1.3 Sound Production Flashcards
What is a mechanical wave?
A mechanical wave is a disturbance that is created by a vibrating object and subsequently travels through a medium from one location to another, transporting energy as it moves.
What is the principle of superposition?
When two or more waves meet their total displacement is the sum of their individual displacements, taking their directions into account. Having met, the waves continue on their way as if they never met at all.
What must waves be to have a consistent pattern?
The waves need to have the same frequency and speed, and ideally the same or very similar amplitudes.
Give the example of standing waves in music.
The vibration of a device, such as a wire, drum, or reed, creates musical sounds.
The instrument causes the surrounding air to vibrate at the same frequency as the device.
Musical instruments usually fall into the categories of percussion, string or wind.
What us the sound of an acoustic guitar a result of?
The shape and body of the guitar, the strings themselves do no produce a very loud sound because of their small amplitudes and the fact they cannot move a large volume of air.
How does the string of a guitar create a larger amplitude of sound?
It effectively transfers its vibration of the body of the guitar which vibrates as a whole and makes much more air move.
Why is the top plate and air within a guitar important?
The top plate is only a few millimetres thick and can vibrate easily. The air inside the body is important since it can vibrate just like the air in a bottle when a person blows across the top.
What three things does the pitch of a vibrating string depend on?
The mass of the string: more massive strings vibrate more slowly.
The tension in the string: the tighter the string the higher the pitch.
The length of the string: shortening the strings gives a higher pitch.
How are standing waves on strings formed?
They’re formed from the superposition of two identical waves travelling in opposite directions.
Where do travelling waves move from in string instruments?
In string instruments travelling waves move from the point where string is plucked or bowed. They then travel to the ends of the string where they reflect and undergo a phase change of 180 degrees where a crest becomes a trough.
In travelling waves, there are points when the total displacement always remands zero. What are these points called?
Nodes.
In travelling waves, there are points when the displacement varies between a maximum in one direction and a maximum in the other direction, what are these points called?
Antinodes.
Explain the harmonics of a string.
A string has a number of frequency at which it will naturally vibrate. These natural frequencies are known as the harmonics of string.
What is the natural frequency at which a string vibrates at dependent on?
The tension of the string, the mass per unit length and the length of the string.
What does doubling the frequency of a vibration do?
It halves the wavelength and means that two loops are formed.
What does three times the fundamental frequency of a standing wave give?
The third harmonic.
Where does the rich sound come from in a musical instrument?
Several harmonics, occurring at the same time, and it is the balance of their amplitudes that determine the timbre.
Where does the vibration usually begin at in wind instruments?
The mouth piece- this is the device which classifies the wind instrument.
How do reed instruments work?
Air is blown through pieces of reed plant which causes the reed to vibrate quickly. This opens and closes the pipe. The pulses of air passing through the reed cause the air in in the pipe to vibrate in sympathy and sets up a standing wave.
How is sound produced in flutes and recorders?
They have a sharp ‘edge mouth piece’. The sound is produced by blowing a stream of air at this sharp edge. The stream of air vibrates above and blow the edge setting up a standing wave in the pipe.
How is sound produced in brass instruments?
The players lips vibrate against each other and against each other and against the rim of the mouthpiece.
How does the displacement of air particles work in a pipe?
There is a displacement antinode at the open end and there will be a displacement node at the closed end. Displacement rather than pressure is used to represent the standing waves in a pipe.
What are beats?
If two musical notes are sounded together, when the waves meet they will superpose and form an overall wave in a similar way to how standing wave is set up- the frequencies are similar but not identical a regular rising and falling of amplitudes of the resultant wave will be heard.
When the strings are not in tune what happens to the beat frequency?
It is equal to the difference between the frequencies of individual strings.
