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

3. 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

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

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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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”

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

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