Determining frequency & wavelength of sound
Sound is a wave we cannot see — but with the right apparatus we can measure it precisely. In this lesson we do two things the NSSCAS syllabus asks for. First we determine the FREQUENCY of a sound: we feed the signal from a microphone into a calibrated cathode-ray oscilloscope (c.r.o.), read the period T straight off the time-base grid, and use f = 1/T. Then we determine the WAVELENGTH of a sound using stationary waves — a resonance tube, a sonometer or tuning forks — where neighbouring nodes sit exactly half a wavelength apart. Finally we tie the two together with the wave equation v = fλ to find the speed of sound, and we look hard at the sources of error and how a careful physicist reduces them.
By the end you should be able to (NSSCAS Physics (AS) 2.3):
- Determine the frequency of a sound using a calibrated cathode-ray oscilloscope (c.r.o.): read the period T from the time-base and use f = 1/T
- Describe the apparatus for the c.r.o. method — a microphone (or signal generator and loudspeaker) connected to the c.r.o. input
- Read the period of a trace off the c.r.o. grid using the time-base setting (time per division)
- Explain how measuring several complete cycles reduces the uncertainty in the period
- Determine the wavelength of a sound using stationary waves (resonance tube, sonometer or tuning forks), using that neighbouring nodes are λ/2 apart
- Describe the resonance-tube method for measuring the wavelength and hence the speed of sound
- Recall and use the wave equation v = fλ to find the speed of sound from measured frequency and wavelength
- Identify the main sources of error in both methods and state how each can be reduced