The Basics of Sound

By Steve Jackson

The air around us is made up of molecules of various atoms. These atoms float around in a seemingly random pattern, spaced out more or less evenly around us. Air, if left to its own devices, would try to expand outward until it was rarified. Rarified means that the air would be very spread out. Space in fact has air, it is just rarified until you would have a hard time finding it without scientific instruments.

The air of the planet is held down by gravity, and is relatively compressed by all of the air on top of it. We refer to the amount of air that exists in a given space as air pressure. Air pressure low in the atmosphere, such as at sea level, is higher (less rarified) than air near space. In fact, people can start having breathing problems as low as 10,000 feet, and anyone who lives near the ocean side that goes to the high mountains can even have problems climbing stairs because of the low air pressure.

Sound is the transmission of kinetic energy through a medium. While any medium, from water to stone can carry sound, we usually discuss sound transmission in terms of sound moving through the air, since our best sound detectors (our ears, which can be found on the side of most peopleÕs heads) are designed to detect sound moving through a gaseous environment and function less well in water or pressed against a solid object.

Sound travels through the air by compression. Take a drumstick and a drum. Tap the drum. You should hear that tap with your ears. What happens is that the drum bends, then bends back, causing the air right next to it to compress. Molecules that are a gas are that way because their atomÕs electron shell is not willing to share any more electrons. Electrons repel each other, and since they cannot form a liquid or a solid, they bounce back when pushed together. Imagine a crowded room at a party. Two people want to get to the buffet table, so they shove between two other people, who press into two other people, who likewise push into another two. The people in the crowded party work very much like air molecules. Note that the molecules do not travel anywhere. They bounce into the next molecule then bounce back into almost the same place. All that has traveled is the energy that started them moving in the first place.

Thus, sounds are just waves of compressed air moving through the atmosphere. But it has to be more than that, since lots of things that compress air cause no sound. Sounds can be measured, and our ear is a very acute measuring device, but to understand sound we have to understand all of its components.

Loudness is the amount of energy carried by a sound wave. A mosquitoÕs wings carry a very small amount of energy; you can hear them if you have good ears (which explains why children complain of their noise more than adults) but only if they are only a few centimeters from your ears. A jet engine is a very loud noise. Even across a landing field the sound of a jet engine can be painful. We measure the loudness of sound in decibels. A decibel is not a measure of sound, but a measure of the ratio between two quantities, in this case between whatever the sound measured and the lowest power sound the ear can hear. In sound measure, the decibel is logarithmic, meaning 2db – 1 db is not the same a 3db – 2db but instead increases by a factor of 10 with each number (other log systems exist, but higher math will be saved for later). This is good because the quietest sound that the ear can hear and the loudest sound it can hear are a million times apart or more, which would be very hard to measure accurately. Besides, the ear, while able to hear inside of the range of power is not able to discern the difference between two sounds that are not at least 10 times louder than each other.

Side Note: Weber-Fechner Law and StevenÕs Power Law

Ernst Heinrich Weber was an early quantitative researcher who tried to understand human senses using mathematics. In a classic experiment, Weber gave a small weight to a blind man and then slowly increased the weight, waiting for when the man would notice the change. Weber determined that humans cannot tell minute differences between small changes in mass, and that the more mass a person held, the more mass that was required to create a detectable change. WeberÕs law might have gone unnoticed, except for another scientist named Gustav Feschner who developed an explanation for why WeberÕs law worked. WeberÕs law is most interesting to visual artists because of its application to how the human ear perceives sound, and how the human eye perceives light.

S. Smith Stevens took WeberÕs Law and expanded it to a wide variety of stimulus, showing that the human body perceives most of nature through logarithmic progression.

To understand the db scale in non-mathematical terms, it is important to understand how loud various sounds are. In a quiet room (one isolated from outside noise) a human sitting 10 feet away doing nothing other than breathing generates about 10db of sound. If they talk to you in a low voice, they might generate 45db. If they raise their voice to conversational levels it might raise to 50db, and if they scream at the top of their lungs they would be hard pressed to make over 90db of sound.

Most sounds are vibrations rather than just a single mass of pressure moving around. A vibration is energy that is released over a period of time as an object snaps back and forth. The energy is carried in a series of compressions of the air called waves. If that sounds like a term best used for beaches, it is because waves of water in the ocean work in a similar manner to waves of sound energy moving through the air. In sound, a vibration in an object (such as the cone of paper in a speaker) causes air to rapidly compress and rarify. The speed at which this rarification and compression occurs is known as the frequency of the sound. If it happens many times a second, the sound is said to be a high frequency (or high pitched) sound. If it happens only a few times a second, it is said to be a low frequency (or low-pitched) sound.

In talking about sound waves, we will be working with something called waveform theory. This is important because the other major means of technological communication used by humans relies on light, and waveform theory works for light as well as sound. For more on this, see the section of this paper called ÒWaveform Theory.Ó

The frequency of a vibration is measured in Hertz (Hz), which is a measure of how many times something repeats each second. Besides affecting how much energy a sound wave can carry over what distance, the frequency affects humans psychologically and intellectually. By varying the frequency of sound (such as we do when we speak) information is encoded by us and carried to a listener by sound waves. The lowest frequency sound that most humans ÒhearÓ is around 20 Hz. We can actually perceive sound that has enough energy at lower frequencies, but we donÕt truly hear it, it causes our bones to shake. Babies are known to hear sounds above 20,000 Hz, but adults rarely can perceive this. Children raised in quiet environments can often hear the whine given off by electronic equipment and by some animals that adults cannot hear. Music and television usually damages their ears that by their teens this is no longer possible.

The frequency of sound is important for musicians in terms of pitch. Pitch is the perception of sound, while frequency is the scientific measurement of sound.

Sidebar: Pitch

Many students who are involved with music want to know what is the difference between frequency and pitch. In reality, there is no difference; a pitch can be expressed as a frequency. Practically though, pitch is a measurement of the human perception of sound, and although pitch has been defined scientifically, in the real world pitch is partially subjective. This comes from the practice of tuning up instruments.

Tuning up instruments is the process of making all of the instruments that have to play together work in harmony. An instrument may sound good alone, but if it is out of tune with another instrument, and they try to play together, the difference will quickly be noted by the people listing to the music. When tuning instruments, the toughest to tune instrument is usually the one everyone else tunes to, but rarely are instruments tuned to scientific instruments.