Mechanical Waves and Sound Physical Science. Mechanical Waves & Properties of Mechanical Waves.

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Mechanical Waves and Sound Physical Science

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Transcript of Mechanical Waves and Sound Physical Science. Mechanical Waves & Properties of Mechanical Waves.

Page 1: Mechanical Waves and Sound Physical Science. Mechanical Waves & Properties of Mechanical Waves.

Mechanical Waves and Sound

Physical Science

Page 2: Mechanical Waves and Sound Physical Science. Mechanical Waves & Properties of Mechanical Waves.

Mechanical Waves&

Properties of Mechanical Waves

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What are mechanical waves?

What do you think waves carry?

Can you see waves? Examples?

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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!

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How are mechanical waves created?

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Creation of mechanical waves:

• Need a source of energy!

• That energy causes a vibration to travel through the medium

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Types of Mechanical Waves• Transverse:

– A wave that causes the medium to vibrate at right angles to the direction of the wave

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Parts of a transverse wave:

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Types of Mechanical Waves, ctd.

• Transverse

• Longitudinal:– A wave in which the vibration of the medium is

parallel to the direction the wave travels

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Parts of a longitudinal wave:

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Remember!

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

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Transverse and Longitudinal Wave

Which is which?

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Types of Mechanical Waves, ctd.

• Transverse

• Longitudinal

• Surface:– A wave that

travels along a

surface

separating two

media

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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)

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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

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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!

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Sound and Hearing

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Properties of Sound Waves

• What type of waves are sound waves?

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Properties, ctd.

Sounds behave in certain ways because of the properties of sound waves. Some properties are:

• Speed

• Intensity & Loudness

• Frequency & Pitch

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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

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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)

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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.

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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

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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!)

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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

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Behavior of Waves

17.3 Physical Science CIA

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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?

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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!

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How?

Page 35: Mechanical Waves and Sound Physical Science. Mechanical Waves & Properties of Mechanical Waves.

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!

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Refraction

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Refraction

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A wave moves around an obstacle?

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

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Diffraction

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A wave meets another wave?

• Interference – when two or more waves overlap and combine together.– Constructive– Destructive

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Constructive Interference

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

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Destructive Interference

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

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Standing Waves

• Occur because of interference!

• A standing wave is a wave that appears to stay in one place.

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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

Page 45: Mechanical Waves and Sound Physical Science. Mechanical Waves & Properties of Mechanical Waves.

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


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