Producing a Sound Wave What happens when you strike an object? It vibrates at its 1 st harmonic frequency.
Using a Tuning Fork to Produce a Sound Wave As the tine swings outward, it forces the air molecules near it closer together This produces a high density area in the air This is an area of compression
Using a Tuning Fork, cont. As the tine moves inward, the air molecules near spread out This produces an area of low density This area is called a rarefaction
Using a Tuning Fork As the tuning fork vibrates, compressions and rarefactions spread out from the fork. Note: The sine wave is not the actual wave; it is a representation. The wave is NOT a transverse wave that goes up and down. A SOUND WAVE IS A LONGITUDINAL WAVE! (Let students feel vibrating tuning forks.)
Using a tuning fork The tuning fork creates a longitudinal wave in the air Note: The air molecules only vibrate, they do not move forward with the wave.
Audibility This is what sound is. Can we detect all of them?
Categories of Sound Waves Audible waves Frequencies within the normal range of hearing of the human ear Normally between ~20 Hz to ~20,000 Hz Infrasonic waves Frequencies too low to hear Earthquakes are an example Ultrasonic waves Frequencies too high to hear Dog whistles are an example Compare to visible, Infrared, and Ultraviolet Light
Sounds have frequency Like all waves, sounds have a frequency. The human brain hears frequency as pitch. Pitch is the frequency of a sound wave. Pitch can also be thought of as the wavelength. High Frequency (or short wave length) is High Pitch. Frequency is determined by the sound generator. Velocity (and wavelength) is determined by the medium. ν=ƒλ Volume is the amplitude of a sound wave. Amplitude does not affect frequency!!!
Some Pitches C – 256 (some sites say 262)Hz D – 294 Hz E – 330 Hz F – 349 Hz G – 392 Hz A – 440 Hz (Middle A on piano) B – 492 Hz C – 512 (Or 524) Hz (Middle C on piano)
Octave A doubling or halving of a frequency moves to the next octave. An A would be 110, 220, 440, 880, ect. Hz. In music theory, an octave is 12 Half steps. So each half step changes the frequency by
Octaves Think about a standing wave. If the frequency doubles, the wavelength halves. Changing octaves is like changing harmonics of a standing wave.
Pure Tone A pure tone is a sound with only one frequency present. Adding other octaves will create a richer tone.
Speed of sound The speed of sound is not constant. Depends on density, pressure, and temperature. All these are properties of the medium. Property Increases Speed Guess Inc or Dec Density Pressure Temperature Humidity (air only)
Different speeds Sound travels fastest in solid. Slightly less in liquid. Much less in gas. (air is about 1/5 th the speed of water.) Because of how far apart molecules are.
Problem The speed of sound in sea water is 1560 m/s. A dolphin shouts at the ocean floor 250 m beneath him. How much time passes before he hears the echo?
Speed of Sound in Air Guess: Does the speed of sound depend on air temperature?
Speed of Sound in Air Does the speed of sound depend on air temperature? Yes Guess: Is it faster or slower when it is hotter outside?
Speed of Sound in Air 331 m/s is the speed of sound at 0° C Speed of sound increases with temp 343 m/s is the speed of sound at room temperature
Speed of sound (How do you convert from C to K?)
Problem During a thunderstorm, lightning strikes 300 m away. If the air temp is 25 Celsius, how much time passes between when you see the lightning and hear the thunder?
Speed of sound The speed of sound is not constant. Depends on density and temperature. These are properties of the medium. Property Increases Speed Density Increases Pressure (almost) No Affect Temperature Increases Humidity(air only) Increases (because of air density increase)