Results of SMO Muffler Test

As you may know from the recent Santa Monica Daily Press article, CASMAT volunteers have been collaborating with the City and Justice Aviation to make sound measurements of a Cessna 172 before and after being fitted with the Gomolzig ‘QuietFlight‘ silencer/muffler.  CASMAT first proposed (see here) the use of another (French) silencer/tuned exhaust combination in May this year.  This proposal was put forward as an alternative to the staff proposal to pay the flight schools to pattern fly elsewhere which was withdrawn in response to public outcry.

The ‘QuietFlight’ muffler

Subsequent study by City staff showed that the suggested device did not have the necessary FAA certification to be operated in this country, however, staff identified the Gomolzig muffler as an alternative that did have FAA certification.  Staff invited Gomolzig, a German company, to present a seminar on mufflers at the Museum of Flying during the annual airport open day.  The city then purchased one of the Gomolzig mufflers and began negotiation with a flight school to conduct a before/after test using one of the flight school aircraft.  The test was delayed for various reasons, so that the first test (without muffler installed) did not occur until December 6th.

As previously agreed with staff, CASMAT volunteers witnessed the City test and data gathering operations, and also took their own measurements of sideline noise in order to get a better picture of what the impact might be in the surrounding neighborhoods (as opposed to the city’s fixed noise sensors which are essentially in a straight line with the runway).  We chose a position on the hill in Sunset Park, unlike the city noise sensors (1, 3, 5, and 4) which are all ‘below’ the end of the runway.  On the hill one hears the complete takeoff path as there is no height drop to suppress the early portion.  This anticipated difference between the actual neighborhood takeoff noise envelope, and that seen by the city’s low lying noise sensors (which are below most of the houses impacted) was a key thing that CASMAT wanted to look at during the test.

CASMAT sound recording ‘gadget’

To take our noise measurements we combined an industrial noise meter, a video camera, and some custom software developed for the test, in order to fully document the flight path while simultaneously recording an accurate sound envelope.  The plane performed a number of previously agreed flights and takeoff maneuvers.  For each, the course was carefully recorded using a GPS device while staff simultaneously recorded power settings and other pertinent data within the aircraft, and also monitored the city noise monitoring stations.  At the same time CASMAT took our own recordings of the flight.

The second test (with muffler) occurred on December 17th and a series of identical flights were recorded once again in order to measure any differences.  The wind, humidity, temperature and other atmospheric conditions were virtually identical at SMO on the two days, thus eliminating these effects from consideration.  The chart below shows the averaged noise envelopes measured by CASMAT before and after the muffler installation.

The summary chart below takes the average smoothed envelope and compares the two directly:

Comparison of sound envelopes before/after muffler installation

The CASMAT results agree closely with the city’s findings, thus validating both.  CASMAT met with airport staff this Thursday to go over both sets of results.  Detailed City results will not be publicly released until they have been confirmed by the City noise consultant, however having gone over the preliminary results extensively, I will go ahead and discuss them here.

The city measured an approximate 4 dBA drop in maximum noise level (LMax) – from 74.7 dBA to 71.4 dbA, combined with a 40% reduction in the width of the noise envelope (roughly from 14 seconds to around 8 seconds).  Initial City figures showed a 5-6 dBA drop in the SENEL measurement.  Staff reported the aircraft was significantly quieter when overflying the noise sensors at 1,000 feet, to the point that it was difficult to detect the aircraft on the sensors.  This observation holds promise for those under the SMO pattern flying loop.  CASMAT did not take this measurement,  so we cannot confirm this observation, though it matches our subjective experience.  Lastly, staff were able to use the elapsed time between peaks on the various noise sensors to measure aircraft speed and performance at identical RPM before/after muffler installation.  The results showed no evidence of any aircraft performance change as a result of muffler installation.

Since CASMATs measurements were of sideline noise taken from higher and further away from the line of flight (which is what most people actually experience), we found a wider envelope (19 seconds vs. 14) for the un-muffled flight, but a lower peak sound level (LMax) of 62.7 dBA (vs. 74.7).  This confirmed our earlier suspicion that the noise period measured on City sensors is shorter than that actually experienced by the community.  As the charts above show, we measured a 4-5 dBA reduction in peak noise, but more importantly, a dramatic thinning of the noise envelope (from 19 seconds to 12 seconds) also approximately a 40% reduction – as found by the city.  In all, the SENEL we computed to be 6 dBA down.  A 6 dBA reduction in SENEL is a very significant result, actually equivalent to making the plane sound twice as far away.  Even a 5dBA SENEL reduction is a lot.  Both the city results and the CASMAT results show that there is a steep drop-off in the noise level immediately after the plane passes the observer whereas the curve is lower but more gradual on the approach side.  This was an unexpected result and as yet we have no explanation.

Subjectively, at the CASMAT measurement site, we noticed a significant reduction in the noise impact with the muffler installed; it was considerably quieter.  More importantly, the noise envelope is ‘thinner’, that is you hear the noise for a much shorter period of time.  Unlike the first test when the sound of the approaching plane could be heard for some time before one was able to see it, with the muffler installed there was very little audible warning before the plane ‘appeared’.  However, the most dramatic thing we noticed, was that without the muffler, one hears the plane continuously, not only as it takes off and passes, but also (and a little quieter) as it executes the return leg of the pattern (over Venice).  With the muffler installed, one could see the plane doing the return leg, but could not hear it above the ambient noise.

This means the muffler has the potential to get rid of the continuous lower level droning of pattern flying for all neighborhoods in the pattern flying loop, as well as reducing the impact of the actual takeoff overpass for the observer; an effect that benefits particularly Sunset and Ocean Park residents.   The combination effect could make a very real contribution towards mitigating the negative noise impacts of propellor plane traffic at SMO.

Measuring LMax alone does not really tell you how ‘annoying’ the noise is, just how loud it’s peak was.  In contrast, the SENEL measurement takes the total noise energy within the envelope out to the point where it falls 10 dBA below the peak.  For this reason SENEL values are generally higher than LMax dBA levels.  The SENEL measure accounts for how long the noise lasts, as well as the actual noise level.  Longer lasting noises are generally more ‘annoying’ than shorter ones, and so any real-world measure of noise impact should include the time for which the noise persists.  The SENEL measure is the best metric we currently have for this purpose.

According to airport staff, SMO is one of only two airports in the U.S. that use SENEL measurements (the other is John Wayne), the rest simply measure values derived from LMax, and thus are failing to measure a major aspect of the noise.  Past U.S. silencer studies have failed to measure or appreciate the impact of muffler sound envelope period reduction, and have focussed simply on LMax reduction.  Moreover, past U.S. studies often include data taken at the airfield itself.  Both the CASMAT volunteer and airport staff independently noted that there appears to be little or no difference after installing a silencer as observed near the actual point of takeoff.

CASMAT believes that the muffler study shows that there is a clear benefit to be gained from the installation of the devices on other propellor aircraft at SMO, particularly those aircraft involved in pattern flying.  As we noted in our earlier post on landing fee exemptions, the City is conducting a study of landing fees at this time, and 2/3 of all takeoffs at SMO currently do not pay any landing fees. The City looses around $800,000 per year subsidizing aviation operations, so it is clear that landing fees must go up, and that they must be paid by all aircraft including those based at SMO.

CASMAT would like to propose that the installation of mufflers on prop aircraft be tied into possible future landing fee revisions, such that by installing an approved muffler device on an aircraft, the operator is able to pay a somewhat lower landing fee than the identical plane without a muffler.  As a matter of fact, this system is already in effect at many European airports.  Such a system would incentivize others at SMO to install the mufflers, and might eventually significantly reduce total neighborhood noise at SMO, and thus take the first baby step towards a ‘greener’ airport.  There is ample evidence from Europe to support these conclusions.

The city is within its rights to raise fees to cover losses (indeed City agreements with the FAA currently obligate it to do so), so there can be no repercussions from the FAA if the City then offers a voluntary reduction to offset raised fees when mufflers are installed.  As far as the flight school planes, which engage in pattern flying, the installation of mufflers we believe should be ‘encouraged’ by any and all means available to the City.

Not all prop aircraft at SMO are Cessna 172’s, and so the City should advocate with the FAA for fast track approval of this or other mufflers that can be installed on the rest of the SMO propellor fleet.  Some of these other planes actually make more noise than the 172, and so might show even greater benefit from an installed muffler.

Lastly CASMAT would like to thank City staff for all their work on the muffler issue, and on this series of tests in particular.  Staff’s willingness to consider outside ideas, and to work together with other groups like CASMAT in this test, represents what we believe is a watershed event in bringing about the ‘openness’ that was outlined in the visioning process.  Hopefully this openness will now extend to looking into other areas where the airport can become a better neighbor, in particular, given the recent CASMAT petition results (which show strong public support for change at SMO), we hope that staff and City Council will now take positive action on the rest of the Airport Commission recommendations, not just the landing fee changes.

Also see today’s story in the SMDP regarding the test results.

UPDATE (1/27/2013):The results of the city tests are now posted.  The results are far more detailed than the preliminary results discussed above and show even greater benefit.  SENEL noise reduction varied from 4.8 dBA to a high of 8.3 dBA when the aircraft follows the ‘fly neighborly’ departure path (which involves a slight turn to follow the golf course after departure rather than straight out flight).  The sound period was reduced in the city results from 17% to 56% depending on the ground maneuver involved.  The city’s stop & go test results were consistently responsible for the low end of the improvement scale because the aircraft came to a complete stop some 2,500 feet down the runway before beginning its takeoff and as a result it was much lower when it overflew the sensors.  It goes without saying that an 8.3 dBA reduction in SENEL for the normal ‘fly neighborly’ departure path could truly be a game changer in terms of mitigating neighborhood noise impacts from prop planes.

UPDATE (1/30/2013): I want to clear up an urban myth that appears to exist that the muffler somehow doesn’t reduce the noise as the aircraft approaches; absolutely it does and it reduces it by between 4-5 dBA, exactly the amount that the LMax value is reduced.  This idea might have come from the staff graphs.  Logically, I think we can all agree that regardless of what is fitted to the exhaust, the plane, according to the laws of physics, must be loudest when it is nearest (or overhead).  This means that to truly compare two charts for effects before/after we must first align the peaks of the charts directly below each other.  Only then is it valid to see what difference happened before and after.  When we do that (you’ll have to imagine shifting the red lines to do it for the city graphs, the CASMAT graph is shown above) we see that the curve on the approach side has the same shape as the un-muffled curve, it is just 4-5 dBA lower.  On the departing side of the curve it drops off much faster and may be as much as 10 dBA or more lower.  This is the asymmetry.  So to be clear, the muffler works in both directions, just more so as the plane moves away from you.  In the city results, since the instruments respond to the rising curve and haven’t been time shifted, it looks like there is no change on approach until about half way up the slope it suddenly drops off and the plane has passed.   I think we can all agree that this is physically impossible, and perhaps the graphs should be time-shifted to correct this impression.