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Live Sound > Wave Theory > Flashcards

Wave Theory Flashcards

1
Q

Wavelength formula

A

Velocity/Frequency

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2
Q

Velocity

A

Feet per second

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3
Q

Frequency

A

Cycles per second, Hz

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4
Q

Velocity for sound at room temperature?

A

1130 feet per second

Rounding to 1000 is typically acceptable

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5
Q

Period or Time formula

A

1/Frequency
1=cycles PER SEC
1000/Frequency
1000=cycles PER MS

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6
Q

What is the wavelength of 1kHz in feet? Use 1000 instead of 1130.

A

V/F

1000/1000Hz = 1ft

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7
Q

What is the wavelength of 500Hz in feet? Use 1000 instead of 1130.

A

V/F

1000/500Hz = 2ft

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8
Q

What is the wavelength of 2000Hz in feet? Use 1000 instead of 1130.

A

V/F

1000/2000Hz = 0.5ft

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9
Q

Multiple arrivals at 0° phase

A

Complete addition

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10
Q

Multiple arrivals at 90° phase

A

Partial addition

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11
Q

Multiple arrivals at 120° phase

A

No addition

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12
Q

Multiple arrivals at 180° phase

A

Complete cancellation

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13
Q

Boundary Reflections

A

1ms time offest between the first and reflected sound causes comb filtering with 1kHz frequency spacing at the listening position
ex. direct path = 50ms, reflected path = 51ms

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14
Q

Speaker interactions, overlap in speaker coverage

A

1ms time offset between the two speakers causes comb filtering with 1kHz frequency spacing at the listening position
ex. upper speaker path = 51ms, lower speaker path = 50ms

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15
Q

Microphone interaction, inaccurate placement of 2 mics

A

1ms time offset between two mic positions causes comb filtering when the mics are summer together in the mixing console
ex. Mic 1 path = 5ms, Mic 2 path = 6ms

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16
Q

Direct and microphone signal interaction, latency differences between mic and DI sources

A

1ms time offset between two direct and mic’d signals causes comb filtering when the channels are summed together in the mixing console
ex. Mic path = 1ms, Direct input path = 0ms

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17
Q

Why is it called comb filtering?

A

Because the frequencies are being cancelled out in intervals, causing the waveform to create spikes going up and down, resembling teeth of a comb

18
Q

Polarity vs Phase

A

Polarity is about + and - of the waveforms, and phase is about time

19
Q

2 speakers in polarity and in phase

A

Both speakers matched polarity +- and are both reaching the listener at the same time
ex.
1st speaker = +-, 1ms
2nd speaker = +-, 1ms

20
Q

2 speakers in polarity yet out of phase

A

Both speakers matched polarity, but are not reaching the listener at the same time
ex.
1st speaker = +-, 1ms
2nd speaker = +-, 2ms

21
Q

2 speakers out of polarity yet in phase

A

Speakers are not matching in polarity, but are both reaching the listener at the same time
ex.
1st speaker = +-, 2ms
2nd speaker = -+, 2ms

22
Q

2 speakers out of polarity and out of phase

A

Speakers are not matching in polarity and are not reaching the listener at the same time
ex.
1st speaker = +-, 1ms
2nd speaker = -+, 2ms

23
Q

Latency in cabling, explain

A

0 latency. Force applied through the cable causing what is going in at the start of the cable to push out what’s built up at the end of the cable. An example would be a pipe full of BBs, if you push in one, it’ll cause one to fall out the other end. The force of pushing is voltage, and the BBs are electrons.

24
Q

Does the length of a cable affect latency?

A

No, because voltage pushing electrons in a cable causes electrons to come out the end the moment one enters the start. There’s no delay at all.

25
Q

Latency in analog processors

A

Rare and usually insignificant

26
Q

Latency in digital processors

A

This occurs because computers utilise logic-based circuits - calculations take place when processing incoming signal. Signal “stops” to be evaluated before getting sent forward and this takes time

27
Q

Latency in speaker design

A

This occurs because of the physical difference in types of speaker (tweet, honk, woof)

28
Q

How is latency considered in speaker design?

A

Inside a speaker, the woof is set furthest back, tweet is set most forward, and honk is set in between. This will account for the time it takes for the soundwaves to hit the air, and by spacing them out accordingly, will allow the sound waves from all three to hit the air at the same time

29
Q

Polar pattern of a speaker

A

The point where a speaker drops 6dB in level, off axis, defines its polar pattern

30
Q

Inverse Square Law

A

States: you will lose 6dB of level per doubling of distance from a speaker

31
Q

Inverse Square Law example

A

Speaker was 12ft away and is moved to 3ft away
1. Divide old distance from new distance: 12/3=4
2. Calculate log of 4 by pressing the LOG button on calculator: Log of 4 = 0.60
3. Multiply the log of 4 by 20: .60 x 20 = 12dB
Moving the speaker from 12ft to 3ft increased the speaker output signal by 12dB

32
Q

Critical Distance

A

The point in a room where the speaker’s energy and the energy in the room are equal to each other

33
Q

What is the period of 1kHz?

A

1/1000=.001 second
or
1000/1000=1 millisecond(ms)

34
Q

What is the period of 2kHz?

A

1/2000=.0005 second
or
1000/2000=0.5 millisecond(ms)

35
Q

Pink noise

A

Equal energy per octave

36
Q

White noise

A

Equal energy every frequency

37
Q

First cancellation in comb filter frequency

A

First cancellation in frequency response is always HALF of the comb filter frequency

38
Q

After the first cancellation in comb filter frequency, in what increments does it continue to cancel after that?

A

Every 1 kHz

39
Q

Where is the first cancellation in a 1 kHz comb filter?

A

500 Hz

40
Q

Where is the second cancellation in 1 kHz comb filter?

A

1500 Hz / 1.5 kHz

41
Q

Where is the first cancellation in a 500 Hz comb filter?

A

250 Hz

42
Q

Where is the second cancellation in a 500 Hz comb filter?

A

1250 Hz / 1.25 kHz

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