2.4.3     Definitions of further terms

Equivalent continuous sound level

The average sound level L_m calculated in the previous section is also called "energy-equivalent sound level" or "equivalent continuous sound level" due to the obligatory energetic averaging. It is commonly abbreviated as L_m or L_eq. 

An equivalent continuous sound level is an A-weighted sound level with a constant amplitude, which transmits the same amount of sound energy in the reference period as a sound level with a fluctuating amplitude corresponding to the interference effect of a steady continuous sound.

The process of averaging is used as analysis basis for noise measurement. If the range of measurement variations stays below 10 dB, the average sound level lies about 1/3 of the range of variations below its upper boundary. The exact procedure of averaging sound levels varying in time is given in DIN 45641.

According to the modes of operation of noise meters described in section 2.4.4, the average sound level can be differentiated according to various time weightings:

"FAST" time weighting: L_AFm
"SLOW" time weighting: L_ASm

"IMPULSE" time weighting: L_AI
"FAST time weighting with takt maximal sound level": L_AFT

Figure 2/11a illustrates the problems associated with the indication of average sound levels. You can see that uniform traffic with 2,000 cars per hour has about the same average sound level as a single train in one hour. This may lead to the conclusion that an adequate assessment of certain sound situations is not possible on the basis of average sound levels. One can reply that this procedure has proved itself in practice and has become a useful instrument of administration. "The special features of railway traffic" are taken into account by adding a bonus of 5 dB in order to avoid serious miscalculations as for the application of the average sound level (section 3.1).
 

The impulse-weighted average sound level

The impulse-weighted average sound level LAIm differs from the strictly physical energy definition of the equivalent continuous sound level as it weights impulse noises (e.g. shooting noises, striking mechanisms etc.) more due to a short rising time constant and a slow falling one.
 

The takt maximal sound level procedure

The average sound level L_AFTm resulting from this procedure weights the impulsive characteristics of noises more than the energy-equivalent averaging does. Similar to the Impulse sound level meter, this procedure assigns a seemingly longer period to short-time level peaks. This is due to the fact that level peaks are scanned in a uniform interval of 3 or 5 seconds and so the actual level curve is replaced by an echelon signal.

The takt maximal sound level is usually higher than the average sound level L_Am and further increases with the impulsive characteristics of the noise. It is therefore not necessary to make an impulse addition.
  

Exceedance level

Exceedance levels are based on the frequency distribution of measured sound levels. They allow for a more detailed description of the basic structure of an existing noise situation than the simple indication of the average sound level could.

L₉₅ is the sound level which is attained or exceeded during 95 % of the measurement period. It is called the "background noise level" and used for the analysis of constantly dominating extraneous sounds.

L₁ is the sound level which is attained or exceeded during only 1 % of the measurement period and is therefore regarded as the "average maximum level".

Rating level

The rating level L_r of a measured or calculated situation serves as a comparison with the immission values (limit values, guide values and reference values). Just like the average sound level, it refers to limited periods of time, e.g. an eight-hour working shift, the time from 6 am to 10 pm (16 hours) or the time from 10 pm to 6 am (8 hours or the loudest hour).

In contrast to the average sound level, the rating level cannot be directly determined through measurements. It is the result of weighting level additions (or deductions), which cannot be deduced metrologically but are applied according to the agreements reached in the various sets of regulations. Level additions are applied for example in the context of more disturbing noises during specified resting periods or of sounds with impulsive characteristics or discrete components or as a result of meteorological corrections.

As for road traffic noise, a level addition is considered which takes into account the increased interference effect of nearby signal-controlled crossings - an addition which would not be absolutely necessary according to comparative measurements. Vice versa, the lower subjective interference effect of railway traffic leads to a level deduction (called railway bonus).