Transcript Chapter 20: Sound
Vibrations, Waves and Sound
Unit 7: Vibrations, Waves & Sound Chapter 20: Sound
20.1 Properties of Sound
20.2 Sound Waves
20.3 Sound, Perception, and Music
20.1 Investigation: Sound and Hearing Key Question:
What is sound and how do we hear it?
Objectives:
Identify the range of frequencies humans can hear.
Describe the how perception influences the sound humans hear.
Make and analyze a histogram of class data.
The frequency of sound
The
pitch
of a sound is how you hear and interpret its frequency. A low-frequency sound has a low pitch.
A high-frequency sound has a high pitch.
Each person is saying “Hello”.
The frequency of sound
Almost all the sounds you hear contain many frequencies at the same time.
Humans can generally hear frequencies between 20 Hz and 20,000 Hz.
The loudness of sound
The
loudness
decibels
(dB). of a sound is measured in The decibel is a unit used to express relative differences in the loudness of sounds.
The loudness of sound
Most sounds fall between 0 and 100 on the decibel scale, making it a very convenient number to understand and use.
The frequency of sound
Sounds near 2,000 Hz seem louder than sounds of other frequencies, even at the same decibel level.
According to this curve, a 25 dB sound at 1,000 Hz sounds just as loud as an 40 dB sound at 100 Hz.
Decibels and amplitude
The amplitude of a sound increases ten times every 20-decibels.
The speed of sound
The speed of sound in normal air is 343 meters per second (660 mi/hr).
Sound travels through most liquids and solids faster than through air.
Sound travels about five times faster in water, and about 18 times faster in steel.
The speed of sound
Objects that move faster than sound are called
supersonic
.
If you were on the ground watching a supersonic plane fly toward you, there would be silence.
The sound would be
behind
the plane, racing to catch up.
The speed of sound
A
supersonic
jet “squishes” the sound waves so that a cone-shaped
shock wave
forms where the waves “pile up” ahead of the plane.
In front of the shock wave there is total silence.
The speed of sound
Passenger jets are
subsonic
because they travel at speeds from 400 to 500 mi/hr.
The Doppler effect
When the object is moving, the frequency will not be the same to all listeners.
The shift in frequency caused by motion is called the
Doppler effect
.
You hear the Doppler effect when you hear a police or fire siren coming toward you, then going away from you.
Recording sound
Most of the music you listen to has been recorded in stereo.
The slight differences in how sound reaches your ears lets you know where sound is coming from.
Recording sound
To record a sound you must store the pattern of vibrations in a way that can be replayed and be true to the original sound.
1.
A microphone transforms a sound wave into an electrical signal with the same pattern of vibration.
Recording sound
2.
An “analog to digital converter” converts the electrical signal to digital values between 0 and 65,536.
Recording sound
3.
One second of compact-disc-quality sound is a list of 44,100 numbers which represents the amplitudes converted sounds.
Recording sound
4.
To play the sound back, the string of numbers is read by a laser and converted into electrical signals again by a second circuit which reverses the process of the previous circuit.
Recording sound
5.
The playback circuit converts the string of numbers back into an electrical signal.
6.
The electrical signal is amplified to move the coil in a speaker and reproduce the sound.
Unit 7: Vibrations, Waves & Sound Chapter 20: Sound
20.1 Properties of Sound
20.2 Sound Waves
20.3 Sound, Perception, and Music
20.2 Investigation: Properties of Sound Waves Key Question:
Does sound behave like other waves?
Objectives:
Listen to beats and explain how the presence of beats is evidence that sound is a wave.
Create interference of sound waves and explain how the interference is evidence for the wave nature of sound.
What is a sound wave?
Sound waves are
pressure waves
with alternating high and low pressure regions.
When they are pushed by the vibrations, it creates a layer of higher pressure which results in a traveling vibration of pressure.
Pressure and molecules
At the same temperature and volume, higher pressure contains more molecules than lower pressure
.
The speed of sound increases because collisions between atoms increase. Therefore, if the pressure goes down, the speed of sound decreases.
The wavelength of sound
The wavelength of sound in air is similar to the size of everyday objects
.
The wavelength of sound
Wavelength is also important to sound.
Musical instruments use the wavelength of a sound to create different frequencies.
Reverberation
The reflected sound and direct sound from the musicians together create a multiple echo called
reverberation
. The right amount of reverberation makes the sound seem livelier and richer.
Sound wave interactions
Like other waves, sound waves can be reflected by hard surfaces and refracted as they pass from one material to another. Diffraction causes sound waves to spread out through small openings. Carpet and soft materials can absorb sound waves.
Ultrasound
The ultrasound image above is a heart.
Ultrasound is high-frequency sound, often 100,000 Hz or more.
We cannot hear ultrasound, but it passes through the human body easily.
Medical ultrasound instruments use ultrasound waves to create images of the human body’s interior for diagnostic purposes.
Standing waves in pipes
A panpipe makes music as sound resonates in tubes of different lengths.
The natural frequency of a pipe is proportional to its length.
Standing waves in pipes
Because frequency and wavelength are inversely related, longer pipes have lower natural frequencies because they resonate at longer wavelengths. A pipe that must vibrate at a frequency 2 times higher than another pipe must be 1/2 as long.
If the long pipe has a frequency of 528 Hz, what is the frequency of the short pipe?
Standing waves in pipes
Blowing across the open end of a tube creates a standing wave inside the tube. If we blow at just the right angle and we match the natural frequency of the material and the sound
resonates
(spreads).
Standing waves in pipes
The open end of a pipe is an open boundary to a standing wave and makes an
antinode
. The pipe resonates to a certain frequency when its length is one-fourth the wavelength of that frequency.
Wave speed and designing instruments
Sounds of different frequencies are made by standing waves. The length of a vibrating system can be chosen so that it resonates at the frequency you want to hear.
Fourier's theorem
Fourier’s theorem
says any complex wave can be made from a sum of single frequency waves.
Sound spectrum
A complex wave is really a sum of
component frequencies
.
A complex wave can be made from a sum of single frequency waves, each with its own frequency, amplitude, and phase.
Unit 7: Vibrations, Waves & Sound Chapter 20: Sound
20.1 Properties of Sound
20.2 Sound Waves
20.3 Sound, Perception, and Music
20.3 Investigation:
Sound as a Wave
Key Question:
How can we observe sound as a wave? How can we use the speed of sound and certain frequencies to build a basic instrument based on wavelength?
Objectives:
Explain how pitch is related to frequency and wavelength of a sound wave.
Determine the lengths of pipe required to produce sounds at certain frequencies at a specific speed of sound.
Construct a basic instrument from PVC pipe and use it to play musical notes.
Sound perception and music
When you hear a sound, the nerves in your ear respond to more than 15,000 different frequencies at once.
The brain makes sense of complex sound because the ear separates the sound into different frequencies.
Sound spectrum
A
frequency spectrum
is a graph showing the different frequencies present in a sound.
Sound containing many frequencies has a wave form that is jagged and complicated.
Sonograms
1.
2.
3.
More information is found in a
sonogram
which combines three sound variables: frequency, time, and amplitude (loudness).
Sonograms
Which letter represents a soft sound lasting 5 seconds?
What is it’s frequency?
How we hear sound
The parts of the ear work together: 1.
2.
3.
When the eardrum vibrates, three small bones transmit the vibrations to the
cochlea
.
The vibrations make waves inside the cochlea, which vibrates nerves in the spiral.
Each part of the spiral is sensitive to a different frequency.
Sound protection
Listening to loud sounds for a long time causes the hairs on the nerves in the cochlea to weaken or break off resulting in permanent damage.
Music
The
pitch
frequency.
of a sound is how high or low we hear its
Rhythm
sounds.
is a regular time pattern in a series of
Music
is a combination of sound and rhythm that we find pleasant.
The musical scale
Most of the music you listen to is created from a pattern of frequencies called a
musical scale
.
Music and notes
Each frequency in the scale is called a
note
.
The range between any frequency and twice that frequency is called an
octave
.
Music and harmony
Harmony
is the study of how sounds work together to create effects desired by the composer.
The tense, dramatic sound track of a horror movie is a vital part of the audience’s experience. Harmony is based on the frequency relationships of the musical scale.
Beats
When two frequencies of sound are not exactly equal in value, the loudness of the total sound seems to oscillate or
beat
.
Echolocation and beats
Bats navigate and find food using echolocation.
The beat frequency is proportional to how far the insect is from the bat.
Consonance and dissonance
When we hear more than one frequency of sound and the combination sounds pleasant, we call it
consonance
. When the combination sounds unsettling, we call it
dissonance.
Making sounds
The human voice is complex sound that starts in the larynx, at the top of your windpipe.
The sound is changed by passing over by expandable folds (vocal cords) and through openings in the throat and mouth.
Making sounds
For a guitar in standard tuning, the heaviest string has a natural frequency of 82 Hz and the lightest a frequency of 330 Hz.
Tightening a string raises its natural frequency and loosening lowers it.
Harmonics and music
The same note sounds different when played on different instruments.
Suppose you compare the note C (262 Hz) played on a guitar and the same note played on a piano.
A single C note from a grand piano might include 20 or more different harmonics.
Harmonics and instruments
The variation comes from the
harmonics
.
The sound from an instrument is not a single pure frequency.
Deafness is poorly understood in general.
For instance, there is a common misconception that deaf people live in a world of silence.
To understand the nature of deafness, first one has to understand the nature of hearing.
Hearing