Fundamentals of Acoustics Flashcards
Noise
Excessive or unwanted sound which potentially results in annoyance and/or hearing loss
Sound
A pressure variation wave that travels through a medium and is detected by the ear
Longitudinal
Particle motion in direction of wave motion
Amplitude
Amount of sound pressure (dB)
Frequency
Rate of vibration per unit time measured in cycles per second (Hz)
Sound Power Level
Total sound power emitted by a source in all directions
Give 3 intuitive facts of sound
- Travels undistorted
- Travels with some time delay
- Travels independently if from different sources
Monopole Source
A source which radiates equally in all directions
Dipole Source
Consists of two monopole sources of equal strength but opposite phase
Locally reacting solid interface
The movement at each point of the surface is in a normal direction to the surface and is independent of both the motions of the other points and the angle of incidence
Lossy medium
A medium where part of the sound energy will be dissipated to viscous and thermal effects
Shear viscosity coefficient
A measure of the diffusion of momentum by molecular collision between regions of the fluid possessing different net velocities
Bulk viscosity coefficient
A measure of some conversions of energy between molecular motion, internal molecular states, and structural potential energy states
Speed of sound
343m/s
Quadrupole
Consists of two out of phase dipole sources
Sound Pressure
Difference (scalar) between the pressure caused by a sound wave and the ambient pressure of the media the sound is travelling through (p_total = p_atm + p_acoustic)
Sound Intensity
Sound energy flow in Watts per square metre, measured in the direction of the sound wave propagation. Proportional to acoustic pressure squared
Sound Power
Total acoustic power (energy per second) radiated by a source in all directions and measured in Watts
Tonal Noise
A sound or noise recognised by its regularity. A pure tone has one frequency.
Broadband Noise
Noise whose energy is distributed over a wide section of the audible range
Propagation Vector
A vector indicating the direction of wave propagation and the phase delay per unit length
Surface of constant phase
A wavefront plane of constant amplitude
Specific Acoustic Impedance
Ratio of effective sound pressure at a point in an acoustic medium to the effective particle velocity at that point
Acoustic Impedance
Ratio of sound pressure to sound volume velocity
Acoustic Impedance (resistance)
Resistance of flow of sound through a medium (real part of acoustic impedance)
Acoustic Impedance (Reactance)
Imaginary part of acoustic impedance
Snells Law
A formula used to describe the relationship between angles of incidence and refraction when referring to waves passing through a boundary between media
Sin(theta_i)/c_1 = Sin(theta_t)/c_2
Low Pass Filter
Signals above cut-off frequency are attenuated. Power transmission outside of this range is approximately 1.
High Pass Filter
Signals below cut off frequency are attenuated. Power transmission outside of this range is approximately 1.
Band Stop Filter
Signals inside of band width are attenuated. Power transmission outside of this range is approximately 1.
Critical angle
Angle of incidence which causes a refraction angle of 90 degrees
Threshold of Audibility
20Hz
Threshold of Pain
130dB
Equal Loudness
A contour of equal loudness over a range of frequencies.
Phon Concept
Two sounds may have the same sound intensity but may not sound equally loud because the human hearing sensitivity varies with frequency
Frequency weightings
Correlate objective measurements with human subjective response
Sound Reflection
when a sound wave reaches the boundary between one medium and another medium, a portion of the wave undergoes reflection and a portion of the wave undergoes transmission across the boundary
Scattering
Irregular diffraction or reflection of sound waves in many different directions
Sound Absorption
The proportion of energy absorbed by a material or media encountered by the sound wave
Room Modes
Decay of sound in a room due to the sound waves interacting with the rooms surfaces
Room Modes (Axial)
Pairs of parallel walls
Room Modes (Tangential)
Four room surfaces
Room Modes (Oblique)
Six Surfaces
Reverberation Time
Time required for the sound pressure level to decrease by 60dB after the sound source has stopped
Sabine Equation
Eq relating reverberation time with sound absorption and room volume. T=0.161V/A
Masking Noise
A noise which is intense enough to render inaudible or unintelligible another sound present
Lateral Quadrupole
4 monopole sources with alternating phases in the corner of a square (clover leaf pattern)
Longitudinal Quadrupole
2 opposite phase dipoles lying on a single line
Near field - 4 maxima, 4 minima
Far field - 2 maxima, 2 minima
Sound Pressure Level (SPL) calculation
L_p = 20log(p/p_0)
Sound Power Level (SWL) calculation
L_w = 10log(W/W_0)
Loudness
Impression of magnitude we hear with our ears
Adding decibel values together
L_total = 10*log( 10^(dB1/10) + 10^(dB2/10) + … )
Omni-directional sound
Source emits sound equally in all directions
Directional sound
Power emitted in each direction is different
What is the absorption coefficient
alpha = (absorbed energy) / (incident energy)
Image Method
Rays are perfectly reflected at boundaries. When a ray is reflected, it spawns a secondary source “behind” the boundary surface, located on a line perpendicular to the wall, at the same distance from it as the original source
A single reflection is a first order source, more than one reflection represents a higher order source
Rayleigh reflection coefficient
Defines the angle of transmission through an oblique incidence
