MTD2 class 2
Contents
Week Two
In Class
- More on the Properties of Sound
- Wave
- Speed
- Frequency/Period
- Amplitude
- Wavelength
- Basic Sound forge Editing Simpsons Project
- Start Simpsons http://iam.colum.edu/MTD2/assignments/class2/editing1/editing1.html
Properties of Sound
- Sound Is a Longitudinal wave
- Speed of sound (measured in m/s)
- Frequency (measured in Hz)
- Period (measured in seconds per cycle)
- Wavelength (measured in meters)
- Amplitude/Pressure (for this class we will only look at dBs)
- Phase (measured in degrees)
Wave
Sound Is a Wave (it needs a medium) Transverse vs. Longitudinal Waves medium http://dictionary.reference.com/search?q=medium
- An intervening substance through which something else is transmitted or carried on.
- move in the same direction as the wave between 0° and 180° to the direction of the wave.
A sound wave is a longitudinal wave
- move at right angles to the direction of the wave.
Water is a transverse wave
Transverse Wave and Longitudinal Waves
http://www.phy.ntnu.edu.tw/java/waveType/waveType.html
Neither the molecules in the air or those in water actually move along with the wave. Air and water are simply a medium for the wave to pass through.
Speed of Sound
The Speed of sound is measured in meters per second.The speed of sound is affected by several factors: temperature, humidity, and elasticity.
SOS ~ 344 m/s or 1130 f/s
NOTE: for class we will assume the speed of sound in air @ 20 degrees Celsius an 75% humidity is 344m/s
Speed of sound Calculator http://www.measure.demon.co.uk/Acoustics_Software/speed.html
Frequency/Period
Sound is a periodic event. This means that there is a fixed amount of time between different events.The Frequency component of a sound wave affects the pitch of the sound.
Frequency is measured in Hertz (cycles per second)
Period = 1 / Frequency in HzT = 1/f
Frequency = 1/ Period f = 1/T
Different types of frequency components periodic/ aperiodic
It is assumed the humans can hear frequencies from 20Hz to 20,000 Hz.
20-20KHz Sweep
- http://iam.colum.edu/sound/media/sweep20-20.wav 41kb,16 bit 3.7megs
Resonance
Tacoma Narrows Bridge
Movie1 http://interactive.colum.edu/sound/media/tacoma3.avi
Movie2 http://interactive.colum.edu/sound/media/tacoma4.avi
Wavelength
Wavelength is measured in distance (ft. of m)
The actual physical length of a single cycle of a wave
Wavelength = speed of sound / Frequency in Hz
- λ = SOS/f
wavelength = speed of sound * period
- λ = SOS*T
The wavelength of a sound wave varies greatly with the frequency of the wave. A 20 kHz wave has a wavelength of 1.72 (0.7 in)cm while a 20 Hz wave has a wavelength of 17.2 m (~56 ft.).
Examples http://iam.colum.edu/MTD2/lectures/week1_examples.htm
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Amplitude/Pressure
Measured in dB Decibels
The softest sound we as humans can hear is about 20 micro pascals (20µPa or 0 dB re 20µPa ) the loudest sound we as humans can tolerate is about 200 Pa(120 dB re20µPa) this is a 10,000,000:1 ratio. This is why Bel labs decided to use a different scale to represent the numbers. They used the deciBel or 1/10 of a Bel.
decibels as a power ratio The Bel was defined as the logarithm of a power ratio and was named after Alexander Graham Bell. The deciBel is 1/10th of a Bel. Since the range of human hearing is so large a logarithmic scale was implemented.
Sound Pressure and common sound sources
| 140 dB SPL | .45 APC Colt pistol (25 ft.) |
| 130 dB SPL | 50 HP Siren (100 ft.) |
| 120 dB SPL | Threshold of pain |
| 110 dB SPL | Typical rock concert |
| 90-100 dB SPL | Platform of subway station as train arrives. Loud classical music |
| 80 dB SPL | Person Shouting |
| 60 dB SPL | Average conversation (5 ft) |
| 40 - 50 dB SPL | Average suburban home at night |
| 30 dB SPL | Very quiet whisper |
| 20 dB SPL | Extremely quiet Recording Studio |
| 0 dB SPL | Threshold of hearing. Anechoic Chamber |
Decibels
Decibels are based on a logarithmic scale.
dB SPL/voltage/current
dB = 10 * log (P1^2 + P2^2) = dB = 20 * log (P1 + P2)
1dB small change in power
6dB doubling of power
20 dB roughly twice as loud ten times the voltage
| Voltage, current or SPL | dB |
| 1 | 0.00 |
| 1.5 | 3.52 |
| 2 | 6.02 |
| 2.5 | 7.96 |
| 3 | 9.54 |
| 4 | 12.04 |
| 5 | 13.98 |
| 6 | 15.56 |
| 7 | 16.90 |
| 9 | 18.06 |
| 10 | 19.08 |
| 20 | 20 |
| 30 | 29.54 |
| 40 | 32.04 |
| 50 | 33.98 |
| 100 | 40 |
| 1000 | 60 |
| 5000 | 73.97 |
| 10000 | 80 |
here are some other good explanations
http://ews64.com/mcdecibels.html
Here is a good dB adding calculator
http://www.mcsquared.com/dbframe.htm this site can also do some other neat o thangs...
Intro To Sound Forge
Review interface of soundforge and look at
sound forge manual (it's in //iam.local/classfolders/MTD2) or available @ http://www.sonymediasoftware.com/download/step2.asp?DID=565
- chapter 3 - getting started
- chapter 4 - navigation zooming and selecting
Basic Sound forge Editing
Simpsons Project
Simple Editing Tutorial for Simpsons Project
HomeWork
Finish Simpsons Project
Read Sound Design for Interactive Multimedia chapter 1 pg 3 - 31
