Instruments

Question 1

The role of the body and cavity of a stringed instrument in sound production

Answer 1

Vibrations of strings lead to vibrations of the body of the instrument and the air inside, which then transmits energy to outgoing sound waves

The natural vibration frequencies of the body and air inside should include a large range of frequencies that match the range of oscillation frequencies produced by the strings

Question 2

Bow driven oscillation of a string

Answer 2

When a bow is dragged across a string, static friction pulls the string along with the bow when the string is moving in the same direction of the bow

The string slips relative to the bow when it is moving in the opposite direction

Therefore, the force from the bow on the string has the same period as the oscillation of the string, thus driving the oscillation

Question 3

Cylindrical open-open tube

Answer 3

Flute

All harmonics are produced

Question 4

Cylindrical closed-open tube

Answer 4

Clarinet

Produces odd harmonics

Question 5

Conical tube

Answer 5

Sax, oboe

All harmonics are produced

Question 6

Cylinder + cone + bell

Answer 6

Brass instruments

Produces harmonics 2, 3, 4, 5, etc, plus unused resonance below the fundamental

Question 7

Pressure variation in wind instruments

Answer 7

Pressure variation is largest where there is a displacement node (eg, the closed end)

Pressure variation is smallest where there is a displacement antinode (eg, the open end, where there is contact with surrounding air)

Question 8

Register hole (woodwinds)

Answer 8

Opened to decrease pressure variation at a specific place

Creates a displacement antinode here, which can suppress the fundamental mode –> next available harmonic is heard

Question 9

Bernoulli principle (& reed instruments)

Answer 9

When air is moving along a surface, the pressure of the air on that surface is less than it would be if the air were still

If you blow across the top of a piece of paper, the paper moves upward

Causes reed to bend, blocking airflow –> Bernoulli effect lessens, so reed bends less –> more airflow = repeating oscillation cycle

Question 10

How are standing wave oscillations driven in a flute or recorder?

Answer 10

An air jet is directed towards the edge of an opening (open end)

The jet is directed up and away from the instrument when it leaves the opening

Air entering directs the jet downward

Question 11

Sound production and harmonic selection in brass instruments

Answer 11

Tension of the player’s lips affects the vibration frequency

Lip vibration frequency can be matched to the frequency of the desired harmonic (locks on to standing wave oscillations)

Question 12

Bell of a brass instrument

Answer 12

Raises lower resonances