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## AP®︎/College Physics 1

### Course: AP®︎/College Physics 1>Unit 9

Lesson 6: Beats and interference of sound waves

# Beat frequency

Beat frequency occurs when two waves with different frequencies overlap, causing a cycle of alternating constructive and destructive interference between waves. Learn how this results in a fluctuation in sound loudness, and how the beat frequency can be calculated by finding the difference between the two original frequencies. Created by David SantoPietro.

## Want to join the conversation?

• The higher a note, the higher it's frequency. We've established that different frequencies when played together creates "wobbles" due to constructive and destructive interference. But normally musicians don't play the same exact note together; they play different notes with different frequencies together. So does that mean when musicians play harmonies, we hear "wobbles", and the greater the difference in interval, the more noticeable the "wobbling"?
• that happens. when you tune a piano, the harmonics of notes can create beats. higher harmonics mean more beats, because the same percentage of difference results in more units difference when scaled up.
• but if the difference in frequency of 2 instruments is really high, so the beat frequency would be really high and human ear would not recognize any wobbling, it would seem that its one continuos note, am I right?
• Hello Dean,

Yes and no. Each of us comes equipped with incredible music processor between our ears, With a little training we are able to detect these beat. We can use this ability to tune an instrument, in fact a trained musician can tune in real time by making thousands of minor adjustments.

Suppose we had two tones. Lets' keep one at a constant frequency and let's let the other one constantly increase. When we start the tones are the same, as we increase we start hear the beat frequencies - it will start slow and then get faster and faster.

Keep going and something interesting happens. We will perceive beat frequencies once again as the tones approach certain mathematic relationships. As an example consider western musical terms. If we start at "C" we will hear strong beats when approaching "E" and again at "G."

Regards,

APD
• So is the amplitude of a sound wave what we use to measure the loudness?
• Audio engineer/music producer here. Yes and no.

Yes amplitude is what we would use to mechanically measure the loudness of a given sound wave.

However sometimes two sounds can have the sample amplitude, but due to their harmonics one can be PERCEIVED as louder than the other. The human ear is more sensitive to certain frequencies than to others as given by the Fletcher-Munson curve.
• Can we create 2 perfectly destructive waves at home?
If so how? If we cannot,why?
• When two instruments producing same frequency sound, there must be a chance that two sound wave are out of phase by pi and cancel each other out.
Why would this seem never happen?
• by 90 degrees off, then you can. it is just that it is too hard to time it right, unless a computer can play 2 equal tones with a set phase interval between them.
• What happens when we use a second sound with a different amplitude as compared to the first one?
• The formation of beats is mainly due to frequency. If the amplitude of the two waves are not equal, than the overall sound will vary between a maximum and a minimum amplitude but will never be zero. The points at which in the equal amplitude case we were getting zero resultant wave, we will have some uncancelled part of the wave with a higher frequency
• What would happen if a wave was overlapped with another wave that had the half of its wavelength? How would that sound?
Hope my question makes sense.
• Hi
I have a question about example clarinet
Iwant to know why don't we tune down 445Hz to 440Hz, i think it very good to do it. But why we use the method that tune up from 435Hz to 440Hz. Because, if you intepret same as this video, I think if we successive raise from 445Hz, it still have more beat per second.
• Hi! I have a question: since the wave travels up and down, what does it mean when the distance from the midline to the trough is negative? is because that the molecule is moving back and forth, so positive means it moves forward and negative means the molecule goes backwards? It doesn't mean that the volume decreases right?? I would rlly appreciate it if someone could clarify this point for me! :)