Noise Calculations assignment

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2020

Homework Assignment 5

Noise

Student Name: Laila Almarzoog

Be specific; show your units and calculations. When in doubt, write it down so I can see your reasoning and follow your process. Remember that expressing an answer in other than full dB values (98 dB not 98.3) is meaningless to the human ear. Also remember that pressures (N/M2) and SPL values are a function of distance; therefore, a correct answer would be expressed in the format of: 26 dB @ 3 M.

1. What is the total acoustic or sound power (in watts) produced when the two specified noise sources are operating simultaneously?

Sound power:

Is the total acoustic output (power) from a sound source.

-The Energy of sound per unit time

Units are Joules/ second (J/s) or Watts (W).

-Independent of environment

Which means it has a constant value.

- it is the rate at which energy is radiated from a source

- it is a fundamental expression of loudness potential.

- Expressed as Watts- a power unit; a watt is a joule per second (energy transfer per time).

Question #1 is asking about the acoustic power, which is the sound power, keep in mind that Sound Power DOES NOT equal Sound Power Level

Sound Pressure DOES NOT equal Sound Pressure Level.

Sound Power Level:

Sound power level is designated in decibels (dB).

- Relates a standard measure to a measure of interest.

Logarithmic based.

SWL=Sound Power Level

W=Sound power of the source

W0=Reference sound power of 1X10-12W

- The sound power level is also independent of environment.

i.e, does not decrease in magnitude with distance

- Sound power reference is 1X10-12W and the auditory threshold of humans at 28cm

- When sound power (W) increases by a factor of 10, the sound power level (SWL) increases by 10dB.

Sound Pressure:

Sound pressure is the force of sound on a surface area perpendicular to the direction of the sound.

- It is related to the displacement amplitude of the vibrating sound source

- Quantifies the intensity of a sound

- Not independent of the environment, it decreases with distance from the source

- Expressed as a force per unit area, which is Pascals (Pa) or N/m2.

Sound pressure Level:

Expressed in dB

Where,

SPL=Sound Pressure Level

P=Sound pressure of the source

P0=Reference sound pressure of 2X10-5N/m2 (or 20μPa)

· We measure SPL

· SPL is always associated with distance

· We can calculate SWL from SPL given the distance from the source

· We can calculate the SPL from SWL for any given distance

· When sound pressure (N/m2) increases by a factor of 10

· The sound pressure level increases by 20dB

Source 1: 0.65 N/M2 @ 3 meters (Sound pressure). 0.3 watts

Source 2: 110 dB SWL (Sound Power Level)

2. A noise source has an acoustic power of 0.45 watts. Estimate the following:

A. Sound power level.

B. Acoustic pressure at 6 feet.

C. Sound pressure level at 1 meter.

3. Calculate the wavelengths of the following noise frequencies.

A. 16,000 Hz

B. 6,000 Hz

C. 500 Hz

· The speed of sound is calculated by:

Where,

C = Speed of sound in m/sec

λ = Wavelength in meters

F= Frequency in Hz (cycles per second)

· The speed of sound in air at normal temperature and pressure is ~340m/s

· Noise control is frequency dependent

· Higher frequencies are easier to control than lower frequencies

Wavelength= C/F

4. You have just measured a noise source with a SPL of 60 dB at 5 meters. Estimate the following based on this measurement.

A. Acoustic power of the source.

B. SWL of the source.

C. Acoustic pressure at the measurement point.

D. Acoustic pressure at 3 meters.

5. A noise source is to be positioned in the corner of a concrete building.

The manufacturer tells you that this machine has a SWL of 96 dB. Estimate the following.

A. SPL produced by this machine at 10 and 30 feet away.

B. SPL at 10 and 30 feet away if the machine was moved out of the corner and placed against a long wall.

This question has 4 answers.

6. If a noise source produces 0 dB at 40 feet, what SPL would be measured at a

distance of 4 feet from the source?

7. A large air compressor is located against a concrete block wall mounted on a

concrete floor. The manufacturer has rated the machine at 100 dB (SPL)

at 2 meters if radiating into free space. Estimate the following.

A. SWL of the machine.

B. Sound power of the machine.

C. Actual SPL you would measure at 2 meters in the situation

described.

8. A noise source produces 4 x 10-1 watts of acoustic or sound power. Estimate the SPL at 2 feet, 32 feet and 5.88 meters if radiating into free space.

10. You have employees exposed to noise at the following levels. For each exposure situation, describe the maximum time of exposure to result in a 100% noise dose using OSHA criteria and using ACGIH criteria.

97 dBA

103 dBA

110 dBA

84 dBA

93 dBA

11. You have just conducted an octave band analysis which produced the

following data.

Frequency (Hz)

Unweighted SPL

125

250

500

1000

2000

4000

108

8000

102

Estimate the overall SPL. Describe the dominant frequencies and explain how they impacted the overall SPL as estimated.

dB P P

LSPL P 0

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dB W W

PWLLSWL W 0

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PWLLSWL

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dB P P

LSPL P 2

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LSPL

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