Making Sounds with Musical Instruments
by Ron Kurtus (revised 8 January 2008)
The regular vibration of common materials results in sound waves. Musical instruments create sounds that can be varied in pitch and volume.
Some give off relatively pure tones, while others consist of a pleasing mix of frequencies.
Questions you may have include:
- What are the different ways musical sounds can be made?
- How are musical sounds amplified?
- What makes musical sounds pleasing?
This lesson will answer those questions. Useful tool: Units Conversion
The vibration of some device, such as a wire, drum, or reed, creates musical sounds. Sometimes moving air can be made to vibrate at certain frequencies.
If you tie a string or wire tightly between two posts, you can start it vibrating by plucking or tapping it. The vibration creates compression waves in the air, resulting in sound, according to the frequency of the waves.
Musical instruments that use vibrating wire to produce sound include the guitar, violin, piano and harp.
Factors determining pitch
The frequency, wavelength or pitch of the sound depends on what the string or wire is made of, its thickness, its length, and how tight it is strung between the posts. Material, length and thickness are usually combined and indicated as the mass of the string in the String Frequency Equation. (See Equation for Sound Created from a String for more information.)
Tuning a string instrument
The frequency from a vibrating wire is relative pure. In other words, it primarily consists of one frequency. This fact is used in tuning the musical instrument. By adjusting the tension of the wire, the frequency can be changed slightly until it is exactly at some set pitch.
Although some experts can tell that it is at the correct pitch with their ears, a more accurate way is to use a tuning fork that is calibrated at the desired frequency. When two sounds are almost at the same frequency, a throbbing sound is heard. It is also called the beat frequency.
This frequency gets slower and slower as the string reaches the same frequency of the tuning fork. When the beat frequency disappears, the string is tuned to the exact pitch of the tuning fork.
A drum is made by stretching some material over a container. Striking the head of the drum causes it to vibrate and make a sound. In reality, the head of a drum is like a string in two-dimensions.
Being in two-dimensions, the vibration of the drum head can be fairly complex. An interesting experiment is to sprinkle some powder on the hear of a drum and then strike it gently. The vibration of the drumhead will create a pattern in the powder.
A relatively small, thin piece of material, held in place, can be made to vibrate and make sounds.
A simple example is putting a blade of grass between your thumbs and blowing on it. Another example of a reed is wrapping some wax paper around a comb, putting it to your mouth and blowing on it to make a sound like a kazoo.
The clarinet and harmonica are musical instruments that use reeds to make their sound. Your vocal cords are also a set of reed-like materials.
The act of blowing through or across a chamber of air can create sound vibrations. The best example of this is whistling. By changing the position of your tongue, you can change the note of your whistling.
If you blow across the mouth of a bottle, you can also make a sound. By putting different levels of water in the bottle, you can adjust the pitch.
Typical musical instruments using this technique for creating sound are the trumpet, flute and organ.
Sounds can be amplified by trying harder, resonance, and electronically.
One way to make the sound louder in a musical instrument is to try harder. You can hit the piano keys harder, strum the guitar harder, or blow harder on the trumpet.
Another method is through using resonance. The reason a bottle makes sound when you blow across the top is that the air bounces back and forth inside, amplifying the sound wave that is the length of the depth of the bottle.
This principle is used in many musical instruments, no matter how the sound in created. For example, a guitar string plucked by itself makes a weak sound. But when added to the hollow body of the acoustic guitar, the sounds resonate and are amplified. The body of a clarinet amplifies the sound of the reed.
Electronic amplifiers use microphones and similar devices to pick up the sound and increase the volume. The electronics can also create special effects, like echoes and frequency changes.
It is difficult, though, for electronic amplification to reproduce the "character" of some resonance amplifications.
What makes a sound pleasing to the ear? A tuning fork has a pure sound of one frequency, but that is not necessarily pleasing or music. Early synthetic or electronic sound was able to create and vary many pure frequencies, but again, it wasn't too pleasing to the ear.
One factor in musical sounds is harmonics, or multiples of the pure tone. These harmonics can add fullness and character to make the tone sound better.
Combinations of notes
Certain combinations of frequencies or pitches are more pleasing to the ear than others that may sound "sour" or off-tune. Some of this is also cultural, because some foreign instruments can sound out-of-tune to someone not brought up with that type of music.
Musical sounds are created by strings, drums, reeds, and blowing. They can be amplified by trying harder, resonance or electronics. Harmonics, combinations of frequencies, and culture play a role in what makes music and is pleasing to the ear.
Resources and references
Musical Instruments - From the high school Physics Classroom
Physics of Music Notes - From Michigan Tech College Physics department
Questions and comments
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