Formula for calculating wavelength of sound waves in interference?

[Myphysicsteachershit]

Homework Statement

We performed an experiment where 2 speakers were set up with 3 chalk lines spaced 1 metre apart from each other in front of the speakers. A 500 hz and then a 1000 hz sound was played through both speakers. We were to walk along the lines and mark the position of where the sound was the quietest (node) for both frequencies. After that we had to measure the distance between these nodes and calculate the wavelength.

Relevant Equations

For theoretical wavelength: lambda=340m/s divided by frequency
For experimental wavelength I used: lambda=2*distance between nodes

The values calculated was nowhere near the theoretical values, though I guessed they won't be as the results recorded was incredibly inaccurate. My teacher acknowledged the fact the final values won't be close to the theoretical ones but also said that my formula was wrong, that it works to find wavelength of the first line and not the other 2. I need help finding what formula to use, I have searched everyone but I don't really know what I'm looking for and our teacher said that it's not in the textbook that we use.

Here is a diagram of experiment:

Here is the results:

	Average distance between nodes (cm)
Frequency (hz)	Line 1	Line 2	Line 3
500	253	176	105
1000	333	438	None

My analysis:
ƛ/2=D
∴ƛ=2D where ƛ=wavelength (cm)
D=distance between nodes/antinodes (the average,cm)

500hz:
Line 1: 506cm
2: 356cm
3: 210

100hz:
Line 1: 666cm
2: 876cm
3: none

As seen above the wavelengths are not same for the different lines, that's because I used the same formula. My guess is that there should be a third value in the equation, eg 'n' where n might equal number of lines/harmonics or whatever. But I'm not sure.

Any help would be appreciated.

 

[BvU]

Hello @Myphysicsteachershit , !

For experimental wavelength I used: lambda=2*distance between node

This is true for two waves that travel in opposite directions. That is not the case in your setup.
There it looks (sounds) more like this: (https://en.wikipedia.org/wiki/Wave_interference)

As you see, the lines through the nodes 'move away from each other' if you go further away from the sources.