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Purolator Acoustic Porous Metals
Products &
Applications
Purolator Facet Inc. offers
an extensive line of porous metal face sheet membrane material for engine
nacelle acoustic treatment. Rayl values from 3 to 140 CGS have been
produced with or without a supporting perforated sheet. Rayl value
is normally defined as a ratio of pressure drop to flow. It is selected
based on sound intensities and pressure levels. Panel sizes are
20" x 30" to 24" x 48" and non-linearity factors are usually less than
1.4 based on 200/20 CGS rayl readings. Thickness varies from .005"
to .60".
Purolator produces three basic types of acoustic face sheet material:
- Poromesh®--Single layers of wire cloth diffusion bonded at points of crossover and calendered.
- Poroplate®--Multi-layers of wire mesh, all diffusion bonded and calendered.
- Poroplate®--Mesh and perforated plate combinations with one or more layers of wire cloth diffusion bonded and calendered to obtain given rayl value and then diffusion bonded to perforated plate.
Purolator acoustic Poromesh® and Poroplate® diffusion bonded porous metals are used to provide acoustic flow resistance for Helmholtz resonator sound absorption applications in jet engine nacelles and other noise abatement applications. The applications include face sheet and septum material for single or two-degree of freedom treatments.
The face sheet or septum porous metal membrane operates as a viscous damper of air particle velocities and is usually located about one fourth of the wave length of a peak or dominant frequency from the wall of the resonator cavity. The flow resistance required for optimum operation is determined by analysis of sound energy levels, complex acoustic impedance requirements for a sound absorbing structure and correlation of these with steady state fluid flow resistance of permeability values.
Porous material permeability is measured by gas flow testing candidate material over the required range of flow rates corresponding to face velocities of 20 to 500 centimeters per second (cm/sec). The resultant data is then reduced to show acoustic resistance in centimeter/gram/second rayls as a function of flow velocity and plotted on standard logarithmic graph paper.
Plotting rayl values as a function of low velocity on linear graph paper normally approximates a straight line, the slope of which can serve as a measure of non-linearity. Lower ratios of non-linearity are preferred since they permit acoustic treatment over a wide band of operating conditions (primarily due to temperature and resultant air density).
Non-linearity factor (NLF) is normally defined as a ratio of two rayl values at two different velocities, i.e., R 200/20.
Selections from the above
combinations are made based upon:
1. Intended applications
(duct work, nacelle, face sheet or septum)
2. Rayl value
3. Linearity
4. Forming over
constant or changing radii
5. Structural strength
(torsional and beam and tensile)
6. Protection from
galvanic corrosion by isolation
7. Bonding to perforated
plate (metallic to metallic)
8. Drum face sonic
load
9. Thermal shock
10. Alloy 304L, 316L, nickel,
Inconel, Monel
11. Weight
12. Thickness
13. Panel size: 18" x
36", 18" x 48", and 28" x 36", 24" or 48"
These alternatives have been
mentioned since each has specific advantages of cost, forming, boundary
layer control and strength. These considerations are important to
most developmental programs.
Acoustic Technical Data Q & A:
What materials (and associated
lead times) are used?
We have currently developed
acoustic face sheet materials in stainless steel alloys of 304L and 316L
with lead times of 6-8 weeks depending on quantity.
What sheet size is offered?
We presently offer acoustic
media in sizes up to 24" x 48".
What time, temperature
and environmental (i.e., atmospheric) controls are employed in heat treatment
of acoustic media?
All Purolator diffusion bonded
and/or annealed products (as well as acoustic media) are manufactured
in a controlled, reduced-atmosphere environment under extreme pressure.
The result is an oxidation-free product without sensitization or delamination
problems. All heat treatment and final inspection is performed in
accordance with rigid Purolator processes and product specifications to
ensure conformance to design and contract requirements by part number
and drawing.
What are the forming capabilities?
Fully annealed acoustic media
is adaptable to all conventional forming and shearing operations within
its metallurgical limits and without significant change to its nominal
rayl value.
What is the rayl number
verification?
Purolator uses a 2" diameter
flow port opening and Meriam laminar flow elements integrated with pressure
transducers and digital readout. Flow data is produced via a computer
program to a graph of rayls vs velocity (CGS).
Acoustic
Sheet Summary
| Type | Part No. | R 100 | NLF 200/20 | #/Ft2 | Thick (") | Alloy | Size (") | Perf. Thk. |
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| Type | Part No. | R 100 | NLF 200/20 | #/Ft2 | Thick (") | Alloy | Size (") | Perf. Thk. |
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without perf |
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| Type | Part No. | R 100 | NLF 200/20 | #/Ft2 | Thick (") | Alloy | Size (") | Perf. Thk. |
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with perf |
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