At Nadmas we have a 80db noise limit which we strictly adhere by. All models have to be tested to make sure they meet the criteria. To help with this, below you will find useful information and links to help you decide on the best engine and prop sizes and related articles

Noise Test
ALL models require airframe inspection, including electric-powered ones.

Ground Test (conducted by committee member)

• METHOD OF MEASUREMENT OF NOISE
  EMITTED BY A MODEL AIRCRAFT
  The model, working at maximum rpm, should be held between 1 and 2 metres off the ground in a bracket or by one person standing upwind of the model. The microphone of the noise meter should, where necessary, be fitted with a windshield. It should be positioned downwind of the model, 7 metres away from it (measured horizontally), and 1.2 metres vertically from the ground. The wind speed should not be over 3 metres per second (force 2).Measurements should be taken in the open air and away from reflecting surfaces such as buildings.
  The noise level emitted, for the purposes of this Code, shall be the maximum value of four readings:
  (I) model pointing directly at the meter,
  (ii) model pointing directly away from the meter,
  (iii) model pointing 90o to left of meter,
  (iv) model pointing 90o to right of meter.
• The noise meter shall be in accordance with BS5969:1980 (‘Specification of sound-level meters’), using the A-weighing response, and with the meter set to ‘slow' response. The meter should
  be calibrated prior to use. Note: Some local authorities may be willing to assist model flying clubs with noise measurements in order to establish the noise levels of aircraft being flown by club members.
  BMFA ADVICE ON THE NOISE TEST
  The noise testing procedure noted in the DoE noise code above should be followed carefully but to get the best results it is strongly recommended that you
  should take special note of the following. Make sure that no noise reflecting surfaces are near the test site. This means not just buildings but cars,
  concrete, models, model boxes and even hard packed earth. Do the test over grass. Do not take measurements when there is any appreciable background noise. Traffic on a nearby
  road, other models flying or being readied for flight and even people talking near the meter can affect the readings.Wind blowing across the microphone has a big
  effect on readings. Do not test on breezy days and when you do test, use a microphone wind shield. Make sure that the actual microphone is over the
  end of the seven metre tape, not your hand or the center of the meter. Think carefully about the four test positions of the model at the other end of the tape. As a suggestion,
  for the sideways-on readings put the fuselage on the seven metre mark, for the nose-on reading put the propeller over the mark and for the tail-on
  reading line the trailing edge of the wing up with it.
  HELICOPTER NOISE TESTING
  Because of the specific problems associated with performing noise tests on helicopters, it is recommended that a revised procedure be adopted.
  Three markers should be laid out in a line on the flying area, one central, one seven metres to one side (crosswind) and one seven metres to the other
  side (crosswind). The helicopter which is being checked is held in a steady hover above the centre marker with the pilot standing downwind of it, as
  normal.
  Noise readings are then taken with the meter positioned over each of the end markers in turn. For safety, when the meter is being carried from one
  end marker to the other, the checker must walk around behind the pilot flying the model. The two readings obtained take the place of the four
  obtained in the fixed wing test and all other criteria are as noted in that test procedure. Note - This method of testing is offered by the
  BMFA as a safe way of obtaining meaningful figures for helicopter noise levels on club sites by club flyers. It is not officially part of the DoE Noise Code.
  

On the test form the following are recorded:

• Name, Model , Date, Engine, Prop, Silencer, Revs. (The tester should also note any Fail-safe test).
• A ground reading of over 80dbA will FAIL the test.

(After testing, advice may be given on how to further reduce noise and reference to records held in the clubhouse would give guidance.)s

Below are a list of links that give examples and tables of model type, engine and prop sizes with the noise readings,

• Noise Reduction Chart
  http://www.flyingsites.co.uk/howto/noisechart.ht
  
• Model aircraft noise reduction- make your own muffler!!!
  If your thinking of making your own rc Muffler take a look here for some designs and methods,
  http://www.geocities.com/milesandjus/intro.html
  http://www.geocities.com/milesandjus/reduction.html quick link to what makes the noise