Notice: A quick lab will require some of you to download a free app on an iPhone or iPod touch. App title: Physics Oscilloscope Size: 309 Kb Compatibility:

Notice: A quick lab will require some of you to download a free app on an iPhone or iPod touch. App title: Physics OscilloscopeSize: 309 KbCompatibility: iOS 4.0 or later

Nature of Sound

Sound Waves

• Humans can hear sound with frequencies between 20 Hz and 20 000 Hz.

• All animals have a different auditory frequency range.

Frequency Range

Producing Sound with a Speaker

• Speaker cones move in and out according to an electric signal.

• Cone moving out = air molecules are pushed (compression)

• Cone moving in = air molecules spread out (rarefaction)

Compression + Rarefaction = Sound Wave

Air Molecule Movement

Sound Perceptions and Wave Behavior

Read p.378 for more on sound quality.

Having Fun with Tuning Forks

Observe the pitch of different tuning forks with different frequencies.

What do you think is the relationship between wave frequency and pitch?

Oscilloscope: Seeing a Sound Wave

Quick Lab: Measuring Sound Frequency

• The free app Physics Oscilloscope acts as a cheap oscilloscope that does the job.

• Find the frequency of a vibrating tuning fork using the app. The 9 division scale can be calibrated by touching the screen.

• Compare the value you calculated to the one given on the fork.

• Repeat for two other tuning forks.

How Do We Hear Sound?

Echolocation

• Read p.381, and answer the following questions:1. What is echolocation?2. Which animals use echolocation?3. What is the sound frequency range of dolphins?4. With echolocation, what are they capable of

identifying?5. Can you use echolocation?

The Speed of Sound

• Sound is the product of mechanical waves, as it needs a medium to propagate.

• Sound must travel a certain distance from the source to a receiver, and does not have lightning speed.

The Speed of Sound in Air

Wherev = speed of sound (m/s)Tc = temperature of air (C)

So, what is the speed of sound at room temperature (20 C)?

ExampleOn a dark and cold September night, you find yourself outside, alone in front of a cliff face. You suddenly feel something brush your left ankle. By sheer terror that it could be a lobster, you let out an unflattering high pitched sound. Amidst all the excitement, you realize that you heard your echo one second after letting out your unforgiving yelp. Regaining your senses you begin to think of what really matters, and come up with this question:« How far am I from the cliff? »

Practice Problems!

p.390 # 1, 2, 3, 4, 5, 6

(Refer to p.387 for some problems…)

Dissipation of Sound

• Friction in a medium slowly decreases the amplitude of a wave, but does not affect its speed.

• Reminder: Sound being a mechanical wave, its speed is determined by the medium through which it travels.

Speed of Sound in Common Materials

• The closer the molecules are to each other and the tighter their bonds, the less time it takes for them to transfer the energy and the faster sound can travel.

• The speed of sound as physical property is then modeled by two properties of matter: Elasticity and Density.

Example 1

How long will it take a sound to travel through 14 km of water? (distance between New Brunswick and P.E.I.)

Example 2

A woman is swimming when she hears the underwater sound wave from an exploding ship across the harbor. She immediately lifts her head out of the water. The sound wave from the explosion propagating through the air reaches her 4.00 seconds later. How far away is the ship? (Assume that the water temperature is 20 C and that the air temperature is 23 C.)

The Doppler Effect• The Doppler effect refers to

the variation of the frequency heard when a source of sound and the ear are moving relative to each other.

• As a car passes by at a high speed sounding its horn, the pitch (frequency) of the sound drops noticeably as it passes the observer.

• This variation in frequency is called the Doppler effect.

Variations in frequency

Doppler Lab Demonstration

• You may need a few things, such as:– A car– A thermometer– A few audio recorders

Apparent Doppler-Shifted Frequency

Wheref’ = Doppler-shifted frequencyf = actual source frequencyv = speed of soundvo = speed of the observervs = speed of the source

• The + sign in the denominator is used when the source is moving away from the observer.

• The – sign is used when the source is moving toward the observer.

Example

On a beautiful sunny day of 20C, an automobile sounds its horn while passing an observer at 25 m/s. The actual horn frequency is 400 Hz.a) What is the frequency heard by the observer

while the car is approaching?b) What is the frequency heard when the car is

leaving?

But, What Happens If…

… the speed of an object surpasses the speed of sound?

Sonic BOOM!

Supersonic Objects

• When moving at the speed of sound, objects create a massive compression against the medium.

• A supersonic jet leaves behind a large cone of change in pressure, also known as a shock wave. This is heard as a sonic boom.

The sonic boom trails behind the jet, growing larger but weaker

as it expands.

Other Examples of Sonic Booms