The nature of a Sound Wave
- Sound is a wave that is created by vibrating objects and propagated through a medium from one location to another.
What is a wave of sound you ask?
- Sound is the vibration of any substance
- It must be made clear that the vibrations to produce sound are not the result of an entire volume moving to and from at once.
- If it were that, the atmosphere would have to rearrange itself for every sound made.
Whether it be air, wood, or any other material.... sound cannot travel inside a vacuum however.
- The vibrations to create sound occur in the individual molecules
Sound Waves and their Perception
- Sound is simply vibrations in matter.
- The sound that we can perceive can travel through three different mediums
- Airborne sounds travel in the form of longitudinal waves
What about transverse waves?
- light and radio waves are transverse
Transverse wave - A wave vibrating at right angles to the direction of its propagation
Notice:
Constructive and Destructive Interference of Sound Waves
Situation
- Two speakers set up at exact same distance away from you.
- Each emitting the same sound.
- Diaphragms are vibrating synchronously.
Condensations of the waves of each speaker meet at the same time in the middle creating one combined wave.
This is called constructive interference.
Slightly changed situation:
- Move on of the speakers .5m away instead of 1m.
- Volume turned up so amplitude remains constant.
- Rarefactions of one wave is offset by the condensation of the other.
- Results in cancellation of both waves.
- Constant air pressure which means you can hear no sound coming from the speakers.
- This is called destructive interference.
Beats
What happens when two sound waves at different frequencies overlap?
- Microphone picks up engine noise
- Component creates inverse of the noise
- Destructive Interference of waves causes quieter background.
Physics-like Terms and Knowledge
Speed of sound travels at 330 meters per second
Snell's Law
A law stating that the ratio of the sines of the angles of incidence and refraction of a wave are constant when it passes between two given media
When a vehicle travels faster than the speed of sound, about 330 meters per second, a sonic boom can be heard. As the vehicle overtakes its own sound, the sound waves spread out behind in a shockwave, or sonic boom.
Radio Waves travel at 300,000 KM per second
A wavelength is the distance between matching points on a wave
( )
is measured in meters!
slow=large wavelength
fast= small wavelength
Speed: Measured by how many meters the wave travels in one second
Frequency is the number of times the waveform passes a point per second
Wavelength is the distance between matching points in a wave
as frequency (f) increases
wavelength decreases
when speed is constant
Therefore!
V=(f)( )
speed is measured in m/s
frequency in hertz
wavelength in m
SOLVE
http://www.acoustics.salford.ac.uk/schools/index1.htm
Inverse square law
A law stating that the intensity of an effect such as illumination or gravitational force changes in inverse proportion to the square of the distance from the source
Intensity of three dimensional waves
Breakdown
Hearing
Pitch: The sensation of a frequency is commonly referred to as the pitch of a sound. A high pitch sound corresponds to a high frequency sound wave and a low pitch sound corresponds to a low frequency sound wave
Timbre: "Timbre is then a general term for the distinguishable characteristics of a tone. Timbre is mainly determined by the harmonic content of a sound and the dynamic characteristics of the sound such as vibrato and the attack-decay envelope of the sound."
Phons
Intesity and loudness are not the same.
- Human sensitivity varies with frequency
Sones
The sone scale was created to provide such a linear scale of loudness. It is usually presumed that the standard range for orchestral music is about 40 to 100 phons. If the lower end of that range is arbitrarily assigned a loudness of one sone, then 50 phons would have a loudness of 2 sones, 60 phons would be 4 sones, etc.
Decibels
Note:
The logarithm involved is just the power of ten of the sound intensity expressed as a multiple of the threshold of hearing intensity. Example: If I= 10,000 times the threshold, then the ratio of the intensity to the threshold intensity is 104, the power of ten is 4, and the intensity is 40 dB:
The difference in decibels between the two is defined to be
10 log (P2/P1) dB where the log is to base 10.
If the second produces twice as much power than the first, the difference in dB is
10 log (P2/P1) = 10 log 2 = 3 dB.
as is shown on the graph, which plots 10 log (P2/P1) against P2/P1. To continue the example, if the second had 10 times the power of the first, the difference in dB would be
10 log (P2/P1) = 10 log 10 = 10 dB.
If the second had a million times the power of the first, the difference in dB would be
10 log (P2/P1) = 10 log 1,000,000 = 60 dB.
Hz
Frequency is measured in hertz
What is frequency?
The number pf waves that pass a point per second
Frequency = cycles / time
Question: If a wave travels 21 cycles over the course of 5 seconds, what is the frequency in hertz?
Micropascals
- We said above that the decibel is a ratio. So, when it is used to give the sound level for a single sound rather than a ratio, a reference level must be chosen.
- For sound pressure level, the reference level (for air) is usually chosen as 20 micropascals (20 μPa), or 0.02 mPa. (This is very low: it is 2 ten billionths of an atmosphere.
- Nevertheless, this is about the limit of sensitivity of the human ear, in its most sensitive range of frequency.
- Usually this sensitivity is only found in rather young people or in people who have not been exposed to loud music or other loud noises.
- Personal music systems with in-ear speakers are capable of very high sound levels in the ear, and are believed by some to be responsible for much of the hearing loss in young adults in developed countries.)
If a fishing boat recieves the echo back 50 ms after sending it. the speed of sound in water is 1500 m/s
the sound took half the time to reach the bottom
therefore!
time = 50/2=25
d=speed x time
what is the depth of the water?
multiple reflection of sound waves in a confined area so that the sound persists after the source is cut off
Resonance:
when one object vibrating at the same natural frequency of a second object forces that second object into vibrational motion.
Acoustics
The interdisciplinary science that deals with the study of all mechanical waves in the three forms of matter; among these vibration, sound, ultrasound and infrasound.
Gases
- Condensations and rarefactions of a sound wave move about when the molecules collide with eachother.
- Lower Temperatures= More molecular movement
- Higher Temperature= Less Molecular Movement
Liquids
Solids
Extra!
Cymatics!
Extra!
Like different musical instruments, different types of stars produce different types of sound waves. Small stars produce a sound with a higher pitch than bigger stars, just like the 'piccolo' produces a higher sound than the cello
Extra
How do we hear?
Extra
Le Beat Applet
Le Wave Applet
Sources
http://library.thinkquest.org/19537/Physics5.html
http://library.thinkquest.org/19537/Physics5.html
The Physics of Sound
What do soundwaves look like?
wave direction
vibration
Note that the molecules themselves do not consistently move in the direction of the wave, but instead oscillate around a fixed point.
http://library.thinkquest.org/19537/Physics5.html
Destructive Interference
Noise Cancellation:
vibration
wave direction
air molecules
long wavelength
short
higher frequency
lower frequency
Quizzzzzzz
What is a wave?
http://www.physicsclassroom.com/class/sound/
http://www.ted.com/talks/evan_grant_cymatics.html
http://www.overgrownpath.com/2009/03/music-and-its-symbols.html
Explain and demonstrate the Doppler effect
http://www.cymatics.co.uk/
http://library.thinkquest.org/19537/Physics2.html
What does a dB measure?
http://www.fossweb.com/modules3-6/PhysicsofSound/index.html
http://www.phys.unsw.edu.au/jw/hearing.html
http://www.silcom.com/~aludwig/Physics/Main/Physics_of_sound.html
What was your favorite thing we learned?
http://www.podcomplex.com/guide/physics.html
http://www.sengpielaudio.com/calculator-soundlevel.htm
http://www.sengpielaudio.com/calculator-distancelaw.htm
http://www.audiocheck.net/
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/soucon.html
Define ultrasonic and infrasonic
Sound Propagation
Sound Measurement
Waves bend when they pass through a boundary
Sound also speeds up when it enters a denser material.
When waves cross a boundary that makes the chnage speed they also change direction, unless they are at right angles to the boundary.
Virtual wombat
sound waves in water
real wombat
Waves bounce off a surface
incident ray
2 laws of reflection:
reflected ray
Echo!!!!!
1. The angle of incidence is equal to the angle of reflection
2. The incident ray, the reflected ray, and the normal all lie in the same place.
Refraction
Reflection
Reverberation
Interference
Diffraction
The variation of wave amplitude that occurs when waves of the same or different frequency come together.
The decibel scale is a reflection of the logarithmic response of the human ear to changes in sound intensity:
http://www.phys.unsw.edu.au/jw/hearing.html
The use of the factor of 10 in the definition of the decibel is to create a unit which is about the least detectable change in sound intensity
That having been established, it can be noted that there are some variations. The jnd is about 1 dB for soft sounds around 30-40 dB at low and midrange freqencies. It may drop to 1/3 to 1/2 a decibel for loud sounds.
Waves bend around obstacles
Waves spread out when they pass through a small gap
1 Hertz = 1 vibration/second
If the size of an object is similar to the wavelength of a wave it bends round it and spreads out as it passes
The natural frequencies of a musical instrument are sometimes referred to as the harmonics of the instrument. An instrument can be forced into vibrating at one of its harmonics (with one of its standing wave patterns) if another interconnected object pushes it with one of those frequencies. This is known as resonance - when one object vibrating at the same natural frequency of a second object forces that second object into vibrational motion.
In a solid the particles are closer together,
Sound travels faster in liquids than in gases because are more tightly packed.
The vibration passes from one particle to the next easily enough to have a sound wave travel through it.
In fresh water, sound waves travel at 1,482 meters per second (about 3,315 mph)
The exact speed of sound in steel is 5,960 meters per second (13,332 mph)! But, this is only for the majority of solids. The speed of sound in all solids are not faster than in all liquids.
At freezing (0º Celcius), sound travels through air at 331 meters per second (about 740 mph). But, at 20ºC, room temperature, sound travels at 343 meters per second (767 mph).
http://www.acoustics.salford.ac.uk/schools/index1.htm