Measuring sound speed between microphones

Part (a) marking points

Final answer: 340 m s⁻¹. Numerical checks assess the final value only; they do not check working or the number of digits typed.

1. Convert gradient to 0.00294 s m⁻¹. (1 method mark)
2. Use v = reciprocal gradient = 340 m s⁻¹. (1 accuracy mark)

No earlier numerical result is required. Award method credit for valid working and accuracy credit for the stated answer. Accept a defensible equivalent method; wrong physics cannot earn dependent accuracy credit.

Common errors: Convert milliseconds to seconds before taking the reciprocal.

Part (b) marking points

Final answer: 0.50 m. Numerical checks assess the final value only; they do not check working or the number of digits typed.

1. Use λ = v/f with the unrounded speed. (1 method mark)
2. Obtain 0.50 m. (1 accuracy mark)

Use the speed from part (a) in wavelength = speed/680 Hz, retaining unrounded working where available.

Common errors: Wavelength is speed divided by frequency, not their product.

Part (c) marking points

1. A fixed relative delay in the microphone/electronic channels can add a constant time offset. (1 independent mark)
2. This shifts the intercept without changing the delay–distance gradient. (1 independent mark)

Award each distinct physical point once. Equivalent scientifically correct wording is creditable; a contradictory statement cannot earn the same point.

Acceptable alternatives: A constant separation zero error can also create an intercept; identify it and explain why slope is unchanged.

Part (d) marking points

1. A fitted gradient uses differences, reducing the effect of a constant offset. (1 independent mark)
2. A larger distance/time span reduces the relative effect of reading resolution/scatter. (1 independent mark)

Award each distinct physical point once. Equivalent scientifically correct wording is creditable; a contradictory statement cannot earn the same point.