Name___________________________________________ Section__________

Please show all of your work!

1. On the axes provided below, graph the following:

a. 100-Hz sine wave with a peak amplitude of 1.0 Pa

b. 200-Hz sine wave with a peak amplitude of 0.5 Pa

c. Sum of the previous two waves

2. Find the equation that describes the waveform that you graphed for part 1c.

!1.5%

!1%

!0.5%

0%

0.5%

1%

1.5%

0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10% 11% 12%

Pressure%(Pa)%

Time%(ms)%

LMSC-209W Midterm Exam Page 2

Ver. A – ELR 12/1/13

3. Calculate the sound pressure level that corresponds to a sound pressure of

5 x 10-3 Pa.

4. Calculate the overall A-weighted level of the spectrum below.

Band 63 Hz 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz 8 kHz

LP 82 dB 77 dB 72 dB 60 dB 64 dB 57 dB 50 dB 40 dB

LMSC-209W Midterm Exam Page 3

Ver. A – ELR 12/1/13

5. Three small fans are located near each other at 1.5 m above the ground, and

produce an annoying noise in the 250 Hz band. A local resident whose bedroom

window is at the same height as the fans and is 80 m away complains about this

noise. Measurements indicate that noise levels in the 250 Hz band at 2 m from

each fan are 85 dB, 92 dB, and 91 dB respectively.

a. Calculate the total level from the three fans at the window. (Note that you

do not need to A-weight anything in this problem)

b. A noise control barrier is to be located 3 m from the fans, and must

reduce the 250 Hz fan noise to 55 dB outside the neighbor’s window.

Measured from ground level, how tall must the barrier be?

(A copy of the nomograph is attached to this exam for reference.)

LMSC-209W Midterm Exam Page 4

Ver. A – ELR 12/1/13

6. The room pictured has the following

absorption coefficients at 500 Hz:

Floor: Carpet, α = 0.14

Ceiling: Acoustic Ceiling Tile, α = 0.70

Walls: Gypsum Wallboard, α = 0.10

a. Calculate the total absorption in this room.

b. Calculate the reverberation time.

c. Calculate the mean free path.

2 m

8 m

6 m

LMSC-209W Midterm Exam Page 5

Ver. A – ELR 12/1/13

0

5

10

15

20

25

30

31.5 Hz 63 Hz 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz 8 kHz

Barrier Insertion Loss (dB)

Octave Band Center Frequency (Hz)

Sound Barrier Calculation

H2/D%

0.125%

0.25%

0.5%

1%

2%

4%

8%

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