# Beat freuqency of a saxophone and trombone at middle A (440 Hz)

1. Jul 7, 2008

### solars

If you are playing your saxophone at middle A (440 Hz) and a trombone also playing middle A (440 Hz) in the open bed of a pickup truck is approaching you. The beat frequency you hear is 4 Hz. What frequency do you hear from the trombone?

The formula I know I am supposed to use is:
fbeat=f1-f2

but for my question what is f1 and f2. two answers i get depending on me putting 440 for f1 and f2 and the fbeat of 4 is 440 and 436.
Which one is right?

2. Jul 7, 2008

### dynamicsolo

Re: frequency

Have you had anything on the Doppler effect yet? The truck with the trombone player is approaching you. Is 440 and 436 the only solution?

BTW, it doesn't actually matter for the calculation which frequency is f1. The beat frequency is generally stated as

b.f. = | f1 - f2 | ...

3. Jul 7, 2008

### solars

Re: frequency

Sorry I meant 444 and 436. They are the only answer choices given, but I cant figure out which one is the correct answer.
I thought of doppler effect but the velocity of the truck is not given.

4. Jul 7, 2008

### dynamicsolo

Re: frequency

You don't need the truck's velocity for this problem, but only the concept of Doppler shift. You are going to hear your own note at A-440, so that is one of the frequencies (doesn't matter which). If the truck is coming AT YOU, will the tone you hear from it be shifted higher or lower from 440 Hz?

5. Jul 7, 2008

### solars

Re: frequency

Oh...it should be higher then, since it's coming closer.

6. Jul 7, 2008

### dynamicsolo

Re: frequency

And there you go!

7. Jul 8, 2008

### solars

Re: frequency

thanks