1. Phase
Two quantities are in phase if they are simultaneously increasing or decreasing, eg V & I are in phase in an AC circuit
Waves in phase

2. Crest of one wave coincide with the trough of the other wave.
2 waves out of phase by ½ a cycle or 180o.
Crest of one wave coincide with the trough of the other wave.

3. Waves out of phase by 90o or a ¼ cycle
If there is a period of time where one quantity is increasing while the other is decreasing they are out of phase.
The two quantities need to have the same frequency (f) but not necessarily the same amplitude to stay in phase.
Waves out of phase by 90o or a ¼ cycle

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5. Beats is a regular pulsing of the loudness of a sound which is heard when two sources produce sounds of slightly different frequency.
fbeat = f1 – f2
For example, when a 552 Hz and a 556 Hz tone are played simultaneously, we hear , or 4 beats per second. The beat frequency is 4 Hz.
Uses ~ Piano Tuning

8. Beats
Let’s say that the tuning fork without a small mass attached has a frequency of 200Hz. If you hear a beat with a frequency of 4 cycles every second, what is the frequency of the tuning fork with the mass on it?
800 Hz
204 Hz
196 Hz
50 Hz

9. Beats
As two notes get closer to being “in tune” with each other (having the same note), what happens to the frequency at which the beats occur?
Increases
Decreases
Stays the same