Basics of Sound

Acoustics - the physical side of music

What is sound?

When something causes air to vibrate, it makes sound waves. When these sound waves reach our ears, we hear the sound.

Can you see sound-waves?

No, but they can be nicely emphasized: throw a stone into a pool or a lake (first, make sure nobody is there!), and watch the water waves expanding in gradually widening circles. A sound source creates sound-waves in the exact same way.

Sound waves that are not regular and periodical are usually perceived as noise. By contrast, a steady frequency of sound-waves creates a set pitch.

Listen well to the sound of a car engine or an electric chain-saw. Since these sounds are cyclic, you will notice you can sing them.

Does thunder have a definite pitch, too? and the waves of the sea, shattered at the beach? The reason those lack a definite pitch is that they do not create a regular sound frequency, meaning they do not generate cyclic sound waves.

The orchestra knows how to use noise as a musical means, too: most percussion instruments usually do not play melodies, but add sounds of noise in different rhythms, thus enriching musical arrangement. In the 20th century, the composer John Cage brought the recognition that every noise is also music, and combined environmental sounds and noises in his musical works.

The production of musical sounds

In the world of musical instruments, there are different forms of sound production. Among these, are:

Woodwinds and brass - operate on a column of air, which, according to its changing length (set by pressing the holes or valves) produces different pitches of sound, for a long column of air generates low frequency sound waves, whereas a shorter column of air generates higher frequency sound waves. The organ also operates on the same principle of the air column.

Percussion - by beating different materials we produce sounds of varying pitch - according to the size of the membrane and the material of which it is made. Drums, the piano, and all sorts of xylophones work on the principle of beating against a body.

Fretted - here we use a principle similar to that of the air column, but by pressing a string (of a guitar, a violin and so on) we shorten it to different lengths. The shorter the string - the higher its pitch, for sound wave frequency will be higher.

The gong is a percussion instrument

A Recorder is a woodwind instrument

The Guitar is a fretted instrument

Pitch

A sound's pitch, therefore, can be examined by its wave form. The more sound waves per second - the higher the pitch.

Wavelength

A sound source generating sound at a frequency of 440 air vibrations per second makes the notes a'.

We measure this with the concept of Hertz, expressing frequency per second. 440hz, therefore, is the frequency of a' in the first octave (the middle one on the piano).

The human ear cannot hear all sound frequencies. Sounds whose frequency is lower than 16hz or higher than 20,000hz will not be heard by the human ear.

Amplitude

The Amplitude

Now, when we know the reason for sound pitch (the higher the frequency of sound waves - meaning a greater number of sound waves per time unit), let us learn the reason for the fact there are stronger and weaker sounds.

A sound source whose vibrations are longer (a string we plucked powerfully or a strong blow into a woodwind instrument) will sound stronger. The bigger the distance between the peak and low point of the sound wave is, the stronger the sound. The scientific term is amplitude.

Low and strong sound

High and weak sound

In music, we call controlling the intensity of playing "dynamics". It is an important component in the musical performance.

Among other things, the conductor guides the players on how to play the dynamics of a musical work

Why is that as we go far from the source of sound, the sound gets weaker and weaker? The reason is that sound waves get weaker because their ability to move through air grows weaker.

Timbre and overtones

Have you ever asked yourselves why different instruments sound different? How do you distinguish the sound of a guitar from that of a mandolin or a piano?

The answer is that, in addition to the basic wave an instrument produces, it also makes many overtones, created because of the compound of the materials of which it consists. These are additional sounds, built on the basic sound. The relative volume of each one has an impact on the instrument's sound timber.

Do only overtones bring the differences the timbre of various instruments create, or are there other elements?

Did you know...

In the apparatus called synthesizer, you can directly manipulate the volume of each overtone, and recreate sounds of different instruments, or make new ones.

An instrument's timber is affected by various factors, such as the type of material which the instruments is made of:

There are metal or brass instruments, such as the trumpet and the tuba, stringed instruments made of wood, like the guitar and violin, instruments made of leather like drums, and so on.

Waveform

þListen to the 3 known waveforms:

Sine

Saw tooth

Square

The regularity of sound pitch

The production of different pitches from a chord has mathematical logic, discovered by the ancient Greek philosopher Pythagoras, in the 6th century B.C. Pythagoras, who was also involved in music theory, found out that between sounds of various heights there are mathematical ratios, which also affect how well they sound together (pure intervals and consonants, and, on the other hand - dissonances - "discordant" intervals, which in certain eras were banned in religious music.

Pythagoras showed the rules he discovered, by pressing the chord at different points:

By dividing a string in half (that is pressing its center - exactly in the middle) - you get a pitch an octave higher than the original sound.

By dividing it in three - you hear in each section of the string a note an octave and a perfect fifth higher.

By dividing it into quarters - a note two octaves higher than the original.

By dividing into five - a note two octaves and a major third higher than the original.

Study some acoustic terms...