In the last part of our Noise Measurement Terms Series we cover Peak and Max, Max Sound Level and other key metrics.
Peak and Max
In environmental measurements the maximum value of the exponential sound level is often requested. It could be with any frequency and time weighting, but possibly the most common is the maximum value of the A-frequency-weighted and S-time-weighted sound level over the measuring period – symbol LASmax.
However, many people mix this up with the Peak value – symbol Lpk. This is easy to do, but they are very different indeed. The Peak value is the highest value of the original acoustic signal and for a sine wave – the signal that comes from a sound calibrator – the peak value must be exactly 3dB higher than the sound level, or 2√2 times more (1.414) in pressure.
Peak sound pressure – in Pascal – is formally defined as:- the greatest absolute instantaneous sound pressure during a stated time interval.
Peak sound level – in decibels – is defined as:- twenty times the logarithm to the base ten of the ratio of a peak sound pressure to the reference sound pressure, peak sound pressure being obtained with a standard frequency weighting.
Max Sound Level
Max sound level however is simply the highest value of the exponential sound level over the period and is usually measured with a max hold function, as defined in IEC 61672. The symbol for the A-frequency and S-time weighted measurement is:-LASmax.
As above, for a sinusoidal sound the peak value is 3dB higher than the Max level and this is the closest they can get to each other. For ‘normal’ environmental noise, the Peak value may be over three hundred times – or 30 dB – higher than the Max level.
There are a few other environmental metrics, some specifically for aircraft measurement and possibly the most common is Ldn(the day-night level). A Californian invention, this metric simply divides each 24 hour period into defined day and night periods and takes the Leq for each period then simply adds a fixed ‘fudge’ correction number to the night-time value to account for the added annoyance of noise at night.
Different workers use different day and night times and even different fudge factors. Perhaps most common are 06:00 to 22:00 and 22:00 to 06:00 as the periods and 10dB the added number. In some countries, California-complication is not enough so they divide the 24 into three periods (Day, Evening and Night) adding a smaller amount in the evening period. This is usually called Ldin (Day, Intermediate, Night). The definition of such things is purely political and determined by the specific users.
One important European political definition is given in EU Directive 2002/49/EC, where Lden is defined for assessing EU environmental noise, including airport noise. Here the 24 hour period is divided into three periods, day (07:00 to 19:00), evening (19:00 to 23:00) and night (23:00 to 07:00) and 5dB is added to the evening Leq and 10dB to the night-time Leq.
The EU have mandated the use of Lden for the compulsory noise maps that should have been produced by 2007. They have also introduced the ‘new’ metric of Lnight ( where Lnight = LAeq +10dB from 23:00 to 07:00) and as well have produced a procedure that converts the LA10 level to Lden. The EU however do not use conventional terminology and simply call it Ldeninstead of the correct Lden.
Some airports invent even more complex metrics, one being the NNI (Noise and Number Index). Despite being widely discredited, some authorities still use it. Other very large airports, mainly those run by government bodies, come up with even weirder and more complex metrics “because our airport is special”. One is perhaps reminded of the old Roman adage “There is no situation so complex that politicians and bureaucrats cannot make worse”.
What to Use
Obviously each airport can decide for itself what to use, but using ‘odd-ball’ metrics will inevitably mean that an aircraft can take off from an airport using simple metrics and “Pass” their noise limits and yet “Fail” at an intermediate airport where they have invented their own metric. For this reason the proposed new IEC standard is intended to mandate that only LAE or a direct derivative is used, possibly with LASmax as the descriptor of each separate aircraft event.
For the overall noise, what is needed is a pair of statistical levels for say Day, Night and any intermediate period, L10 and L90being most common, together with periodical LAeq, say hourly.