Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Seany on 04/04/2011 12:08:44
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There was a really annoying high-pitched noise coming from our house, and I could not find where it was coming from! Now, if it was a low-pitch noise, like a low humming, it is so easy to find its origin. But what makes it different for a high-pitch noise?
Its high frequency and short wavelength? Even so..?
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Normally "high pitched" noise is very directional and, provided you have a direct line-of-sight (maybe sight is not the right word) to the source, it is easily located. Low frequencies, on the other hand, are not very directional, which is why it is common to be able to employ a single sub-woofer in a stereo system. However, locating a non-specific sound does not always follow the same rules, especially inside a room.
I suspect in trying to detect the sources of low frequency sounds you are doing it by moving around until the volume is loudest and thereby locating the source. This does not need the directionality of the sound to provide any assistance. High frequency sound sources can be problematic to trace in the same way because there can be interference effects that can cancel or enhance the sound at different points in a room. This may cause you to move away from the source rather than towards it. Just moving your head would be good if in the open, as human's are good and locating sound sources via their stereo reception system (otherwise known as ears). In a room with lots of objects, reflections may render this less than perfect though.
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As Graham said, the loudness varies all over the place. This is more pronounced with high frequencies because they reflect off of smaller surfaces. The speed of sound is about 343.2 m/s, so 15 khz sound has a wavelength of about 23 mm. Sound whose wavelength is longer than the size of a chair will go around the chair, and shorter wavelengths will reflect off of the chair.
On the other hand, intermittent high frequency sounds, like a ticking clock, are easier to localize because the beginning of each tick arrives at the nearest ear first. The difference of arrival time is constant in seconds, but it is a greater fraction of a wavelength for higher pitched sounds. Birds take advantage of this by using a steady tone for danger above and an intermittent sound for danger below. That way, they don't give away their position to an eagle, and they don't care if a cat knows where they are because it can't reach them.
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An important feature of sound location (as in all location measuring ) is bandwidth A narrowband sound is difficult to locate that is a sinusoidal one if however it is a click or a buzz it is much easier to locate. It is all to do with the information that can be carried by the sound.