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

2. 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

3. 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

4. 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

5. 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

6. 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.

7. 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

8. 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 = ?