Dr. Jie Zou PHY 1071 1

Chapter 4 (Hall)

Sound Propagation

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Outline Reflection

Examples of reflection of sound waves Diffraction

Examples of diffraction of light and sound waves

Outdoor music Three generalizations of outdoor

music Designs for outdoor musical theaters

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Reflection Reflection: When waves encounter an

obstacle we expect them to be reflected. Example: echoes Specular reflection: If the surface is

smooth, the reflection is regular and orderly. Diffuse reflection: A rough surface causes

irregular or diffuse reflection. Being rough or smooth depends on

whether the bumps on the surface are as large as the wavelength of the waves.

Example: A good optical mirror should not have bumps larger than a thousandth of a millimeter. A textured wall that appears rough to your eyes may reflect sound like a smooth mirror.

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Examples of reflection of sound waves

Reverberation of sound in an enclosed room.

You can hear people talking in another room even though you cannot see them due to multiple reflection.

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Diffraction Diffraction: Waves bend or

diffract, when they pass by a barrier or through an opening. The divergence of wave from its initial line of travel or the spreading of wave is called a diffraction.

Examples of diffraction: Sound can travel around corners Ocean waves passing through a

narrow opening in a breakwater into a harbor spread out and reach into every part of the harbor.

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Diffraction and the width of an opening

How much waves are diffracted depends on the relation between the wavelength and the width of an opening.

If wavelength is about the same size as the width of an opening, or if it is larger, the waves spread out with comparable strength in all directions.

If wavelength is much smaller than the width, there is only a little spreading.

Examples: Diffraction of light under

everyday conditions is so weak that we do not ordinarily notice it.

Strong diffraction of sound around objects of ordinary size, especially for bass notes.

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Outdoor music: three generalizations Generalization one:

Any source sounds weaker outside: Whatever sound misses your ears as it first moves outward is lost forever, while an enclosed room will trap it and send it by repeatedly.

A practical consequence: outdoor musical performances require sources that can put out a lot of energy, e.g. large groups, brass bands.

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Outdoor music: three generalizations (cont.)

Generalization two: The “deadliness” of outdoor sound:

The first sound you receive directly from the sources is not followed by a series of overlapping reflected sounds, and thus it lacks the reverberation and lasting warmth of indoor sound.

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Outdoor music: three generalizations (cont.)

Generalization three: It is hard to distribute sound evenly over

a large audience outdoors: The reflections inside a room come from many different directions, and will average out in a well-designed hall so that all listeners receive nearly the same sound levels. Outdoors, the single outgoing wave weakens rapidly as you go father from the source.

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Outdoor musical theater designs

Have some reflection structure or partial enclosure near the performers to redirect some of the sound. Example: Hollywood Bowl.

Placement of the performer: The performer should be a little less than halfway out from the shell to its center of curvature.

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Homework

Ch. 4 (Hall), P. 69, Exercises: #1, 4.