Help Needed: Wave Frequency & Wavelength vs. Air Temp

[Dark Visitor]

A sound wave in air has a frequency of 500 Hz and a wavelength of .68 m. What is the air temperature?

* -18 degrees C
* 0 degrees C
* 15 degrees C
* 27 degrees CI can't find an equation for this, and I don't know how to solve it. Anyone know anything about this? Please help me. Thanks.

 

[denverdoc]

Try googling for speed of sound. It varies with temp. So compute the speed from the data and compare with what you find online. Google is your friend.

 

[Dark Visitor]

Not that your suggestion is wrong, but when looking in my book, it says "use the speed of sound in air at 20 degrees C, 343 m/s, unless otherwise specified."

So I guess I use that. But where does that help?

 

[denverdoc]

The velocity of C=lambda*f. You are given both above--so what is C. It is not 343. Still do a google, meanwhile do you think the speed of sound increases or decreses with temp?

EDIT: I'll spare you the math as its been a long day: here is a table: http://www.sengpielaudio.com/calculator-speedsound.htm

 

[Dark Visitor]

When I Googled it, the first thing that popped up was "speed of sound is 340.29."

And then if I multiply:

500 Hz * .68 m, I get 340. How does that connect?

 

[denverdoc]

When I Googled it, the first thing that popped up was "speed of sound is 340.29."

And then if I multiply:

500 Hz * .68 m, I get 340. How does that connect?

Go back to my earlier post, I edited it, and remember google is your friend. There are lots of time when animations and what not can make a difficult concept clear. Books cannot do this!

 

[Dark Visitor]

Thanks for the link, but what do I type in? I don't know a temperature.

 

[denverdoc]

Find the temperature that corresponds to 340m/s and compare with your answer choices. Scroll down to the bottom of the page and find TABLE--I see your confusion.

 

[Dark Visitor]

Okay. I did that, and 15 degrees C is the closest (it was 340.27). But how would I show my work for that? Unfortunately, my teacher would not accept "I used Google to get my answer" as an acceptable excuse.

 

[denverdoc]

Okay. I did that, and 15 degrees C is the closest (it was 340.27). But how would I show my work for that? Unfortunately, my teacher would not accept "I used Google to get my answer" as an acceptable excuse.

Well, then look up the math. I don't know what you're using for a book or your notes, but there are several formula of increasing complexity that allow you to calculate C as a function of T. If your book says use 343, I'm assuming there is no discussion in it re variability. Wiki has a decent discussion with two fairly simple equations uou can solve in a couple of minutes.

 

[Dark Visitor]

I don't see any way to show any math, so I just put a note on the side saying (343 is the speed of sound in 20 degrees C).

 

[denverdoc]

As i said, its on wikipedia, or any of a 1000 other sites. But your call.BTW look at wiki under practical formulas for dry air.

C=331+0.6*c where c=temp in degrees c. Plug in 340 and you'll get 15 degrees c!

 

[Dark Visitor]

I know, but I don't know which one to use, or how to use it. I'll keep looking. Can you finish helping me on "Find The Wavelength" please? It doesn't have to be tonight, but I need all my posts done by tomorrow and I have 3 left, including that one. Or help me on any others (the other 2 are "Kinetic Energy at the Bottom" and I am going to post another one in a second.) Thanks so much.

 

[denverdoc]

look at the formula above on my last post. That is very simple.