Music and Noise: the Science of Sound
It's Christmas season, and it sure sounds like it! It seems like the moment Thanksgiving ends, you hear Christmas music everywhere.
Whether you celebrate Christmas or not, everyone is familiar with the classic Christian hymns, even those matched to popular music. But it’s not just during Christmas season when music seems to matter so much. Music has been a powerful part of cultural events and entertainment for as long as we’ve existed, reaching all corners of the globe throughout the millennia.
Music is made up of sounds, and sound is everywhere in our life. It wakes us up in the morning, accompanies us on road trips, and helps us communicate. But what is sound, and how does it work?
Sound is energy that travels as a wave.To create sound, we have to put matter – whether it’s a gas like air, a liquid, or even a solid material – in regular motion. The vibration or disturbance creates a wave of a specific frequency, which we hear as a sound of a particular pitch. We don’t see these vibrations because they are usually too small to see.
We have a sound-making instrument in our own bodies: the set of vocal cords within the voice box, or larynx, in our throat. Can you feel the vibrations of your vocal cords when you place several fingers against the front of your throat? Your vocal cords tense up and draw close together when you are about to speak or sing. Msucles push air up from your lungs and through the narrow opening between the vocal cords. The force of the air causes the vocal cords to vibrate. The vibrating vocal cords produce sound waves.
Sound travels from one place to other as a machinal wave. It’s a bit different than an electromagnetic wave like light. Whereas light waves can travel in a vacuum, sound waves need a medium to travel. That’s why, contrary to what movies may depict, outer space is completely silent (Sorry, Star Wars fans!)
And there’s a difference in speed too. You may realize during fireworks displays or summer thunderstorms that you always see the explosion or lightning bolt before you hear the sound. Sound travels much slower than light.
The speed of sound depends on the temperature of the air. At normal temperature, sound travels close to 1,125 feet per second. Light, on the other hand, is just a bit faster: 186,000 miles per second!
Of course, we do have some jet planes and rockets that can travel faster than the velocity of sound. Speeds faster than sound are usually called supersonic.
We also have sound-receiving instruments in our body: our ears.Sound waves come in contact with our ears first by being gathered, reflected, and delayed by the pinna. The sound waves then travel into the ear canal to the eardrum. Directly connected to the other side of the eardrum are the tiniest of bones which transmit the kinetic energy absorbed by the eardrum to the cochlea. Auditory nerves carry these pulses to our brains, where they are interpreted.
The number of vibrations produced per second is called frequency. Frequency varies for each sound and is measured in hertz. One hertz is equal to one vibration per second. A sound with a low frequency will have a low pitch, such as a human's heartbeat. A sound with a high frequency, like a dog whistle, will have a high pitch. Humans cannot hear sounds of every frequency. The range of hearing for a healthy young person is 20 to 20,000 hertz. The hearing range of humans gets worse with age. Yes, contrary to what my uncle assures me, all of us lose the ability to hear sounds of high frequency as they get older.
Human hearing also depends on the intensity of sound, commonly called loudness. Human ears can register sounds over many orders of magnitude of intensity. A logarithmic scale rather than linear scale is used to measure sound intensity. This scale is comely known as decibels (dBs). The quietest sound heard in this scale is 0 dB. Washing machines produce 60 dBs of sound. Exposure to more than 90 dBs of sound is harmful to our ears.
Musical sounds are periodic and somewhat regular. They are pleasing to our ears and minds, and sound has been one of the greatest forms of expression since the beginning of time. Unpleasant, irregular sound is often described as noise, which is what I’d tend to call most “music” on today’s radio stations.
When looking at waves of sound, music has more patterns and gradual variances in volume, but noise is jagged and sporadic.
Sound is also used in medical technology. Ultrasound imaging, also called ultrasound scanning or sonography is a type of imaging technique to look at organs and structures inside the body.
This technique involves the use of a small probe and ultrasound gel placed directly on the skin. High-frequency sound waves are transmitted from the probe through the gel into the body. The probe also collects the sounds that bounce back and a computer then uses those sound waves to create an image. Unlike X-rays or CT scans, an ultrasound examination does not use radiation, which is why it’s safe to use on pregnant women.
Sound truly is marvelous. Its use and importance in our lives today really can’t be overstated. And a brief study of how exactly sound works helps the holiday music seem that much more holly.
Merry Christmas everyone, and happy listening!
(Dr. Yadav Pandit is an experimental nuclear physicist currently working at Allen Community College as a physical science instructor. He writes a column of general interest in science for The Register.)