acoustics

Question 1

define wavelength

Answer 1

distance across a space covered by one complete cycle of a waveform

Question 2

how many deg in a phase

Answer 2

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

Question 3

what is the speed of sound in air? (m/s)

Answer 3

344 m/s

Question 4

define sound

Answer 4

a longitudinal-wave disturbance in any compressible substance

longitudinal meaning with a direction parallel to the thing that made it happen

Question 5

define frequency

what is the perceivable freq range of humans?

moving an octave does what to frequency?

Answer 5

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

Question 6

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?

Answer 6

sin(degrees of phase)

Question 7

give the equation linking wavelength, speed and freq

Answer 7

W=S/F

a wave with freq 100Hz travelling through air has a wavelength of = 344/100 = 3.44 m

Question 8

compression a rarefaction are…

Answer 8

equal and opposite

Question 9

define displacement amplitude

Answer 9

distance each bit of air moves to either side of its normal position during its vibration

Question 10

define pressure amplitude

Answer 10

max increase of air pressure in a sound wave compression which is measured by microphones

Question 11

what’s the freq of concert pitch?

Answer 11

A2 = 440Hz

therefore A3 = 880Hz and A1 = 220Hz

Question 12

define waveform

how are waveforms of musical instruments produced?

Answer 12

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

Question 13

define periodic sound

Answer 13

perfectly steady sound, repeats same vibrational sound for a long time, eg synths

Question 14

define transient sound

Answer 14

a sound that is not sustained, dies away quickly, eg drum kick
has attack and decay

Question 15

define sound propagation

Answer 15

sound travelling through a compressible media like air or water

Question 16

how does absorption of sound happen?

Answer 16

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

Question 17

how does reflection of sound happen?

Answer 17

hard materials with densely packed particles are not easily made to vibrate so energy just bounces off these surfaces

Question 18

what is the law of reflection?

Answer 18

the angle of incidence is equal to the angle of reflection to the normal of the surface
i=r

Question 19

for diffuse reflection to occur, the irregular surface must have…

Answer 19

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

Question 20

a sound directed at concave wall, how does distance from the wall affect where the sound can reach?

Answer 20

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

Question 21

define refraction

Answer 21

change of wave direction due to difference in speed from one place to another

Question 22

define diffraction

Answer 22

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

Question 23

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?

Answer 23

spreads out equally in every direction

remains largely confined to a beam continuing in the original direction

Question 24

describe the inverse square law

Answer 24

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

Question 25

define intensity

Answer 25

measurement of energy acting on a given areag

Question 26

give the 5 classifications of instruments according to curt sachs

Answer 26

- 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

Question 27

define sympathetic vibration

Answer 27

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

Question 28

describe the natural modes of an ideal string

Answer 28

they form a pattern of standing waves with nodes and antinodes fundamental first harmonic second harmonic and so on

Question 29

define harmonic series

Answer 29

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...

Question 30

define resonance

Answer 30

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

Question 31

how does dissipition affect resonance?

Answer 31

weak dissipation gives strong resonance

Question 32

describe the law of superposition

Answer 32

if multiple sine waves happen simultaneously their waves can be combined to form a new wave shape alternatively they can cancel each other out

Question 33

when a wave on a fixed string hits a fixed point, what happens?

Answer 33

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

Question 34

define standing wave

Answer 34

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

Question 35

which harmonic frequencies will propagate on a string?

Answer 35

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

Question 36

in air columns, which part is analogous to the fixed points on a string (nodes) and antinodes?

Answer 36

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

Question 37

in an open ended air column (both ends are open) describe how a sound wave would reflect

Answer 37

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

Question 38

in an open end (both ends open) air column, what is the wavelength of the fundamental compared to column length ?

Answer 38

fundamental's wavelength is double that of the column length

Question 39

in a closed end air column (one end is closed one open) which harmonics will be present?

Answer 39

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

Question 40

define node and antinode

Answer 40

node - 0 amplitude, medium does not move antinode - point of max vibration (peaks and troughs)

Question 41

how does change in temp affect speed of sound?

Answer 41

±6m/s change in speed for each 1deg rise or drop in temp