Survey of Sound class 2

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.

Longitudinal Waves

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

Transverse Waves

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.

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 http://www.bell-labs.com/org/1133/Research/Acoustics/AnechoicChamber.html

Ear Wikipedia

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...

Adding Sound Waves Together

Demo

• Add simple sine wave and observer what happens
  • invert the phase of one wave and add it again

Sine wave with the same frequency will add together and the amplitude will increase by 6dB. If the are inverted the will cancel each other out. If they are out of phase the will add or subtract relative to the degree of the phase shift.

Be careful when you are adding waves together. Since the amplitude. YOu never want the amplitude to increase above 0dB. This is cause clipping.

RMS

root mean square Abbr. rms, RMS Mathematics. The square root of the average of the squares of a group of numbers. A useful and more meaningful way of averaging a group of numbers.

from http://www.rane.com/par-r.html

The RMS averaging method is a better method for determining the amplitude of sound. Dynamic Range Definition @http://www.rane.com/par-d.html

The dynamic range of an audio system or and audio performance is the difference between the peak noise level and the noise floor.

Dynamic Range

Describes the difference between the Maximum and Minimum Values. in an Audio system it the the measure if the quitetest sound tha the system cam make (Noise Floor) a the loudest if can get(Peak Level).

Dynamic range = (Peak Level - Noise Floor)

in Digital audio it is determined by the number of bits.

Head Room

Definition @http://www.rane.com/par-h.html

The head room of an audio system is is the difference between the nominal level and the Peak level (or clipping point) Frequency Response Definition @http://www.rane.com/par-f.html

The range in frequency that an audio system or program contains or can pass between certain deviation.

HomeWork

Finish Simpsons Project and post a link on your webpage

Simple Editing Tutorial for Simpsons Project