Where might I obtain (buy not borrow) a db meter that would be of satisfactory quality but not outrageously expensive to check planes at our field?

radio shack has sevveral models, from cheep, to definately not cheep

I have a Radio Shack unit identified as Cat. No. 33-2055. Purchased about four years ago I think for around 59 dollars. It was the only one carried locally and has served well ever since.

John

District 5 has a dB monitor that was purchased from Radio Shack. Very accurate and dependible.

Thanks guys. Guess there is a trip to Radio Shack in my future! We are beginning to have sound level issues at our field and a few of the guys don't have a clue how sound is measured, let alone knowing the difference between 94 db and 100db. Our by-laws currently allow for a sound limit of 100 db and to be honest I don't think any of the members have a clue how loud that really is.

Nick,
How do you verify the accuracy of your meter and what is your calibration process. Unless the meter you purchased is a high end meter your statement that it is very accurate is at odds with all that I have been able to research. The repeatability and accuracy of a dB meter is, at present, directly proportional to price. The lower end Radio Shack meters are incapable of accurately measuring sound pressure levels (SPL) in a repeatable manner.

Anyone wishing to be able to accurately measure SPL for the purposes of comparing noise levels from different sources on different days is going to have to spend $400 to $1000 on a meter and about the same amount for a calibration attachment. Unfortunately, there are no cheap sound level meters that are repeatable and accurate.

Robert,

That is a very interesting statement. Since I started this thread you can safely assume that until today I was totally ignorant as to the quality (or lack thereof) of commercially available civilian meters. In some other instances though I am quite experienced. I read with interest (and copied) the instructions for measuring sound levels at RC aerobatic events as specified by the AMA in their Rules for 2004/2005. As near as I can tell, the rules are written around the very meter I just purchased from Radio shack. We have a Calibration facility where I work so I will take this handy-dandy $49.00 meter by there tomorrow and get it checked out. For what I paid, it seems to be a reasonably good quality piece of gear. I would be willing to bet it is as accurate as any of those infra-red digital temp guages I see advertised on here, certainly close enough for our efforts.

Not knocking what you say, as I am sure it is sometimes true. but as a Government elec. eng. I have found that some of the cheaper openly available T/E is every bit as accurate (within reason) as some of the "precision" stuff we buy to support our development efforts. Not comparing the $15,000 computerized Techtronics 400 mhz storage scope to a Rat Shack $299 special but apples to apples the chaper civilian ones have proven to be amazingly accurate. Enough so that the US military frequently does not require calibration for meters unless a certain accuracy is required.

Nick/John, Thanks for the heads up and for $49.00 it is a nice little meter. I got the 33-2055.

[quote]ORIGINAL: cstevec

<snip> As near as I can tell, the rules are written around the very meter I just purchased from Radio shack. We have a Calibration facility where I work so I will take this handy-dandy $49.00 meter by there tomorrow and get it checked out. For what I paid, it seems to be a reasonably good quality piece of gear. I would be willing to bet it is as accurate as any of those infra-red digital temp guages I see advertised on here, certainly close enough for our efforts.

/quote]

Right on. cstevec -

Noise annoyance is entirely subjective, and not surprisingly there is a considerable variance in noise level that annoys different people. Most people can't discern a difference of less than about 2 dB in sound level and that is about the tolerance of the Rat Shack meter, which is probably considerably better than what makes any difference to people whose activities may be disrupted by noise. It's a good value, the only real shortcoming being that it is only capable of instantaneous measurements. Noise annoyance is cumulative, and objective measurements that attempt to correlate to subjective annoyance need to take this into consideration. Many well-formulated community ordinances specify limits on sound emissions in terms of an integral over some time period (Leq), most often one hour. To get accurate measures in such cases, an integrating sound level meter is needed. That's an expensive item, but can readily be obtained for the short time it is needed in the instrument rental marketplace. Our club does a sound survey every few years with an integrating meter at points on the field perimeter, and relies on the RS meter and simple math for model qualification in the interim periods.

Abel

cstevec
I’m not at all sure what the comparison of dB meters and infra-red sensors has to do with this discussion. They use entirely different components and techniques to make their measurements that have no relationship to each other. Anyway, it’s just not relevant to this discussion.

Yes, the meter you purchased is what is basically suggested by the AMA test procedures. These procedures are essentially those developed by the pattern groups for their purposes and then adopted by AMA for lack of anything else. The following are quotes from IMAC (that uses the same procedures) documents concerning sound level measurements as performed using the method you referenced;

“So what we are left with are commercially available consumer retail units from Radio Shack (RS). Initially we tested various RS units in the lab (at General Dynamics) and found them to be within one dB of each other. So we quite happily employed them in the field as Pattern has done in years past. Unfortunately, as is the case with many systems, once employed in the field the inaccuracies started cropping up. It was not uncommon to find the RS meters consistently 2dBA apart on readings, and in many cases we have found units to be as much as 3dBA apart! When one considers the fact that the intensity of sound doubles for every 3.01 dBA, the use of the RS meters can (and has) given widely varying readings during the ground test.�

Would you accept a class of instruments that measure 2X range for the same inputs?

If, using your meter, you measure your sound level as 96 dBA, and at a meet are told your sound level is 99 dBA and unacceptable, how would you feel? Is a test reasonable if you can’t reliably determine a repeatable value from one tester to another?
Further, IMAC doesn’t even use the ground level test as the final arbiter, they require the judges to evaluate the sound level of the contestant and give them a subjective loudness score that can result in point deductions. Why bother with the SPL measurement if ultimately you’re going to revert to subjective measurements anyway.

Also from the same IMAC document;
“So you ask, knowing all of this what do I as a CD do with the Ground Test at a contest? First keep in mind that we do not intend it to be the "end all test." Folks come to fly at our events, and we want to give them every reasonable opportunity to do so. After all, if someone is Too Loud, the In-Flight Judging Criteria will "catch" them. So in answering this question, we should again consider the inherent inaccuracy of the RS sound meters. Since there can be as much as a 3dBA difference in readings from meter to meter, you as a CD don’t know if your unit is on the high side or the low side. In my view, no one should be turned down from flying if they test within 3dBA of the limit. When this problem first surfaced IMAC began recommending that the use of ATV was appropriate for the ground test. Therefore use the Ground Test for it’s true purpose, giving competitors an expectation of what the plane will do in the air. Our recommendation: Never ground a competitor unless they are substantially (3dBA) over the limit.�

If you’re going to fudge the number by +3dBA why not set the absolute limit to 99dBA and be done with it. And the IMAC document goes on to say that ground level measurements have no relationship to sound levels generated while in the air.

As to the RS meter, according to the specs on your model are ± 2 dB at 114 dB SPL. From the following chart you can see that this meter is not acceptable for even general noise surveys as accepted by standards bodies.

International Standard Classes of Accuracy: Almost all SLM’s are designed and specified to perform to one of four internationally standardized levels of accuracy:
Class Sound Level Meters Purpose Calibrators
0 +/-0.4dB Lab reference standard +/-0.15dB
1 +/-0.7db Precision SLM +/-0.3dB
2 +/-1.0db General purpose SLM +/-0.5dB
3 +/-1.5dB Intended for noise surveys --------------------
(Table showing permitted tolerances as defined by the IEC 60651 and ANSI S1.4-1983.)

I’m saying that the hobby’s approach to sound level determination is extremely poor and promotes a procedure and instruments that result in measurements that have essentially no value in repeatability (which is a must if the measurement is going to be used for anything other than its feel good value) or comparability. Just understand, your measurements don’t mean much.

By the way, you shouldn’t have to take your meter in for calibration, the calibration devices are quite portable, why not borrow it and calibrate just before you make your measurements, which is what all test procedures (except AMA/IMAC) I am familiar with require anyway.
Sincerely,
Robert Camp

I'm not 100% sure about the IMAC and the judges discretion deal...but I can see why. It's no secret anyone with a computer radio can literally pass any dB test if they know ahead of time it will be conducted at engine run up and not during actual flying.(*)

Our club has a set rule and if they tried to ban a $4000 plane from a $49 instrument I'm sure some sort of "discussion" would thus ensue. The last time they actually got it out some guys got ticked that their "quiet sounding" to them engines read louder on the meter than some of the more "annoying" ones...well that's the way it works...so it hasn't been out since.

*- don't know if people still do it, but people use to put a "step" on the throttle and when the guy said "go to full throttle" in the pits to check the sound you pushed the stick forward and actually went to 75% or whatever you needed to get under the sound limit. Soon as that was done you flipped a switch and bingo you actually go to full throttle at full stick again.

yes, that still happens

rcamp, the point was that this isn't engineering and 100% accuracy isn't required. Hence the comparison between types of meters. Our club doesn't require 100% accuracy, just a reference, something to show members an indication of sound levels and a way to identify the obviously too loud planes to members who say things like "How can you say my plane is too loud when there is not a reference to go by" If the $49.00 Rat Shack meter is good enough for the AMA, and the AMA guidelines are in place on how to perform the test using that meter, (and they are) then I am sure that it is good enough for us. I appreciate your viewpoint but our needs are more then satisfied by the one I purchased.

By the way, out of the box & measured against a lab standard, it was within +/- 1 db from 75 to 110 db consistantly. No adjustment required. Good enough for me.

*edited 'cause I spelled your name wrong.*

We bought one of the "Cheap" RS instruments about 8 years ago. After a complaint from a large scaler about an airborne reading on his Byron P51 of 118 dbA at 100 feet, I took it into my lab and checked it against our B&K lab standard, used for our ISO 9000 compliance calibration of some expensive lab equipment. At a lab temperature of 79 degrees, the RS checked out within a +/- 0.5 dbA, fast response, from 70 to 120 dbA The cheaper price just means it can't be banged around the way the $1500.00 B&Ks we use in the lab can.

There's also a lot of subjective "Perception" about how loud one engine is compared to another, and a lot of people will trust their subjective opinion over an actual objective measurement. Regardless of facts! Just like people saying their brand X 60 turning an 11-8 prop at 10500 rpm puts out more power than a brand Y 60 turning the same prop at 11200 rpm. Folks, it ain't necessarily so!

50+AirYears-

Thanks for bring this topic into the real world. The RS meter is good enough. Its tolerance is well within the variance in levels that cause people to be annoyed, and also the error band that accumulates from other conditions. Here are a few:

* By convention in the US we measure at a standard distance of 9 ft (old AMA rule) or 3 Meters (later change in competition rules to get with the rest of the world). This is on the order of a wavelength of significant engine and exhaust harmonics, so direct and reflected waves sum as vectors to produce a resultant that can be significantly different from the direct path level that is of primary concern (as it is expected to dominate while the model is airborne).
* The level of reflected waves is an uncontrolled variable, dependent on the surfzce over which the measurment is taken. In the US, a hard reflective surface is specified by ANSI (and in turn AMA and many others), which is unfortunate as this is the most likely to produce corrupted readings in the case of our primary concern, which is the direct path to the receptor while the model is flying.
* Applying sound level rules intended for competition to the sport flying situation is mixing apples and oranges. In aerobatic competition, one model is flying at a time. Normally sport flying has several in the air simultaneously. Equal sounds summing incoherently increase by 3 dB for every doubling in the number of sources. For example, 4 sport models at 90 dB produce about the same perceived level as one competition model at 96 dB.
* The human hearing apparatus tends to integrate annoyance level with sound over some time period. Standards set by agencies concerned with environmental quality (HUD, ANSI, ISO, et al) usually specify tolerable sound levels as a running average over some interval, usually one hour, as an equivalent level Leq, the propensity to disturb being 'equivalent' to constant noise at the specified level. Most localities that specify objective levels follow the guidance of these agencies, and the bottom line is that compliance with the law requires giving the time factor due consideration. Example: 90 dB instantaneous level that persists for a total of 1/2 hour in any hour is 84 dB Leq.
* Sound emissions from model airplanes are directional. Prop emissions are greatest in magnitude a few degrees aft of the plane of the prop arc. If the prop emissions are dominant, then the AMA specified procedure of measuring broadside to the model produces reasonably accurate results. With glow engines fitted with stock mufflers, however, the exhaust and intake sound levels are usually above the prop generated levels by several dB, so the maximum level could well be emitted from another aspect.

There are many more variables that are ususally uncontrolled outside of the laboratory, but these should be sufficient to conceptualize the relative accuracy of the procedures used vs the accuracy of the meter. You're kidding yourself when you use a micrometer to measure the length of a rubber band.

Abel

In our previous field, the RS readings would have stood up in the local municipal court during a situation with a neighbor, according to several diffferent lawyers, including the law director. Situation was rectified by working together to achieve a mutually agreeable solution, although we had a couple hot heads who could have screwed the situation up.

Using the pamphlet that came with my employer's B&K, we even were able to calculate the effective sound level since the engine sound level at the neighbor's property was less than 8 dbA above the local ambient noise. With the original field layout, we were boarderline by the local statute, and by rotating the flightline 12 degrees, we lowered the sound level at the neighbor's property line to below the limits on a calm day. This actually meant that on a windy day, the local wind through the trees was illegal. Raising our starting time by an hour eliminated the complaints. In fact, he used to bring his kids to the field to watch us fly, although he stayed pretty much out of sight because he realized there were some who held a grudge against him. In fact, our handling of noise issues got us a couple local neighbors joining our club.

The big gas engined guys did continue to be a problem, because of few effective mufflers at the time, and a "Perception" that the big engines were not as loud because of their lower tone. We had a couple people who used what could best be described as deflector manifolds. Relatively quiet with the plane on the ground at idle, but when the plane banked and pointed the outlets at you at as much as 100 feet away, your whole body hurt. What is the threshold of pain anyway, 120, 130 dbA?

When we had a IMAA meet at our new field a couple years ago, the improvements in mufflers on these engines was impressive. Hope they keep improving.

I've used the cheap (analog) radio shack meter for years doing a poor man's RTA on my car audio system, and have never taken lower then second place. (frequency spreadsheet, test disc, and patience) The results with that compare very closely to a professional RTA also. (used to buy rta time before events, until I figured out how accurate the radio shack method was...)

When I bought my Radio Shack $49 meter I was still working for the Army at Ft. Bliss. I took it to our calibration lab and had the guys spot-check it. As you found, it was pretty darned accurate.

I'd say the most probability for error will be in not always using the same procedure for the sound level measurements.

CR