By Anjum Altaf
We are almost there, within striking distance of our primary goal. If you would bear with me just a little longer and not get psyched out by the reference to physics, you would find yourself the proud owner of a number of important insights and you would wonder why you had not been aware of them all along. Believe me, this is a short tunnel and there is a searchlight at the end of it.
We had concluded the last part knowing how sound is created and how it travels from the source to the human ear. We also described the shape of an ideal sound wave and I would urge you to take a look at the graphic if you have not done so already (just observe the shape, ignore everything else).
Almost everything we need to know is hidden in the shape of this wave. Let us unravel it now. First, the easy part related to an aspect of sound we know already: The height of the wave along the vertical axis (technically called the amplitude of the wave) is an indicator of the volume of the sound that reaches the ear. If you take a pencil and paper you can easily convince yourself that you can reproduce a wave pattern that passes through exactly the same points on the horizontal axis but with greater or lesser amplitude. This is what you do when you thrown a stone with more or less force into a pond of water or when you turn the volume dial up or down on the radio to give a louder or softer sound to the same voice (here an amplifier helps you vary the size of the wave).
It is the horizontal axis that contains the new concept. Note that the wave pattern repeats itself. Take any point on the horizontal axis and move to the right to find the point where the wave pattern begins to repeat itself (or begins to re-cycle). The easiest points to identify visually are the points where the wave crosses the middle horizontal line. For example, find two adjacent points where the wave begins to rise up above the horizontal line. You can measure this distance – it would be the same if you measure the distance from one peak of the wave to the next peak or one trough to the next trough. This constant distance is the second characteristic of the wave (the height is the first) and is called its wavelength – it represents one cycle of the wave. (Old radio buffs might recall that some stations could be heard on medium wave while others could be heard on shortwave – we will leave this as a bit of nostalgia but also a reminder that we have heard these terms before without having paid much attention to them.)
Using the pencil and paper you can just as easily convince yourself that you can keep the height of a wave constant but increase or decrease its wavelength (e.g. you can make two waves fit into the same distance that represented one wavelength previously – the wavelength would be halved). So, you can have wave patterns with longer or shorter wavelengths.
Now you need to recall an important fact that was mentioned in the last installment: Sound travels through the air at a constant speed that does not depend on the wave pattern or how hard the source is vibrated – the sound could be loud or soft but the time it takes to reach the ear would remain the same.
With this knowledge of constant speed you can conclude that in a given interval of time (say a second or a minute) there would be fewer cycles of a wave passing a point if the wavelength is long and more cycles of the wave passing the point if the wavelength is short. Now the number of cycles passing a point in one second is called the frequency of the wave. And you have arrived at an important relationship: Wavelength and frequency are inversely related – the longer the wavelength the lower the frequency and the shorter the wavelength the higher the frequency. (Again radio buffs would recall the term frequency which they used to tune in to different radio stations; those who have a particularly good memory might remember that stations received on medium wave had a lower frequency than those received on short wave.)
This is really all we need for now. We know that a wave has a frequency and that different waves have different frequencies. Frequency is the term used by physicists. Musicians refer to the same concept as ‘pitch’ – high frequency is high pitch; low frequency is low pitch.
And now the secret is unraveled – a longer wavelength (equivalent to lower frequency or pitch) sounds flat to the ear; a shorter wavelength (equivalent to higher frequency or pitch) sounds sharp to the ear. Now when someone describes a voice as high-pitched or low-pitched you would know clearly what it is that is being talked about. And just to recapitulate from the previous installments, a typical female voice has a higher pitch than a typical male voice. Recall also that when a boy’s voice changes at puberty it goes from a higher to a lower pitch. And recall also that you would undoubtedly have met a male who you thought sounded like a female.
That’s all for this installment. You can put your newfound knowledge to the test immediately. Listen to aaj jaane kii zid na karo by Asha Bhosle and by Farida Khanum and be able to distinguish between them by the difference in the pitch of their voices (have fun with this piece as it has been sung my many artists including children in the program Chhote Ustaad – Does Aishwarya’s voice have a higher or lower pitch than Asha’s, i.e., is it sharper or flatter?). Of course, there are other differences that characterize voices (breath control is one) but you are on the way to becoming a discerning listener. Congratulations.
You can also now go back to installment 5 and revisit the experiment with the seven children behind the curtain. You can tell them apart because their voices have different pitches and, because they are your children, your ears have become familiar with (attuned to) the differences over time.