Transcript Sound
Chapter 26 Sound Sound waves are longitudinal waves. These waves (energy) travel parallel to the medium. The medium is forced into compression and expansions (also called rarefactions) Sound The Origin of Sound All sounds are produced by the vibrations of objects. (A knock on the door, produces a sound) The original vibration stimulates the vibration of something larger. (Hand knocking door) This vibrating material then sends a disturbance through a surrounding medium (like air) in the form of a longitudinal wave. (This wave reaches your ears) The frequency of the vibrating source equals the frequency of sound waves produced. The Origin of Sound Player plucks string causing string to vibrate. String sends a disturbance through the surrounding air. The frequency of the vibrating string is equal to the frequency of the wave produced. Pitch The wave that reaches our ears carries a certain frequency. How we hear that frequency, is what we call pitch. Low pitched sounds have low vibration frequencies. A fog horn or tuba. High pitched sounds have high vibration frequencies. A piccolo or flute. Pitch Sound waves with frequencies below 20 Hz are called infrasonic waves. Sound waves with frequencies above 20,000 Hz are called ultrasonic. Humans can hear pitches ranging between 20 to 20,000 Hz. Pitch Higher frequency = Higher pitch Notice the shorter wavelengths Notice the longer wavelengths Lower frequency = Lower pitch Pitch Sound Wave Travel Sound waves need a media (medium, matter, etc.) to travel through. They can’t travel through a vacuum. They travel fastest in solids and slowest in gases. The vibrating molecules and atoms can easily vibrate surround molecules and atoms if they are closer together. Speed of Sound In room temperature air, under standard air pressure, the speed at which sound will travel is approximately 340 m/s. If pressure decreases (top of a mountain), the speed will decrease. If temperature increases, the speed will increase as well. Remember it always has to do with the proximity of the molecules and atoms. The closer they are the easier it is for the energy to reach them. Loudness Loudness of sound is a physiological sensation sensed in the brain. In other words loudness is specific to your own ears. Age can affect loudness. Genetics can affect loudness. Damage can affect loudness. Loudness of sound is measured in decibels (dB). Forced Vibrations When you force an object to vibrate by using another vibrating object you are creating a forced vibration. Forced vibrations are useful when you want to hear the frequency of a vibration (make it louder). Instruments force vibrations into other objects so you can hear the sound of the vibration. The vibration of guitar strings are made louder because vibrations are forced into the body of the guitar. Natural Frequency Natural frequency refers to an objects ability to vibrate and the frequency it produces as a result of that vibration. All objects, from atoms to planets have there own frequencies. An objects natural frequency comes from its elasticity and shape. The natural frequency an object produces must not require more energy to continue this vibration. Resonance When a dramatic increase in amplitude occurs as a result of the natural frequency of an object forcing a vibration on an other, it is called resonance. Resonance means to resound or sound again. http://youtu.be/iyw4AcZuj5k http://youtu.be/JDnNmLkQ3Bc Video Follow Up Many students ask "What's resonance?". Resonance is the phenomena that occurs when the frequency at which an object is made to vibrate (called the driving or forced frequency) is equal to its natural frequency (a frequency which the object would naturally vibrate on its own), causing the object to oscillate with a large or maximum amplitude. In this video, you can see that each of the structures made to vibrate, simulating buildings in an earthquake, reaches resonance differently because different structures possess different natural frequencies which explain why in an earthquake, not every building will collapse. Beats When two tones (a sound of distinct pitch) of slightly different frequency are sounded together, a fluctuation in the loudness of the combined sounds is heard. The sound will become loud, then faint, then loud again. This periodic variation in the loudness of sound is called beats. http://youtu.be/IQ1q8XvOW6g