How Many Sound Intensity Minima Are Detected as Frequency Increases?

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Homework Statement

Two sources S₁ and S₂ of sound are situated 80cm apart in air, as shown in the image provided. The frequency of vibration can be varied. The two sources always vibrate in phase but have different amplitudes of vibration. A microphone M is situated a distance 100cm from S₁ along a line that is normal to S₁S₂. As the frequency of S₁ and S₂ is gradually increased, the microphone M detects maxima and minima of intensity of sound.
The speed of sound in air is 330ms^-1. The frequency of the sound from S₁ and S₂ is increased. Determine the number of minima that will be detected at M as the frequency is increased from 1.0kHz to 4.0kHz.

Homework Equations

Frequency=Velocity/Wavelength
(n-½)(λ/a) = x/D where λ is wavelength, x is separation between adjacent maxima and minima and D is the perpendicular distance from the zero order maximum.

The Attempt at a Solution

Using the two ranges of frequency 1.0kHz and 4.0kHz, the wavelengths must be between 8.25cm and 33cm but not sure what to do next. Any help would be greatly appreciated. Thank you.

 

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Picture of question attached here

 

[seiei]

Is anybody able to help me? My physics exam is tomorrow...

 

[Rayquesto]

what do you think the amplitude of waves will affect? will it affect frequency or period?

 

[seiei]

The amplitude will affect the intensities, and the frequency will affect the wavelength of the sound waves