Reflection Of Sound Part 2

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REFLECTION OF SOUNDREFLECTION OF SOUND• The reflection of sound follows the same laws

as for reflection of light– angle of incidence = angle of reflection– the incident wave, the reflected wave and the

normal lay in the same plane.• Sound waves can be reflected by large, hard,

smooth, vertical and faraway surfaces, e.g. walls, buildings and cliffs.

• Echoes are produced by the reflections of sounds from such surfaces.

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REVERBERATIONREVERBERATION

• This occurs when the original and reflected sounds are so close that the two cannot be heard as separate sounds. The original sound just seems prolonged.

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Applications of SoundEcho is a reflection of sound.

Echoes are used to find objects under water. This is called echolocation.

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Applications of Echo

1 To find submarines from surface ships.

2 In fishing boats, to find large shoals of fish.

3 To measure the depth of the sea.

4 to locate a sunken shipwreck or cargo

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1 To find submarines from surface ships.

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2 In fishing boats, to find large shoals of fish.

3 To measure the depth of the sea.

Refer to TB pg. 216

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Nature of sound waves:1 are longitudinal waves

2 need a medium to travel

3 can be reflected

4 travel faster in a denser medium than in the less dense medium.

5 travel at a speed of 330 m/s in air at 25C

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Pitch and Frequency

The sounds produced by the mouse and the girl are high pitch sound.

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Pitch

In musical scales, doubling the frequency of a note produces a sound one octave higher than the original sound. Halving the frequency produces a sound one octave lower.

In general, higher frequency sound gives rise to higher pitch note.

Refer to TB pg. 220

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Compare the frequencies of sound with same loudness:

Lower pitch sound with lower frequency

Higher pitch sound with higher frequency

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Range of Audio Frequency

20 Hz to 20 kHz

Frequency Pitch

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AUDIBLE FREQUENCIESAUDIBLE FREQUENCIES

• This range decreases as we get older and our ears lose their sensitivity to both high and low frequencies.

• Animals have different ranges of audible frequencies. Dogs and bats have a much higher upper audible limit. Bats can produce and detect very high ultrasonic sounds. This enables them to avoid obstacles, even when flying in the dark.

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PITCHPITCH• The pitch of a sound depends on its

frequency. The higher the frequency, the higher the pitch.

• To obtain a note of higher pitch from a vibrating string, we can – increase the tension– shorten the length – use a thinner string of smaller mass per

unit length

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Compare the loudness of sound with same frequency

louder sound with greater amplitude

softer sound with smaller amplitude

Louder musical notes can be produced by hitting the instruments harder so as to produce vibrations of greater amplitude.

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Loudness of sound Loudness is related to the objective

quantity amplitude,

The larger the amplitude the louder the sound.

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Noise• Noise causes stress, fatigue,

headache and decreases concentration. It makes people more irritable and aggressive.

• Prolonged exposure to sounds exceeding 85 dBexceeding 85 dB can permanently damage the ears.

• Sounds exceeding 130dBexceeding 130dB are above what the human ear can withstand and may result in serious pain and permanent damage.

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LOUDNESSLOUDNESS

• If the volume control of a radio is suddenly turned up, the sound suddenly becomes very loud.

• The air particles in front of the speaker gain more energy and begin to vibrate with greater amplitude.

• The maximum displacement or amplitude increases.

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LOUDNESSLOUDNESS

• The intensity or loudness of the sound has increased.

• The unit for measuring noise level is the decibel (dB). If the sound exceeds 80 dB, ear protectors must be used to prevent permanent ear damage.

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Noise and MusicNoise is sound with irregular waveform.

Musical note is sound with regular waveform.

sound from piano sound from violin

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Comparing sound waves and light waves

Feature Sound waves Light waves

transfer energy yes yes

can travel through solids, liquids and gasesbut need a medium

vacuum, gases, some solidand liquids, no mediumneeded

speed 330 m/s in air

1500 m/s in water

5 000 m/s in steel

3 x 108 m/s in air

2.2 x 108 m/s in water

2 x 108 m/s in glass

type of wave longitudinal transverse

typical frequency

typical wavelength

1 kHz

10 cm or 1 m

5 x 10 Hz

5000 nm = 5 x 10-7 m

part of the electromagneticspectrum

no yes

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Qus 3

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Qus 4

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Ultrasound

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Ultrasound

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RecapRecap• What can you say about these sounds? What can you say about these sounds?

Which factors are Which factors are differentdifferent??

PitchPitch

LoudnessLoudness

QualityQuality

Pitch & LoudnessPitch & Loudness

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Resonance

• Principle.• Play short clip on

glass breaking. • Play clip on

resonance later (glass).

• Bridges.

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• Tacoma Narrows bridge in Washington