Sound26.4-26.7

Objectives

1. Factors that affect the speed of sound

2. Loudness vs. sound intensity

3. Examples of forced vibration

4. How a sounding board works

5. Describe natural frequency

What’s Faster:Sound or Light?

• Light• Any examples?• What’s the speed of

sound in normal air temp (@ 20C or 68F)?

• 340m/s– 1 millionth the speed

of light

How Far Away is the Storm?

• How far away is a storm if you note a 3-second delay between a lightning flash and the sound of thunder?

• d = vt– 340 m/s x 3s = 1020m

Is the speed of sound always constant in air?

• Speed of Sound changed by:1. Water vapor

• Why?

• H2O molecules (less mass) move faster than O2 or N2; shortens collision time2. Increased temperature

• Why?• Warm air --> faster moving

particles

What does the speed of sound depend on when traveling

through materials?• The speed of sound depends on the elasticity of a

material. What is elasticity?• Elasticity:

– The ability of a material to change shape and then resume it’s initial shape

• Atoms close together & respond quickly to each others motion.

• Elasticity is NOT “stretchability.”– Is this rubber band elastic?– Steel is one of the most elastic material around

Speed of Sound

• 4 x’s as fast in water than in air

• 15 x’s as fast in steel than in air

• Just to keep things in perspective: light (in air) is 1million x’s faster!!!

Checkpoint

1. Is sound going to travel faster on a foggy day or a clear day, given all other conditions are the same? Why?

Foggy day. Moisture in the air, water molecules move faster than O2 and N2 molecules

Intensity vs. Loudness• Sound Intensity:

– Objective• Measurable data

– measured by instruments (oscilloscope)

• Loudness: – Subjective

• opinion

– sensation sensed by brain

Sound IntensitySound Intensity

Source of SoundSource of Sound Level (dB)Level (dB)

Jet engine 140

Threshold of pain 120

Loud Rock Music 115

Subway Train 100

Average Factory 90

Busy Street Traffic 70

Normal Speech 60

Library 40

Close Whisper 20

Normal Breathing 10

Hearing threshold 0

Forced Vibrations & Sound Boards

• Acoustic guitar– String --> saddle --> soundboard--> ear

• Soundboard:– Larger surface whose vibrations reinforce sound of

instrument

• Vibration of string sets larger surface in motion• Acoustic guitar vs. electric guitar (unplugged).

Which is louder? Why?

Demos

• Demos:– tuning fork on table– Music box– Record player with paper/pin– Talkie strips: How can we make this louder?

• I want you to write down in your notes how each of these things work