BASICS OF SOUND
SOUND PRESSURE VERSUS
SOUND POWER
There are two common measures to
quantify the level of sound: sound
pressure and sound power. Both are
often presented as levels using decibel
(dB) units; however, there is a significant
difference between sound pressure and
sound power. Sound pressure is
measured with a microphone. The
measured sound pressure level from a
generator set will depend heavily on the
distance between the microphone and
the source and the surrounding
environment. In a free-field environment,
basically no reflecting surfaces near the
generator, the sound pressure level
will reduce 6 dB for every doubling of
distance. Sound power is a calculated
measure. Specifically, it is not measured
directly but calculated from sound
pressure measurements. While sound
pressure levels are heavily dependent on
the distance between the microphone and
the generator, the calculated sound power
is constant as it factors the measured
distance into the calculation.
Since sound pressure is dependent on
the measurement distance, one must
ensure that the distance from the
generator set is considered when
comparing levels between various
products and reviewing noise ordinances.
INTRODUCTION
When selecting a generator, there are many factors to consider so as not to negatively impact the existing environment. Today, noise is considered a pollutant and a negative health factor. This paper discusses the basics of sound, how it is measured, and defines enclosure sound levels. Those responsible for generator design and installation have to consider the local environmental impact noise will have on populated areas, and their considerations will have to consider noise regulations set by Occupational Safety and Health Administration (OSHA) and state and municipal noise ordnances.
Across North America, local, state or federal zoning ordinances limit the noise that can be measured at lot lines. Within working environments, OSHA states workers cannot be subjected to more than 80 dBA for more than a set period. This paper discusses the basics of sound, its definitions, what the human ear hears, sound measurement and solutions for reducing noise transmitted to the environment.
Measuring and Understanding Sound
WITHIN GENERATOR APPLICATIONS
POWER SYSTEMS TOPICS 119
BASICS OF SOUND
DB SCALE
The dB scale was developed to define the effect
of sound on the human ear. It is logarithmic, as
the scale of sound the ear first detects 0 dB(A)
to a level that is intolerable 120 dB(A), is
approximately 10 million to one. A base 10
logarithmic scale as depicted in Figure 1 covers
the range of human hearing from 0 dB (barely
audible) to 140 dB (painful on the ear).
DIFFERENCE BETWEEN DB(A) AND DBB
Loudness is the human ear’s impression of the
strength of a sound.
The human ear is more sensitive to sound in the
1 to 4 kHz frequency range than to sound at very
low or very high frequencies. To compensate
for human hearing, sound meters are normally
fitted with filters that adapt the measured sound
response to the human sense of sound.
The A scale and B scale were developed for
different levels of sound. The most commonly
used scale is the A scale with units of dB(A).
When measuring generator sound on the human
ear, the A scale is used.
See Figure 2 with AB scales, note the A scale.
FREQUENCY OF SOUND OCTAVE BANDS
Octave Bands offer a filtering method of splitting
the audible spectrum into smaller segments
called octaves, allowing you to identify different
noise levels across individual frequencies.
Octave Band measurements are used when the
frequency composition of a sound field is needed
to be determined. Octave analysis is often used in
noise control, hearing protection and sometimes
in environmental noise issues.
Common octave frequency bands are: 63 Hz,
125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz and
8 kHz. Their composition is made up of the Lower
Band Limit, Centre Frequency and Upper Band
Limit. Figure 3.
140
135
130
125
120
115
110
105
100
95 90
85 80
75 70
65 60
55 50
45 40
35 30
25 20
15 10
5 0
Gunshot
Jackhammer
Thunder
Chain Saw
Car Horn
Snowmobile
Gas Lawn Mower
Street Traffic
Busy Urban Area
Commercial Area
Quiet Urban Daytime
Quiet Urban Nighttime
Raindrops
Quiet Suburban, Nighttime
Quiet Rural Nighttime
Rustling Leaves
Threshold of pain
Rock Band
Hair Dryer
Garbage Disposal
Factory Noise
Vacuum Cleaner
Dog Barking
Normal Speech
Business Office
Dishwasher
Average Home
Theater
Living Room
Library
Quiet Conversation
Bedroom
Soft Whisper
Recording Studio
Threshold of hearing
Outdoor Sounds dB(A) Indoor sounds
Sound Pressure Level Chart
10 -45
-40
-35
-30
-25
-20
-15
-10
-5
-0
5 100 1000 10000
dBA dBB
Relative Response (dB)
Frequency (Hz)
dB AB Criteria
Figure 1
Figure 2
2 / Generator Sound Attenuation and Measurement
IMPORTANCE OF SOUND FOR
POWER GENERATION
WHY MEASURE SOUND
Regulatory bodies such as OSHA mandate no
more than 80 dB(A) to the ear for more than
8hours in a day. Local ordinances also set noise
limits within populated areas.
Generator systems are supplied in an open
configuration for inside installations and in
weather-protective enclosures for outside
installations. In noise-regulated areas, fully sound
attenuated enclosure will keep sound emissions
within given limits.
Kohler measures the sound emitted by all its
generator systems, whether open or fully sound
attenuated, to ensure the generator selected is
within site location noise emissions.
HOW SOUND IS MEASURED
Sound measurements, to international standards,
are taken for all three generator configurations:
• Weather-protective enclosure
• Level “1 to 2” sound enclosures
• Open generator set
Sound measurements can be affected by
surrounding structures, humidity, topography,
etc. For consistency, measurements are taken in
an open area to set industry standards.Figure 4.
MEASURING SOUND OF ENCLOSED SET
Fully enclosed generator sets, whether in
a weather protective canopy or fully sound
attenuated enclosure, are tested unloaded
and loaded up to 100% of rated load. The
measurements will include all sound sources
including the exhaust. Sound readings, with an
approved decibel meter set to the “A” weighted
scale, are taken at eight equidistant measurement
locations 7 meters (23 feet) from the profile. The
microphones are set at 1 meter high as detailed in
Figure 4.
HEMISPHERICAL FREE — FIELD SOUND ENVIRONMENT
7 m
1 m Spaced at eight 7m intervals:
1) Noon
2) 1:30
3) 3:00
4) 4:30
5) 6:00
6) 7:30
7) 9:00
8) 10:30
8 Microphones set
at same height R e fle c t i v e S u r f a c e A s p h a l t / C o n c r e t e 5 4
2 1
3 6
7 8
G r o u n d : L e v e l G r a d e
Figure 4
Figure 3: Sound Pressure Levels dB(A) – Enclosure Level-1 Sound Measurement
Clock Position
Octave Band Frequency (Hz) Overall
Level63 125 250 500 1000 2000 4000 8000
3:00 67.8 75.4 76.3 77.6 76.0 76.1 68.5 62.7 83.6
1:30 68.1 78.5 86.4 90.9 85.7 82.6 81.9 72.7 93.9
Noon Engine 67.8 81.6 87.8 90.8 89.3 83.2 78.6 72.0 95.3 10:30 66.0 74.8 84.6 87.7 82.2 78.7 78.8 70.6 90.9
9:00 60.6 70.7 74.2 76.2 69.9 67.0 61.6 60.8 79.9
7:30 61.5 74.0 77.2 78.8 77.0 77.9 69.9 60.3 84.4
6:00 64.7 71.7 72.8 69.2 67.7 65.2 57.0 46.3 77.4
4:30 63.8 74.7 75.9 76.3 76.1 75.0 69.1 63.9 82.9
8-pos. log avg. 65.8 76.5 82.7 86.1 82.8 78.8 76.2 68.2 90.0 Per Figure 4 Measurements are taken in an open area with eight microphones set in clock positions indicated above.
The chart depicts sound measurements with the 100% generator loaded.
POWER SYSTEMS TOPICS 119
Generator Sound Attenuation and Measurement / 3
MEASURING SOUND OF OPEN SET
Open generator sets with a mounted silencer are
tested similar to enclosed generators. Figure 4.
MEASURING OPEN SET SOUND &
ISOLATED EXHAUST
As many interior installations are an open
generator set with the exhaust piped to an
externally mounted muffler, sound measurements
of an open set, per Figure 4, are taken, but with
the muffler remotely mounted to ensure negligible
exhaust noise emission within the test area.
Customers purchasing open units for installation
within a building use these noise measurements
to determine if additional attenuation is required.
In some cases, even with a remotely mounted
silencer, additional engine room attenuation is
required to be within the specified noise limit.
MEASURING RAW EXHAUST SOUND
Raw exhaust refers specifically, and only, to the
noise emitted by the engine exhaust system when
no silencer is present. It does not include noise
from other sources on the generator set.
Sound data, at one location, is collected at the
end of a straight pipe connected directly to the
engine exhaust as indicated in Figure 5. The
measurement is taken at a close distance of
1 meter (3.3 ft.) to ensure only the exhaust noise
is significant in the noise measured.
Close proximity raw exhaust measurements
are equally useful in determining silencers for
mounting to, or remotely from, the generator.
The raw exhaust sound data per Figure 6 is
provided as engineering data for use in silencer
design or selection.
IMPORTANCE OF SOUND FOR
POWER GENERATION
CALCULATING THE SOUND AT VARYING
DISTANCES FROM SOURCE
While standardized sound measurements taken
by the manufacturer are set at 7 meters (23 ft),
frequently, to meet sound ordinances at any
particular location, sound readings have to be
taken at varying distances.
When there are no reflecting surfaces to amplify
the noise of the generator set, the lot line sound
(distance from noise source) reduces by 6 dB(A)
each time the distance is doubled, or the square
of the distance from the source Figure 7. This
formula assumes the space to the lot line sound
source is free and clear.
From Engine at 100%
Load
Flow
0.707 m
0.707 m
1 m
Microphone
Measurement
Location
Straight-Pipe
Exhaust
Raw Exhaust Sound Test
Sound Pressure Levels dB(A) – Raw Exhaust (No Silencer) Measurement
Position
Octave Band Frequency (Hz) Overall
Level63 125 250 500 1000 2000 4000 8000
Pipe End 93.1 106.6 116.8 120.4 121.5 122.4 122.4 123.0 129.7 Figure 5
1 5
1 10
15 20
25 30
35 3 5 7 10 15 20 30 40 50 60 70 80 90 100
Reduction of sound (dB) over distance
Distance (meters)
Attenuation (dB)
Figure 7
Figure 6
4 / Generator Sound Attenuation and Measurement
118 Feet= 35.96 Meters
MECHANICAL NOISE VERSUS EXHAUST NOISE
A generator system is a complex piece of
equipment made up of many moving parts.
Movement of each part creates vibration, which
then interacts with the air to create sound at
various frequencies. Figure 6 details various
noise sources generated by mechanical vibration
measured in dB(A) at 1meter.
AMBIENT NOISE AND OTHER STRUCTURES
AROUND A GENERATOR ENCLOSURE
While testing enclosed generator systems in a
free-field environment away from surrounding
structures provides an accurate measurement
of sound pressure received at a set distance,
the actual sound pressure perceived by the
ear will vary from location to location.
Many locations will be in a reverberant field where
sound pressure can be amplified as it is reflected
from a building or other obstacle.
To meet local ordinances, sound measurements
may also have to be taken at the final location.
7 1
3 56
Sources of noise within a
generator system.
1 - Fan
2 - Raw Open Exhaust
3 - Air Inlet
4 - Alternator
5 - Generator Fan
6 - Turbochargers
7 - Engine Block
8 - Valve Covers
2 8 4
Figure 6
POWER SYSTEMS TOPICS 119
Generator Sound Attenuation and Measurement / 5
DEFINING ENCLOSURE SOUND LEVELS
TYPES OF ENCLOSURE
The majority of generator systems installed
and/or used in an outdoor location, whether
mobile or fixed, are enclosed. All types of
enclosures will limit the noise sources to some
degree, but the final degree of audible sound
permitted will dictate the type of enclosure.
Enclosures usually fall into three categories:
• Weather-protective enclosure
• Sound attenuated enclosure to level 1
and 2 (dB(A) reduction)
• Custom enclosures – level 3 and above
WEATHER-PROTECTIVE ENCLOSURES
A weather protective enclosure, usually sheet
steel or aluminum, provides some limited
reduction in noise. Vibration isolators between
major components provide further attenuation
by reducing operational vibration that creates
noise. Redirection of air before it exits the sound
enclosure also helps reduce sound levels.
When the applicable installation noise
ordinances are no lower than 80 dB(A) at 7
meters, consideration should be given to the
exhaust silencer before adding further sound
attenuation material.
See Figure 8. This chart details the comparative
reductions in exhaust noise for each silencer
type. However, final ambient noise will be
influenced by the characteristics of the engine
and silencer placement.
Generator Silencer Grades
Silencer Type Exhaust Noise
Reduction, dB(A)*
Site Attenuation
Requirement
Hospital 32 – 42 Very High
Critical 25 – 35 High
Residential 18 – 25 Medium
Industrial 12 – 18 Low – Medium
*Listed exhaust noise reductions are typical; actual performance may vary.
ENCLOSURES LEVEL-1 AND LEVEL-2 ATTENUATION
1 - Typical sheet metal level-1 enclosure provides
limited noise reduction
2 - Sheet metal enclosure
3 - To attain level-2, sides are lined with sound
absorbing material
4 - Silencer in enclosure will be critical grade for
level 2
1 2
3 4
Figure 9
Figure 8
6 / Generator Sound Attenuation and Measurement
SOUND ATTENUATED ENCLOSURE
LEVEL 1 AND 2
When a basic weather-protective enclosure
has insufficient attenuation to meet site noise
ordinances, the next stage is to line the inside
panels of the enclosure sides with sound
absorbing material. A level 1 enclosure will reduce
noise in the order of 3 dB(A). See
Figure 9 for details.
Additional absorbing material over louvers
through which air flows in and out will achieve
level 2 noise reduction of around 10 dB(A). It
is common to apply this type of enclosure in
critical environments such as healthcare facilities
and populated work areas. Also, levels 1 and 2
enclosures are usually fitted with residential or
critical exhaust silencers.
CUSTOM ENCLOSURES LEVEL 3 AND ABOVE
Some generator systems, such as those
supplying power for film production, live concerts,
or nighttime use, have to meet significantly lower
noise levels. In these applications, the enclosure
will be to level 3 and above reduction, with at
least a 14 dB(A) drop in measured sound.
(Note: Sound is measured on a logarithmic scale,
this is significant attenuation.)
Sound pressure is transmitted through air.
The further the air travels through outlets lined
with noise-absorbing material, the greater the
attenuation before air leaves the enclosure.
However, air is used for combustion and cooling,
any redirecting has to avoid restriction to avoid
overheating. The solution in a level 3 type of
enclosure is not to restrict cooling but redirect
air via sound outlets covered in sound absorbent
material before it exits the enclosure.
With this technology, when applied with
increased vibration isolation and a higher-grade
exhaust silencer, it is possible to reduce noise
emitted from the enclosure to a level that one can
have a discussion next to the generator running
on full load. See Figure 10. The same type of
technology can be applied to open sets installed
within a building.
SAMPLE DESIGN FEATURES TO ATTAIN LEVEL-3 SOUND ATTENUATION 1 - Sound attenuated louvers 5 - Noise absorbent lining on ducting 2 - Ducted air redirected at right angles reduces noise 6 - Secondary silencer 3 - Spring vibration isolators with rubber pads 7 - Noise absorbent lining on internal siding 4 - Flexible exhaust bellows 8 - Sheet metal enclosure 8 22
4 6 7
1 35
POWER SYSTEMS TOPICS 119
Figure 10
Generator Sound Attenuation and Measurement / 7
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POWER SYSTEMS TOPICS 119
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