Sound wave
Also called Sound
A sound wave is modelled as a mechanical longitudinal wave: it needs a medium, and the disturbance is a pressure variation along the direction of travel. Its frequency relates to pitch and its amplitude to loudness.
Two essential knowledge statements fix the model. EK 14.1.A.5.i: sound waves are modelled as mechanical longitudinal waves. EK 14.1.A.2: mechanical waves require a medium in which to propagate. Sound in a vacuum is not quiet, it does not exist.
The two readings a listener makes.
| What you notice | What it measures | CED |
|---|---|---|
| Pitch | frequency | 14.2.A.1.v |
| Loudness | amplitude | 14.1.A.6.ii |
Amplitude needs its own definition here, because a pressure wave has no visible crest. EK 14.1.A.6.i allows it to be given as the maximum increase or decrease in pressure from equilibrium pressure. The longitudinal wave entry has the compression and rarefaction geometry.
Speed is a property of the air, not of the note. A high note and a low note reach you together, which is the only reason music survives the trip across a room. EK 14.1.A.3.iii gives the one dependence the CED states: in a given medium, the speed of sound increases with the temperature of the medium.
No value for the speed of sound is printed. It is absent from the AP Physics 2 constants table and from the Waves, Sound, and Optics equations, so a question that needs a number supplies one. Do not carry 343 m/s in from a textbook and use it against a problem that quoted something else.
Relative motion between source and listener shifts the frequency you measure, which is the Doppler effect.