Light and Sound
Light is generated when mather heats up or when electrons fall from excited to ground state. Sound is generated when matter vibrates.
This vibration can be seen as the back and forth movement of an object. For instance, when we speak, our vocal cords vibrate irregularly to produce sounds. To be heard by another person, these vibrations must travel as sound energy carried as sound waves from the speaker and the listener. Generally speaking, we have two types of waves based on their mode of travel. Recall the electromagnetic spectrum in which we identified light as an electromagnetic wave. Well, it turns out that sound is also a wave with a slightly different set of frequencies and a different mode of propagation. While light travels as a transverse wave, sound goes as a longitudinal wave.
Transverse Waves
These are waves in which the direction of oscillations of the wave is perpendicular to the direction of energy transfer or the direction of propagation of the wave.
Propagation, in this case refers to the transfer (or dissemination) of the sound energy through a larger area. If a transverse wave is moving, say in the positive x-direction, its oscillations are in up and down directions that lie in the y–z plane. As mentioned earlier, light is an electromagnetic wave which has both electric and magnetic components. The direction of oscillation of light waves is perpendicular to the direction of propagation. Therefore it is an example of a transverse wave, while sound is a longitudinal wave.Longitudinal Waves
Longitudinal waves are waves which direction of propagation is parallel to the direction of the vibrations or oscillations of the wave.
Sometimes in a longitudinal wave, the direction of propagation is opposite but parallel to the direction of oscillation. Sound waves are propagated longitudinally. Perhaps, the best way to visualise how sound travels is through the use of a slinky shown in the picture to the left. Please click on the link above to see a youtube demonstration video using the slinky to show how sound propagates through a medium. As can be seen in that demonstration, the spring of the slinky does not move as it transfers the force (or energy) from one end to the other end. This is what happens when sound propagates through the air. The air does not travel with the sound. The molecules of air run into each other throughcompression and rarefaction of the air molecules as the sound energy propagates along. Propagation of sound through a medium is a little bit like a relay race in which there are so many people involved in the race such that if they stand one in front of the other, they can stretch from one end of the race course to the other. In essence, all they have to do to get the baton to the finish line is simply to pass it on from the first person to the second, third, fourth and so on. The participants do not have to move before passing the baton, unlike a real relay race.
Properties of Sound Waves
We may already have gotten used to representing all types of waves using a sinusoidal waveform in which the wave moves up and down as energy is propagated along the shape of a sine wave.
So even though sound waves propagate through rarefaction and compression, we can still represent the wave properties of sound waves using a sine wave that represents rarefaction and compression. Consider the horn loudspeaker to the right. The distance between the beginning of the dark portion and the end of the brighter portion represents a full cycle the measure of the wavelength of the sound wave. The number of times the sound wave completes a full cycle within a space of one second is called the frequency of the wave. In other words, the frequency of the wave is the number of times the sound energy completes a rarefaction and a compression within a second. The frequency of a wave is measured in Hertz. It is the frequency which determines whether or not we would "hear" the sound energy when they finally reach our ears.
Our ears can generally hear sounds with frequency ranges from 20 Hertz to 20,000 Hertz. The frequency of the sound also determines the pitch. The amplitude of the wave is heard as the loudness or volume. Another important property of sound waves is the phase. The phase means the starting point of the wave. For instance, in our loudspeaker image, the speaker was standing at a spot when he started speaking.
If the listener started hearing the speaker from a compression point, that is the phase of the sound. If, however, the speaker moves slightly forward or backwards, the listening may begin hearing the speaker starting from a rarefaction. The change in angle of the wave, in this case, is called the phase difference.
Having examined some of the basic properties of sound waves, we are now equipped to examine how sound waves, from different musical instruments, can be assembled to make music.
Musical Sounds
Making beautiful music usually requires multiple instruments, even though music can be made with only one instrument. The reason for using multiple instruments is perhaps because of the quality of musical instruments called timbre. A middle A, on a piano, which corresponds to 440 Hz has a much different sound from when the same note is played on a saxophone or a trumpet. The reason is that each instrument has different and unique sets of overtones that make them different from every other instrument.
String instruments like the guitar produce different sounds based on the length of the string. When played by a skilful guitarist, he knows where and where to place his finger on the guitar strings to produce different frequencies of sounds by shortening the length of the string to a particular fraction. Also, to produce different harmonics of the same sound, for instance, to produce 880 Hz, 1760 Hz and so on from a particular string, he dampens the string at precise spots producing thinner, finer sound than the fundamental note.
Wikipedia Creative Commons: Fundamental Tone and Harmonics
The way these sounds reach the ear of the listener is the same only the frequency, amplitude and phase differ. There are several aspects of sound that were intentionally missed out in this post such as reflection, echo, speed of sound, speed of sound in different medium, sound engineering or electroacoustics. Those perhaps would make up the content of another post, some other day. As you appreciate how the fundamental note changes frequency to give a different sound in the form of the first, second and third harmonics, you may click the image of the harmonics to hear Carlos Santa do magic with the guitar. Thank you for visiting my blog.
References
- Sounds of Science | The laws of sound physics (acoustics) or How does it work?
- Wikipedia | Slinky
- Method Behind the Music | Physics
- Wikipedia | Pitch
- Mathematics of Music | Sound Waves
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