Chapter 26 Sound & Music

Sound …...a longitudinal wave in air caused by a vibrating object

Origin of Sound Infrasonic frequencies < 20 Hz Ultrasonic frequencies > 20,000 Hz Human hearing range frequencies between 20 Hz and 20,000 Hz

Audible Frequencies Sound waves with frequencies between 20 Hz and 20 kHz 200 Hz 500 Hz 1000 Hz 2000 Hz 5000 Hz wiki Sounds Ultrasound

Nature of Sound in Air Sound requires a medium. solid, liquid or gas Sound waves have compression and rarefaction regions.

Speed of Sound in Air 331 meters/second 760 miles/hour Mach 1

What is the approximate distance of a thunderstorm when you note a 3 second delay between the flash of the lightning and the sound of the thunder? Answer: 3 seconds  331 m/s = 993 meters

Acoustics Acoustics......the study of sound properties. When a sound wave strikes a surface it can be.… (a) reflected. (b) transmitted. (c) absorbed. (d) all of these. 

Reflection of Sound e.g. an echo Reverberation - re-echoed sound, multiple reflections of sound waves from walls Compare reflections from a hard wall with that from a carpet wall.

Refraction of Sound Refraction - the bending of a wave Sound waves bend toward cooler air.

Forced Vibrations... …the setting up of vibrations in an object by a vibrating force. Examples of Forced Vibration: A tuning fork touching a wood surface Sounding boards for stringed instruments Matching tuning fork boxes

During forced vibration sound is intensified because a larger surface area is available to vibrate air molecules.

Natural Frequency... …the frequency at which an elastic object naturally tends to vibrate. At this frequency, a minimum energy is required to produce a forced vibration. The natural frequency of a body depends on its elasticity and its shape.

Natural Frequency Examples Dropping Aluminum Rods Ringing Small and Large Bells Xylophone Rubbing a Wine Glass Mass on a Spring

Resonance Resonance... …is the result of forced vibrations in a body when the applied frequency matches the natural frequency of the body. The resulting vibration has a high amplitude and can destroy the body that is vibrating.

Examples of Resonance Breaking a wine glass using sound Mass on a spring at resonance A singing rod caused by forced vibration A tuning fork exciting a guitar string In 1940, the Tacoma Narrows Bridge was destroyed by wind-generated resonance.

Resonance allows energy to be transferred to a vibrating object efficiently if the energy is delivered at the natural frequency of vibration.

Sound Interference Overlapping crests of a wave will result in an increased amplitude. Overlapping a crest and a trough results in a decrease in amplitude.

Beats - the periodic variation in loudness of two sounds played together The beat frequency is equal to the difference in the frequency of the two sounds. What is the beat frequency when a 262 Hz and a 266 Hz tuning fork are sounded together? 400 Hz and 403 Hz 400 Hz and 410 Hz

Radio Broadcasts AM - Amplitude Modulation 535 kHz to 1605 kHz FM - Frequency Modulation 88 MHz to 108 MHz Modulation - an impression of the sound wave on a higher frequency radio wave

Noise Versus Music What is the difference between noise and music?

Pitch... … is the "highness" or "lowness" of a tone. Pitch corresponds to frequency. Concert A on the Musical Scale has a frequency of 440 Hertz.

Major Scale Letter Frequency Frequency Note Name (Hz) ratio Interval do C 264 9/8 Whole re D /9 Whole mi E /15 Half fa F 352 9/8 Whole sol G /9 Whole la A 440 9/8 Whole ti B /15 Half do C 528

Sound Intensity and Loudness Intensity of Sound refers to the amplitude of the pressure variations in the sound wave

Loudness The physiological sensation directly related to the sound intensity Measured in bels (10 bels = 1 decibels)

Loudness A sound of 10 decibels is or 10 1 or 10 times as intense as 0 decibels. 20 decibels is 10 2 or 100 times the intensity 0 decibels. How much more intense is sound at 100 dB than sound at 50 dB? Answer: times

Source of SoundLoudness (db) Threshold of Hearing0 Conversation60 Ear Damage Begins85 Amplified Music110 Jet Airplane at 30 meters140

Common Sound Intensities

Quality... …is the characteristic sound that allows us to distinguish between two musical instruments. Partial Tones - one of the many frequencies present in a complex tone

Fundamental Frequency the lowest frequency of vibration a.k.a. the first harmonic Harmonic a partial tone that is an integer multiple of the fundamental frequency

Same Note - Different Instrument

Harmonics Harmonics on a Guitar String Harmonics in an Organ Pipe Open on one end, close on the other Open on both ends

Fourier Analysis... …is a mathematical method that will resolve any periodic wave form into a series of simple sine waves.

COMPACT DISCS  Phonograph players give analog signals.  Digital signal is in binary code.  CD has flats and pits and is sampled 44,100 times per second.

1. The quality of a musical note can be changed by simply playing the note on a different instrument. (a) True (b) False

2. The three characteristics of a musical tone are loudness, quality, and timbre. (a) True (b) False

3. Frequencies of sound that are too high for the human ear to hear are called... (a) faster than the speed of sound (b) supersonic (c) infrasonic (d) ultrasonic (e) subsonic (d) ultrasonic

4. For the same temperature for air, does sound travel faster in humid Galveston or in dry El Paso? (a) Galveston (b) El Paso (c) same speed in either city (a) Galveston

5. The bending of sound through air of uneven temperature is called (a) reflection (b) refraction (c) interference (d) reverberation (e) resonance (b) refraction

6. Lightning is seen, then ten seconds later thunder is heard. Approximately, how far away in meters is the thunder cloud? (a) 10,500 m (b) 5280 m (c) 1050 m (d) 3400 m (e) 680 m (d) 3400 m

7. A 250 Hz tuning fork and a 260 Hz tuning fork are vibrating near each other. How many beats per second are heard? (a) 255 (b) 510 (c) 10 (d) 250 (e) 260 (c) 10