Mechanical Waves and Sound

There are Different Types of Waves The two basic types of waves are Mechanical waves: examples-sound, water waves, a pulse traveling on a spring, earthquakes, a “people wave” in a football stadium, etc. Electromagnetic waves: examples-visible light, radiowaves, infrared waves, etc. For now, we’re going to focus on Mechanical Waves

What are mechanical waves? A mechanical wave is a disturbance in matter that carries energy from one place to another. The material through which a wave travels is called a medium. Mechanical waves require a medium to travel through. Solids, liquids, and gases all can act as mediums. They can not travel through a vacuum (like space)

A mechanical wave is created when a source of energy causes a vibration to travel through a medium. In a wave pool, the waves carry energy across the pool. You can see the effects of a wave's energy when the wave lifts people in the water.

Types of Mechanical Waves There are three main types of mechanical waves: transverse waves longitudinal waves surface waves. Mechanical waves are classified into these 3 types by the way they move through a medium.

Transverse Waves When you shake one end of a rope up and down, the vibration causes a wave. Vibrates in a direction perpendicular to the direction the wave travels. Direction wave travels Direction wave vibrates

Longitudinal/Compression Waves A longitudinal wave is a wave in which the vibration of the medium is parallel to the direction the wave travels. Compression An area where the particles are spaced close together is a compression. An area where the particles are spread out is a rarefaction. Direction of wave and vibration Rarefaction

Surface Waves A surface wave is a wave that travels along a surface separating two media. Ocean waves are the most familiar kind of surface waves.

Rest/equilibrium position Properties of a Wave Amplitude (A) – Maximum displacement of particle of the medium from its equilibrium point. The bigger the amplitude, the more energy the wave carries. Measured from crest/peak to rest position OR from rest position to trough Wavelength () –For a transverse wave, wavelength is measured between adjacent crests or between adjacent troughs. For a longitudinal wave, wavelength is the distance between adjacent compressions or rarefactions. Wavelength Crest / Peak Rest/equilibrium position Amplitude Trough

Amplitude The more energy a wave has, the greater its amplitude. Low amplitude & low energy High amplitude & high energy

More Wave Terms Period (T) – Time it takes consecutive crests (or troughs) to pass a given point, i.e., the time required for one full cycle of the wave to pass by. Calculated by T = 1 / f. Frequency (f ) – The number of cycles passing by in a given time. The SI unit for frequency is the Hertz (Hz), which is one cycle per second.

Frequency A wave vibrating at one cycle per second has a frequency of 1.0 Hz. A wave vibrating at two cycles per second has a frequency of 2.0 Hz. Frequency = 1.0 hertz One cycle per second Frequency = 2.0 hertz Two cycles per second

Wave speed (v) – How fast the wave is moving Wave speed (v) – How fast the wave is moving. Speed depends on the medium. Calculated by v =  f. The speed of a wave can change if it enters a new medium, or if variables such as pressure and temperature change.

Wave Terms and Concepts Reflection occurs when a wave bounces off a surface that it cannot pass through. Reflection does not change the speed or frequency of a wave, but the wave can be flipped upside down.

Refraction is the bending of a wave as it enters a new medium at an angle. Refraction occurs because one side of the wave moves more slowly than the other side. Example: A lawnmower turns when it is pushed at an angle from the grass onto the gravel. The wheel on the gravel slows down, but the other wheel is still moving at a faster speed on the grass.

Diffraction is the bending of a wave as it moves around an obstacle or passes through a narrow opening. A wave diffracts more if its wavelength is large compared to the size of an opening or obstacle. This wave diffracts, or spreads out, after it passes through a narrow opening. Diffraction also occurs when a wave encounters an obstacle.

Interference occurs when two or more waves overlap and combine together. Two types of interference are constructive interference and destructive interference. When waves collide, they can occupy the same region of space and then continue on. Constructive interference occurs when two or more waves combine to produce a wave with a larger displacement. Destructive interference occurs when two or more waves combine to produce a wave with a smaller displacement.

Constructive Interference Two waves that are in phase (have equal frequencies with crests and troughs that line up) When these waves meet and overlap the result is a wave with an increased amplitude.

Destructive Interference Two waves that are out of phase (do not have equal frequencies with crests and troughs that line up) When a crest meets a trough, the result is a wave with a reduced amplitude.

A standing wave is a wave that appears to stay in one place—it does not seem to move through the medium. A node is a point on a standing wave that has no displacement from the rest position. At the nodes, there is complete destructive interference between the incoming and reflected waves. An antinode is a point where a crest or trough occurs midway between two nodes.

Sound Waves

Sound is a longitudinal or compression wave Sound is a longitudinal or compression wave. Sometimes called a pressure wave. If a sound wave is moving from left to right through air, then particles of air will be displaced both rightward and leftward as the energy of the sound wave passes through it. The motion of the particles are parallel to the direction of the energy transport = longitudinal wave.

Speed of Sound It takes time for sound to travel from place to place. In general, sound waves travel fastest in solids, slower in liquids, and slowest in gases. Particles in a solid tend to be closer together than particles in a liquid or a gas.

Sound Intensity Intensity is the rate at which a wave’s energy flows through a given area. Sound intensity depends on both the wave’s amplitude and the distance from the sound source. The decibel (dB) is a unit that compares the intensity of different sounds. For every 10-decibel increase, the sound intensity increases tenfold.

Lengthy exposure to sounds more intense than 90 decibels can cause hearing damage. Did you know that MP3 players such as iPods can reach the 120-decibel level when they are at maximum volume?

Sound Loudness Loudness is a physical response to the intensity of sound, modified by physical factors. The loudness depends on sound intensity. Loudness also depends on factors such as the health of your ears and how your brain interprets sound waves.

Frequency & Pitch The frequency of a sound wave depends on how fast the source of the sound is vibrating. Pitch is the frequency of a sound as you perceive it. High-frequency sounds have a high pitch, and low-frequency sounds have a low pitch. Pitch also depends on other factors such as your age and the health of your ears.

The Doppler Effect As a source of sound approaches, an observer hears a higher frequency. When the sound source moves away, the observer hears a lower frequency. The Doppler effect is a change in sound frequency caused by motion of the sound source, motion of the listener, or both. Dial 911