Sound: section 2

How does a trombone (or other instrument) make sound?

What is usually the source of a sound?

Resonance

• “Occurs when forces are applied to a vibrating or oscillating object at time intervals equal to the period of oscillation”
• Examples: “pumping” on a swing, rocking a car free of a snow bank
• Causes larger and larger displacement, amplitude

Natural Frequency

The frequency or set of frequencies with which a system (object) naturally vibrates

Ex: tuning fork, ruler, string

Resonance

Resonance - skip to 48:56

Resonance in Air Columns

• Instruments can be thought of as air columns
• Air columns have natural frequencies which are determined by the length of the column

Resonance in Air Columns, cont.

• Resonance increases the amplitude of the sound waves occurring at the natural frequency of the air column
• And the amplitude of a sound wave is what?

So how does a tuba make sound?

• The player’s lips (creepily) vibrate, creating sound waves. The sound waves that resonate with the air column’s natural frequency are amplified. The player can change the natural frequency by changing the length of the air column and hence change which frequency (pitch) will be amplified.

A standing sound wave in a closed pipe resonator:

A standing sound wave in a closed pipe resonator:

END RESONANCE

Open end, closed end, totally closed pipe

http://newt.phys.unsw.edu.au/jw/flutes.v.clarinets.html

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Take aways:

• At an open end, the inverse is reflected
• At a closed end, compressions reflect compressions, and so too rarefactions.
• At an open end, pressure is average (atmospheric)

Standing pressure waves in open and closed resonators (pipes)

• open end will always act as a node (pressure is always average)
• closed end will always act as an anti-node

Standing wave and resonance length

“Length-wavelength relationship”

• Resonance lengths are found at half-wavelength intervals

Example problem

A tuning fork with a frequency of 392 Hz is used with a closed-pipe resonator. The loudest sound is heard when the column is 21 cm and 65.3 cm long. What is the speed of sound in this case? What observations about the temperature can you make?

“fundamental”

• Fundamental - the lowest frequency (longest wavelength) that resonates with/in/on an instrument
• Harmonics - whole number multiples of the fundamental frequency (these will also resonate)

Resonance on a String: ends function as nodes (secured in place)

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Can we change the speed of the waves on a string?

With a partner

Blue: # 13 - 16 on page 424

Red: # 18 - 21 on page 416