Sound Notes

Standing Waves A standing wave is produced when a wave that is traveling is reflected back upon itself. There are two main parts to a standing wave: Antinodes – Areas of MAXIMUM AMPLITUDE Nodes – Areas of ZERO AMPLITUDE.

Sound Waves Sound Waves are a common type of standing wave as they are caused by RESONANCE. Resonance – when a FORCED vibration matches an object’s natural frequency thus producing vibration, sound, or even damage. Example - shattering a wine glass by hitting a musical note that is on the same frequency as the natural frequency of the glass. Example – Tacoma bridge collapse

Resonance Example Tacoma Narrows Bridge Collapse http://www.youtube.com/watch?v=3mclp9Q mCGs

Electromagnetic vs Mechanical Waves Electromagnetic wave - capable of transmitting its energy through a vacuum (or empty space). Produced by the vibration of charged particles. Ex. All light waves Mechanical wave - not capable of transmitting its energy through a vacuum. Require a medium in order to transport their energy from one location to another. Ex. Sound, slinky, water, standing waves. Two ways to categorize waves – Transverse vs Longitudinal and Electromagnetic vs Mechanical

Sound Waves Longitudinal waves produced by a vibration Changes in pressure (high to low) Molecules in the air vibrate about some average position creating the compressions and rarefactions. a wavelength is the distance from a compression to the next compression (or from a rarefaction to the next rarefaction)

Frequency Wavelength of a wave tells you the frequency Frequency is measured in Hertz (Hz) Frequency is the pitch of a sound High pitch = high frequency/ Low pitch = low frequency Doubling the frequency = octave

Frequency High frequencies are high sounds (whistle) with high pitches Low frequencies are low sounds (tuba) with low pitches. Who has higher frequencies, women or men?

Loudness Amplitude of a wave, tells you the loudness Measure in decibels (dB) Increase of 20 dB = twice the volume Loud Soft

Sound Levels

Loudness of Sound in Decibels Loudness (dbs) Hearing Damage Average Home 40-50 Loud Music 90-100 After long exposure Rock Concert 115-120 Progressive Jet Engine 120-170 Pain

Human Hearning Below 20 Hz – Infrasonic Elephants communication, Average human hearing (audible sound) ranges from 20 Hz to 20,000 Hz Above 20,000 Hz – Ultrasonic Ultasounds

Sound medium Most sounds are transmitted through air In general, sounds are transmitted easily in solids, then liquids, and slowest is air (gases.) Ex. Hold your ear to the ground to hear horses off at a distance. Speed of Sound in Various Media Medium Speed of Sound (m/s) Air (0°C) 331 Air (20°C) 343 Helium (0°C) 972 Water (25°C) 1493 Seawater (25°C) 1533 Copper (25°C) 3560 Iron (25°C) 5130

Sound Wave Interference Two speakers (green dots) generating sound. Listeners at the red points will hear the sound better than listeners at the blue points. Why? Thus, constructive interference occurs wherever a thick line meets a thick line (or thin line meets a thin line) this type of interference results in antinodes (red dots). Blue dots are the nodes, places of destructive interference.

Doppler Effect phenomenon observed whenever the source of waves is moving with respect to an observer. observed to occur with all types of waves - water waves, sound waves, and light waves.

Doppler Effect Ex. A police car traveling towards you on the highway. As the police car approached with its siren blasting, the pitch of the siren sound was high; and then suddenly after the car passed by, the pitch of the siren sound was low. 

Speed of Sound To calculate the speed of sound, use the equation: speed = distance/time s = d/t speed = frequency)(wavelength) v = fλ Speed depends on the medium. Will be given a chart of the various speeds in different mediums.

Speed of Sound Sound waves with a constant frequency of 250Hz are traveling through air at 0 C. What is the wavelength of the sound waves? Speed of Sound in Various Media Medium Speed of Sound (m/s) Air (0°C) 331 Air (20°C) 343 Helium (0°C) 972 Water (25°C) 1493 Seawater (25°C) 1533 Copper (25°C) 3560 Iron (25°C) 5130