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)
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
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.
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 are based on a logarithmic scale.
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|
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...
Adding Sound Waves Together
- 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.
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.
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.
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.
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