SOUND 24.2

Chapter Twenty-Four: Sound

24.1 Properties of Sound

24.2 Sound Waves

24.3 Sound Perception and Music

Chapter 24.2 Learning Goals

Justify the classification of sound as a wave.

Analyze sound interactions at boundaries.

Explain how factors like temperature and pressure affect the behavior of sound waves.

Investigation 24B

Key Question: How can

resonance be controled to make the sounds we want?

Resonance in Other Systems

24.2 What is a sound wave?Sound waves are pressure waves with

alternating high and low pressure regions.

When they are pushed by the vibrations, it creates a layer of higher pressure which results in a traveling vibration of pressure.

24.2 What is a sound wave?

At the same temperature and volume, higher pressure contains more molecules than lower pressure.

24.2 The wavelength of soundThe wavelength of sound in air is

similar to the size of everyday objects.

24.2 The wavelength of soundWavelength is also important to sound.

Musical instruments use the wavelength of a sound to create different frequencies.

24.2 Standing waves

A wave that is confined in a space is called a standing wave.

A string with a standing wave is a kind of oscillator.

24.2 Standing wavesThe lowest natural frequency is called the fundamental.

A vibrating string also has other natural frequencies called harmonics.

24.2 Standing wavesThe place on a harmonic with the greatest amplitude is the antinode.

The place where the string does not move (least amplitude) is called a node.

24.2 Standing waves

It is easy to measure the wavelength of a standing wave on a string.

Two harmonics equals one wave!

24.2 Standing waves in pipesA panpipe makes music as sound resonates in tubes of different lengths.

The natural frequency of a pipe is proportional to its length.

24.2 Standing waves in pipesBecause frequency and

wavelength are inversely related, longer pipes have lower natural frequencies because they resonate at longer wavelengths.

A pipe that must vibrate at a frequency 2 times higher than another pipe must be 1/2 as long.

If the long pipe has a frequency of 528 Hz, what is the frequency of the short pipe?

24.2 Standing waves in pipesBlowing across the open end of a tube

creates a standing wave inside the tube.

If we blow at just the right angle and we match the natural frequency of the material and the sound resonates (spreads).

24.2 Standing waves in pipesThe open end of a pipe is an open

boundary to a standing wave and makes an antinode.

The pipe resonates to a certain frequency when its length is one-fourth the wavelength of that frequency.

24.2 Sound wave interactionsLike other waves, sound waves can

be reflected by hard surfaces and refracted as they pass from one material to another.

Diffraction causes sound waves to spread out through small openings.

Carpet and soft materials can absorb sound waves.

24.2 ReverberationThe reflected

sound and direct sound from the musicians together create a multiple echo called reverberation.

The right amount of reverberation makes the sound seem livelier and richer.