# 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.

# 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

### 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...

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.

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.