Noise restrictions for track days

The question of noise restrictions is one that cannot be ignored.
This FAQ goes through the main issues and how to solve them.

Noise restrictions

If you take you car on a track day, it is almost certain to be subject to noise restrictions. It is fair to say that noise restrictions are getting tighter and cannot be ignored. With many tracks having to either comply or stop operation, noisy cars are no longer tolerated. This means that if your car turns up to participate on a track day and cannot comply, it will not be allowed out. Track day fees are usually not refundable and thus not only can the day be ruined but you can be out of pocket as well! This article describes how to prevent this from happening.

How loud is your car?

The starting point has to be with establishing how loud your car is. Ideally this will require a noise test. The normal test specified by the RAC is a static test running at 2/3 the maximum engine rpm measured at 45° and 0.5 m from the exhaust pipe. These tests can provide slightly different values depending on the proximity of other objects such as cars and so on. The measurements use the dB(A) weighting and not the (C) one. This is important as the C weighting will often give a different (higher) value than the A weighting which makes the test comparisons invalid. These tests are carried out on all cars during sprints and can provide an approximate guide to how loud a model actually is. The information in the table has been collected from these measurements and gives a pretty good indication of how loud a particular model would be.

1 Please note that these figures are typical examples only and the actual values can vary with factors such as the environmental conditions, the age and state of the exhaust system and so on.

It must be stressed that this only a guideline. The values in this table are approximate and can vary. Replacing the exhaust system can change the value by a couple of dB. There is also variation between cars and so on. So ideally, the car should be measured before taking any action to ensure that the results will meet the required level. If that is not possible then this table will give you a rough guide to what may be required.

In summary, the modern cars should have no problems with most track noise restrictions providing they have been left as standard. Fitting sports exhausts, removing the cat or exhaust boxes and so on will add at least 10 dB of noise. Older cars will probably at best be borderline with a 100dB static limit but can be extremely loud and way above this limit. The exotic Wedges such as the SE and SEAC models are probably the loudest TVRs around and can have real problems meeting track day regulations.

For reference, the RAC sprint regulations give a static limit at 0.5 m of 113 dB. Virtually all road cars with the exception of some SEACs meet this and have no problems competing. As a result, the fact that a car competes in sprints does not mean that it can meet the noise restrictions on the same circuit for a track day.

Static vs. drive-by tests

The problem for many owners is relating one set of noise regulations to another. If the regulations are given as a static test figure then it is easier to establish if they can be met. Static tests are relatively easy to reproduce, especially the 0.5 metre one and providing care is taken to remove nearby obstacles. A neighbouring car or object can dramatically increase the value through reflections and echos. This can also affect the drive-by test. The drive-by test is a reading taken as the car is driven by on the circuit. In practice, it can really only be reproduced on site because it is dependent on the environment and other factors. If you have a drive-by figure to meet and all you have are static figures, how can you compare them to see if you have a problem?

I would suggest talking to someone at the track who can suggest a static measurement that in their experience meets a drive-by requirement. At Thruxton, the limit is 80 db(A) at 20 metres. The circuit manager reckons that any car with a static reading of about 100dB or more will fail to meet the drive by requirements. If you now have a static figure, you can work out if you need to silence the car and by how much.

Drive-by tests can be more favourable to the driver as they allow the driver to lift off or reduce the revs and noise. However, other factors can also come into play. With a very quiet exhaust, the induction and other engine noise, along with tyre noise can be louder than the exhaust. It has been known for TVRs to fail drive-by tests not because of a loud exhaust but because of the noise created by the engine.

If you think you may have a problem, how do you find out exactly how noisy your car is? You will need to get access to a noise meter to measure the sound level from the car. They vary considerably in price from several hundred pounds to about £30. The more expensive the meter, the more accurate the reading will be. The cheaper meters tend to be slightly inaccurate but can give a very good indication of the success or failure of specific techniques.

I use a £35 digital noise meter from Tandy and found it to be excellent value. It appears to be about 2 dB out at around 110dB but considering the cost, I am quite pleased with it. My particular unit tends to read high compared to the more expensive RAC equipment but that does give me a little more leeway which is not a bad thing. Demon Tweeks sell meters for around £100 and up. The RAC approved units can cost upto £600.

If you don't have a meter, then it is possible to get the car measured at a sprint or race meeting. Most noise scrutineers are more than willing to do this after they have cleared the competitors and this will give an accurate reading on which to base further work.

Reducing noise levels

Given that you need to quieten down the car, what techniques are there and how effective are they? The table lists the typical methods and the resulting noise reduction. If a car is marginal, then deflecting the exhaust or even reducing the revs may be enough to get you through. After that, additional silencing is needed. Again, the choice of a successful strategy depends on the how much the car must be quietened.

Re-directing the exhaust

Exhaust noise is directional and therefore deflecting it away from the noise meter should reduce the readings. I found that deflecting the exhaust downwards quietened the readings by about 1-2 dB in some cases, depending on the surface. Using angled trims that direct the exhaust (and sound) to one side or downward may help but this is not very reliable. It can work on drive-by tests if the exhaust is pointed away from the tester but if the tester moves and the exhaust points towards it, it will increase the value.

Rev reduction

Both my 390SE and Cerbera gain about 10 dB when revved to about 3/4 maximum revs compared to the reading at ideal. As a rough rule of thumb, reducing the revs by 1000 rpm will reduce the noise by 2-3 dB. Some tracks will accept this method but other do not. It is also limits the way the car can be driven which may be unacceptable to the driver. Not being able to drive at all is far worse however.

Using a tail pipe insert

This is the first of the silencer based techniques. Jetex make a Decibel Reducer Insert that are inserted into each tail pipe and reduce the noise level by about 5 dB. They are removable and cheap but do require that the exhaust pipe is straight for about 12 inches. The unit fits inside and is retained by a bolt at the tail pipe end. The units need plenty of a copper loaded grease like Copperslip so that they can easily be removed and if very tight, may need rubbing down with emery paper or a file. There didn't appear to be any power loss when they were fitted to my Cerbera. They took the noise down from 99 dB to 94 dB. They are available from and Europa by mail order. You will need to specify the tailpipe diameter.

Fixing a decibel insert into the tail pipe.

If you don't have enough straight pipe or don't want to drill holes in the tailpipe, the unit can be mounted in a sleeve that slips over the tailpipes and is externally clamped. Dave Wallis at Tower View can provide these for virtually any TVR.

Replacing the silencer

If the silencer system is old, it may be losing its silencing capability and simply replacing the exhaust system can reduce the noise level. It is possible in many cases to ask for increased silencing when the system is ordered.

The down side is the cost, especially if the system has no problems except that it is too loud.

If the exhaust system is modified by removing a box or fitting a sports version, refitting the original quieter system may be another option, providing that the original has been kept. This will require a bit of planning to arrange for the changeover to be made and will incur a labour cost if the work is done by a garage.

Adding an additional silencer

In extreme cases, adding an additional silencer to the tailpipes may be the only way forward. This can be done in one of two ways: the first method is to use a traditional silencer box in some way. Tim Lamont has such a system for many twin pipe Wedges that routes the exhaust from the tail pipes into a silencer box at the back of the car. The exhaust box is supported by the tail pipes and by a nylon strap attached to the spare wheel mount in the boot.

Tim Lamont's add-on silencer for twin tailpipe wedges.

The second method is to use the Supertrap Muffler system which is a small unit, consisting of several perforated disks that go over the exhaust pipe. These need to be welded to the tail pipe to stop them being a ballistic weapon. Dave Wallis at Tower View is also looking at alternative solutions that require a larger noise reduction using Supertrap mufflers. He has come up with a solution that can be mounted and removed. This would allow him to hire out the units as and when people needed.