10th International Meeting
Low Frequency Noi and Vibration and Its Control
York, England 11-13 September 2002
Engine Test Cell Noi Emission Design
With Performance Validation Results
Jack B. Evans, P.E.
JEAcoustics
E ngineered V ibration A coustic & N oi S olutions
Abstract: An existing industrial test facility was propod to be relocated from a plant site in a high ambient noi environment to a community with low ambient noi. The existing facility contained engine test cells and support equipment with loud noi emissions. The original location was in an industrial park, adjacent to a high-speed multi-lane divided highway, but residential communities were
nearby. Few noi complaints had been received at the existing location. The propod future site was in a mi-rural area, near a two lane, moderate speed roadway. Although some moderate environmental noi emissions existed from an industrial installation on the existing site, the potential was recognized for community noi disturbance from introduction of a new noi source. JEAcoustics was retained to asss the ambient noi environments and noi emission characteristics for the purpo of determining attenuation requirements for the new facility. A consultant’s confidentiality agreement with the owner prevents disclosure of the facility name, plant locations, discussion of plant process or revelation of other proprietary information. This ca study prents the findings of that effort and the noi criteria that were established. Noi control designs and product applications are discusd with results of post-construction noi validation measurements.
Noi Control Design Issues
For business reasons, it became necessary to relocate an
industrial test facility to another existing plant location.
The propod relocation site is a mi-rural community
with moderate ambient noi. Community acceptance of
the facility required its environmental impact be minimal.
Among other issues, the noi contribution to the
environment could not be allowed to cau annoyance to
residents in the area. In addition, compliance with the
building code was required, including land u
compatibility and noi regulations. To achieve the
requirements, acoustical design criteria were required to
satisfy all parameters.
Allowable Noi Criteria
Noi measurements were conducted during late evening and morning hours in the community surro
unding the propod relocation site, for the purpo of establishing acceptable noi levels. Measured ambient sound levels included contributions from the existing plant facility. Noi reinforcing effects due to weather1 were taken into consideration. The findings were compared with the building code to determine a single noi criterion that would satisfy all requirements.
锲而不舍>什么狗适合家养Reference Day Night Factor Allowable
SBCCI Standard for Sound Control 2
SSTD 8-87, Table 303, Residential (R1) 60 dBA -5 dBA Tonality -5 dBA 50 dBA
Average Measured Community Ambient Noi, Evening (assume = nighttime) - - - 53 dBA Weather -3 dBA
50 dBA
DESIGN CRITERION: Property Boundary (night) 50 dBA (55 dBA if non tonal)
Noi Sources to be Mitigated
The noi sources to be relocated included test cell exhaust discharges from diel engines that might vary in size from 500 to 2000 horpower, and depending on the testing requirements, might operate continuously at a constant speed, or operate over a range of rpm’s. Other test cells contain apparata that discharge hot compresd gas (can not describe in detail due to confidentiality agreement). A group of (very tonal) helical screw air compressors provided process air for the test facility. A fabrication and support machine shop inside the building could produce transient impact and machine noi. Anticipated sources also included building air handling and exhaust fans, which were to be roof mounted.
The nighttime permissible noi criterion for tonal sources of 50 dBA controls at the facility property boundaries, which are at least 60 m in any direction from the propod site. At least 27 dB of distance loss could be expected, if the sound is not reinforced by large reflecting surfaces or atmospheric conditions. The existing building at the relocation site is larger and taller than the propod test facility, and conquently, acts as a barrier 3 to noi propagation in one direction. To be conrvative, 25 dB of distance loss was assumed at worst ca. Given a 50 dBA allowable at the property boundary, attenuation is required for noi sources on the site in excess of 75 dBA to assure compliance with the building code and the design criterion.
Sound Levels at Original Installation
炎热拼音Noi measurements were conducted at the original facility to determine source levels and spectra. A Larson-Davis 2900 two channel real-time FFT spectrum analyzer with precision microphone and pre-amp (ANSI Type I, + 1 dB)4 was ud to acquire and analyze data. Outdoor measurements were made with a windscreen. Measurements were conducted within the building, on the roof, near test cell exhaust discharges, and adjacent to the compressor room air inlet. Since the facilities were to be replicated at the new site, the measurement results were considered very reliable indicators of future conditions. The engine and hot gas test cell exhaust pipes incorporated mufflers, who inrtion loss would have to be factored out of the raw data for analysis to determine the true source levels. Sound level measurements were normalized to 3m (10’) from the sound sources (exhaust terminations and inlet air louvers). Data was acquired in 1/3 octave bands over short durations, 30 – 60 conds for continuous sources, and up to 3 minutes for varying level sources. The 1/3 octave spectra were saved for equivalent level or integrated average, Leq, the minimum, Lmin, and maximum, Lmax, levels during the sampled period. In general the Leq values were utilized as the reference source levels, with Lmin to Lmax values ud to determine deviation from the integrated averages.
* Measurements of Engine and Hot Gas discharges included attenuation from existing mufflers, estimated > 30 dBA.
Noi Source , r=3m (10’) LAmin LAeq LAmax ∆L Dominant A-wt. Octave
Diel Engine Test Cell* 83 84
86 3 250 – 500 Hz Hot Compr. Gas Cell* 82 88
92 10 500 – 2000 Hz Screw Air Compressors 71 76
78 9 Tones @ 200 & 400 Hz Avg. on Roof Perimeter : 3 Eng + 3 GS + 4 Compr 69 76
83 14
500 – 2000 Hz Ambient : Roof ~ 9 am 58 61
65 7 250 - 500 Hz
The noi spectra for various sources were analyzed for sound level, balanced spectrum, variability (difference between Lmin and Lmax), and tonality. Sideband differentials of 6 dB or more between 1/
烟花英语3 octaves are considered tonal5. In addition, the A-weighted octave spectra were studied to determine principal contributing frequencies to overall A-weighted level. In other
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words, the octave levels, decread by A-weighting factors, were plotted on level versus frequency charts to determine which frequencies contributed the most to dBA levels or audibility. For example, the engine noi, above, is greatest in the 125 Hz octave, but with A-weighting, the 250 – 500 Hz frequency span contributes the most to the A-weighted sum. The 1/3 octave spectra were then reviewed for tonality (large sideband differentials) and peak frequencies, such as the 200 Hz and 400 Hz helical screw compressor tones in the chart above.
Noi Attenuation Design Concepts
A multifaceted design approach was developed to address the various types of noi sources, and to achieve low noi levels with smooth, balanced spectra. Each type of noi source had distinct spectral, temporal and directional characteristics. Design concepts were developed to match attenuation frequency respons to noi source spectra, and to reduce tonal and intermittent (temporal) sources below the ambient levels at nsitive receivers.
Beginning within the building, absorption was specified to reduce build-up of reverberant sound within test cells and support equipment spaces. Wall, door, window and roof asmblies were designed to contain sound within the building, including vibration isolation and decoupling of element
s to reduce exterior surface radiated noi. Silencers were lected for air inlets, engine exhausts and hot compresd gas discharges. Bad on known locations of residential, commercial, and light industrial zoning, the direction of least nsitivity was determined, so that exhaust pipe terminations could be pointed that way. A roof parapet wall was designed to surround the other three more nsitive sides of the loudest noi sources. With all of the concepts combined, in addition to the estimated 25 dBA of distance noi reduction, the design approach included: (a) room acoustics attenuation, (b) barrier attenuation, (c) building noi containment, and (d) inlet/exhaust silencing.
For each noi source group, the following attenuation measures were recommended:
Engine Test Cells: Diel engines produce broadband noi. With A-weighting applied, dominant octaves are in the 250 - 500 Hz octave bands. Attenuation requirement: > 40 dBA.
Room Acoustics: Perforated metal panels with acoustically absorptive fiber fill (encad in vinyl) on walls and ceilings of cells. Estimated reverberant reduction: 4 - 7 dBA.
Noi Containment: CMU Walls enclosing test cells within building walls. Concrete deck and support equipment mezzanine above. Sound rated doors and windows. Internal duct liner or silencers below exhaust fan and air handler roof penetrations.
Barrier Attenuation: Exhaust pipe terminations on roof point in the direction of least nsitivity (towards fewer, more distant homes). Parapet wall taller than exhaust pipe terminations on the three, more nsitive sides of roof exhaust discharges.
Silencers: Straight perforated pipe silencer (with acoustically absorptive fiber filler in body), within test cell, in ries with 3-chamber reactive muffler located in mezzanine above.
Combined inrtion loss is greatest over 250 - 1000 Hz frequency span, matching maximum A-weighted engine exhaust octaves (e "Test Cell Muffler Concepts" below). Hot Gas Cells: Hot compresd gas discharge produces a broad tonal noi. Dominant A-weighted octaves are in the 500 - 2000 Hz bands. Attenuation requirement: > 43 dBA.
Room Acoustics: Perforated metal panels with acoustically absorptive fiberfill on walls and ceilings of cells. Estimated reverberant build-up reduction: 4 - 7 dBA.
Noi Containment: CMU Walls enclosing test cells within building walls. Concrete deck and support equipment mezzanine above. Sound rated doors and windows.
abac式的成语Barrier Attenuation: Exhaust pipe terminations and parapet wall enclosure as above.
Silencers: Straight perforated pipe silencer (with acoustically absorptive filler in body), within test cell, in ries with larger absorptive muffler with "bullet" inrt, located in mezzanine above. Combined inrtion loss is greatest over 1000 - 2000 Hz frequency span, matching maximum A-weighted hot gas exhaust octaves (e " Test Cell Muffler Concepts " below). Air Compressor Room: Helical screw compressors produce strong tones. For this installation, peak tones are at 200 and 400 Hz octave bands. Attenuation requirement: Minimum > 1 dBA overall, but to assure tonality is reduced below ambient, > 6 dBA.股票技巧
Room Acoustics: Slotted concrete masonry units (CMU), lected for maximum absorption in 250 Hz octave plus vinyl covered, expod insulation below corrugated metal roof deck.
Estimated reverberant build-up reduction: 4 - 7 dBA.
Noi Containment: CMU Walls enclosing compressor room. Concrete on corrugated metal roof deck. Sound gasketed doors.
Silencers: Acoustical louver in exterior wall air inlet, lected for > 7 dB at 500 Hz.
Roof Mounted Air Handler and Exhaust Fans: Radiated noi levels at perimeter of roof were estim
ated to be less than 75 dBA, and therefore required no additional attenuation. Internal duct liner and/or duct silencers were specified below roof penetration for test cell noi (e "Test Cells Noi Containment", above).
Test Cell Exhaust Terminations on Roof (Engine Test Cell and Hot Gas Cell Barriers, above): Exhaust pipes were designed to terminate in a "goo-neck," partially to prevent rain capture.
To benefit from directionality of mid- to high-frequency noi (which has greater affect on A-weighted overall level), terminations point in the direction of least nsitivity (fewer, more distant homes). A parapet wall, designed to be slightly higher than pipe terminations enclod the exhaust terminations on the other three, more nsitive directions (the open, less nsitive side provides a fume dilation draft).
Test Cell Muffler Concepts
Dissipative versus Reactive Mufflers
A silencer design approach was lected to match attenuation spectrum with source spectrum, i.e., maximum silencer inrtion loss in the maximum A-weighted noi source octave. In the cas whe
re a single silencer could not achieve compliance with the allowable noi criterion, two silencers were applied in ries. In tho cas, the silencer types were lected bad on composite inrtion loss spectrum. Two primary types of silencers are common for engine exhaust, dissipative (absorptive), and reactive. It is not the intent of this paper to discuss the “how” and “why” of silencer physics, but instead, to discus the applications.
Dissipative silencers are double wall vesls with perforated inner walls. The annular space is usually filled with acoustically absorptive fibers. Some attenuation occurs from Helmholtz resonance, but most of the broadband attenuation is from the acoustic filler. The most simple designs have a straight perforated pipe as the inner wall, and have virtually no pressure drop.
Others have a greater diameter inner wall, with a perforated "bullet" inrt inside the pipe. The can have somewhat greater attenuation, but at the cost of slightly greater pressure drop. Both variations have good mid- to high-frequency attenuation, but poor low frequency attenuation. (Illustrations courtesy of Burgess Manning6)
Reactive silencers are vesls that attenuate noi by the expansion chamber principle7. Reactive
mufflers generally have at least two chambers, connected by small pipes. The pipes may be perforated to diffu airflow. The frequency respon and amount of attenuation is proportional to the volume and number of chambers. Reactive mufflers have good low frequency attenuation (peak frequency depending on length and diameter), but typically have much greater pressure drop than dissipative silencers.
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