How to measure sound frequency

How to measure sound frequency

On a rope wave, you can easily see what the frequency is. Just look at one point on the rope as the wave passes over it. Count how many times the point goes up and down, divide by the time taken, and you get the frequency.

How do we measure the frequency of sound wave? We cannot see the air molecules moving or the air pressure changing.

One way is to use a microphone that is connected to an oscilloscope. This assumes that you have these instruments. We don't normally keep an oscilloscope at home, though school labs may have them. Anyway, lets try and understand how these instruments work.

When you use a microphone, you usually sing into it and hear your song become very loud. In the microphone, there is a special crystal, called a piezo electric crystal. It has the very special property that it produces an electrical voltage if you apply pressure to it. When the air pressure changes rapidly over the crystal as a result of the sound wave, the crystal produces a tiny voltage. This voltage oscillates with the pressure. For example, as the pressure goes up and down, the voltage goes up and down together. So this voltage has the same frequency as your voice. The microwave is connected by a wire to an amplifier, which is in turn connected to a loudspeaker. The loudspeaker then blares out your song.

In this experiment, we connect the microphone to an oscilloscope instead of the amplifier and loudspeaker. When the tiny voltage reaches the oscilloscope, the oscilloscope can plot a graph of the oscillating voltage on its screen.

An oscilloscope looks like a television, with a screen in front. There is a glass tube inside the oscilloscope, and the screen is just one end of this tube. This tube is air tight, with a vacuum inside. On the other end of the tube is an electron gun. The gun produces a beam of electrons that flies through the vacuum and hit the the screen from the inside. The inner wall at the screen is painted with a layer of special chemical that glows when the electron beam hits it. So on the screen, we would see a small, bright spot. No TV shows unfortunately.

The voltage from the microphone is passed on to to two parallel plates beside the beam - one above and one below. The oscillating voltage then causes the beam, and therefore the bright spot, to move up and down. In order to draw a graph, we have to make the beam move from left to right at the same time. This is done by applying another voltage to two other parallel places, one left and one right of the beam. This second voltage is called the time base. It is generated by the electronic circuits in the oscilloscope. It pushes the beam from left to right, thus sweeping the bright spot over the screen. When this happens very fast and is repeated many times, our eyes see a wave form being traced out on the screen.

There are vertical and horizontal lines printed on the screen. This gives a grid of squares, just like on a graph paper. The horizontal axis is the time. The time for one square, or division, depends on how fast the time base voltage sweeps the spot horizontally. It can be set using one of the knobs beside the screen. Knowing the time for one division, we can read off the time for one wavelength of the wave on the screen. This is the period T. We can then calculate the frequency f, using f = 1/T.

The Physics Notes