 # Chapter 10: Sound Section 1: The Nature of Sound

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Chapter 10: Sound Section 1: The Nature of Sound
Section 2: Properties of Sound Section 3: Music Section 4: Using Sound

Section 1: The Nature of Sound
Sound waves All sound waves are created by something that vibrates A stereo speaker: The vibrating diaphragm of the stereo speaker collides with nearby air molecules, transferring some energy to them These molecule then collide with other molecules and pass the energy to them The process of molecular collision and energy transfer form a sound wave A sound wave is a compressional wave Requires a medium through which to travel Sound can travel through solids, liquids, or gases The speed of sound is different in different media Solids  fast, liquids  slower, gases  slowest The speed of sound in air varies with temperature At 0oC, Vsound = m/s As the temperature increases, speed increases As temperature falls below 0oC, speed decreases

Section 1: The Nature of Sound
The equation for speed of sound: Example 1: If the temperature is 15oC, what is the speed of a sound wave? Solution Where: V = speed of sound (m/s) T = change in temperature (from 0oC to actual temperature) use “+” if the temp. is greater than 0oC use “-“ if the temp. is less than 0oC T = 15oC V = ? ∆T= 15 o C − 0 o C ∆T= 15 o C 𝑉=331.5 𝑚 𝑠 ±0.6 𝑚 𝑠 𝑜 𝐶 ∆𝑇 𝑉=331.5 𝑚 𝑠 +0.6 𝑚 𝑠 𝑜 𝐶 ( 15 𝑜 𝐶) 𝑉=331.5 𝑚 𝑠 +9.0 𝑚 𝑠 𝑽=𝟑𝟒𝟎.𝟓 𝒎 𝒔

Section 1: The Nature of Sound
Example 2: What is the speed of sound when the temperature is –10oC? Solution Example 3: The temperature is 10oC, how far will a sound travel in 10.0-s? 𝑇=− 10 0 𝐶 𝑉= ? ∆𝑇= 0 𝑜 𝐶 −(− 10 𝑜 𝐶) ∆𝑇=10℃ 𝑉=331.5 𝑚 𝑠 ±0.6 𝑚 𝑠 𝑜 𝐶 ∆𝑇 𝑉=331.5 𝑚 𝑠 −0.6 𝑚 𝑠℃ 10℃ 𝑉=331.5 𝑚 𝑠 −6.0 𝑚 𝑠 𝑽=𝟑𝟐𝟓.𝟓 𝒎 𝒔 𝑇=10℃ 𝑡=10.0𝑠 𝑑= ? ∆𝑇=10℃−0℃ ∆𝑇=10℃ 𝑉=331.5 𝑚 𝑠 ±0.6 𝑚 𝑠 𝑜 𝐶 ∆𝑇 𝑉=331.5 𝑚 𝑠 +0.6 𝑚 𝑠℃ 10℃ 𝑉=331.5 𝑚 𝑠 +6.0 𝑚 𝑠 𝑉=337.5 𝑚 𝑠 𝑉= 𝑑 𝑡 𝑡𝑉= 𝑑 𝑡 𝑡 𝑑=𝑡𝑉 𝑑=10.0𝑠(337.5 𝑚 𝑠 ) 𝒅=𝟑,𝟑𝟕𝟓.𝟎𝒎

Section 2: Properties of Sound
Intensity and Loudness The amount of energy carried by a wave corresponds to the wave’s amplitude In a compressional wave, amplitude is related to the density of the particles in the compressions and rarefactions Low energy = low density High energy = high density Intensity – the amount of energy that flows through a certain area in a specific amount of time Intensity influences how far away a sound can be heard Intensity influences how far a wave will travel because some of the wave’s energy is converted to other forms of energy when it is passed from particle to particle Loudness is the human perception of sound intensity The scale for measuring intensity (or loudness) of a sound is the decibel scale Pitch and Frequency Pitch – how high or low a sound seems to be Pitch and wave frequency are related The higher the frequency of a sound wave, the higher the pitch The human hearing range: 20-Hz to 20,000-Hz Ultrasonic waves –waves with frequencies greater than 20,000Hz Infrasonic waves –waves with frequencies less than 20-Hz

Section 2: Properties of Sound
The Doppler Effect Doppler Effect – the change in pitch or frequency due to a moving sound source or a moving observer Example: a police car siren: The siren from a stationary police car will sound the same regardless of where you are in relation to the car. When the car is in motion, a person standing in front on the car will hear the siren at a frequency higher than it actually is while a person behind the car will hear the siren at a frequency lower than it actually is. This occurs because the sound waves are compressed in front of the car (higher frequency) , and stretched behind the car (lower frequency) That same compression and stretching occur if an observer is moving in relation to a stationary source: moving towards the source seems to compress the sound waves while moving away from the source seems to stretch the sound waves.

Section 2: Properties of Sound
The Doppler Effect (continued) There are four (4) equations used to describe the Doppler Effect: Source moving toward observer: Source moving away from observer: Observer moving toward source: Observer moving away from source: Two questions must be asked and answered before you can solve a Doppler Effect problem. The questions: What is moving, the sound source or the observer? If the source is moving use Eqs. 1 or 2. If the observer is moving use Eqs. 3 or 4. What is the direction of motion, toward or away from the observer or source? If the direction of motion is toward the observer or source use Eqs. 1 or 3. If the direction is away from observer or source use Eqs. 2 or 4 Where: fo = frequency observer hears fs = frequency of sound source V = speed of sound Vs = speed of source Vo = speed of observer

Section 2: Properties of Sound
Example: Beth is standing on the corner of Main and Jackson when she hears the sound of an ambulance approaching her at 35 m/s. The temperature is 25oC, and ambulance siren sounds a steady 900 HZ. At what frequency does Beth hear the siren? Solution Solution Steps: List the variables you know and the variable you are solving for Solve the equation for the speed of sound at that temperature (V) Ask and answer the two questions (What is moving? Direction of motion?) to determine which Doppler Effect equation to use. In the example, the source is moving toward the observer (use Equation 1) Solve the problem showing all work and dimensional analysis. Vs = 35.0m/s T = 25oC fs = Hz fo = ?

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