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# Sound 13-3 PowerPoint PPT Presentation

Sound 13-3. A “physical phenomenon that stimulates the sense of hearing.”. What do you think?. A violin, a trumpet, and a clarinet all play the same note, a concert A. However, they all sound different. What is the same about the sound? Are the frequencies produced the same?

Sound 13-3

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## Sound13-3

A “physical phenomenon that stimulates the

sense of hearing.”

### What do you think?

• A violin, a trumpet, and a clarinet all play the same note, a concert A. However, they all sound different.

• What is the same about the sound?

• Are the frequencies produced the same?

• Are the wave patterns the same?

• Why do the instruments sound different?

### Standing Waves

• Standing waves are produced when two identical waves travel in opposite directions and interfere.

• Interference alternates between constructive and destructive.

• Nodes are points where interference is always destructive.

• Antinodes are points between the nodes with maximum displacement.

### Standing Waves on a String

• There is a node at each end because the string is fixed at the ends.

• The diagram shows three possible standing wave patterns.

• Standing waves are produced by interference as waves travel in opposite directions after plucking or bowing the string.

• The lowest frequency (one loop) is called the fundamental frequency (f1).

### Standing Waves on a String

• To the left is a snapshot of a single loop standing wave on a string of length, L.

• What is the wavelength for this wave?

• What is the frequency?

### Harmonics

• n is the number of loops or harmonic number.

• v is the speed of the wave on the string.

• Depends on tension and density of the string

• L is the length of the vibrating portion of the string.

• How could you change the frequency (pitch) of a string?

### Fundamental Frequency

Click below to watch the Visual Concept.

Visual Concept

### Standing Waves in an Air Column

• Wind instruments also use standing waves.

• Flutes, trumpets, pipe organs, trombones, etc.

• Some instruments have pipes open at both ends while others have one end closed.

• Air is free to move at open ends so antinodes occur.

• Closed ends are nodes.

• The velocity of the wave is now the velocity of sound in air (346 m/s at 25°C).

### Wind Instruments

• Wind instruments are not as simple as organ pipes.

• The shape is not always cylindrical.

• The holes change the wave patterns as well.

• The size of the “pipe” varies along the length.

### Practice Problems

• One string on a toy guitar is 34.5 cm long.

• What is the wavelength of the first harmonic or the fundamental wavelength?

• Answer: 69.0 cm or 0.690 m

• The string is plucked and the speed of the waves on the string is 410 m/s. What are the frequencies of the first three harmonics?

• 590 Hz, 1200 Hz, 1800 Hz

• Note: The use of significant figures causes the multiples of 590 to be 1200 and 1800 because only two significant figures are present in the answer.

### Practice Problems

• An organ pipe open at both ends is 34.5 cm long.

• What is the wavelength of the first harmonic or the fundamental wavelength?

• Answer: 69.0 cm or 0.690 m

• What are the frequencies of the first three harmonics if the air temperature is 25.0°C?

• Answers: 501 Hz, 1000 Hz, 1500 Hz

• Answer the same questions if the pipe is closed at one end.

• Answers: 251 Hz, 753 Hz, 1250 Hz

### Practice Problems

• A violin string that is 50.0 cm long has a fundamental frequency of 440 Hz. What is the speed of the waves on this string?