CHAPTER 13 Sound

SECTION 13-1 Sound Waves

- The Production of Sound Waves

- The Production of Sound Waves Sound Waves are longitudinal.

- The Production of Sound Waves Sound waves cause compression and rarefaction of air molecules as they travel through air

- Characteristics of Sound Waves Audible sound waves (that humans hear) range between 20 to 20,000 Hz

-The Human Ear

-Hearing Loss

- Characteristics of Sound Waves Frequency determines pitch (how we perceive the sound to be).

- Characteristics of Sound Waves Frequency determines pitch (how we perceive the sound to be).

- Characteristics of Sound Waves Speed of Sound depends on the medium. Speed also depends on the temperature of the medium: vsound = (331 + 0.6Tc) m/s

- Characteristics of Sound Waves Sound waves propagate in three dimensions.

- Characteristics of Sound Waves Because of sound’s spherical nature, we can examine intensity levels from a point source center

- The Doppler Effect Relative motion creates a change in frequency.

- The Doppler Effect Relative motion creates a change in frequency. fo = f(v+ vo) / v + vs) Highest Sound? Observer and source running toward each other Lowest Sound? Observer and source running away from each other

- 13-1 Important Vocabulary Compression: the region of a longitudinal wave in which the density and pressure are greater than normal. Rarefaction: the region of a longitudinal wave in which the density and pressure are less than normal. Pitch: how high or low we perceive a sound to be, depending on the frequency of the sound wave. Doppler Effect: frequency shift that is the result of relative motion between the source of waves and an observer.

Sound Intensity and Resonance SECTION 13-2 Sound Intensity and Resonance

- Sound Intensity Intensity and frequency determine which sounds are audible.

- Sound Intensity Intensity is the rate of energy flow through a given area. I = ∆E/∆t per unit area J/s/area → Power/area → Watts/m2 For a SPHERICAL WAVE, energy propagates in all directions. The Spherical surface is 4πr2

where r = distance from source - Sound Intensity I = ∆E/∆t per unit area J/s/area → Power/area → Watts/m2 For a SPHERICAL WAVE, energy propagates in all directions. The Spherical surface is 4πr2 Power / 4πr2 where r = distance from source

- Sound Intensity Power / 4πr2 where r = distance from source

- Sound Intensity The softest sound at 1000 Hz = threshold of hearing I = 1.0x10-12 W/m2 The loudest sound the ear can tolerate = threshold of pain I = 1.0x100 W/m2

- Sound Intensity Relative intensity, or decibel level, is measured in decibels. dB = 10 log I/Io I = intensity (W/m2) of sound being heard Io = threshold of hearing for the same frequency

- Sound Intensity – the Decibel Scale

- Sound Intensity – the Decibel Scale

- Forced Vibrations and Resonance A forced vibration at the natural frequency produces resonance.

- 13-2 Important Vocabulary Intensity: rate at which energy flows through a unit area perpendicular to the direction of wave motion. Decibel Level: relative intensity, determined by relating the intensity of a sound wave to the intensity at the threshold of hearing. Resonance: a condition that exists when the frequency of a force applied to a system matches the natural frequency of vibration of the system.

SECTION 13-3 Harmonics

- Standing Waves on a Vibrating String Harmonics are integral multiples of the fundamental frequency.

- Standing Waves on a Vibrating String Harmonics are integral multiples of the fundamental frequency.

- Standing Waves on a Vibrating String Harmonics are integral multiples of the fundamental frequency.

n=1,2,3,… - IMPORTANT EQUATION Fn = n(v/2L) Harmonic Series of Standing Waves on a Vibrating String Fn = n(v/2L) n=1,2,3,… frequency = harmonic # x (speed of waves on string) (2)(length of vibrating string)

- Standing Waves in an Air Column If both ends of a pipe are open, all harmonics are present. If one end of a pipe is closed, only odd harmonics are present. Harmonics account for sound quality, or timbre. Fundamental frequency determines pitch.

- Standing Waves in an Air Column

Fn = n(v/2L) n=1,2,3,… - IMPORTANT EQUATION Harmonic Series of a Pipe open at both ends Fn = n(v/2L) n=1,2,3,… frequency = harmonic # x (speed of waves in pipe) (2)(length of vibrating air column)

- Standing Waves in an Air Column

Fn = n(v/4L) n=1,3,5,… - IMPORTANT EQUATION Harmonic Series of a Pipe closed at one end Fn = n(v/4L) n=1,3,5,… frequency = harmonic # x (speed of waves in pipe) (4)(length of vibrating air column)

- Standing Waves in an Air Column

- Beats Sound waves at slightly different frequencies produce beats. The number of beats per second corresponds to the difference between frequencies.

- Beats

- LET’S PRACTICE Harmonics (Open Pipe) Given: L= 2.45 m, v =345 m/s, f1-3=? Step 1: Choose OPEN equation: Fn = n(v/2L) Step 2: Fn = (1)(345/2(2.45)) = 70.4 Hz (2)(345/2(2.45)) = 141 Hz (3)(345/2(2.45)) = 211 Hz

- LET’S PRACTICE Harmonics (Open Pipe) Given: L= 4 m, v = 777 m/s, f1-3=? Step 1: Choose OPEN equation: Fn = n(v/2L) Step 2: Fn = (1)(777/2(4)) = 97.1 Hz (2)(777/2(4)) = 194 Hz (3)(777/2(4)) = 291 Hz

- 13-3 Important Vocabulary Fundamental Frequency: the lowest frequency of vibration of a standing wave. Harmonic Series: series of frequencies that includes the fundamental frequency and integral multiples of fundamental frequency. Timbre: the quality of a steady musical sound that is the result of a mixture of harmonics present at different intensities. Beat: interference of waves of slightly different frequencies traveling in the same direction, perceived as a variation in loudness.