Beats Video Lecture | Physics Class 11 - NEET

Hi. It’s Mr. Andersen. And this is AP Physics
essentials video 117. It is on beats. Beats
are changes in amplitude when you have two
wavelengths that have a frequency that is
very close together. And you can actually
play around with this if you are connected
to the internet. And so I will put a link
to this down below. But is you go to this
website right here and you launch it, what
you can do is you can play a tone. And so
what I am going to do is play one that is
at 440 Hertz. I am then going to launch another
tab. And in that tab I am going to briefly
change what note I am playing. So that is
440 Hertz. That is also 440 Hertz. But listen
what happens when I change one. Those are
beats. Now they are gone. Now as I go above
it you can hear the beats again. And as I
bring them back to the same value, then it
goes away. Now when would this be useful?
Well let’s say you are tuning your guitar.
What you can do is tune one of the strings.
So make sure you tune that to a tuner or piano
or something like that. Once you have that,
then you can play another string, you have
to move up on the fret, but play another string.
And if it is close you will hear the beats.
And if it is exactly the same you can adjust
the one, then the beats will go away. And
then you can move on to the next string, and
the next string and the next string. And so
waves, when they come together, will interact.
And as they interact they cause interference.
Now the interference can be constructive,
sometimes building up the wave or the summation
of the waves. And sometimes it is destructive.
It is tearing it down. But if those waves
are very similar in frequency, what you get
are these beats. And beats are going to be
amplitude changes. In other words the volume
is going to go up and down and up and down
due to this constructive and destructive interference.
Now it is really easy to calculate the frequency
of the beats. All you do is, the frequency
of the beat equals the absolute value of the
difference between the two frequencies. So
if I know frequency one and two I simply subtract
one and it is the value, the absolute value
of those. And so again in review, if you do
not know what constructive and destructive
interference are, if we have two waves that
are perfectly the same frequency and they
are right next to each other, what we will
get is constructive interference. If they
are out of phase with one another we will
have destructive interference. But if the
frequencies are slightly off, and if we look
at this animation down here, these are frequencies
that are close, but not exactly the same,
what you get is as you have this constructive
and destructive interference you get the blue
summation of those two waves and you can see
that we are seeing these changes in the amplitude.
In other words the amplitude is decreasing
and then it is increasing and then decreasing.
Now if I could make those frequencies exactly
the same, then the beats are simply going
to go away. How do you calculate it? Again
it is frequency one minus frequency 2. Absolute
value of. And let’s do a sample of that.
So I am using audacity. It is a little music
software. And so what I am going to do is
I am going to generate some tones. And so
I am going to generate a 440 Hertz tone. So
that will be in one track. And then I am going
to add another track down here and we are
going to generate another tone. So we will
make this one slightly different. It is 444
Hertz. Like that. Now let me mute the bottom
one and play the top. And now let me mute
the top and play the bottom. They sound almost
the same. But let’s play them together now.
So I will unmute them. So now you can hear
those beats or those changes in the amplitude.
Now let’s calculate what the frequency of
the beats should be. So frequency of the beat
is going to be f1 minus f2. And so what were
the two frequencies? Well we had 440 and 444.
And so if I put those two values in what is
my frequency of the beat going to be? It is
simply going to be 4 Hertz. So now let’s
check that with this model. And so what I
am going to do is now mix these two together
and we are making a single track of the two.
And so let me get rid of those first two tracks.
You can already see the beats. And now let’s
check our numbers. So let’s look at 1 second
in time. It is exactly 4 beats. The frequency
of the beats equals the difference between
the two frequencies. So did you learn to use
a visual representation to explain how slightly
different frequencies cause beats?
I hope so. And I hope that was helpful.