sound question

[wetlightning]

ok,

can anyone help me with this:

i think there was a discrete on aamc 8 about a reflected sound wave and what does not change (and the answer was that everything (frequency, amplitude, wavelength) but the speed changes).

i've been trying to do some research on sound, and can't find out any info, so a quick run-down would be helpful. i know sound travels fastest in solid>liquid>gas, but when does frequency remain constant/speed remain constant/wavelength remain constant, etc.

is it not like light, where the freq. remains constant but speed and wavelength alter?

HELP!

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[Shrike]

Velocity is constant -- that's a property of the medium and the wave (here, sound in air), not frequency or anything else. You never get to change v on MCAT problems. (Bonus points -- what is the only exception on the MCAT, though it's never stated as one, to the principle that wave velocity is independent of frequency?)

The other answer choices (not sure I recall them correctly, as I've never seen the problem but have been asked about by students): frequency changes because of Doppler effect -- the moving wall is a moving source. Wavelength changes because frequency changes. Amplitude changes because the wall was an imperfect reflector -- some of the sound energy went into the wall.

 

[iwutitan]

Velocity is constant -- Bonus points -- what is the only exception on the MCAT, though it's never stated as one, to the principle that wave velocity is independent of frequency?)

???? so what is the exception?

 

[laya533]

is the exception dispersion ?

 

[wetlightning]

but i thought that, with light, frequency remains constant and velocity and wavelength adjust accordingly....

so with sound, the velocity is a prop. of the medium and its temp?

 

[Shrike]

Laya is right. With light, velocity in some media is (weakly) dependent on wavelength, hence dispersion. Frequency never changes in MCAT waves (except for Doppler, as above, but that's really creating a new wave), and velocity never changes unless either the medium changes, or the exception applies. Essentially, wavelength is the only thing you get to screw with.

Wetlightning, yes, sound velocity is dependent on the wave type (there's only one kind for sound), and the medium; relevant properties of the medium include state (here, gas), temperature, density, and molecular composition of the gas (e.g., moist vs. dry air). Luckily, most of these factors have little effect within the confines of the atmosphere we're used to.

 

[laya533]

Shrike, could you please describe the relationshipp between indicators and changing colors (giving a complementary color in the visible light)?I am not sure . Does it increase the wavelength? I am confused