Creating musical sounds
Creating musical sounds

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Creating musical sounds

3 Sound production in musical instruments

Musical instruments come in all shapes and sizes and produce an enormous variety of different sounds. Yet, with the exception of certain electronic instruments, the basic physical principles by which sound is produced are the same for all instruments – including the human voice. In this section, I shall introduce some of these principles. These will then be expanded upon over the rest of the unit.

Remember I told you that when a musician plays an instrument they cause it to vibrate. Therefore, for every musical instrument there must be a means of initiating or exciting the vibration. In other words, there must be a mechanism by which the player can put energy into the instrument to make it sound.

When excited, an instrument doesn't just vibrate randomly. Instead it vibrates particularly strongly at certain frequencies. Exactly what these frequencies are and just how strongly the instrument vibrates at them plays a large part in determining the properties of the sound generated by the instrument: in other words, the timbre and the pitch, if any, of the note produced.

So, why is it that an instrument vibrates strongly at certain frequencies? To help us to answer this question, the first thing to consider is that most instruments are made up of several component parts. For example, the violin comprises a set of strings that are stretched over a hollow, air-filled box. In this case, there are three component parts: the strings, the violin body and the air contained within the body.

Now, when a note is played on an instrument, each of these component parts has certain frequencies at which it prefers to vibrate. These frequencies are known as the component's natural frequencies. In some instruments, all that is needed is to give one of the components an initial excitation and then let the resultant vibrations die away. In such cases, the component will vibrate in some combination of its natural frequencies. The kinds of instruments I'm thinking of here are, for example, the guitar where a string is plucked and then allowed to vibrate freely, or the xylophone where a wooden bar is struck with a hammer. More often than not, however, we want to be able to sustain the sound an instrument makes. To be able to do this, we need to feed in energy continuously, for example by using the bow on the violin. When we do this we are forcing the component to vibrate. That is, we are driving the component. As you might expect, this has most effect – that is, the component vibrates most strongly – when the frequencies at which the component is being driven correspond to the component's natural frequencies. This phenomenon is known as resonance. Because of this, these frequencies are often referred to as resonance frequencies. Indeed, the terms natural and resonance frequencies tend to be used interchangeably.

Remember though that an instrument usually consists of not one but several component parts. For example, when we bow a violin we force one of the strings to vibrate, which in turn forces the body of the violin, and the air contained within the body, to vibrate as well. So, how do the natural frequencies (or, if you like, the resonance frequencies) of these other components affect the way in which the overall instrument vibrates? Well, this is where things get a little complicated because the different parts of the instrument are not independent of each other. That is, each vibrating component influences the behaviour of the other vibrating components – the components are said to be coupled together. The coupling may be strong, in which case the influence of one component on another will be large; or it may be weak, in which case the influence will be small.

Analysing the collective behaviour of several coupled vibrating components can require quite a lot of complicated mathematical calculations. Fortunately, in most instruments, the collective behaviour of the vibrating components is dominated by just one component – for example, the string in the case of a stringed instrument, or the column of air in the case of a wind instrument. The frequencies at which the instrument as a whole vibrates strongly then closely match the natural frequencies of that dominant component. I'll call this component the primary vibrator.

The other vibrating components – I'll call them secondary vibrators – still have an influence on the sound produced by the instrument. They either play a part in the excitation of the instrument, determining what frequencies the primary vibrator is driven at, or they affect just how strongly the whole instrument vibrates at each of the natural frequencies of the primary vibrator. Either way, secondary vibrators influence the loudness and timbre of notes produced by the instrument.

In summary, when a note is played on an instrument, the primary vibrator is responsible for determining the frequencies at which the instrument vibrates strongly and consequently the pitch, if any, of the note. Secondary vibrators affect how strongly the instrument vibrates at each of these frequencies and consequently they influence the loudness and timbre of the note.

Again the violin provides a good example of this. The string that is being excited is the primary vibrator, while the violin body and the enclosed air are the secondary vibrators. The instrument vibrates strongly at the natural frequencies of the violin string. How strongly it vibrates at these frequencies depends to a great extent on the violin body and the air contained within it. While the string is responsible for determining the pitches of the notes that the violin produces, the violin body and enclosed air play a large part in determining both the tonal quality and the loudness of the notes.

In the case of a woodwind instrument that is excited by blowing through a reed, such as a clarinet or bassoon, the column of air contained within the tubing of the instrument is the primary vibrator and the reed is the secondary vibrator. The air column is responsible for determining the pitches of the notes that the instrument produces. The reed does, however, affect the tonal quality of the notes because it plays a part in determining what frequencies the air column is driven at.

For a brass instrument such as a trumpet or euphonium, the primary vibrator is again the air column but now it is the player's lips that act as a secondary vibrator. The air column determines the pitches of the notes produced by the instrument. Meanwhile, the player's lips help in determining the frequencies at which the air column is driven. In this way, they influence the loudness and timbres of the notes that the instrument produces.

I should point out that when any wind instrument is blown, the walls of the instrument also vibrate. In reed woodwind instruments, these vibrations are tiny in amplitude and it is generally accepted that they have no noticeable effect on the sound produced. In brass instruments, though, the wall vibrations are a little larger in amplitude. Whether they significantly affect the sound produced by the instrument is a subject of much debate.

Activity 3

Identify the primary vibrator and any secondary vibrators in each of the following instruments:

  1. guitar

  2. trombone

  3. piano

  4. oboe

  5. xylophone

Discussion

Comment

  1. Each string of the guitar is a primary vibrator. The body of the guitar and the air contained within the body are secondary vibrators.

  2. The trombone is a brass instrument and so it is played in the same way as the trumpet, by the player blowing air through their lips into the instrument. Thus the primary vibrator is the air column and the player's lips act as a secondary vibrator.

  3. The piano is a stringed keyboard instrument. When a note is played, a string is struck, and the string is therefore the primary vibrator. The soundboard is a secondary vibrator.

  4. The oboe is a woodwind instrument that uses a reed. Thus the primary vibrator is the air column. The reed is a secondary vibrator.

  5. The xylophone comprises a number of wooden bars that are struck with a hammer. Hanging under each bar is a vertical tube. Each bar is a primary vibrator and the air columns contained within the tubes are secondary vibrators.

Let me summarise all these points. When a musician plays an instrument, they cause it to vibrate. Therefore, for every musical instrument there must be a means of initiating or exciting the vibration – in other words, a mechanism by which the player can feed energy into the instrument to make it sound. The input of energy will cause the instrument to vibrate strongly at certain frequencies. These frequencies correspond to the natural (or resonance) frequencies of the primary vibrator. Other parts of the instrument – the secondary vibrators – influence just how strongly the instrument vibrates at these frequencies. Finally, all instruments must efficiently radiate their vibrations to the surrounding air so that they can be heard by a listener.

Activity 4

Below are three true statements (1 to 3).

  1. Each component part of a musical instrument influences the vibrational behaviour of the other component parts.

  2. The primary vibrator is responsible for determining the pitch, if any, of the note produced by a musical instrument.

  3. Secondary vibrators influence the loudness and timbre of the note produced by a musical instrument.

Here are three explanations (a to c) of the above statements. These are in the wrong order. Which explanation goes with which statement?

  • (a) Because they affect just how strongly the whole instrument vibrates at the natural frequencies of the primary vibrator.

  • (b) Because they are all coupled together.

  • (c) Because the instrument as a whole vibrates strongly at the natural frequencies of the primary vibrator.

Answer

The correct pairings are given below.

1 and (b) – Each component part of a musical instrument influences the vibrational behaviour of the other component parts because they are all coupled together.

2 and (c) – The primary vibrator is responsible for determining the pitch, if any, of the note produced by a musical instrument because the instrument as a whole vibrates strongly at the natural frequencies of the primary vibrator.

3 and (a) – Secondary vibrators influence the loudness and timbre of the note produced by a musical instrument because they affect just how strongly the whole instrument vibrates at the natural frequencies of the primary vibrator.

In the remaining sections of this unit we shall be looking in more detail at the mechanisms for exciting vibrations in musical instruments, at the phenomenon of resonance and why instruments vibrate particularly strongly at certain frequencies, and at the role played by the primary and secondary vibrators in the effective radiation of the sound vibrations.

TA212_2

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