OEM Applications
Before Selecting Acoustical Materials:
- Identify noise source components and
where possible determine the relative overall
dB levels and frequency distribution.
- Relocate or remove noisy components
away from operator areas.
- In air flow systems utilize proper aerodynamic
principles to minimize noise
generated by turbulence.
- Reduce operating speeds of rotating
components.
- Alter operating speeds to avoid coincidence
with equipment resonant frequencies.
- Isolate and decouple rotating components
from the supporting structure.
- Utilize flexible connections on all equipment
intakes, exhaust lines, electrical
conduit and other service or utility lines.
- Where practical, manufacture OEM
component parts where impingement or
impact takes place out of plastics or
other non-metallic materials with better
inherent damping qualities. Examples
include gears, rollers, stops and guides.
- Minimize the use of thin gauge sheet
metal for large surface area equipment
panels and as necessary use stiffeners
and bracing to limit structural resonance
that generates airborne noise.
- Minimize the percentage of open area in
panels designed to contain noise.
- Utilize closed cell gasketing and seals
around doors and removable panel
sections.
- Reduce drop heights on parts or material
impacting hoppers, chutes and parts bins.
- Where practical, utilize expanded metal
instead of sheet metal for belt guards.
Product Cross
Sectional View | Component
Layers | Recommended Uses |
| Adhesive Absorber Facing | Used to line the inside of cabinets, compartments and
panels where the existing skin provides the necessary
transmission loss. Increasing the thickness and/or density
increases low frequency acoustical absorption. |
| Adhesive
Damping
Absorber
Facing | Used to line the inside of cabinets, compartments and
panels where it is necessary to add mass to the existing
skin, reduce structural resonance and absorb airborne
sound. The absorber layer faces the noise source. |
| Adhesive
Absorber
Barrier | Used to line the outside surfaces of equipment housings,
connected ducts, pipes and other guards and panels
where inside surfaces are not practical to treat. The
absorber layer acts as a decoupler/spacer to enhance the
transmission loss of the barrier. |
| Adhesive
Absorber
Barrier
Absorber
Facing | Used to line the inside of cabinets, compartments and
panels where the existing skin is light gauge and not capable
of providing enough transmission loss. To get the full
benefit from these decoupled absorber/barrier composites
the percentage of open area in the cabinet or panel
should be 10% or less of the total surface area. |
| Adhesive
Damping | Used to line inside or outside surfaces of thin metal or
other rigid surfaces to reduce noise generated by structural
resonance. This product group can be applied in sheets
or using a liquid damping compound. |
| Typical source control treatment at left. QWYATTCORE
acoustical materials are used to line OEM portable air
compressor housings. |
Fire Safety
The most commonly utilized acoustical foams for
OEM applications are polyester and polyether
polyurethane materials rated at UL94-HF1. These materials
will burn in the presence of a flame and give off
toxic combustion products. Although the UL94 rating
carries a "self-extinguishing" designation, this terminology
is not intended to reflect properties of the material
under actual fire conditions. Many of the fiberglass
based products and quilted blankets carry class A
ratings as per the stringent ASTM E-84 tunnel test. The
E-84 class A rating conforms to most fire hazard building
code regulations. Some specialty foams have been
developed to meet this class A rating.
Density
Performance of absorber products is directly related
to the density. In many cases increased density for
a given thickness results in increased absorption
ratings most dramatically in the low frequency range
and reduced performance in the high frequencies.
This begins when the absorber product becomes so
dense that it begins to take on characteristics of a
barrier thus reflecting some of the short wavelength
high frequency noise. Density of damping treatments
does not usually have a well defined effect on performance
although adding more mass to the surface will
ultimately change its natural frequency. In barrier
materials, doubling the density of the barrier increases
the transmission loss by 6dB.
Thickness
Material thickness of absorber products has much the
same effect as increasing product density with increased
performance in the lower frequencies. Degradation of higher
frequency absorption with increased thickness is not typical.
Increasing absorber thickness yields a small incremental
increase in absorption in high frequencies compared to the
increase in low frequencies. In damping materials, thickness
of the coverage as it relates to the treated surface thickness
will affect performance. As a general rule the damping material
should be at least equal to the thickness of the surface it
is applied to and two or three times if high loss factors need
to be attained. In barrier materials, increasing the thickness
changes performance as defined by the mass law which
states that transmission loss will increase by 6dB for each
doubling of the mass or frequency.
Coverage
Performance of absorber products is not coverage dependent.
The function of the absorber materials is to dissipate
acoustic energy and limit reverberant build-up. As a general
rule 75% of the noise build-up can be eliminated inside an
enclosure or compartment with as little as 50% coverage of
inside reflective surfaces. Likewise, damping treatments and
coatings are not coverage dependent. Attacking surface areas
where vibrational motion is most prevalent is more important
than 100% coverage. A40% to 60% coverage is usually sufficient.
To the contrary barrier materials rely on complete
coverage as close to 100% as possible to realize their full
acoustic performance. Potential practical limitations for various
coverages are as follows: maximum 10dB reduction for
90% coverage, 15dB reduction for 98% coverage and 20dB
reduction for 99% coverage.
Facings
Usually it is necessary to incorporate some type of thin
membrane facing to cover the absorber layer exposed to
the noise source inside a cabinet or enclosure. This protects
the product from contamination and provides a surface that
can be wiped down. As long as the film facing is in the 1 to
4 mil thickness range there will be a minimal effect on
acoustic performance. Many times there is only a frequency
shift with slightly lower absorption in higher frequencies
and slightly higher absorption in lower frequencies.
Damping and barrier materials are many times part of
composite products not exposed to the environment and
are not covered with facings. Some typical facings are
Tedlar, mylar and urethane.
Installation
Most products are available in standard rolls, sheets or die
cut to meet OEM specifications. The products can usually be
hand cut using a utility knife, scissors, band saw or other
common cutting tools. Attachment is recommended with solvent
based contact adhesives for the urethane foam absorbers and
composites. Adhesive recommendations should be reviewed in
detail at the time of application as special considerations must be
made depending on the surface shape and preparation, whether
the surface is oriented horizontal or vertical and what working
time is required. Pressure sensitive adhesive systems (PSA) are
available for most products and are highly recommended. High
tack acrylic based PSA backings are preferred to assure the best
bond. Stick clips, insulation pins and other mechanical fasteners
may be necessary in addition to the adhesives.
| Sizing Machinery Mufflers
Many OEM process systems utilize rotary-positive blowers that will require
mufflers or silencers on both the intake and discharge. Without such treatment
noise levels could easily be in the 110 to 115dBA range. Blower sizes are
described in inches (for example 12x25) where the first number represents the
timing gear diameter and the second number represents the length of the rotor.
The product of the gear circumference in feet and the blower RPM is the peripheral
velocity of the timing gear. Noise and pulsation produced by rotary-positive
blowers inherently reaches a critical level at gear pitch-line velocities of about
3300 feet per minute (FPM) for intakes and 2700 FPM for discharges. These critical
gear pitch-line velocities are commonly referred to as the blower transition
speeds. Muffler selections shown in the QWYATT FLOW muffler product section
(page 154) make reference to the blower transition speed. Consult QWYATT Sales
Engineers on muffler applications for process systems and for other equipment
such as compressors, engines, generators, turbines, etc... |
Designing Sound Blankets
Custom fit removable thermal/acoustic blankets such as the one
shown at right are designed based on the equipment or component field
measurements and/or housing/casing drawings. For proper construction
and fit it is necessary to determine the equipment casing surface temperature.
The inner and outer jacketing should be completely water resistant
and suitable for both caustic and acidic environments. For durability,
all blanket construction should be double sewn lock stitch with a
minimum of 7 stitches per inch. All mating match seam blankets should
have an overlapping flap cover which is also an extension of the loaded
vinyl flexible sound barrier layer (internal to the blanket). This is essential
for minimizing noise leaks. Stainless steel quilting pins should be
utilized no greater than 18" apart to prevent shifting of the insulation.
Hog ring construction should be avoided wherever possible in order to
assure the highest blanket design quality possible. | |
|
Selecting Ventilation Silencers
Fan and HVAC silencers are commonly used on low pressure air handling
systems and equipment. Acoustic louvers are utilized in conjunction with
enclosure or mechanical room ventilation where noise isolation is important.
The fundamental tone or frequency that a fan produces is a function of each
blade on the fan wheel passing the cut-off sheet on the fan housing. This is
commonly referred to as the blade passage frequency (BPF). The first and
second multiples of the BPF (harmonics) are prominent but less critical.
Calculating the BPF is done by multiplying the fan RPM times the number of
blades on the fan wheel, divided by 60 (converts to cycles per second or
Hertz). Silencer performance can be approximated by finding the rated insertion
loss of the silencer in the octave band where the BPF falls. |
Designing Acoustical Enclosures
Noisy equipment and systems may require
path control modular acoustical enclosures
where source treatments are not practical or
effective. Sound enclosures can be freestanding
structures, partially integrated or fully integrated
with the equipment. Key design considerations
for OEM acoustical enclosures
include durability, aesthetic appeal, accessibility
and visibility, acoustic performance, cost
of installation and material manufacturing
costs. Depending on the type of equipment,
system or process; lighting and ventilation are
also important considerations. The goal of the
product engineer is to meet the acoustic
requirements with a design that incorporates
all the necessary features.
Enclosure Design Rule #1: Air Leaks
The chart at right shows how actual
sound transmission loss relates to the
enclosure wall transmission loss potential
(from lab test data) and the percent open
area. As shown in the example, as little as
2% open area over the entire surface area
of an enclosure reduces the 38dB potential
transmission loss to only 18dB actual
transmission loss. Air leaks occur around
windows and doors, at panel joints,
around cutouts to accommodate pipes,
ducts, utility lines and other obstructions
and where the enclosure does not seal
against the floor. Where enclosure ventilation
is required, openings that are not
acoustically baffled will further reduce
the actual dB transmission loss.
Effective Enclosure Design Will Assure
the Lowest $ per dB Cost |
Enclosure Design Rule #2: Ventilation
|
Heat build-up inside
acoustical enclosures can be
dissipated using forced ventilation
fans and blowers in a draw
through (see right) or blow through
design. The intake and discharge of
forced ventilation systems must be
acoustically treated with louvers, lined
baffles, hoods or duct silencers. The
owner or designer of the enclosed equipment
needs to determine what is sufficient
air flow to dissipate heat. The formula at
left is a good guideline. Proper placement
and location of intake and discharge
openings should ideally bring in air low on one side or end
of the enclosure and draw air over the equipment before
exiting high on the opposite end or side. |
Enclosure Design Rule #3: Access
|
Removable panel sections
(bottom left) can allow for infrequent
maintenance access in a
limited area. Hinged doors (at
right) provide for more frequent
access but require clearance for
the door swing outside the enclosure.
Sliding doors (see left) also
provide easy access without
protruding into work area space
outside the enclosure. Acoustical
performance of sliding doors is
less than that of hinged doors.
Double doors provide access to a
larger area. Sliding or removable
roof panels and sections are best
for major repairs requiring a crane. Small enclosure
designs can be a knock down design where all panels are
latched together for quick and easy disassembly. |
Near
Field |
The near field is the region close to a sound source usually defined as 1/4 of the longest wavelength
of the source. Near field noise levels are characterized by drastic fluctuations in levels as
much as 10dBA for small changes in distance from the source. Near field references can pertain
to both indoor and outdoor environments. |
Far
Field |
The far field describes a sound field beyond the near field limits described above where the sound
pressure level (SPL) drops off at the theoretical rate of 6dB for every doubling of distance from the
source. This rule of thumb is called the Inverse Square Law. Please note that if the far field does
not meet the criteria for a free field as described below, then less than the theoretical drop rate will
pertain. In such case doubling the distance from the source may yield a drop rate of 3-4dB. |
Free
Field |
To be considered free field there can be no obstructing surfaces in the sound path of spherical
wave propagation. Free field conditions are characterized by SPL loss rates following the Inverse
Square Law. Free field references pertain to large open outdoor spaces or in rooms where walls
and other surfaces are almost completely absorptive. Anechoic (without echoes) acoustical test
chambers simulate free field conditions where omnidirectional sound wave propagation exists. |
Direct
Field |
The direct sound field is also used to describe far field conditions that follow the Inverse Square
Law SPL loss rate of 6dB for every doubling of the distance. The actual formula used to make
calculations at various distances in the far/direct field is as follows: SPL1 [20x log (d2/d1)] = SPL2
where SPL1 is the noise level at the location closer to the source at a distance of d1 from the source
and SPL2 is the noise level at a location farther from the source at a distance of d2. |
Diffuse
Field |
In a diffuse field there are so many reflections contributing to the total sound field that sound levels
measured virtually anywhere in the sound field are the same. Diffuse fields usually pertain to
indoor environments. Rooms that are categorized as "live" have larger diffuse fields than free
fields. "Dead" rooms have much larger free fields than diffuse fields. |
Reverberant
Field |
The reverberant field is essentially the same as the diffuse field. For indoor sound field discussions
it is used to contrast direct fields. Reverberation test chambers have all room surfaces almost
completely reflective so that total sound energy remains constant throughout the environment and
sound levels can be measured independent of location and distance. |
Please refer to the figure below which shows the relationship
between sound fields.
Sound Fields Relative To Distances
From A Source
|