Noise Calculations assignment

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9.2ERHS350Online_Noise_FA2020NOAUDIO.pptx

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Ear Anatomy and Physiology & Noise Monitoring

ERHS 350 Online

Module 9

Section 2

Objectives

When you have completed this module, you should be able to:

Define noise

Calculate a variety of noise parameters given equations

Define and calculate sound power and sound pressure level

Define and calculate octave bands

Objectives

Continued

Add decibels given a list of noise source levels

Describe the types of noise

Define the types of sensorineural hearing loss

Define exchange rates

Describe weighting filters

Important Equations for Homework

Sound

Propagation of Sound

Molecules in the medium

Compress

Rarify

Particle-to-particle interaction

Rarefaction

Compression

Acoustics

Sound can be illustrated by waves:

Frequency is related to pitch

Amplitude or intensity is related to loudness

Wavelength (λ)

Frequency (CPS or Hertz)

Amplitude

Above

AAP

Below ambient

air pressure

Acoustics

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

Acoustics

It is important that we know how to calculate the frequency and wavelength of noise

Noise control is frequency dependent

Higher frequencies are easier to control than lower frequencies

For example, when you are in traffic, do you typically hear low frequencies (bass) or higher frequencies (treble) coming from the car next to you that is playing loud music?

Acoustics

Example

Calculate the wavelength of a pure-tone1000Hz noise source at NTP

Acoustics

Example

Calculate the frequency of a 2’ long wavelength

Sound Power

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

Constant value

Acoustic or Sound Power

Rate at which energy is radiated from a source

Fundamental expression of loudness potential

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

Other Components of Noise

Acoustic Power = Sound Power

Acoustic Pressure = Sound Pressure

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

Where,

SWL=Sound Power Level

W=Sound power of the source

W0=Reference sound power of 1X10-12W

Sound Power Level

Also independent of environment

i.e., does not decrease in magnitude with distance

Sound power reference

1X10-12W

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

Source	Sound Power (W)	Sound Power Level (dB) re: 10-12W
Large chipping hammer	1	120
Chain saw	0.1	110
Air chisel	0.01	100
Lawn mower	0.001	90
Vacuum cleaner	0.00001	70
Hair dryer	0.000001	60
Refrigerator	1X10-8	40
Ticking watch	1X10-10	20
Threshold of hearing	1X10-12	0

Sound Pressure

Sound pressure

The force of sound on a surface area perpendicular to the direction of the sound

Related to the displacement amplitude of the vibrating sound source

Quantifies the intensity of a sound

Not independent of the environment

Decreases with distance from the source

Expressed as force per unit area

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)

Sound Pressure Level

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	Sound Pressure (N/m2)	Sound Pressure Level (dB) re: 2X10-5N/m2
Threshold of discomfort	20	120
Textile loom	2	100
Garbage disposal @ 1m	0.2	80
Conversational voice	0.02	60
Quiet room	.002	40
Soft whisper	0.0002	20
Threshold of hearing	2X10-5	0

Sound Pressure Level

The SPL decreases with distance

Where,

SPL = Sound Pressure Level

SWL = Sound Power Level

r = Distance to the source

Sound Pressure Level

SPL changes with an increase or decrease in distance to the noise source

We can estimate this change by:

Where:

SPL2 = New sound pressure level

SPL1 = Known sound pressure level

d1 = Distance associated with SPL1

d2 = New distance associated with SPL2

Sound Pressure Level

We measure a noise source at 3 meters at 95 dB, what is the estimated SPL at 6 meters?

Sound Pressure Level

In the previous example

We doubled the distance from the source (3 meters to 6 meters)

The SPL was reduced by 6 dB

Whenever we double the distance to or from a noise source, the SPL either increases or decreases by 6 dB (in a free-field)

Remember the 6 dB doubling rule! Double the distance, decrease the SPL by 6 dB.

Sound

Source: OSHA

Sound Pressure Level

In addition, noise sources have a directionality component

Reflective planes in the environment can increase the SPL

E.g., floors and walls

The Directivity Factor (Q) accounts for reflective planes of a sound pressure wave

Sound Pressure Level

To Account for directionality

Where,

Q=Directionality Index

Q=1 for spherical radiation

Q=2 for 1 reflective plane (floor)

Q=4 for 2 reflective planes (floor and wall)

Q=8 for 3 reflective planes (corner)

Sound Pressure Level

What is the SPL of a machine @ 3 meters and a SWL of 93dB?

What if the machine is against a wall?

Increase of 6dB!

End of Session 2

Application of concepts: Homework 5 (100 points)

log

SPL

SWL

SPL

log

SPL

log

SPL

log

PWL

SWL

log

1000

340

1000

340

305

557

340

SPL

log

SWL

SPL

log

SPL

)

log(

SPL

SWL

SPL

log

)

log(

SPL