Cover Letter for Nasa Report
October 29, 1999
RE: Acoustic Testing of 2 air motors, the DVA series and the DV10 series
After reviewing your hardware and your objectives, Johnson Engineering performed a series of tests to assist you with acquiring engineering data for your motors.
Attached to this document is the test report. We took 1/3-octave band center frequency measurements along with Narrow band measurements. The 1/3 octaves are industry standard, and the narrow band measurements gave us the tonal properties of the air motors. Within the test plan, it was noted that the operating speeds should be varied throughout. The tests were generally run at two different rpms; the first one was usually the operating rpm, and the second one was full air pressure.
The tests were performed on-site at PSIA in the shop area after the air conditioners were shutdown and while the compressor was off. Low background levels were achieved (30 dB less than the measurements) before the testing began. The test set-up consisted of an analyzer, 2 microphones - one placed at 1 foot away, one placed at 3 feet away, and the necessary cables for the preamplifiers.
This report shows the acoustical spectrum for two different series of motors and the attenuation achieved by connecting mufflers from two different manufacturers to the air exit. The lowest noise levels range from 78 dBA for the DVA series to 83 dBA for the DV10 series with PSI Safety Silencers. This is a drastic improvement over the original 115 -120 dBA, measured by the microphones without any mufflers.
If the dBA level with the PSI Safety Silencer is acceptable from a noise and cost standpoint, then that is the solution. If not, additional work targeting the rotor blade tones may be appropriate as noted in the test report.
Feel free to contact us with any questions or concerns you may have.
cc: Karen Saltsman
To View Test Pictures Please Click Here
Johnson Engineering performed a series of acoustical tests on two types of air motors, the DV10 and the DVA, built by PSI Automation. These motors were tested with different types of mufflers to understand the attenuation possibilities. The data suggests that the combination of PSI Safety Silencer and Diffuser, reduces the noise emission up to 35 dB. Also noted, the blades inside are the major noise source from the motors. The test results: tabular data, graphs, and pictures are included in this report.
TABLE 1: Air Motor Test Support Equipment
Table 1 shows a list of the equipment used to perform the tests. All of the acoustical equipment is made by Larson Davis and is calibrated. A two channel sound analyzer was used with pre-amplifiers and microphones. A pocket rpm meter was used in conjunction with the sound testing to determine operating speed. The microphones were calibrated to a single tone of 94 dB at 1000 Hz. The background noise was recorded and then subtracted from the air motor data. The microphones were placed in the flow of noise at a distance of 1 foot and 3 feet. Other distances in between can be interpreted. Windscreens were used to eliminate excess wind noise. The air motors and piping were isolated from the test stand by a piece of foam to eliminate vibration noise through the stand. A set-up diagram can be seen in picture 1:
The DV10 Series is the larger of the two motors tested. It's 10 hp and includes a #405012 safety silencer. The primary and secondary exhausts were connected together. The first set of tests was at 4500 rpm. The same DV10 motor was used in all tests with different muffler combinations for comparison. Graph 1 shows the results of the two types of mufflers that were tested. The dark blue line is the 405012 Safety Silencer Combination. The pink line represents the current muffler made by Allied Witan, series S-2.
Graph 1: DV10 Series Motor at 4500 rpm
Quick View Results - 4500 rpm
Two Observations: 1) There is a tone problem between 300 and 400 Hz. 2) This graph shows that the 405012 provides the best attenuation for the configurations tested, reducing the noise to 83 dBA. The Allied Witan muffler reduces the noise to only 106 dBA.
Graph 2, shows the same series of motors at 8000 rpm. Based on observed results, the change in rpm doesn't seem to affect the data. The 405012 muffler reduces the noise to 84 dBA, and the Allied Witan remains at 106 dBA. There is a 35 dBA improvement in the air motor noise when using the PSI Safety Silencers.
Quick View Results - 8000 rpm
This series is smaller and quieter overall. The DVA series motors are 2 Hp motors with the #405008 Safety Silencer. They are also configured the same way as the DV10 motor with the primary and secondary exhaust connected together. Four different muffler combinations were tested for this motor. Graph 3 shows the DVA motor at 7200 rpm:
Graph 3: DVA Motor with 4 Configurations
The points where the graph drops to zero represent noise levels equal to or less than the background. Because the dBA weighting represents what the ear hears, it may not be obvious from the graphs, which is the best. The graph results are as follows:
Quick View Results - 7200 rpm
Without any mufflers, the DVA series is at 117 dBA. The PSI Combo reduces the overall noise by 39 dBA.
An additional test set of 1/3-octave band measurements that were taken compared two different types of designs of the DVA motor. One included a redesigned manifold. There was virtually no difference between the two. The last set was the same DVA motor as the others, but it had a block (plug) in and was run at half the rpm with an Allied Witan S-1/2 muffler. The overall dBA is 78.
Graph 4: DVA at 5100 rpm with Block
Narrow Band Data
Narrow band data were taken to look at the spectrum more closely and try to diagnose the noise problem. Upon looking at the data, it's evident that the noise is motor blade noise. Somewhere between 587 and 593 Hz, occurs the first tonal spike. Comparing that frequency to the operating speed and the number of rotor blades:
From the output of the equation, one can tell that it's blade noise dominated. Each of the smaller spikes, and even the larger spikes, represent harmonics of the same tone. The one possible solution to this tone problem is through the use of a Helmholtz resonator, specifically tuned to that frequency.
Graph 5: DVA Narrow Band Data - 7400 rpm
Design Solutions (Results)
The PSI Safety Silencers provide the best noise attenuation. The combination of the expansion chamber and diffuser attenuate the noise by 35 - 40 dBA. If this is acceptable from both a noise and a cost standpoint, then the problems are solved. If not, then what else can be done? One of the tests looked at the redesign of a manifold to see how it affected the noise, but that was unsuccessful. For follow on work, it makes sense to target the rotor blade tones before the main noise even hits the mufflers. If a Helmholtz resonator was placed in front of the main muffling system, the tonal properties of the wind noise could be eliminated.
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