What is the Speed of Sound in this Lab Setup?

[lightofthemoon]

Homework Statement

In this part of the lab you tracked a single peak as you moved a microphone in order to get a good value of the speed of sound. This question will lead you through a similar process with just two measurements.

(The lab set up: speaker emitting the signal faced one microphone a certain distance x away. Measure Δt of the same peak in both positions)

The graphs below show two sine-wave signals like you saw from a microphone positioned in front of a speaker in Part 3 of the experiment. The two dashed vertical lines represent the cursors of the oscilloscope, with the one on the right side (colored orange) tracking a peak of the sound wave as the microphone is moved. The position x of the microphone in front of the speaker and the time between the cursors Δt is shown below each graph.

Mic position: x = 5.00 cm. Cursor difference: Δt = 0.0828 ms

Mic position: x = 20.0 cm. Cursor difference: Δt = 0.511 ms
Answers have to be correct within 2%

Homework Equations

velocity = Δx / Δt

The Attempt at a Solution

convert all units to standard units
1) mic position = .05 m , cursor difference = 8.28 * 10^-5 s
2) mic position = .2 m , cursor difference = 5.11*10^-4 s

velocity = (.2 - .05) / (4.282 * 10^ -4)
velocity = 350.3 m/s

I input this answer online, but it says it is wrong... check rounding or significant figures. I tried slightly different numbers, like rounding 4.282 * 10^-4 to 4.28 * 10^-4, giving me an answer of 350.5 but it is still wrong... rounding it up to 351 doesn't work either and I suspect rounding down to 350 also will not work because it is within 2% of my initial answer... did I make some kind of mistake elsewhere?

 

[mfb]

I get the same result.

There is no obvious modification that would lead to an answer more than 1% away but still within realistic values for the speed of sound.