Physics of Sound

Physics of Sound

• The vibration of a source causes pressure changes in air which result in pressure waves
• Perceived sound is comprised of numerous pressure waves of varying characteristics
• Pressure wave characteristics
• Amplitude—The amount of sound pressure measured in decibels (dB)
• Frequency—The rate of vibration per unit time measured in cycles per second, more commonly known as hertz (Hz); 
• range of normal perception for young person is 20–20,000 Hz

Octave Bands

• Quantifies effective frequencies without looking at each frequency one at a time
• Standardized notation used to characterize the frequency dependence of noise
• Characterized by center frequency
• Covering range of human hearing(20–20,000 Hz)
• fc= (f1f2)1/2–Where: fcis center frequency and f1and f2are lower and upper band edges respectively
•                                               Hz                                      

                     31.5, 62, 125, 250, 500, 1K, 2K, 4K, 8K, 16K



Sound Pressure
Pressure is fundamental to acoustics

-Definition

Pressure = force per unit of area
-Units
  Newtons per square meter (N/m2)—Called a Pascal(modern unit)
  Dynes per square centimeter (D/cm2)—Not commonly used



Human hearing covers a wide range of sound pressures
–Threshold of hearing: 0.00002 Pa
–Loud noise: 200 Pa



Decibel (dB) scaleis a log-based scale developed to quantify sound
–Compresses range to 0–140 dB
–Scale starts at zero when sound pressure equals the threshold of human hearing



Sound Pressure Level (SPL) and Sound Pressure (Pa)

Decibel scale–
Reference energy is the threshold of human hearing
10 * Bel = decibel (dB)
Sound pressure level (SPL)

for more calculative forms...mail me.i will sent you important pdf..