You definitely heard from the EK physics book, pg 103/104 about average freq vs beat frequency (from here on out, I'm just gonna call this the beat). here's a summary of that, plus some stuff I've picked up from other sources (waves/sound was my weakness; I've accumulated quite a lot of stuff on it):
(I looked @ (berkley review physics book 2, pg 38, question 16).
*PITCH= FREQUENCY=LOUDNESS=AVERAGE FREQUENCY I prefer think of pitch as loudness, as well as freq, for a more intuitive way to remember it. Think of the range for hearing; it's always given in Hz
So anyways, EK phy pg 104 gives a scenario of tuning a piano with a tuning fork. The person listens to the beat. For perfect tuning, the beat is zero.
"What the person actually hears is the pitch. The freq creating the pitch would be an average of freq's from the piano and the tuning fork. Pitch correlates with freq, a high note has high freq and pitch."
what the person hears when tuning instrument. beat is alternating increase and decrease in intensity.
Pg 103: beat frequency is when 2 waves with slightly different freq's are superimposed. @ some pts, there's constructive interference, and destructive @ other pts.
Those points alternate with the beat= absolute value of f1-f2
I mean, we all know that formula.
Summary:
Constructive and destructive interference between 2 waves create sounds with intensities that cycle from loud to quiet.
*What do we hear? pitch=average of 2 frequencies.
*beat=variation of intensity=difference in frequency between 2 waves.