in the reflections. At the so called critical distance, the direct sound and the reflections have the same level of energy. The critical distance is a property of the room and the shape and size of the sound source. In most rooms, the critical distance is between 0.75 and 1.5 meters.
When the distance is larger, the precedence effect comes into play. This effect explains how the mechanism in your auditory system localizes sound when the sound source is beyond the critical distance. But, this does not mean that there is a lot of directional information in the integrated signal that reaches your ear. Most of the energy comes from all directions, through reflections. This is very similar to noise, which is usually also distributed fairly evenly throughout a room. Noise will reach your ears from all directions, unless you are very close to the noise source.
The precedence effect masks the auditory bubble that we live in. It hides the fact that speech coming from 3 or more meters across the room often has no directional information left in it once it arrives, apart from that soft first wavefront. Don’t be tricked by this. The fact that you can accurately localize sounds coming from beyond the critical distance does not mean that directional microphones are beneficial when distance from the sound source is large. Amplifying sound coming from one direction more than sound coming from another direction will not enhance the signal-to-noise ratio. If that were true, we would not need advanced wireless microphones.
Directional microphones in hearing aids are a great innovation and have helped hearing aid users understand speech much better at short ranges. An improvement of up to 6 dB in signal-to-noise ratio is huge. But that value is only true in anechoic chambers or in rooms that are well within the critical distance. At the critical distance, the maximum improvement which is theoretically possible is just 3 dB.
Beyond the critical distance, you need an advanced wireless microphone to achieve a better signal-to-noise ratio. In these wireless microphone systems we see more and more directional microphones, as they are so effective at a short range. A wireless microphone, whether it is a ‘Madonna-mic’, a lapel mic or a table microphone, is often placed well within the critical distance to the talker.
At Phonak, in Naída V and Sky V, we combine the best of two worlds – directional microphones in the hearing aids (1), with directional wireless microphones (2). For this, we have three analog-digital (AD) convertors inside our hearing aids, which is pretty unique. We call this ‘Roger and Directional’. This combination allows for optimal speech understanding in the near field and far field, at the same time. More on that in a future blog post.
To read more about the precedence effect, critical distance or Roger and Directional:
Litovsky et al.: Precedence effect. J. Acoust. Soc. Am., Vol. 106, No. 4, Pt. 1, October 1999.
Brian Moore, The Psychology of Hearing. 6th edition.