# need help with my physics homework

Discussion in 'Off Topic [BG]' started by AmazingGracePlayer, Dec 9, 2006.

1. ### AmazingGracePlayer

Oct 8, 2006
Summit, NJ
Hey everyone, I'd really appreciate it if someone could help me with my homework, because I have no idea how to do it, well, here it is...

Your teacher gives you two speakers that are in phase and are emitting the same frequency of sound, which is between 5000 and 10,000 Hz. She asks you to determine this frequency more precisely. She does not have a frequency or wavelength meter in thel ab, so she asks you to design an interference experiment to determine the frequency. The speed of sound is 340 m/s at the temperature of the lab room.

a) From list below, selection additional equipment you'll need for your experiment: speaker stand, meterstick, ruler, tape measure, stop watch, sound-level meter (don't know what that does)

b) describe how the experiment would be set up

c) briefly outline the procedure that you would use to make the needed measurements

d) using equations, show explicitly how you would use your measurements to calculate the frequency of the sound produced by the speakers

What I'm thinking is to have speaker 1 in position A, and move back speaker 2 x meter such that the two speakers' sound waves are destructive... but not sure if that's right.

Thanks for looking and helping!

2. ### dcartoon

Jan 8, 2006
San Francisco, CA
Hey,

I hope that this helps if you haven't already solved your problem. I have been driving for a while, so in my present state I've probably forgotten something important.

Anyways, here goes. First off, it's useful to know what magnitude wavelengths you will be dealing with. At 5,000 Hz, you are looking at wavelengths around 6cm, and 3cm for 10,000 Hz.

The sound level meter basically tells you the loudness of sound at the position of the meter. i.e. if the meter reads 0db, then you have silence at that location, and if the meter reads 140 db, you will probably blow out your eardrums if you stick your head there.

On to the experiment. As you mentioned, destructive interference is one way to go, and that is probably how I would proceed.

Think about what happens if you put the speakers exactly one wavelength apart, and with each speaker generating a sinusoidal wave. Speaker A generates a sine wave that goes positive, passes through 0 amplitude, goes negative, and then returns to 0 amplitude when the wave reaches speaker B. Because the speakers are in phase, Speaker B also generates a sine wave that also goes positive and then negative.

Given the positions of the speakers and their phase, the waves that they produce will cancel out at all points. So, if you pass your sound level meter between the speakers, you should get a 0 reading at all points (nothing works out this well in real life, especially given all of the wave reflections ).

There are actually multiple speaker positions where the sound waves will cancel out perfectly, and multiple positions where the sound waves will reinforce each other. You can probably come up with the equations that will give you these positions.

So, if you pick a starting position (I might pick ~3cm, corresponding to 10 kHz), and take measurements and work your way outwards, you can figure out what wavelength, and in turn, frequency you are dealing with.

If you want to get more sophisticated, you could put your stopwatch to use and measure the beat frequency produced at the center point between the speakers, and use that to help you work out what frequency sound you have.

Feel free to contact me with any questions, or, let me know if I've made some silly mistakes or left out something important (more likely).

Dan

3. ### AmazingGracePlayer

Oct 8, 2006
Summit, NJ
Yeaup, that helped a lot, thanks man!