# 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

# 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.).

# 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

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

Simpsons Project

# HomeWork

Finish Simpsons Project
Read Sound Design for Interactive Multimedia chapter 1 pg 3 - 31