1. An expert's commentary:

‘Noise’ is defined as “sound, especially when it is unwanted, unpleasant or loud”. 

The term itself is a subjective judgment. The sound of motorsport is likely to be enjoyable to participants and spectators, however for those living close by and wanting to enjoy the countryside near to circuits at exactly the same time, this sound may be unwanted, unpleasant or annoying.

While it would be desirable to completely separate the two parties, sound is a form of energy which is transmitted through the air and as such is difficult to control.  In transmitting sound, the air particles vibrate, causing rapid cyclic pressure changes.  Sounds are characterized by frequency and level. For all environments there will be a significant variation in sound with both time and level.  The sound pressure level is a measure of the magnitude of sound.

The human ear is more sensitive to certain frequencies.  The ear can detect mid and high frequency sounds (such as human speech) a lot more easily than low frequency sounds (bass frequencies in music). To take into account this sensitivity, noise levels are normally assessed using the A-weighting scale and presented in terms of dB(A).

An example of some typical sound levels is presented below.

With regards to changes in sound level some basic simplified rules of thumb can be applied.  If a particular sound (for example that of continuous road traffic sound) increases by less than 3 dB(A) most people would not notice the change. A change of 10 dB(A) corresponds to a subjective doubling (or halving) of loudness.

The sound level in dB(A) only provides information concerning a sound level at one position at one point in time.  In reality sound levels will vary, sometimes considerably with time while the distance away from the sound source clearly affects how loud the level is.  Another simplified rule of thumb is that in the open air sound will reduce at a rate of 6dB(A) per doubling of distance.  As an example an exhaust level of 92 dB(A) at 2m would reduce to 80dB(A) at 8m, 68 dB(A) at 32m and 50 dB(A) at 200m.  Wind speed and direction will influence the propagation of sound.  Any barriers, either man made (fences/buildings) or natural (earth bunds), will reduce sound propagation.  Typically, reductions in the order of 5-15 dB(A) can be achieved depending on the height of the barrier, distance to the source of sound and distance to the receiver of sound.

In order to describe how sound varies with time there are many, many different parameters that can be used. The most common is the continuous equivalent sound level or the dB LAeq, T parameter. This essentially provides an average of the sound level over an averaging period T.  This period may be a 16 hour daytime, a 1 hour sample during the day, or 8 hours through the night etc. etc.

There are very few sound limits.  There are however many sound guidelines some of which become mandatory limits through the use of planning conditions.

There are recommended sound limits for cars/motorcycles competing in various sporting events.  These are usually set by the sporting organization such as the ACU or MSA.  These sound limits do not correspond to an agreed acceptable noise level at communities around motorsport venues.  These limits, now rigourously enforced, are designed to prevent excessively noisy (subjective term)vehicles competing in events thereby reducing the risk of disturbance.

There are recommended guideline values for acceptable sound levels.  However these guideline values are usually intended to be used for anonymous transportation noise and their use for noise emanating from sporting activity is questionable. The measurement of sound is a straightforward objective procedure.  The assessment of noise is something of an art.  The effect of noise is possibly something altogether imponderable.

For example, the following list sets out some variables which would affect the acceptability of a sound to somebody;  the level of sound, the time of day the sound occurs, whether windows are open or closed, whether people are in their gardens/bedrooms/living rooms, the existing background sound level, how the sound varies with time, how often the sound occurs, how long the sound lasts, whether the person has any personal connection with those making the sound (e.g. long standing disagreement with their neighbours over other non sound related issues), whether the person works for the person/business making the sound, habituation to the sound over many years, whether the sound is intrinsic to the local area, what the recipient's present activity happens to be and the nature of the sound (whine, drumming, buzz, throb, whistle) etc. etc. etc.

Some quotes from the UK government regarding noise:

“Noise from recreation and sporting activities — For these activities (which include open air pop concerts), the local planning authority will have to take account of how frequently the noise will be generated and how disturbing it will be, and balance the enjoyment of the participants against nuisance to other people.” (Guidance Note No.PPG24)

“The Government recognises that motor sports are enjoyed by a great number of people across the country and does not seek to curtail that enjoyment if a site operator operates within the planning conditions.  However, if the Local Authority finds that a nuisance interferes with a person’s reasonable use of their property or is prejudicial to their health the operator can be told under the Environmental Protection Act (EPA) 1990 to abate a reported nuisance.” (No.10 Downing Street petition response) “

2. A Layman's Pragmatic Commentary:

It is clear that noise is a complex, partially scienctific, partially subjective topic, one that is largely misunderstood by the man in the street.  It's logarithmic scale is a mystery and what the ordinary person is familiar with is simply the effect, not the beast itself.

As science tends to be mathematically exact and the law tends also to work within absolutes where proof or reasonable doubt may be established, these two disciplines may make comfortable bedfellows.

Setting parameters which can be stated in terms of decibels and hours or days and where levels and distances can be measured and expressed numerically is simple enough and this indeed is a logical part of the normal planning approval process.

However, the effects of noise are anything but scientifically calculable.  Perception and reaction are wholly subjective topics where the rock music fan may find the sound of a baby's cry repellant and the twitcher enjoying the call of the lapwing may be disturbed by the buzz of the honey bee, while the Mozart aficionado may also love the sound of a racing engine on full song as it hurtles past.  Each of us is different and our feelings shift with our moods, our experiences and our activities of the moment.

The following short statement is taken from a government response to another recent Downing Street petition:
“In relation to the noise sources specifically mentioned, it seems unlikely that an Environmental Health Department would consider church bells or children playing in a playground to be considered a statutory nuisance as they very well may be considered reasonable for the area”.
Contrastingly, here we have a scenario whereby an officer of a local authority must make a decision as to what ‘reasonableness' is.  No longer does science apply, no more are finite values the measure of the matter, nor can meters or calculators assist.  The graphs and charts are jettisoned in exchange for a cut-off point somewhere in an elastic philosophical range.
So, where a dispute over noise arises (even where planning permissions have been strictly adhered to), a motorsport facility operator, having invested in a business that exists courtesy of those all-important planning permissions - decided by a local authority, based upon fixed mathematical measures of time, distance and decibel levels - is now judged by the self-same local authority on the basis of opinion as to 'reasonableness' of effect.  It's like oil and water, chalk and cheese - the two don't mix.  Something that has been allowed on a scientific basis should surely not be disallowed on an arbitrary basis.  Anything agreed methodically should not be disapproved irrationally.
If such an action for statutory noise nuisance has been brought not through a local authority but directly to litigation by an individual, it is a Judge who decides the strength of the plaintiff's case once again on the entirely subjective effects of noise (requiring little burden of proof).  Some would argue that for important decisions such as this to have validity, a complex and all-encompassing matrix of interlinked variables would have to support such an important calculation, taking into account survey results, expert witness reports and a further in-depth analysis of all significant circumstances and changes since the initial permission was granted.  In other words, back to the scientific.  But no, it seems that simply to express an opinion that a noise is a nuisance is enough on its own to establish the case.
If the effect of noise is to be considered subjectively (encompassing feelings, opinions, phobias, sensitivities and past experiences) in the legal setting, then at least let it be adjudicated in a commensurate manner.  The only way to achieve this is through a 'jury' of neutral observers who may reliably provide an average human opinion after witnessing the noise in question and translating this to effects upon themselves before advising the decision makers.
To reiterate, the following is taken from Planning and Policy Guidance Note PPG24:
" ... the local planning authority will have to ..... balance the enjoyment of the participants (of the noise-producing activity) against nuisance to other people".
Again this illustrates the impossible position for a local authority where, as 'gamekeeper', they must use finite science and mathematics to establish a motorsport circuit's operational permission in the first place, then as 'poacher' can only use elastic opinion to compare the enjoyment of thousands against the annoyance of a few.  No formula has yet been written to reliably allow such a calculation .....