Vibro-Acoustics®

GUIDELINE MASTER
SILENCER SPECIFICATIONS

A.      GENERAL

1.      Silencers shall be of the size, configuration, capacity and acoustic performance as scheduled on the drawings.  All silencers shall be factory fabricated and supplied by the same manufacturer.  Silencers shall be Vibro-Acoustics (basis of design). 

2.      Alternate manufacturers must request and obtain written approval by the Engineer to bid the project at least 10 days prior to the bid due-date.  As a condition of pre-approval, alternate manufacturers must submit to the Engineer a minimum of twenty (20) different HVAC silencer test reports.  Each report shall be for a silencer tested in full accordance with the ASTM E-477-99 silencer test standard in an aero-acoustic test facility which is NVLAP accredited for the ASTM E-477-99 standard.  Each test shall have been conducted within the last 12 month period.  A copy of the laboratory’s NVLAP accreditation certificate must be included with the submitted reports.  Any changes to the specifications must be submitted and approved in writing by the Engineer at least 10 days prior to the bid due-date.

3.      Silencer inlet and outlet connection dimensions must be equal to the duct sizes shown on the drawings.  Duct transitions at silencers are not permitted unless shown on the contract drawings.

B.     MATERIALS

1.   Rectangular Silencers:  All rectangular silencers, including models RMB, RD, RFMB, RFL, RNM, EX-RMB, EX-RD, EX-RFMB, EX-RFL, EX-RNM, RLP-MB, RLP-D, RLP-FMB, RLP-FL, RLP-NM, SRMB, SRD, SRFMB, SRFL and SRNM shall be constructed with a 22 gauge galvanized steel outer casing and 26 gauge galvanized perforated steel.

2.   Circular Silencers:  All circular silencers, including types CMB, CD, CFMB, CFL, CLP-MB, CLP-D, CLP-FMB, CLP-FL, CLP-NM, AC-MB, AC-D, AC-FMB, AC-FL and AC-NM shall be constructed with a galvanized steel casing as noted below and 22 gauge galvanized perforated steel.  All casing seams and joints shall be lockformed and sealed or stitch welded and sealed.

                                                Casing Diameter                                 Casing Gauge

                                                Less than 30"                                                  20

                                                30" to 54"                                                         18

                                                Over 54"                                                          16

      All CNM and CENM circular silencers shall be constructed with a 22 gauge galvanized steel casing and 26 gauge galvanized perforated steel.  All casing seams and joints shall be lockformed and sealed.

                                                Connection Diameter                          Casing Gauge

                                                Less than 18"                                                  22

                                                18" to 30"                                                         20

                                                30" to 54"                                                         18

                                                Over 54"                                                          16

3.   Elbow Silencers:  All elbow silencers, including models REMB, RED, REFMB, REFL, RENM, EX-REMB, EX-RED, EX-REFMB, EX-REFL, EX-RENM shall be constructed with an 18 gauge galvanized steel outer casing and 22 gauge galvanized perforated steel.  All acoustical splitters shall be internally radiused and aerodynamically designed for efficient turning of the air.  Half and full splitters are required as necessary to achieve the scheduled insertion loss.  All elbow silencers with a turning cross-section dimension greater than 48” shall have at least two half splitters and one full splitter.

4.   Transitional Silencers:  All transitional rectangular silencers, including models TMB, TD, TFMB, TFL and TNM shall be constructed with a 22 gauge galvanized steel outer casing and 22 gauge galvanized perforated steel.  Transitioning shall occur internal to the silencer such that the height of the gap or air passage is uniformly changing with the length of the splitters.

5.      Acoustic Media:

5.1    MoldBlock Media^TM silencers and Film lined MoldBlock Media^TM silencers, including models RMB, RFMB, CMB, CFMB, REMB, REFMB, TMB, TFMB, EX-RMB, EX-RFMB, EX-REMB, EX-REFMB, RLP-MB, RLP-FMB, CLP-MB, CLP-FMB, AC-MB, AC-FMB, SRMB and SRFMB:

         Media shall be Vibro-Acoustics^® MoldBlock Media^TM containing 100% natural cotton fibers treated with an EPA registered, non-toxic borate solution, “flash dried” to provide resistance to mold, mildew and fungi.  Media shall comply with UL181 and NFPA 90A.  MoldBlock Media^TM shall be packed with a minimum of 15% compression during silencer assembly.  Media shall not cause or accelerate corrosion of aluminum or steel.  Glass fiber, fiberglass and rockwool will not be permitted as a substitute for MoldBlock Media^TM .

5.2    Dissipative and Film Lined silencers, including models RD, RFL, CD, CFL, RED, REFL, TD, TFL, EX-RD, EX-RFL, EX-RED, EX-REFL, RLP-D, RLP-FL, CLP-D, CLP-FL, AC-D, AC-FL, SRD and SRFL:

         Media shall be of acoustic quality, shot-free glass fiber insulation with long, resilient fibers bonded with a thermosetting resin.  Glass fiber density and compression shall be as required to insure conformance with laboratory test data.  Glass fiber shall be packed with a minimum of 15% compression during silencer assembly.  Media shall be bacteria and fungus resistant, resilient such that it will not crumble or break, and conforming to irregular surfaces.  Media shall not cause or accelerate corrosion of aluminum or steel.  Mineral wool will not be permitted as a substitute for glass fiber.

5.3    No-Media silencers, including RNM, CNM, RENM, CENM, TNM, EX-RNM, EX-RENM, RLP-NM, CLP-NM, AC-NM and SRNM:

         All No-Media silencers shall not contain absorptive media of any kind.  Attenuation shall be achieved with controlled impedance membranes and broadly tuned resonators.

6.      Media Protection:

6.1    Dissipative silencers, including models RD, CD, RED, TD, EX-RD, EX-RED, RLP-D, CLP-D and SRD:

         Where indicated on the silencer schedule, media shall be encapsulated in glass fiber cloth to help prevent shedding, erosion and impregnation of the glass fiber.  All AC-D Axial Cone silencers shall have a glass fiber cloth liner.

6.2    Film Lined silencers, including models RFMB, RFL, CFMB, CFL, REFMB, REFL, TFMB, TFL, EX-RFMB, EX-RFL, EX-REFMB, EX-REFL, RLP-FMB, RLP-FL, CLP-FMB, CLP-FL, AC-FMB, AC-FL, SRFMB and SRFL:

The acoustic media shall be completely wrapped with Tedlar film to help prevent shedding, erosion and impregnation.  The wrapped acoustic media shall be separated from the perforated metal by a factory installed ½” thick acoustically transparent spacer.  The spacer shall be flame retardant and erosion resistant.  A mesh, screen or corrugated perforated liner will not be acceptable as a substitute for the specified spacer.

7.      Combustion Ratings: 

7.1    MoldBlock Media^TM silencers, including models RMB, CMB, REMB, TMB, EX-RMB, EX-REMB, RLP-MB, CLP-MB and SRMB:

          Silencer materials, including acoustic media shall have maximum combustion ratings as noted below when tested in accordance with ASTM E84, NFPA 255 or UL 723.

Flamespread Classification:               10

            Smoke Development Rating:              50

7.2    Dissipative silencers, including models RD, CD, RED, TD, EX-RD, EX-RED, RLP-D, CLP-D and SRD:

          Silencer materials, including glass fiber shall have maximum combustion ratings as noted below when tested in accordance with ASTM E84, NFPA 255 or UL 723.

Flamespread Classification:               15

            Smoke Development Rating:              5

7.3    Film Lined silencers, including models RFMB, RFL, CFMB, CFL, REFMB, REFL, TFMB, TFL, EX-RFMB, EX-RFL, EX-REFMB, EX-REFL, RLP-FMB, RLP-FL, CLP-FMB, CLP-FL, AC-FMB, AC-FL, SRFMB and SRFL:

Silencer materials, including acoustic media, Tedlar film and acoustical spacer shall have maximum combustion ratings as noted below when tested in accordance with ASTM E84, NFPA 255 or UL 723.

Flamespread Classification:               20

                  Smoke Development Rating:              45

8.   HTL Casings:  Where indicated on the silencer schedule, silencers shall have high transmission loss (HTL) walls externally applied and completely sealed to the silencer casing by the silencer manufacturer to assure quality controlled transmission loss.  The HTL walls shall consist of media, airspace, mass and outer protective metal skin, as required, to obtain the specified room noise criteria.  Standard acoustical panels will not be accepted as HTL walls.  If requested by the Engineer, breakout noise calculations for each air handling and fan system shall be provided with the silencer submittal to insure compliance with the room noise criteria.  Breakout noise calculations shall be based on the sound power levels of the specified equipment. 

9.      Access Doors:  Where indicated on the silencer schedule, silencers shall be supplied with an access door(s) to permit fire damper service.  Access doors shall be supplied as an integral part of the silencer by the silencer manufacturer.  Where HTL walls are also supplied, the access doors shall not reduce the effectiveness of the HTL walls.

C.     CONSTRUCTION

1.      Silencers shall be constructed in accordance with ASHRAE and SMACNA standards for the pressure and velocity classification specified for the air distribution system in which it is installed. Material gauges noted in “Section B Materials”, are minimums.  Material gauges shall be increased as required for the system pressure and velocity classification.  The silencers shall not fail structurally when subjected to a differential air pressure of 8 inches water gauge.

2.      Casings shall be lockformed and sealed, except as noted in Section B Materials, to provide leakage-resistant construction.  Airtight construction shall be achieved by use of a duct-sealing compound supplied and installed by the contractor at the jobsite.

3.      All perforated steel shall be adequately stiffened to insure flatness and form.  All spot welds shall be painted.

D.     ACOUSTIC PERFORMANCE

1.      Silencer dynamic insertion loss shall not be less than that listed in the silencer schedule.

2.      Silencer generated noise shall not be greater than that listed in the silencer schedule.

3.      Acoustic performance shall include dynamic insertion loss and generated noise for forward flow (air and noise in same direction) or reverse flow (air and noise in opposite direction) in accordance with the project’s air distribution system requirements.

4.      All silencer ratings shall be determined in a duct-to-reverberant room test facility which provides for airflow in both directions through the test silencer in accordance with the ASTM E-477-99 test standard.  The test set-up, procedure and facility shall eliminate all effects due to flanking, directivity, end reflection, standing waves and reverberation room absorption.

E.     AERODYNAMIC PERFORMANCE

1.      Silencer pressure drops shall not exceed those listed in the silencer schedule.  Silencer pressure drop measurements shall be made in accordance with the ASTM E-477-99 test standard.  Tests shall be conducted and reported on the identical units for which acoustical data is presented.

2.      The manufacturer shall provide Computational Fluid Dynamics (CFD) aerodynamic analysis of the silencer as indicated in the silencer schedule. The analysis shall include the attached ductwork, a minimum of 5 equivalent duct diameters up and downstream of the silencer, as shown on the drawings, to determine silencer pressure drop, including system effects, at design airflow. The manufacturer must report and validate a converged solution domain of the CFD analysis to show the solution is independent of mesh refinement such that two models of different mesh refinement levels produce equivalent results, each with a maximum residual tolerance of 0.001.  The minimum cell count shall be 200,000 and the validation model shall have a cell count at least 50% higher.   The manufacturer must report the selection of CFD parameters, including mesh type, mesh size, boundary conditions, convergence criteria, and turbulence model. Each CFD analysis shall also include additional post-processed information including number of iterations, convergence status, and resulting y+ values.

F.      SUBMITTALS

1.      Provide acoustical system calculations for all duct systems with silencers to demonstrate that the resultant ductborne sound levels of the equipment as measured in the occupied spaces meet the specified criteria.  In the absence of specified background sound level criteria, the guidelines as expressed in Table 34 of Chapter 47, “Sound and Vibration Control” of the 2003 ASHRAE Handbook - HVAC Applications, shall be used.

2.      The manufacturer shall supply certified test data for each scheduled silencer.  The data shall include dynamic insertion loss, generated noise and pressure drop for forward or reverse flow, matching the project’s air distribution system requirement.  All ratings shall be conducted in the same facility and shall utilize the same silencer.

3.      The manufacturer shall test the silencer(s) as indicated in the silencer schedule.  The engineer shall be notified of the test date at least two weeks in advance and the test may be witnessed by the engineer.  Test shall show compliance with the project criteria and is subject to engineer approval.

4.      Test facilities and test reports shall be open to inspection upon request from the Engineer. Silencer performance must have been substantiated by laboratory testing according to ASTM E-477-99 and so certified when submitted for approval.  The aero-acoustic laboratory must be NVLAP accredited for the ASTM E-477-99 test standard.  A copy of the accreditation certificate must be included with the submittals.  Data from non-NVLAP accredited test facilities will not be accepted.