Chapter TwentyFour Sound 24 1 Properties of Sound

Chapter Twenty-Four: Sound Ø 24. 1 Properties of Sound Ø 24. 2 Sound Waves Ø 24. 3 Sound Perception and Music

24. 1 The frequency of sound ØThe pitch of a sound is how you hear and interpret its frequency. ØA low-frequency sound has a low pitch. ØA high-frequency sound has a high pitch. Each person is saying “Hello”.

24. 1 The frequency of sound ØAlmost all the sounds you hear contain many frequencies at the same time. ØHumans can generally hear frequencies between 20 Hz and 20, 000 Hz.

24. 1 The loudness of sound ØThe loudness of a sound is measured in decibels (d. B).

24. 1 The frequency of sound ØSounds near 2, 000 Hz seem louder than sounds of other frequencies, even at the same decibel level. ØAccording to this curve, a 25 d. B sound at 1, 000 Hz sounds just as loud as an 40 d. B sound at 100 Hz.

24. 1 The speed of sound ØThe speed of sound in normal air is 343 meters per second (660 miles per hour). ØSound travels through most liquids and solids faster than through air. ØSound travels about five times faster in water, and about 18 times faster in steel.

24. 1 The speed of sound ØObjects that move faster than sound are called supersonic. ØIf you were on the ground watching a supersonic plane fly toward you, there would be silence. ØThe sound would be behind the plane, racing to catch up.

24. 1 The speed of sound ØPassenger jets are subsonic because they travel at speeds from 400 to 500 mi/hr.

24. 1 The Doppler effect ØWhen the object is moving, the frequency will not be the same to all listeners. ØThe shift in frequency caused by motion is called the Doppler effect. ØYou hear the Doppler effect when you hear a police or fire siren coming toward you, then going away from you.

24. 1 Recording sound 3. One second of compact-disc-quality sound is a list of 44, 100 numbers which represents the amplitudes converted sounds.

24. 1 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.

24. 1 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.

24. 2 What is a sound wave? ØSound waves are pressure waves with alternating high and low pressure regions. ØWhen they are pushed by the vibrations, it creates a layer of higher pressure which results in a traveling vibration of pressure.

24. 2 What is a sound wave? ØAt the same temperature and volume, higher pressure contains more molecules than lower pressure.

24. 2 The wavelength of sound ØThe wavelength of sound in air is similar to the size of everyday objects.

24. 2 The wavelength of sound ØWavelength is also important to sound. ØMusical instruments use the wavelength of a sound to create different frequencies.

24. 2 Standing waves ØA wave that is confined in a space is called a standing wave. ØA string with a standing wave is a kind of oscillator.

24. 2 Standing waves ØThe lowest natural frequency is called the fundamental. ØA vibrating string also has other natural frequencies called harmonics.

24. 2 Standing waves ØThe place on a harmonic with the greatest amplitude is the antinode. ØThe place where the string does not move (least amplitude) is called a node.

24. 2 Standing waves Ø It is easy to measure the wavelength of a standing wave on a string. ØTwo harmonics equals one wave!

24. 2 Standing waves in pipes ØA panpipe makes music as sound resonates in tubes of different lengths. ØThe natural frequency of a pipe is proportional to its length.

24. 2 Standing waves in pipes ØBecause frequency and wavelength are inversely related, longer pipes have lower natural frequencies because they resonate at longer wavelengths. ØA 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?

24. 2 Standing waves in pipes ØBlowing across the open end of a tube creates a standing wave inside the tube. ØIf we blow at just the right angle and we match the natural frequency of the material and the sound resonates (spreads).

24. 2 Standing waves in pipes ØThe open end of a pipe is an open boundary to a standing wave and makes an antinode. ØThe pipe resonates to a certain frequency when its length is one-fourth the wavelength of that frequency.

24. 2 Sound wave interactions ØLike other waves, sound waves can be reflected by hard surfaces and refracted as they pass from one material to another. ØDiffraction causes sound waves to spread out through small openings. ØCarpet and soft materials can absorb sound waves.

24. 2 Reverberation ØThe reflected sound and direct sound from the musicians together create a multiple echo called reverberation. ØThe right amount of reverberation makes the sound seem livelier and richer.

24. 3 Sound perception and music ØWhen you hear a sound, the nerves in your ear respond to more than 15, 000 different frequencies at once. ØThe brain makes sense of complex sound because the ear separates the sound into different frequencies.

24. 3 Sound perception and music ØA frequency spectrum shows the amplitudes of different frequencies present in a sound.

24. 3 Sonograms Ø More information is found in a sonogram which combines three sound variables: 1. frequency, 2. time, and 3. amplitude (loudness).

24. 3 Sonograms Which letter represents a soft sound lasting 5 seconds? What is it’s frequency?

24. 3 How we hear sound Ø 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.

24. 3 Sound protection ØListening 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.

24. 3 Music ØThe pitch of a sound is how high or low we hear its frequency. ØRhythm is a regular time pattern in a series of sounds. ØMusic is a combination of sound and rhythm that we find pleasant.

24. 3 The musical scale ØMost of the music you listen to is created from a pattern of frequencies called a musical scale.

24. 3 Music and notes ØEach frequency in the scale is called a note. ØThe C major musical scale that starts on the note C (262 Hz).

24. 3 Music and harmony ØHarmony is the study of how sounds work together to create effects desired by the composer. ØThe tense, dramatic sound track of a horror movie is a vital part of the audience’s experience. ØHarmony is based on the frequency relationships of the musical scale.

24. 3 Superposition ØThe superposition principle states that when sound waves occur at the same time they combine to make a complex wave. ØWhen two frequencies of sound are not exactly equal in value, the loudness of the total sound seems to oscillate or beat.

24. 3 Music and harmony ØWhen we hear more than one frequency of sound and the combination sounds pleasant, we call it consonance. ØWhen the combination sounds unsettling, we call it dissonance.

24. 3 Making sounds ØThe human voice is complex sound that starts in the larynx, at the top of your windpipe. ØThe sound is changed by passing over by expandable folds (vocal cords) and through openings in the throat and mouth.

24. 3 Making sounds ØFor a guitar in standard tuning, the heaviest string has a natural frequency of 82 Hz and the lightest a frequency of 330 Hz. ØTightening a string raises its natural frequency and loosening lowers it.

24. 3 Harmonics and music ØThe same note sounds different when played on different instruments. ØSuppose you compare the note C (262 Hz) played on a guitar and the same note played on a piano. ØThe variation comes from the harmonics in complex sound. ØA single C note from a grand piano might include 20 or more different harmonics.

24. 3 Harmonics and music ØA tuning fork is a useful tool for tuning an instrument because it produces a single frequency