- Current section: Introduction
- Learning outcomes
- 1 Aims of Creating musical sounds
- 2 What is a musical instrument?
- 3 Sound production in musical instruments
- 4 Excitation
- 5 Primary Vibrators
- 5.1 Standing waves
- 5.2 Vibrating string: speed of wave propagation
- 5.3 Vibrating string: standing waves on a string
- 5.4 Vibrating string: normal modes of vibration
- 5.5 Vibrating string: pitches of notes produced by stringed instruments
- 5.6 Vibrating air column
- 5.7 Vibrating air column: reflection at the end of an air column
- 5.8 Vibrating air column: standing waves in a cylindrical tube open at both ends
- 5.9 Vibrating air column: standing waves in a cylindrical tube closed at one end
- 5.10 Vibrating air column: end effects
- 5.11 Vibrating air column: standing waves in a conical tube
- 5.12 Vibrating air column: pitches of notes produced by wind instruments
- 5.13 Other primary vibrators
- 5.14 Response and damping
- 5.15 Summary of Section 5
- 6 Radiation
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Creating musical sounds
How do different instruments produce the sounds we classify as music? How do we decide...
How do different instruments produce the sounds we classify as music? How do we decide whether something – a piano, a vacuum cleaner – is actually a musical instrument? In this unit we investigate the way vibrations and sound waves are harnessed to create music.
By the end of this unit you should be able to:
- Explain correctly the meaning of the emboldened terms in the main text and use them correctly in context;
- Identify whether a given sound source can be classed as a musical instrument and explain why (Activity 2);
- Identify the primary vibrator and any secondary vibrators in the most common types of instrument (Activity 3);
- Appreciate that, when a note is played, a musical instrument vibrates strongly at certain specific frequencies and that these frequencies correspond to the natural frequencies of the primary vibrator; (Activity 4);
- Determine whether the sound from a given instrument is transient or sustained (Activity 5);
- Identify the mechanisms by which different instruments are excited (Activity 6);
- Explain how a standing wave is formed and locate the positions of the nodes and antinodes (Activity 9);
- Calculate the speed of a wave on a string given the appropriate formula (Activity 12);
- Derive an expression for the wavelengths of the normal modes of vibration of a string fixed at each end; from this calculate the string's natural frequencies given the formula relating wavelength to frequency, and thus show that the natural frequencies form a harmonic series (Activity 14);
- Perform simple calculations involving fundamental frequency, length, mass per unit length and tension of a vibrating string given the appropriate formula (Activities 15 and 16);
- Derive an expression for the wavelengths of the normal modes of vibration of an air column that is open at both ends; from this calculate the column's resonance frequencies given the formula relating wavelength to frequency, and thus show that the resonance frequencies form a harmonic series (Activity 18);
- Derive an expression for the wavelengths of the normal modes of vibration of an air column that is open at one end and closed at the other; from this calculate the column's resonance frequencies given the formula relating wavelength to frequency, and thus show that the resonance frequencies form a harmonic series but with the even harmonics missing (Activity 19);
- Describe what is meant by end correction in relation to vibrating air columns and take these into account when calculating an air column's resonance frequencies, given the formula relating end correction to column radius (Activities 20 and 23);
- Describe the factors that affect the fundamental frequency of an air column and calculate the fundamental frequency given the appropriate formula (Activities 21, 22 and 23);
- Describe some of the normal modes of vibration of rectangular bars, circular membranes and circular plates and know that the natural frequencies of these primary vibrators do not form harmonic series;
- Explain the results of driving a primary vibrator over a range of frequencies and explain the effect damping has on the resultant vibrations;
- Describe how in some instruments the primary vibrator enlists the help of secondary vibrators to improve the radiation of sound, and explain the properties of such secondary vibrators in terms of their frequency response and damping.
Creating musical sounds
This unit is concerned with the tools required to perform music, namely musical instruments. When you see the term musical instrument, you probably automatically think of the instruments found in an orchestra such as the violins, trumpets and flutes. Maybe you think of keyboard instruments like the piano or the organ. Or perhaps you visualise more modern instruments such as the electric guitar or the electronic synthesiser. You may even think of the human voice. These are all certainly examples of what we traditionally consider to be musical instruments (Figure 1). But what is it about these examples that makes us classify them as musical instruments?
On the face of it, they all seem bewilderingly different. Some involve plucking or bowing a string, others involve blowing air through a tube, and still others involve pressing keys. As if this weren't enough, consider the difference in size between some of these instruments. A flute is small enough to be able to be carried comfortably in one hand, while a pipe organ may be so large that it has to be permanently installed in a spacious building like a church or concert hall. However, despite all these differences, there are some features that are common to all musical instruments. Here we shall examine the general principles of sound production that apply to all instruments.
This unit is an adapted extract from the Open University course