1. Sound waves How they are generated and travel
Physics of Sound Part 1
Sound waves
How they are generated and travel

2. Sound Waves Generation and Propagation
Sound wave = changes in pressure caused by vibrating object
Compression = High pressure
Rarefaction = Low pressure
Sound needs a medium to “vibrate”
Usually air, but could be anything
Speed of sound depends upon the medium
Air = 1130 ft/sec
Water = 5000 ft/sec
Steel = ft/sec

3. Measuring sound waves Sound waves are longitudinal waves
Vibrating object compresses the air around it.
Pushes air away leaving an area of low pressure
Vibrating object then compresses more air to create a “chain”

4. Measuring methods Cycle A single push and pull of the vibrating object
One are of compression followed by one area of rarefaction
An initial increase in atmospheric pressure from the norm, followed by a drop below the norm and then a return to normal
Mathematically displayed by a sine curve
Pressure on Y axis
Time on X axis

5. Measuring methods Period (T) and Frequency (f)
Period - The time it takes to create one cycle
Frequency - The number of cycles in one second
Measured in Hertz (Hz) or cycles per second

6. Measuring methods Example
It takes ¼ sec to create one cycle. What is the sound wave’s frequency?

7. Measuring methods Frequency will determine pitch
High frequency = high pitch
Low frequency = low pitch
Octave – a doubling of halving of the frequency

8. Measuring methods Human hearing range Low range between 15 to 30 Hz
With enough power lower than 15 Hz can be felt, buy not heard as “sound”
High range varies with age and gender
Women - up to 20 kHz
Men – between 15 to 18 kHz
High frequency range will lower with exposure to high levels of sound and age

9. Tuning Traditional orchestra would tune First Chair Violin A first.
Remaining instruments would tune relative to that
A above middle C was tuned to about 420 Hz
As halls grew larger it was found to be desirable to tune sharper
1939 A was established to be 440 Hz
Corresponds to the 49th key on a full size piano
Tuning is not a science. The relative frequency difference is what is important

10. Measuring methods Wavelength
The distance from one area of compression to the next or one area of rarefaction to the next
l=wave length
V = velocity of sound in medium
usually 1130 ft/sec
f = frequency

11. Measuring methods Amplitude
How high the pressure goes above and below normal atmospheric pressure
Corresponds to how loud the sound is
“loudness” is relative to frequency and dependant on the listener.

12. Timber and Harmonics Harmonics – multiples of a base frequency
Timber – the characteristics of a particular sound or instrument
Different harmonics combined in different levels

13. Basic Acoustics Inverse square law Reinforcement/cancellation
Physics of Sound Part 2
Basic Acoustics
Inverse square law
Reinforcement/cancellation

15. Interference
Phase
measurement of where the amplitude of a wave is relative to another wave
A cycle can start at any point in a waveform
Two waves with the same frequency can start at different times
Measured as an angle in degrees
Related to the sine wave representation of the wave

16. Interference Constructive of destructive interference
Waveforms will add by summing their signed amplitude at each instant in time

17. Beats Happens when two slightly different frequencies interfere
Often used in tuning

18. Standing waves
When sound waves bounce off of obstructions, they can interfere with themselves
Tends to reinforce some frequencies and attenuate others
Prevented by using
Non- Parallel walls, ceilings
Convex surfaces
Multi-level ceiling sections

19. Reverberance (Reverb)
Consisting of multiple, blended sound images caused by reflections from walls, ceilings and other structures which do not absorb sound
NOT echo
Echo consists of individual, non-blended sound images
Reverb time is related to
The time it takes for a sound to reduce to an inaudible level
Loudness of sound relative to background noise
Ratio of loudness of reverberant to direct sound
Short reverb time (less than 1.5 sec) is better for speech or drama
Long reverb time (more than 1.5 sec.) is better for music

20. Absorption Controlling reflections can reduce or increase reverb time
Air tends to absorb frequencies above 2K Hz
Sight line obstructions
Frequencies above 10 kHz tend to not bend around corners well or other obstructions
l=1.3 inches for 10 kHz tone
Frequencies below 1kHz do very well
l=5.65 feet for 200 Hz tone
Specialists are often hired to “tune” a space acoustically

21. Acoustic attributes
Defined by Leo Beranek after a 6 year study of 54 concert halls
Used to define acoustic properties in terms that other trained professionals can understand

22. Acoustic attributes Intimacy – Indicates the size of a room
How it sounds to the listener, not actual size
Determined by the initial-time-delay-gap (ITDG)
Interval between the sound that arrives directly at the ear and the first reflection
Usually considered to be the most important attribute

23. Acoustic attributes Liveness Warmth Related to Reverberance
Room size is related
More reflections is live. Less reflections is dry or dead
Warmth
More low frequency sound relative to mid frequency
Too much low frequency sound is said to be “Boomy”

24. Acoustic attributes Loudness of direct sound Definition or Clarity
Inverse square law
Loudness of sound will decrease by one quarter every time the distance from the source is doubled
Definition or Clarity
Good definition when sound is clear.
Related to intimacy, liveness, loudness of direct and reverberant sound

25. Acoustic attributes Brilliance Diffusion
A hall that has liveness, clarity and intimacy
Diffusion
Relates to the orientation of reverberant sound
Where is the reflected sound coming from
It is preferable to have reverb sound coming from all directions

26. Intensity
Like pitch, loudness is a sensation in the consciousness of a listener
To produce a sound twice as loud requires 10 times the power
Inverse square law
Sound level is reduced by a factor of the square of the distance away from the source
If you move double the distance from the source, the sound intensity will by one quarter

27. Intensity Intensity is a measurable quantity
SPL – Sound Pressure Level
dB – deciBel
A system of measuring a ratio between two powers
dB (spl)=20log(P1/p0)
p0 is a reference level, usually taken to be the minimum intensity audible to an average person – “Threshold of Hearing”
3 dB change is hardly noticed as a change
6 dB changes is heard as twice as loud

28. Thunderclap, Air Raid Siren 1 Meter 120 dB Jet takeoff (200 ft) 110 dB
dB SPL
Sound
150 dB
Jet engine at 1m
140 dB
Rock and Roll stack at 1m
130 dB
Thunderclap, Air Raid Siren 1 Meter
120 dB
Jet takeoff (200 ft)
110 dB
Rock Concert
100 dB
Train passing up close
90 dB
Heavy traffic
80 dB
Hair Dryer
70 dB
City street
60 dB
Noisy bar or restaurant
50 dB
Open plan office environment
40 dB
Normal conversation level
30 dB
Library, Soft Whisper (5 Meter)
20 dB
Quiet domestic environment
10 dB
Broadcasting Studio, Rustling Leaves
0 dB
Threshold of hearing in young adult

29. Sound Envelope Listener does not hear individual cycles of sound waves
Attack – Time it takes for sound to rise from nothing to its greatest intensity. Usually short.
Decay – Time it takes for a sound to fall from its attack level to its sustaining level. Decay time is usually short
Sustain – The time during which the initial vibrating source continues to supply energy to the sound. Usually perceived as the duration and intensity of the sound
Release – Time it takes for the sound to drop from its sustain level to inaudibility after vibrating object stops supplying energy

30. For Next Class Read Basic Electricity, Kai’s Sound Handbook
P , Leonard