ACOUSTICS 2 Flashcards

0
Q

The year can hear between what and what Hz? But it is most sensitive between what and what Hz?

A

20 - 20,000 Hz

125 - 6,000 Hz

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1
Q

Pitch corresponds to…

A

Wavelength

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2
Q

Loud sounds represent a larger amount of energy stored in the wave. What can you measure them in?

A

Watts/cm²

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3
Q

Ratio between the quietest sound the human ear can hear and the loudest sound that does not cause pain…

A

1 : 10,000,000,000

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4
Q

Basic unit of sound intensity is called the decibel, and as expressed by the formula…

A

IL = 10 log (I/I_o)

Where

IL is the intensity level in decibels
I is the intensity of the sound being measured
I_o is the reference intensity of 10^-16 W/cm², which is the quietest sound that we can hear

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5
Q

If you happened to be measuring sound power level instead of sound intensity level, the equation it’s very similar:

A

PWL = 10 log W/W_o

Where

PWL is the sound power level
W is the power at the source measured in watts
W_o is the reference wattage, 10^-12

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6
Q

What is the sound pressure level?

A

This is a weird one. This is the pressure exerted by the soundwave on a surface at a given location. It deals with barometric pressure in addition to sound intensity level.

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7
Q

What is the formula for sound pressure level?

A

SPL = 20 log P/P_o

Where:

SPL is the sound pressure level
P is the pressure at the measured point, in newtons over m²
P_o is the reference pressure, 2 x 10^-5 N/m²

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8
Q

Is the relationship between sound intensity level and sound pressure level for the purposes of this exam?

A

They are the same

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9
Q

How do you find the sound power of a source in Watts if you have the intensity at any given point? I mean what is the formula.

A

I = W / (4πd²930)

Where:

I is the intensity in W/cm²
W is the power at the source in Watts
d is the distance to the source in feet
930 is the conversion factor to translate between feet and W/cm²

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10
Q

For sound, give me the inverse square law has a formula

A

I_2 = I_1 (d_1/d_2)²

I_2 Second intensity
I_1 first intensity
d_2 second distance
d_1 first distance

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11
Q

If a material has an absorptivity of 1.0 Sabin, what does this mean?

A

All sound is absorbed

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12
Q

Formula for total absorption

A

A = S1a1 + S2a2 + … + Snan

Where:

A is total absorptivity
S1 surface area of material 1 in ft.²
a1 absorptivity of material 1 in Sabins
S2 surface area of material 2 in ft.²
a2 absorptivity of material 2 in Sabin
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13
Q

What is the NRC or noise reduction coefficient?

A

This is the average of the absorptivity from all the different frequencies from 250 Hz to 2000 Hz

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14
Q

What is the formula to calculate the volume or amplitude of the sound in and and closed reverberant space… If you know the power in watts at the source

A

I = P/930A

Where

I is the intensity throughout the space
P Power at the source, and watts
A total absorptivity of the space, in Sabins

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15
Q

How do you find the noise reduction, and decibels, between a sound in one room compared to the sound in another?

A

NR = 10log(A_2/A_1)

NR is the noise reduction
A_2 is the total absorptivity of room 2
A_1 is the total absorptivity of room 1

16
Q

Definition of reverberation time. Formula for reverberation time.

A

The time it takes for a sound to reduce 60 dB

T_R = .049V/A

Where
T_R is reverberation time in
V volume of the space in cubic feet
A is total absorptivity in Sabins

17
Q

When a space is larger, the reverberation time _____.

A

Increases

18
Q

What is the rule of thumb formula for establishing the height of a room in order to achieve a desired reverberation time?

A

H = 20xT_R

19
Q

What is the formula for transmission loss between two non-reverberant spaces?

A

TL = 10 log I/theta

20
Q

What is the formula for noise reduction between reverberant spaces?

A

NR = IL_2 - IL_2
= TL - 10 log S/A_r

Where:
TL = transmission loss of the wall
S = of the separating wall, in ft.²
A = absorptivity in the receiving room, in Sabins