Mechanical Waves and Sound
Ch. 17
Physical Science

Mechanical Waves
&
Properties of Mechanical
Waves
17.1 / 17.2
Physical Science CIA

What are mechanical waves?
What do you think waves carry?
Can you see waves?
Examples?

Mechanical Waves
• Mechanical waves are disturbances in matter that carry energy from one place to another.
– Usually require matter through which to travel
– The matter a wave travels through is called a medium .
• Medium can be a solid, liquid, or gas
• Some waves can travel through space…with no medium!

How are mechanical waves created?

Creation of mechanical waves:
• Need a source of energy!
• That energy causes a vibration to travel through the medium

Types of Mechanical Waves
• Transverse:
– A wave that causes the medium to vibrate at right angles to the direction of the wave

Parts of a transverse wave:

Types of Mechanical Waves, ctd.
• Transverse
• Longitudinal:
– A wave in which the vibration of the medium is parallel to the direction the wave travels

Parts of a longitudinal wave:

Remember !
• A wave doesn’t move the medium…it’s just energy traveling through the medium!

Transverse and Longitudinal Wave
Which is which?

Types of Mechanical Waves, ctd.
• Transverse
• Longitudinal
• Surface:
– A wave that travels along a surface separating two media

Period vs Frequency
• Period – (T) – The time it takes for one cycle (= how long?)
– Measured in seconds
• Frequency – (f) – The number of cycles in a given time (= how many?)
– Measured in Hertz (Hz)
• Frequency is the inverse of the Period f = 1 / T

Speed of a wave
• We find speed by distance divided by time.
– The same holds true for waves!
• Speed of a wave = Wavelength/Period
• Speed of a wave =
Wavelength*Frequency

Speed of a Wave
• The speed of a wave is constant within a medium.
• The speed can change when a wave enters a new medium
• All waves of the same type travel at the same speed
– This means wavelength is inversely proportional to frequency!
• If the wavelength increases the frequency has to decrease!
• If the wavelength decreases the frequency has to increase!

Practice Problem #1
A wave in a spring has a wavelength of 0.1 meters and a period of 0.2 seconds. What is the speed of the wave?
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Practice Problem #2
Find the wavelength of a wave in a rope that has a frequency of 2.0 Hz and a speed of
0.4 meters/second.
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Behavior of Waves
17.3 Physical Science CIA

What happens when…
• A wave meets a hard surface like a wall?
• A wave enters a new medium?
• A wave moves around an obstacle?
• A wave meets another wave?

A wave meets a hard surface like a wall?
• Reflection
– A wave bounces off a surface that it can not pass through
• Reflection does not change the speed or frequency of the wave, BUT the wave can be flipped upside down!

How?

A wave enters a new medium?
• Refraction
– The bending of a wave as it enters a new medium
• The wave bends because as it enters a new medium it either speeds up or slows down!

Refraction

Refraction

A wave moves around an obstacle?
• Diffraction – the bending of a wave as it moves around an obstacle or passes through a narrow opening

Diffraction

A wave meets another wave?
• Interference – when two or more waves overlap and combine together.
– Constructive
– Destructive

Constructive Interference
• Two or more waves combine to produce a wave with a larger displacement

Destructive Interference
• Two or more waves combine to produce a wave with a smaller displacement

Standing Waves
• Occur because of interference!
• A standing wave is a wave that appears to stay in one place.

Parts of a Standing wave
• Node – Where there is no displacement of the medium in a standing wave
• Antinode – Where there is maximum displacement of the medium in a standing wave

How do standing waves fit on a string?
• Standing waves only form if a half a wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating string

Sound and Hearing
17.4 Physical Science CIA

Properties of Sound Waves
• What type of waves are sound waves?

Properties, ctd.
Sounds behave in certain ways because of the properties of sound waves. Some properties are:
• Speed
• Intensity & Loudness
• Frequency & Pitch

Speed
• Speed of sound = 342 m/s
…THAT’S 765 mph!
• Sound speed varies, depending on medium
– Travel fastest in solids, slowest in gases
– Travel fastest in the most dense media
– Speed of Sound animation

Intensity
• Intensity: the rate at which a wave’s energy flows through an area
• Sound intensity depends on
– Amplitude
– Distance from source
• Measured in decibels (dB)

Loudness
• Subjective ! (This means it depends on the person who is hearing it.)
• Loudness is a personal, physical response to the intensity of sound.
• As intensity increases, so does loudness, but loudness also depends on the listener’s ears and brain.

Frequency & Pitch
• Frequency of a sound wave depends on how fast the source of the sound is vibrating.
• Pitch is how we hear frequency of sound waves
• Pitch depends on frequency…high frequency sounds are high pitched, and low frequency sounds are low pitched.
• Pitch also depends on age and health

Frequency and
Pitch animation

Ultrasound
• Most people hear sounds between 20 and
20,000 Hz.
– Infrasound – sound at frequencies lower than people usually hear
– Ultrasound – sound at frequencies higher than people usually hear
• Used in technologies such as sonar and ultrasound imaging

Ultrasound, ctd.
• Sonar – a technique used to determine the distance to an object under water.
• Ultrasound – medical technique used to take pictures of different organs (or a fetus!)

The Doppler Effect
• Where have you heard of the term
“Doppler”?
• Doppler Effect : a change in sound frequency (pitch) caused by the motion of the sound source, the listener, or both
• Why do we observe this?
• Doppler Effect animation

Hearing and the Ear
• Your ear has a membrane that vibrates when sound waves hit it…what is this membrane?
• The Ear animation

Hearing & the Ear
Ear consists of 3 main parts
• Outer Ear – gathers and focuses sound
• Middle Ear – receives and amplifies vibrations
• Inner Ear – uses nerve endings to sense vibrations and send signals to the brain

Reproduction of Sound
• What are some ways that sound is stored?
• How do you think sound is stored, and then played back?

Reproduction of Sound
• To record: sound waves must be converted into electronic signals that can be stored
• To reproduce: electronic signals are converted back into sound waves

Music
• Musical instruments change their pitch by changing the frequency of the waves they produce… how do they do this?
• Reflection animation