For what frequency does the intensity have a minimum?

[Firben]

Two speakers A and B are separated by 1 meter, the point P is 4 m away from speaker B. If P is a person for what frequency does he hear that the intensity have a minimum?. (The listener hear that the sound intensity reduces and increases).I don't know the distance from P to B. (They form a right-angled triangle)

B------A
.
.-------P

(1.0 m)^2+(4.0 m)^2 = BP^2

BP = 4.123 m

The distance from Poit P to B is 4.123 m

how can I determine the wavelength ?

The answer should be: The minimum occurs for 1.3 kHz, 4.1 kHz, 6.9 kHz,...

Should i use this formula ?

f(n) = n*v/d

The path difference is d=(4.123-4.00)= 0.123 m

 

[cepheid]

For destructive interference to happen, the path difference between the sound wave from A and the sound wave from B should be an odd multiple of half the wavelength. You know what this path difference is, so you can find the wavelengths that work.

 

[Firben]

Estimate the relationship between the maximum and minimum intensity. Assume that there is a spheric wavepropogation and the effect from the surrounding kan be neglected. The speed of sound can be set to 340 m/s. The intensity decreases with 1/r^2 ie the intensity on the wave r m from the source is I0/r^2 where I0(zero) are the intensity on 1m from the distance from the source.

How can i found the intensity and what's aboute the max and min intensity, what value should i use for that ?