4.2 Recording sounds
Interactive 1 will allow you to see what ‘real’ sounds that you make yourself look like as waves. To get started, right-click on the link below the image to open the interactive in a new tab, then click on the large microphone symbol at the centre of the interactive. The first time you do this, you may see a pop-up window asking for permission to use your microphone – click on the appropriate button to allow this. Then try making some sounds. You will see a pink line moving.
To record a sound, click on the record button (the circle) at the bottom left of the interactive. As an example, Figure 19 shows the wave produced by humming a low note.
To make the picture, the microphone on the computer ‘sampled’ the sound 44 100 times per second and drew a pink line from 0 V to the value sampled. Because there are so many samples and the screen has limited resolution, some of the lines get overdrawn by others and merge to form solid blocks of colour.
It is quite hard to see the details of the wave at this resolution, but the interactive allows you to click on the wave and move the ‘zoom’ slider from left to right. Figure 20 shows the result of doing this to the wave in Figure 19. Notice that at the bottom of the wave, there are 24 long downward spikes with a shorter downward spike in between each pair. The long spike represents what is called the fundamental frequency of the recorded voice, and the shorter spike is what is called a harmonic.
The interactive can also be used to measure the time between the spikes and therefore calculate the frequencies. To do this, click your mouse on the tip of the leftmost spike and note the numbers at the bottom of the interactive to the right of the loudspeaker icon. The first of these numbers is the time that the spike was recorded. Then click on the tip of the rightmost spike and note this number again. The difference between the two times divided by the number of gaps between the spikes gives you the time period between any pair of spikes, from which you can calculate the frequency.
For example, you can see from Figure 20 that clicking on the leftmost spike shows the numbers 0:00:877 / 0:02:519 at the bottom of the interactive. Therefore that spike was recorded at 0:00:877. Clicking on a spike at the right of the interactive, as shown in Figure 21, shows that it was recorded at 0:01:099. There are 22 gaps between the long spikes in this example, so the time period between any pair of spikes is (0:01:099 − 0:00:877)/22 = (1.099 − 0.877)/22 = 0.01 seconds. Thus, since the period for one oscillation of the long spikes is 0.01, there are 1/0.01 = 100 spikes per second and the frequency is 100 vibrations per second. Engineers use the term hertz (Hz) for ‘times per second’, so the fundamental frequency of this recorded hum is 100 Hz.
A recording of an electric toothbrush shows the time at the top of a peak on the left as 0.219 seconds (Figure 22(a)) and the time at the top of a peak on the right as 0.452 seconds (Figure 22(b)). How many gaps between peaks occur between these two times? What is the period of this wave (that is, the length of time between any two consecutive peaks)? What is the frequency of vibration of this electric toothbrush?
There are 61 inter-peak gaps between the two measured times, so the period for one vibration of the electric toothbrush is (0.452 − 0.219)/61 = 0.003820 seconds. The frequency is the reciprocal of this, 1/0.003820 = 262 Hz. So the toothbrush vibrates 262 times per second.
Keep the interactive open, because you will need it in the next section.