Increasing environmental awareness means noise emissions limits are getting more restrictive. Here are a few comments about noise testing and ways to help this.

1. Silencer wadding deteriorates with usage. Don't assume that because you passed last year that you will automatically pass again this year; we have opened some silencers and found a teaspoon of soot inside! Also check that the limits for a particular circuit/track day venue haven't changed. If you have a repackable silencer then it might be worth a quick check inside to see if the wadding is still present in a reasonable quantity.

2. Noise sources. Not all the noise from your car is coming out the tailpipe. On cars with modified cams and/or induction system the amount of noise from the induction system can be significant. If you have changed from a stock exhaust manifold to a tubular design the this too can radiate more noise. Consider adding some damping material under the bonnet to prevent the sound  resonating around this area. Fitting special exhaust wrap may also help - this is definitely a benefit anyway because any reduction in intake air temperature is a bonus.

3. If you still need to reduce the noise further then you will have to physically change something to increase the noise dissipation. Remember noise is acoustic energy and the only way to reduce its amplitude (loudness) is to convert it into another form of energy or spread it out into a larger volume. Noise is measured in decibels which is a logarithmic scale (a noise check should use an 'A' weighting which is more representative of human hearing range than a 'C' weighting, it should also use a slow response time.). The logarithmic scale means that a lot of energy has to be removed to make a small difference to the value.  The exception to this is resonance, where the amplitude increases very rapidly over a very small range of frequencies. If your vehicle gets excessively loud at a particular rpm then you most likely have a problem with a resonant effect in one part of your system; changing tail pipe length or silencer length may improve the noise significantly. If your vehicle is loud at all revs then you will need to increase the silencer effectiveness. Several factors affect the performance of a silencer - the volume (size) of the space available to absorb sound, the material used for packing the silencer and the type of perforated tube in the middle of the silencer.