Vibrations, Waves and Sound

Vibrations, Waves and Sound

Unit 7: Vibrations, Waves & SoundChapter 20: Sound • 20.1 Properties of Sound • 20.2 Sound Waves • 20.3 Sound, Perception, and Music

Key Question: What is sound and how do we hear it? 20.1 Investigation: Sound and Hearing 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 loudnessof a sound is measured in decibels (dB). • The decibelis 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 waveforms 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 subsonicbecause they travel at speeds from 400 to 500 mi/hr.

The Doppler effect • When the object is moving, the frequency will notbe 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. • A microphone transforms a sound wave into an electrical signal with the same pattern of vibration.

Recording sound • An “analog to digital converter” converts the electrical signal to digital values between 0 and 65,536.

Recording sound • One second of compact-disc-quality sound is a list of 44,100 numbers which represents the amplitudes converted sounds.

Recording sound • 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 • The playback circuit converts the string of numbers back into an electrical signal. • The electrical signal is amplified to move the coil in a speaker and reproduce the sound.

Key Question: Does sound behave like other waves? 20.2 Investigation: Properties of Sound 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 waveswith 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 • 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. The ultrasound image above is a heart.

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.

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? 20.3 Investigation: Sound as a Wave 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 • More information is found in a sonogramwhich 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: • 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.

Vibrations, Waves and Sound