ACOUSTICS 2 Flashcards
The year can hear between what and what Hz? But it is most sensitive between what and what Hz?
20 - 20,000 Hz
125 - 6,000 Hz
Pitch corresponds to…
Wavelength
Loud sounds represent a larger amount of energy stored in the wave. What can you measure them in?
Watts/cm²
Ratio between the quietest sound the human ear can hear and the loudest sound that does not cause pain…
1 : 10,000,000,000
Basic unit of sound intensity is called the decibel, and as expressed by the formula…
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
If you happened to be measuring sound power level instead of sound intensity level, the equation it’s very similar:
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
What is the sound pressure level?
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.
What is the formula for sound pressure level?
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
Is the relationship between sound intensity level and sound pressure level for the purposes of this exam?
They are the same
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.
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
For sound, give me the inverse square law has a formula
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
If a material has an absorptivity of 1.0 Sabin, what does this mean?
All sound is absorbed
Formula for total absorption
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
What is the NRC or noise reduction coefficient?
This is the average of the absorptivity from all the different frequencies from 250 Hz to 2000 Hz
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
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
How do you find the noise reduction, and decibels, between a sound in one room compared to the sound in another?
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
Definition of reverberation time. Formula for reverberation time.
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
When a space is larger, the reverberation time _____.
Increases
What is the rule of thumb formula for establishing the height of a room in order to achieve a desired reverberation time?
H = 20xT_R
What is the formula for transmission loss between two non-reverberant spaces?
TL = 10 log I/theta
What is the formula for noise reduction between reverberant spaces?
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
