acoustics Flashcards
define wavelength
distance across a space covered by one complete cycle of a waveform
how many deg in a phase
a complete cycle has 360° of phase counting from the start of the cycle.
eg from 0 amplitude to the first peak is 90°, then to the 0 point again is 180°, to the trough is 270° then 360° to complete the cycle
what is the speed of sound in air? (m/s)
344 m/s
define sound
a longitudinal-wave disturbance in any compressible substance
longitudinal meaning with a direction parallel to the thing that made it happen
define frequency
what is the perceivable freq range of humans?
moving an octave does what to frequency?
number of times per second a wave completes a cycle, repetition rate of a vibration
measured in Hz or kHz
humans can perceive 20Hz to 20kHz, approx 10 octaves
moving an octave doubles or halves frequency
how do you calculate the value of a normalised (amplitude between 1 and -1) sine wave with peak value of +1 and trough value of -1?
sin(degrees of phase)
give the equation linking wavelength, speed and freq
W=S/F
a wave with freq 100Hz travelling through air has a wavelength of = 344/100 = 3.44 m
compression a rarefaction are…
equal and opposite
define displacement amplitude
distance each bit of air moves to either side of its normal position during its vibration
define pressure amplitude
max increase of air pressure in a sound wave compression which is measured by microphones
what’s the freq of concert pitch?
A2 = 440Hz
therefore A3 = 880Hz and A1 = 220Hz
define waveform
how are waveforms of musical instruments produced?
a waveform is a graphical representation of pressure as a function of time
fluctuations in pressure in the air are measured by a microphone and converted to an equivalent electrical voltage and displayed on an oscilloscope
define periodic sound
perfectly steady sound, repeats same vibrational sound for a long time, eg synths
define transient sound
a sound that is not sustained, dies away quickly, eg drum kick
has attack and decay
define sound propagation
sound travelling through a compressible media like air or water
how does absorption of sound happen?
particles of a soft or flexible surface can easily be made to vibrate by a sound wave so the energy is drained into these surfaces
how does reflection of sound happen?
hard materials with densely packed particles are not easily made to vibrate so energy just bounces off these surfaces
what is the law of reflection?
the angle of incidence is equal to the angle of reflection to the normal of the surface
i=r
for diffuse reflection to occur, the irregular surface must have…
irregularities that are roughly on the same scale as the wavelength of the sound wave
irregularities that 1/4 of of wavelength will begin to cause diffusion
a sound directed at concave wall, how does distance from the wall affect where the sound can reach?
if the source is far form the wall, the sound reflects right back to the source
if the source moves a bit closer to the wall, the sound spreads out when it reflects
define refraction
change of wave direction due to difference in speed from one place to another
define diffraction
a behaviour of waves in which they will spread outward, filling regions rather than leaving a shadow
eg, sounds can be heard from around a corner
diffraction
when sound comes thru an opening narrow compared to its wavelength how does the wave behave?
when it goes thru an opening that is wide compared to its wavelength how does it behave?
spreads out equally in every direction
remains largely confined to a beam continuing in the original direction
describe the inverse square law
sound spreading from a source into unobstructed air, the sound intensity decays at a rate inversely proportional to the square of the distance travelled thru the unobstructed space
if sound travels double the distance, the intensity decreases by a factor of 4, distance 2d gives intensity i/4
define intensity
measurement of energy acting on a given areag
give the 5 classifications of instruments according to curt sachs
- idiophones - entire instrument moves to produce sound, eg cymbal
- membranophones - vibrating membrane stretched over an opening, eg drums
- chordophones - sound produced by vibrating string, eg piano
- aerophones - sound produced by vibrating column of air, eg brass and wind
- electrophones - instruments requiring voltage
define sympathetic vibration
when an object cannot produce sound on its own and uses a larger surface area to create loudness and pass their vibrational energy into the air
eg guitar strings using the body to create sound
describe the natural modes of an ideal string
they form a pattern of standing waves with nodes and antinodes
fundamental
first harmonic
second harmonic and so on
define harmonic series
a series of musical tones whose frequencies are integer multiples of the fundamental frequency, wavelength is twice the length of the string
harmonics are considerably lower in volume than fundamental
eg, 110Hz, 220Hz, 330Hz 440Hz…
define resonance
resonance is the large amplitude vibration that is caused when an object is vibrated sympathetically by some source at a frequency that closely matches the object’s natural oscillation frequency
how does dissipition affect resonance?
weak dissipation gives strong resonance
describe the law of superposition
if multiple sine waves happen simultaneously their waves can be combined to form a new wave shape
alternatively they can cancel each other out
when a wave on a fixed string hits a fixed point, what happens?
the wave changes direction
inverts phase: the reflected wave is an 180° phase change of what the wave would’ve been if it continued past the barrier/fixed point
superimposes: so the reflected wave interferes constructively with the incident wave (they are the same) and the resultant wave is larger sum of these two due to law of superposition
define standing wave
a wave formed when an object vibrates at a frequency whose wavelength is related to the dimensions of that object
so the nodes and antinodes are at fixed points along the object, these points cannot move so the whole pattern appears to standing still, hence a standing wave
which harmonic frequencies will propagate on a string?
the harmonics that “fit” or are related to the length of the string
the ends of the string are fixed so only harmonics that fit in between those fixed points can propagate
in air columns, which part is analogous to the fixed points on a string (nodes) and antinodes?
nodes are the closed end of a tube
and antinodes would be the open end of a tube because the air is free to undergo longitudinal motion
in an open ended air column (both ends are open) describe how a sound wave would reflect
blowing into the tube introduces pressure in the tube, when it reaches the end it experiences a change in air density and reflects and flips phase too
in an open end (both ends open) air column, what is the wavelength of the fundamental compared to column length ?
fundamental’s wavelength is double that of the column length
in a closed end air column (one end is closed one open) which harmonics will be present?
fundamental, whose wavelength is 4 times the length of the tube
and odd harmonics , because the closed end is confined to being a node, it prevents from positive harmonics forming
define node and antinode
node - 0 amplitude, medium does not move
antinode - point of max vibration (peaks and troughs)
how does change in temp affect speed of sound?
±6m/s change in speed for each 1deg rise or drop in temp
