Q about this strange wave thing

[JFK90787]

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This thing. Please tell me if if my facts are right, since none of my material really covers it and I have had to piece this together from practice problems.

1. This is formed when 2 sound waves encounter each other
2. The distance between the two purple dots is the wavelength of the beat
3. The distance between the green dots is the wavelength of sound

I am a little confused because I thought the distance between the purple dots would be the 'wavelength'. But in EK they describe the conditions I stated above. Are these correct? TY

 

[chiddler]

1. Yes.
2. Yes.
3. Hmm...i think so. I would like somebody else to answer.

wiki has some nice media to help intuit it. listen to the audio clip on their page, too.

 

[dsoz]

This looks more like amplitude modulation (AM), like in AM radio. The smaller waves have their amplitude changed to make the "carrier wave" that has a "beat" wavelength equal to the "fuchsia dots."

If this were a sound wave, the sound volume of the small wavelength would increase and decrease in time as the carrier wave for the larger wave of sound. In essence, it is easier to create sound waves with a shorter wavelength and adjust the volume of those sounds to create a lower sound (the beat frequency) of the longer wavelength.

The distance between THREE green dots would be the wavelength of the smaller wave. (two green dots = 1/2 wavelength) To have a wavelenth, you need both an "up" part and a "down" part.

Hope that clears some things up.

dsoz