Engine sound/noise

We recently renewed our flying field lease. One of the stipulations was a maximum sound level of 90 dB at 9 feet. No other conditions, quantitative or qualitative were implied.
The AMA rule book (pg 202) states dBA weighting at slow response.
My question - have any of you had encounters regarding (measured) noise levels? What restrictions must you live with?
There does not appear to be any reference to sound levels in our specific sections - control line, general or p/a. Did I miss something?
As more and more fields seem to go away, noise must be a large contributing factor.
So, what, if any guidelines are around to address this?
Let's start some dialog.
Bob Z

The AMA does not have sound restrictions. You can fly at our nationals without a muffler if you wish. (Your fellow competitors may desert you, however, with their hands over their ears.) The FAI does have sound limitations, and, surprisingly, they are not difficult to meet.

If you don't have one, get a sound meter from Radio Shack. That is what we have. These are inexpensive and work quite well. I have tested a number of engine/muffler combinations, and, would you believe, some of the pipe systems are noisier (on the meter) than a tongue muffler? A lot is perception. The tongue muffler gives a sharper, higher pitched sensation, while the pipe system gives a deeper "roar". The sound meter does not measure pitch, however, but merely the sound itself. And the human ear has far less tolerance for the higher pitches than the lower pitches (take note of the teen-agers driving by with the 400 megawatt bass boomer in the back seat. All you can hear from any "song" is "thump, thump, thump.") And this is also why lawn mowers, with their higher db levels are tolerated in communities when your neighbor may object to your starting up your half-A engine in the garage.

Anyway, your sound levels are quite reasonable and easily attained. Simply fork out $35 (or whatever) for the sound meter, check a few of your planes, and make sure everyone is aware of the requirements. (No, an open exhaust McCoy 35 is not allowed here.)

You might also note that the prop noise is about half of the total noise produced by engine with muffler and that 3 decibles is twice the power and 10 decibles is ten times the power. Thus 87 decibles is half the sound level of 90, so don't expect to go a lot below this.

We keep a dB meter and a log book at the field. The log includes info about the particular plane and engine. This is a pre-emptive/ protective move because we have had some issues with a neighbor. It's a way of demonstrating that we are keeping up our end of the deal.

The issue of the pitch/tone of the sound vs the actual sound pressure is very interesting. Four stroke motors often have higher dBa levels than comparable two strokes, yet the sound is generally more toler-able to the ear because of the low pitch. Does anyone have information on how we can quantify this pitch/tone business? "Tuning" your exhaust might take on new meaning if we were trying to find an ear-friendly pitch.

What attitudes are measured?

>What attitudes are measured?
>
>Al

Not specified. There's some interesting features to this topic, but I think I will wiat until after next week to discuss them.

>The issue of the pitch/tone of the sound vs the actual sound
>pressure is very interesting. Four stroke motors often have
>higher dBa levels than comparable two strokes, yet the sound
>is generally more tolerable to the ear because of the low
>pitch. Does anyone have information on how we can quantify
>this pitch/tone business? "Tuning" your exhaust might take
>on new meaning if we were trying to find an ear-friendly pitch.

I started a post which I never quite got finished on experiments I did with mufflers. I remember asking Ted a question at the Nats a number of years ago as to why the pipe engines sounded deeper, even though they were running at a higher pitch. Although he didn't have an answer at the time, it is really quite simple--it is the length of the "muffler" (pipe, in this case).

Giving a real short comment from some of the stuff I was going to put in this post on mufflers, when we were experimenting with the Magnum 53, I didn't know what size muffler to use. The original mufflers that Art had made for us for the modified 35 FP didn't work. We had to add 1/4 inch to the body length to keep them from restricting the power. So Art made up a muffler for me for the 53 with a longer body with holes drilled in various places where the "end" of the muffler could be fixed. I thus had an adjustable size chamber.

Well, I tried this on the plane in a couple of spots and noticed the difference in pitch, but nothing as far as engine characteristics were concerned. I then put the engine on the test stand and left the end loose where I could slide it in and out. It made a real nice trombone. Although there is a limit where going smaller makes a difference, once this length is exceeded I could hold the tach on the engine and move the end in and out and play a tune with no noticeable effect on the engine. It also made no effect on the db meter--just pitch.

So, if you want a more "pleasant" sound to the ear, you can simply use a longer chamber in the muffler. I have had some longer mufflers made up for one of our guys for this very reason. The additional length has no effect on the run of the engine and does add some weight, but it makes for a deeper, and more pleasant sound for some people. As you say, the 4-cycles are generally not quieter. They just produce a deeper sound, which, for more people, is more tolerable to their ears. And a good tongue muffler can be as quiet or quieter than a tube muffler, but some people will object to it simply because of the pitch.

Get yourself a db meter and check this out. Oh, and Dan, I never found any evidence with this muffler that at any length it would resonate to act like a pipe.

HERE in Portland Oregon the CITY Ordinance is 95db at 50’ sunup to sundown QUITE LOUD. So My McCoy 35 is well below the law. Our club keeps the peace when we can. HOWEVER, at the flying field we can exceed 95db’s.

Scott(WHAT!!!)Riese

I am still new at this muffler stuff. When you say chamber, I assume you are talking about the the larger diameter part of the muffler system??? And if so, you are saying that I could elongate that part for a change in pitch w/o engine compromise??? Also is that the part that is called the expansion chamber???

SoH

>I am still new at this muffler stuff. When you say chamber,
>I assume you are talking about the the larger diameter part
>of the muffler system??? And if so, you are saying that I
>could elongate that part for a change in pitch w/o engine
>compromise??? Also is that the part that is called the
>expansion chamber???
>
>SoH

I am talking about the length, not the diameter, and, yes, that is what is called the expansion chamber. The ones Art makes are of a uniform diameter so can be made to any length, but the length will definitely affect the pitch of the sound coming out of the muffler and making the length longer has no affect on the running of the engine. There can be a point where they are too small, however, and these will then compromise the running of the engine.

most of the "pitch/tone" differences are in the harmonics(multiples) of the base frequency and the shape of the sound wave. If you look at spectrographs of musical instruments and similar things they all have the basic pitch, say 440 cps for a standard A note, but the shape of the wave can be wildly different due to the instrument. A Fox Mk IV sounds much louder and harsher than an LA 40 running the same prop and rpm. There are also harmonics of the base frequency that Len's "tuning" muffler idea will change. This could be an excellent, cheap way to get a nicer sound. Probably more points too. Judges are human like everybody else. If it sounds better it might score better.

So the expansion chamber is like a wave guide of sorts for sound. Would muffler material have an effect on the sound as well as shape and size??? It seems to make a difference with quality acustic guitars versus cheapies.

SoH (disclaimer: spelling & grammer)

>So the expansion chamber is like a wave guide of sorts for
>sound. Would muffler material have an effect on the sound
>as well as shape and size??? It seems to make a difference
>with quality acustic guitars versus cheapies.
>
>SoH (disclaimer: spelling & grammer)

Since we aren't trying to amilify (just the opposite, in fact), we aren't too concerned about the affect on the quality of the sound. Ideally we want these thick enough that they do not resonate on their own. Then what happens is that it dampens the wave inside and the pulses coming out the exhaust will have less amplitude, hence less sound. But the longer muffler body will deepen the sound, just like the tronbone when you push the slide out.

What would be some good materials to make mufflers with or does it matter as long as they are thick enough to absorb as to prevent resonance??

SoH

>What would be some good materials to make mufflers with or
>does it matter as long as they are thick enough to absorb as
>to prevent resonance??
>
>
>SoH

We usually just use aluminum tubing. It doesn't matter and it actually can be quite thin although something a little thicker does not resonate as much. The pipes are made out of carbon fiber mat and high temp resin. Now if you could figure out a way to mate a carbon fiber tube muffler to an aluminum exhaust stack, that could be ideal. There may be a problem with it burning, however.

The Radio Shack meter ($40 list price) has both a dBA and a dBC scale. Also a slow response and a fast response scale. More expensive, industrial meters can measure dB at specific frequencies like 500 hz,1000 hz 2000 hz etc. But then you have to do some fancy logarithmic math to add up the individual frequencies. Stick with Radio Shack meter for now.
That regulation should have specified either dBA or dBC. Also, many regulations are written as 3 meters or 10 ft. That extra foot can help. Sound dissipates with distance.
A good chamber muffler should keep you under the limit. On smaller less powerful engines, a tongue muffler will be OK.

LAST EDITED ON Jun-30-04 AT 04:20 PM (CDT)
 
>We recently renewed our flying field lease. One of the
>stipulations was a maximum sound level of 90 dB at 9 feet.
>No other conditions, quantitative or qualitative were
>implied.
>The AMA rule book (pg 202) states dBA weighting at slow
>response.
>My question - have any of you had encounters regarding
>(measured) noise levels? What restrictions must you live
>with?
>There does not appear to be any reference to sound levels in
>our specific sections - control line, general or p/a. Did I
>miss something?
>As more and more fields seem to go away, noise must be a
>large contributing factor.

Not really. The noise is one of several excuses. Around here, if there is a noise complaint, the cops bring out a DB meter and measure at the edge of the property, and if it's below, too bad, no noise complaint. And it's never going to be close to the limit unless you are running unmuffled MCoys, Orwicks, etc.

That won't keep people from complaining, however. At a local school, there have been noise complaints on *gliders*. It's not the noise, it's that people don't want model airplanes in their neighborhoods. So pursuing noise reduction, beyond legal limits, is a complete red herring in field retention.

The big issue, of course, with field retention and acquisition, is *lawsuits*. A two million dollar site insurance program is not going to cover the preliminary hearings. When I was trying to get a place to fly at work, I mentioned the two million dollar insurance to the Lockheed legal department, and they just laughed. A typical house in a nice neghborhood is well over a million, two million of coverage for a "deep pockets" site owner is a joke. Same with the city governements. The fact that model aircraft have not been classified as a hazardous activity" means that the city can be sued for injuries - unlike soccer, which has been classified of hazardous, meaning that there is an "assumption of risk" and thus the site owner has a reasonable chance of avoiding or winning the suit.

The other interesting item on the topic of noise is how much noiser a 4-stroke in stock trim is, than a piped engine - at least measured sound level. I have been flying my PA1 airplane and had it *drowned out* by an OS52 surpass running in the pits. But I would have to agree that the noise from a 4-stroke tends to be much more acceptable. If the noise was a real issue, that would probably help, until the DB meters come out.

Funny you should mention it, however. Just last night, here in the Signature Inn parking lot, some helicopter guys were test-flying their helicopters. I heard a faint whirring of gears, and looked over, assuming it was electric. But no, there was a huge cloud of smoke. Turns out it was a YS 91 4-stroke. But it was darn near silent, as far as the engine was concerned. Truly amazing.