Acoustics Fundamentals Quiz

Acoustics and Sound Measurement

• Acoustics is the science of sound, a branch of physics
• Basic physical quantities are mass, time, and length
• MKS (meter-kilogram-second) system is internationally accepted
• Scalars have magnitude only, vectors have magnitude and direction
• Velocity (v) = displacement (x) / time (t)
• Acceleration (a) = change in velocity (v) / time (t)
• Force (F) = mass (M) × acceleration (a)
• 1 newton (N) = 1 kg × m/s²
• 1 dyne = 1 g × cm/s²
• The net force is the sum of all forces acting on an object
• Friction opposes motion
• Friction depends on the nature of materials and the velocity of motion
• Ff = Rv where Ff is force of friction, R is the coefficient of friction, and v is the velocity of motion
• Elasticity is the tendency of a body to return to its original form after being deformed
• Restoring force (FR) = stiffness (S) × displacement (x)
• Pressure (p) = force (F) / area (A)
• 1 pascal (Pa) = 1 N/m²
• 1 dyne/cm²
• Work (W) = force (F) × displacement (x)
• 1 joule (J) = 1 N·m
• 1 erg = 1 dyne·cm
• Energy is the capacity to do work
• Potential energy is stored energy
• Kinetic energy is energy of motion
• Power (P) = work (W) / time (t)
• 1 watt (W) = 1 J/s
• Intensity (I) = power (P) / area (A)
• 1 W/m²
• Intensity decreases with the square of the distance from the source (inverse square law)

Types of Waves

• Transverse waves - particle movement is perpendicular to wave direction
• Longitudinal waves - particle movement is parallel to wave direction
• Sound waves are longitudinal waves
• Waves are produced by vibrations that transmit energy through a medium

Sound Waves

• Sound is a form of vibration propagating through the air
• Sound radiates outward from its source
• Vibrations in a tuning fork transmit to surrounding air particles
• Compression is higher air pressure, rarefaction is lower air pressure
• Wavelength (λ) is the distance from one point on a wave to the next corresponding point
• C is the speed of sound = λ×f where f is the frequency

Combining Sinusoids

• Two or more sine waves can be combined algebraically
• Combining waves in phase creates a combined waveform of the same frequency, twice the amplitude
• Combining 180 degrees out of phase creates a waveform of zero amplitude
• Combining different frequencies create non-sinusoidal waves, with harmonics (multiples of fundamental frequency)

Complex Periodic Waves

• Complex waves are made up of multiple periodic waves, their fundamental frequency is the lowest frequency present
• Complex waves are repetitive, a complex periodic waveform contains all of its harmonic frequencies at the fundamental's frequency

Aperiodic Waves

• Transient sound is short in duration, its waveforms are not repeated
• Random noise has a range of frequencies and is continuous
• Constant amplitudes are present across all possible frequencies