What is sound? (XV) Footsteps and head scratching

When one thinks of sounds, the image that comes to mind is a speaker playing back a sound wave, which travels through air to the ears of the listener. But not all sounds are like that. I will give two examples: head scratching and footsteps.

When you scratch your head, a sound is produced that travels in the air to your ears. But there is another pathway: the sound is actually produced by the skull and the skin, and it propagates through the skull directly to the inner ear. This is called “bone conduction”. A lot of the early work on this subject was done by von Békésy (see e.g. Hood, JASA 1962). Normally, bone conduction represents a negligible part of sounds that we hear. When an acoustical wave reaches our head, the skull is put in vibration and can transmit sounds directly to the inner ear by bone conduction. But because of the difference in acoustical impedance between air and skin, the wave is very strongly attenuated, on the order of 60 dB according to these early works. It is actually the function of the middle ear to match these two impedances.

But in the case of head scratching, the sound is actually already produced on the skull, so it is likely that a large proportion of the sound is transmitted by bone conduction, if not most of it. This implies that sound localization cues (in particular binaural cues) are completely different from airborne sounds. For example, sound propagates faster (as in water) and there are resonances. Cues might also depend on the position of the jaw. There is a complete set of binaural cues that are specific of the location of scratching on the skull, which are directly associated with tactile cues. To my knowledge, this has not been measured. This also applies to chewing sounds, and also to the sound of one’s own voice. In fact, it is thought that the reason why one’s own voice sounds higher when it is played back is because our perception of our own voice relies on bone conduction, which transmits lower frequencies better than higher frequencies.

Let us now turn to footsteps. A footstep is a very interesting sound – not even mentioning the multisensory information in a footstep. When the ground is impacted, an airborne sound is produced, coming from the location of the impact. However, the ground is not a point source. Therefore when it vibrates, the sound comes from a larger piece of material than just the location of the impact. This produces binaural cues that are unlike those of sounds produced by a speaker. In particular, the interaural correlation is lower for larger sources, and you would expect that the frequency-dependence of this correlation depends on the size of the source (the angular width, from the perspective of the listener).

When you walk in a noisy street, you may notice that you can hear your own footsteps but not those of other people walking next to you, even though the distance of the feet might be similar. Why is that? In addition to the airborne sound, your entire skeleton vibrates. This implies that there should be a large component of the sound that you hear that is in fact coming from bone conduction through your body. Again these sounds should have quite peculiar binaural cues, in addition to having stronger low frequencies. In particular, there should be different set of cues for the left foot and for the right foot.

You might also hear someone else’s footstep. In this case there is of course the airborne sound, but there is also another pathway, through the ground. Through this other pathway, the sound reaches your feet before the airborne sound, because sound propagates much faster in a solid substance than in air. Depending on the texture of the ground, higher frequencies would also be more attenuated. In principle, this vibration in your feet (perhaps if you are bare feet) will then propagate through your body to your inner ear. But it is not so clear how strong this bone conducted sound might be. Clearly it should be much softer than for your own footstep, since in that case there is an impact on your skeleton. But perhaps it is still significant. In this case, there are again different binaural cues, which should depend on the nature of the ground (since this affects the speed of propagation).

In the same way, sounds made by touching or hitting an object might also include a bone conducted component. It will be quite challenging to measure these effects, since ideally one should measure the vibration of the basilar membrane. Indirect methods might include: measurements on the skull (to have an idea of the magnitude), psychoacoustic methods using masking sounds, measuring otoacoustic emissions, electrophysiological methods (cochlear microphonics, ABR).

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