Stupid Physics question. Transmitting sound through different media.

[Dylab]

This question is bothering me. If I transmit a sound wave through a medium, ie air, and it reaches a denser medium, ie water, what percentage is transmitted and what percentage is reflected.

 

[Schneibster]

Well, the simple answer is, it depends on the angle of the direction of propagation of the wave to the interface between the media, and the characteristics of each medium. The dependence is mathematically complicated. How much do you want to know?

 

[Dilb]

The equations I used for that sort of problem in my intro physics course were

[latex]$$ \frac{I_r}{I_i} = \biggl (\frac{z_1-z_2}{z_1+z_2} \biggr )^2 $$
$$ \frac{I_t}{I_i} = \frac{4z_1z_2}{(z_1+z_2)^2} $$[/latex]

With I being intensity (power/area) and z being the acoustic impedance (which is density times speed of sound). For air and water, it works out to something like 99.89% reflected, or 0.108% transmitted. This is for a plane sound wave meeting an infinite plane boundary at normal incidence (i.e., hitting a large, flat object straight on).

Other situations are probably hideously difficult to calculate.

 

[Schneibster]

I'd expect a trigonometric adjustment to those equations, let's see, my guess off the top of my head would be a sine or sine-squared function of the angle from the normal to the interface since that will increase to 1 at 90 degrees, parallel to the interface. Carefully consider the likely effect on a spherical wave front.

 

[MRC_Hans]

Yes, impedance and angle are important. Also, whenever the wave hits at an angle different from 90deg, refraction occurs. Basically, it is somewhat like the way light behaves when passing between mediums, although impedance is less important for light (or different transparant mediums have more similar impedances).

Hans

 

[Dylab]

I was curious on whatever or not it was similar to the Fresnel Equations. I guess it was pretty obvious since they both follow the conservation laws.

 

[MRC_Hans]

Light waves and sound waves behave in a somewhat similar way, but one should be very careful about making assumptions from one to the other, since they are two very different phenomenon, and there are also some distinct differences.