Sound
The Fundamentals of Sound
All sound comes from vibrations. These vibrations move through the air as waves. Imagine waves in water, ripples moving through a pond. Instead of these waves moving through the water, imagine them being pushed through the air. This is how sound would appear, if we could see it.
Sound waves are longitudinal waves, with alternating levels of compression and rarefaction. We can see these in graphic forms called waveforms.
The movement of sound is called vibration. Sound is pushed and pulled (compression and rarefaction), causing it to vibrate back and forth. The sound vibrates around the place where it initially was before the sound wave arrived. This original place of origin is called the equilibrium.
Sound has the ability to move through any form; gases, liquids or solids. The object which the sound is traveling through is the medium. Sound cannot exist if it has nothing to travel through, no medium. For this reason, sound couldn’t exist in outer space, as this is a vacuum, containing no gases, liquids or solids, therefore nothing to carry sound.
How do we create sound? Sounds are formed by vibrating an object. For example, when we strike a guitar string with a plectrum, it vibrates up and down. The vibration causes a sound by moving the air around it. As the string moves up, the air above it is compressed, and when the string moves down, the air moves with it and expands. The compressing and expanding of the air produces differences in air pressure. The differences in pressure in the air, moving away from the guitar string, create a wave of sound. This is how the guitar produces a sound that we can hear. The sounds we can hear vary as our ears are subject to various characteristics.
Often we are tasked with characterizing sounds. When describing sounds, we often use words like; loud, quiet, soft, harsh, high and low. We characterize sounds in terms of volume and pitch. The scientific equivalent of this is: amplitude or intensity and frequency. There are some characterizations that the human ear cannot detect. For example, the human ear is unable to detect the wavelength of a sound. Despite us not being able to hear the wavelength of a sound, we can see it when the sound is represented as a wave.
The amplitude of the wave is the difference in pressure as the sound wave passes. As we increase the amplitude of a sound, we increase the volume of the sound, making it louder, just as we would increase the volume on a radio. Similarly, as we decrease the amplitude, we are making the sound quieter. Amplitude can be expressed in a visual form.
Waves carry energy, this is how they move. So it’s logical to assume the higher the energy of the wave, the higher the amplitude, and thus, the louder the sound. Correct! Therefore the lower the energy of the wave, the lower the amplitude, and the quieter the sound. The intensity of the sound dictates the perception of its loudness. Relative sound intensities are often measured in units named decibels (dB).
As we’ve seen in pictures of waves, a wave is essentially made up of a repeating shape, or cycle. Frequency refers to the number of cycles of a sound in a second. If a sound wave has a high frequency, then the sound will have a high pitch. Conversely, if a sound has a low frequency, it will be low pitched. Just as we measure sound intensity in a unit called decibels, we measure sound frequency in Hertz.
One Hertz refers to the number of cycles per second. Increasing the frequency of a sound, thus increasing the amount of cycles per second, leaves us with a higher pitched sound. As we decrease the frequency, decreasing the amount of cycles per second, we are left with a lower pitched sound. The human ear can only detect sounds within our range. Human hearing is generally limited to frequencies between 12 Hz and 20 kHz (20,000 Hz). The limits, however, are not definite and are subject to variation. The upper limit generally decreases with age, meaning; the older we get, the less likely we are to detect high pitched sound. The limits of human hearing are not shared throughout all species. Dogs are able to detect sound vibrations higher than 20 kHz. This is why humans are unable to hear the sound that a dog whistle produces.
The nearer we are to the source of a sound, the louder we perceive the sound to be. If a sound is to travel a long distance, then the sound needs to start loud. As the sound wave moves through the air, the sound wave gets smaller as it moves away from the origin (equilibrium). The wave gets continuously smaller because the wave is spreading out; this is called spreading loss.
This loss occurs because the total amount of energy in a wave remains the same as the wave spreads from the source. When the wave gets bigger, and the sound increases in volume, the energy of the wave must spread to fill it. Therefore, the energy per unit length of the wave must get smaller. The height of amplitude decreases as the energy per unit length of the wave gets smaller.
Spreading loss isn’t the only reason that sound weakens as it moves. There’s also sound absorption. Just as a sponge can absorb a liquid, sound gets absorbed as it travels through a medium. Sound is caught by the molecules within the medium. The amount of absorption depends on the frequency of the sound. A high frequency sound has many cycles per second, therefore the particles in the medium are vibrating at a more rapid pace.
As we’ve discovered, waves vibrate and sound moves. But how fast does sound move? The answer is relative to the medium of the sound. Sounds travel faster through water than they do through air. Sound travels through water at approximately 1500 metres per seconds, but it travels through air at approximately 344 metres per second. Simple? Unfortunately not. The answer is realistically not that simple. The speed of sound is subject to many variables and is always subject to change.
The temperature affects the movement of sound. Heat, just like sound, is kinetic energy. Molecules at higher temperatures contain more energy and vibrate faster. Because the molecules vibrate faster, they allow sound waves to travel quicker. This means the higher the temperature, the faster sound can travel through it. Theoretically, if you were placed equidistant from a sound source, in the desert, and in the Arctic, you’d hear the sound quicker in the desert.
Temperature isn’t the only variable to affect sound. Humidity also affects the speed of sound. Humidity makes the air denser. When the air is dense, the air particles are tightly packed together. The particles are so tightly packed, that they vibrate more rapidly, allowing sound to be able to travel through the dense air quicker. Again, this is why there is no sound in space; because the air has no density, the sound has nothing to travel through.
In the simplest situations, sound travels in a straight line. However, interactions between sound and medium can make the transmission of sound more complicated, resulting in three effects; reflection, refraction and scattering.
You have experienced sound reflection, commonly attributed to an echo. An echo is simply a reflection of sound. Hard, smooth surfaces can reflect sounds; the reflection we hear is an echo. We are only able to hear an echo in specific environments, for example in a canyon. We hear an echo here because we’re surrounded by hard surfaces to reflect the sound. When in a field we cannot hear an echo, as there’s nothing to reflect the sound.
Sound refraction occurs when a sound wave enters a medium where the speed is different. This causes sound waves to bend from their original direction. Refraction is caused by sound entering the new medium at an angle. Because of the angle, part of the wave enters the new medium first and changes speed. The difference in speeds causes the wave to bend.
That concludes our introduction to sound. Hopefully now you’ve gained a more detailed understanding of what sound is, and how sound works.
Article first published as The Fundamentals of Sound on Technorati.
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