# :eek: standing waves where to begin

Two loudspeakers face each other, vibrate in phase, and produce identical 438-Hz tones. A listener walks from one speaker toward the other at a constant speed and hears the loudness change (loud-soft-loud) at a frequency of 3.3 Hz. The speed of sound is 343 m/s. What is the walking speed?

Have you studied beats yet?

beat frequency

we are just starting beats now but my homework assignment is due sunday night, which is the day before the lecture on standing waves and beats...so far i've been reading and understand that 3.3Hz are the beats per second which is the beat frequency... and i probably need to use the equations f1=v/2L and v = lamda f to calculate something but i'm just not sure where to put everything...

turin
Homework Helper
mayo2kett said:
Two loudspeakers face each other, vibrate in phase, and produce identical 438-Hz tones. A listener walks from one speaker toward the other at a constant speed and hears the loudness change (loud-soft-loud) at a frequency of 3.3 Hz. The speed of sound is 343 m/s. What is the walking speed?
Use doppler shift. The frequency is frame dependent. The difference in the observed frequencies of the two speakers will be the beat frequency. The frequency of the speaker towards which the person is moving will have an observed increase, and the other will have an observed decrease, from the nominal 438 Hz.

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any help would me much appreciated... the assignment is due tonight
thanks :)

i think i got the answer using the equation f' = ((v + vo)/(v- vs))*f which is the same as the equation f'=(1+u/v)f since in my case the source isn't moving... i was wondering if someone could check this for me to see if i was right... i have 3.3Hz=(1+u/(343m/s))438Hz which when i multiply it out gives me 340.4m/s for the walking speed...