loudspeakers

Question 1

what is a loudspeaker?

Answer 1

an electro-acoustic transducer
turns electrical signal into acoustic energy

Question 2

how does a dynamic speaker work?

Answer 2

electrical signal makes a coil move back and forth due to Faraday’s law of induction which causes a diaphragm to move back and forth and make sound waves

Question 3

what were problems of early loudspeakers?

Answer 3

lots of added unwanted harmonic/freqs

Question 4

why is a cabinet needed in loudspeakers?

Answer 4

when the diaphragm is in motion, positive in front of the speaker and negative pressure behind the speaker created from the motion will cancel out, sounds will dissipate naturally

more so for low freqs because they happen more slowly and is have more time to move a dissipate

could mount it on a plate but freqs with wavelengths larger than the middle of the speaker to the end of the plate will dissipate so less effective as wavelength increases, would need an infinitely large boundary aka a baffle

Question 5

what is a baffle?

Answer 5

a box/case around a loudspeaker

Question 6

what are 2 problems with a baffle?

Answer 6

• if the box is airtight, the speaker becomes less efficient because there is more resistance to changes in pressure inside the box
  analogous to decreasing sensitivity of microphones, they respond less well to changes in pressure, but it measured in mV of electrical input per dB of acoustic energy and for speakers its dB of acoustical energy per watt of energy input
• creates a resonating body, waves inside the box will create an amplified standing wave

Question 7

give 4 ways baffle problems are solved

Answer 7

bracing - making the baffle stiffer and less prone to movement itself

mass - adding mass to it, using heavier materials

absorbing materials inside like fibreglass

internal shape - make internal shape irregular to create diffusion, eliminate parallel surfaces

Question 8

what is a ported cabinet?

Answer 8

aka vented boxes, phase inverters, reflex cabinets

its a speaker cabinet/baffle with a hole in it

Question 9

what are the benefits of a ported cabinet

Answer 9

increases ability of speaker to produce and amplify lower freqs by lowering the drop off (of the frequency response chart) to one octave below than in the equivalent sealed box, eg halving the freq of the bit where the freq response drops off

mid freq response is more consistent

increases sensitivity by 3-6dB

port forms a Helmholtz resonator at a given freq, determined by the dimensions of the port and cabinet

Question 10

what are multi-element speakers? what happens as speaker size increases?

Answer 10

boxes with more than one loudspeaker in them, can be 2-way, 3-way etc…

speakers become less directional as size increases, because sound coming from different sides of the speaker to the same place can be out of phase, want them to be sending sound to the same place at the same time

Question 11

describe how the freq range is distributed in a 2-way speaker

Answer 11

each loudspeaker can effectively project 5 octaves, 10 octaves is the audible range, so one will cover the high end and the other the low end
a crossover is used to separate the input signal into two frequency ranges sending one to each speaker that is optimised for that range

the projection of each frequency will be constant (at max eg 1dB) until the frequency coverage meets the mid point in which case each loudspeaker will project half the dB (0.5dB each) of that so altogether they are projecting the full amount

Question 12

describe how the freq range is distributed in a 3-way speaker

Answer 12

each loud speaker will cover 3.3 octaves, two on either end and one in the middle with the crossovers happening like the 2-way but in two different places
the crossover splits the input signal into 3 octaves and sends each one to its appropriate loudspeaker

Question 13

give 5 changes that can be made using physical intervention

Answer 13

grater sensitivity
greater efficiency
more even freq response
more or less directionality
more even phase response, like when speakers are combined

Question 14

horns, phase plugs and compression drivers are all examples of what

Answer 14

physical intervention

Question 15

what is the flare rate of a horn?

Answer 15

the degree of change of the cross section as we move from the throat (the inner most part of the horn) to the mouth

can be conical, constant or exponential, variable rate of change

Question 16

give the benefits of using a horn with flare rate

Answer 16

increased efficiency, especially lower freqs that are constrained to the front of the speaker, they can n longer move the to back and dissipate

efficiency rises to 10-50% and increases sensitivity by 10dB

Question 17

what is the cut off freq of a horn?

Answer 17

the freq at which the horn becomes effective/adds efficiency, fc

proportional to the flare rate, which is a function of mouth size and length of horn, all of these factors impact efficiency change and are all different for different frequencies:
- for low frequencies, these changes must be physically large to avoid a standing wave

Question 18

give 2 types of horn

Answer 18

folded horn that are inside the baffle

vestigial horns that are shallow

Question 19

what is a compression driver and what does it do?

Answer 19

a chamber between the diaphragm and the throat of the horn where air is compressed through a smaller opening

it increases efficiency of a speaker
when air moves through the smaller space it is compressed, giving a gain in efficiency

Question 20

what is compression ratio of a compression driver?

Answer 20

the ratio between the size of a diaphragm and the size of the opening of the driver (where its being compressed into)
eg. a 400% compression ratio just means the diaphragm is four times are the size of the opening of the chamber

Question 21

what is a phase plug and what does it do?

Answer 21

when you have an array of speakers, eg. sound coming from multiple different speakers to the same point, a phase plug helps direct the sound waves along the same path so they are arriving at the same time and in phase

especially when sound is coming thru the opening of the compression chamber, it can happen out of sync due to the dimensions of the chamber, so a small object can be place at the opening to ensure all sound exits in sync, equalises the path length between the edge and centre of comp. driver

normally made of stiff lightweight materials