Sound Intensity and Resonance : Sound Intensity and Resonance Intensity : Intensity Intensity is the rate at which energy flows through a unit of area perpendicular to the direction of wave motion
Intensity= Power/ Area
Area of a spherical wave = 4πr2 Thresholds : Thresholds Intensity and frequency determine which sounds are audible
As you have learned, humans can hear 20-20,000 Hz.
The softest sound a human can hear is at a frequency of 1000Hz and an intensity of 1x10-12 and is called the threshold of hearing.
The loudest sound a human can tolerate has an intensity of 1 and is known as the threshold of pain Intensity Continued… : Intensity Continued… Relative intensity is measured in decibels
The intensity of a wave determines the loudness.
Relative intensity is the human perception of loudness
A difference in 10db means the sound is twice as loud Guitars : Guitars When an isolated guitar string is held tight and plucked, hardly any sound is heard.
When the same string is placed on a guitar and plucked, the intensity of the sound increases dramatically. This is called forced resonance.
The vibrating of the strings of a guitar force the bridge of the guitar to vibrate
The forced vibrations are called sympathetic vibrations. Resonance : Resonance All objects have natural frequencies
Every object will vibrate at a certain frequency
Resonance- a condition that exists when the frequency of a force applied to a system matches the natural frequency of vibration of the system. Resonance Demos : Resonance Demos Go to the following link to watch a resonance demo by Paul Hewitt
http://www.freesciencevideo.com/conceptual-physics-demo-of-resonance.html Examples of Resonance : Examples of Resonance Example #1- Tacoma narrows bridge. The wind blowing through the canyon watched the natural frequency of the bridge and caused the bridge to oscillate and eventually crumble Examples of Resonance : Examples of Resonance Example #2- A kid on a swing, pumps their legs at the same frequency each time to cause them to swing higher each time. They are matching the natural frequency of the string. Examples of Resonance : Examples of Resonance Example #3- A wine glass has a natural frequency. A singer can sing at the same frequency and can cause the glass to vibrate until it shatters Harmonics : Harmonics The fundamental frequency is the lowest possible frequency of a standing wave.
The series of frequencies of a standing wave are called the harmonic series
Frequency= harmonic number x (speed/2 length)
F= n(v/2L) Guitars Continued… : Guitars Continued… When a guitar player presses down on a guitar string at any point, that point becomes a node and only a portion of the string vibrates
As a result, a single string can be used to create a variety of fundamental frequencies
L in the previous equation would represent the portion of the string that was vibrating Standing Waves in a Pipe : Standing Waves in a Pipe Standing waves can also be set up in a tube of air and not just on a string.
Harmonic series of a pipe if both ends are open is different on a pipe if only one end is open
Both ends open:
Frequency= harmonic number x (speed/2L)
One end is closed:
Frequency= harmonic number x (speed/4L) Music : Music In music, the mixture of harmonics that produces the characteristic sound of an instrument is referred to as the spectrum of sound, which results in a response in the listener called sound quality or timbre. Interference : Interference When two waves of the same frequency interact, you get either constructive or destructive interference
If the waves are opposite each other they are set to be out of phase and destructive interference occurs. No sound is heard.
IF waves match up it is in phase and constructive interference occurs. The sound gets louder
However, if waves with slightly different frequencies interact, a variation creates a soft to loud sound called a beat.