Re: Can sound waves be polarised?

Hi Zuya,

That is a very interesting question... and one that prompted me to bring out my edition of Halliday, Resnick, and Krane's Physics. I found two references to the polarization of sound waves in there that I think will validate your physical intuition on the subject.

Before we address the polarization question, let's just briefly figure out what polarization is. We'll be dealing with simple, idealized waveforms as in the figures 1 and 2 below. Figure 1 shows a transverse wave. As you can see, a transverse wave is one where the movement is in a direction perpendicular to the direction of propagation. E.g., if you have a bob in a lake and a wave comes by, the bob moves up and down. Although the wave is moving across the lake, the bob moves in a direction perpendicular to the direction of propagation of the wave (namely, vertically). As you no doubt know, light is a transverse wave (an electromagnetic wave) which can have it's electric field in a vertical or horizontal (or inbetween) direction (please see Figure 1 below).

Figure 1

Another simple kind of wave is a longitudinal wave, like sound. A familiar way to visualize sound waves is to imagine a slinky, as in Figure 2, below. Longitudinal waves differ from transverse waves in that their displacement is in a direction parallel to the direction of wave propagation. That is, if the wave is moving forward, a particle in it's path will be pushed forward and backward as the wavefront passes it by.

Figure 2

So now that we've defined transverse and longitudinal waves sufficiently for our purposes, let's tackle polarization. A polarized wave is one in which the displacement pattern is in a single plane. An unpolarized wave is one with no preferred plane of polarization. E.g., if a light wave encounters an electrically charged particle and it moves up and down, then the light wave is said to be vertically polarized. If the charged particle moves left and right, then it's said to be horizontally polarized. In either case, the light wave is then said to be plane-polarized. If you take a vertically polarized wave and put an object with a vertical slit in it's path, it will still go through. However, if you rotate the object with the slit so that the slit is now horizontal (as in Figure 3, below), the vertically polarized wave will be blocked and won't get through the filter. This is true for transverse waves but since longitudinal waves only vibrate along the direction of motion, no orientation of the slit would stop them and so longitudinal waves are unpolarized!

Figure 3

This is why it's particularly curious to hear Halliday & Resnick mentioning that. I have a (old) copy of H&R and I browsed through it briefly and the only 2 references I've been able to find for sound waves' polarization are 2 "thinking" questions: one where it asks if sound waves can be polarized and another (on p. 1018) where it asks why sound waves are not polarized. My guess is that maybe you just misread these questions and hence the confusion? However, if you really do have a reference in H&R where they talk about sound wave polarization, I'd appreciate it if you could drop me a line at rickys@sethi.org with the relevant passage. I asked some of my colleagues about this and the closest thing to that anyone's heard of is an acoustic-optico effect in which sound waves are used to control light waves. Irrespective, however, longitudinal waves do not display polarization.

Hope that helps,


Rick.