MEASURING SOUND
Frequency Filtering: dBA
dBC
Fixed Period
Averaged Sound Levels: Leqs
Exponentially Averaged Sound Levels: dB
"fast" or dB "slow"
Peak Sound Pressure
Percentile Measurements: L10 etc.
Noise Regulation: Interpreting Metrics
Noise regulations for concert music may use different ways to quantify sound. Typically metrics employ definitions that include both sound frequency and time. Grozier systems can measure sound in all the ways discussed below, and many more besides.
Metrics can be divided into two groups
according to sound frequency filtering. These are dBA and dBC. dBA
emphasizes the mid frequencies and dBC additionally includes bass frequencies.
(Note: The use of octave bands in concert noise regulation is rare and
is not normally recommended) The preferred frequency metric is dBA , there
may be some justification for using this metric in special
situations.
Fixed Period Averaged Sound Levels
Within each of these two groups, a time-averaging metric is also incorporated. It is very important to understand which time-average applies to any decibel level that is quoted. A time-average that is “fast” registers the sound pressure of every drumbeat. When an averaging time period is specified, it is usually defined using Equivalent Sound Level (written as Leq) along with a defined period. E.g. 1 minute or 30 minutes. Thus the average sound level for a 10-minute period might be quoted as 80dBA Leq 10-min or 95 dBC Leq 10-min. An example is given here.
. 
Exponentially Averaged Sound Levels: dB "fast" or dB "slow"
Sometimes you you see sound levels quoted as dB "fast" or dB "slow". The terms refer to the rate of exponential averaging. These were popular during the days of sound meters with analog displays. "fast" is an exponential rate that approximates to the human ear, 125 msec. "Slow" has an exponential decay rate of about 1 second. The filter characteristic must also be specified.
The proper definition of this is the maximum instantaneous sound pressure. This means the absolute value of the pressure wave during a stated interval. However, it is often misused to mean the maximum indicated dB level on a meter, without defining the average period. So take care with this one.
Another metric uses time
and sound level in an additional way. The concept is to define
the sound level that is exceeded for a defined percentage of the
time. For example, if the sound level is exceeds 91 dBA for 10
percent of the time, the metric would be L10 91 dBA. The
understanding of this metric also requires close attention, because the
time averaging must additionally be given. So, in full, the above
example should read L10 of 91dBA for Leq 1-min data over a period of 1
hour. In the above example, this means that the 1-minute average (blue
line) exceeds 91 dBA for a total of 6 minutes.
Noise Regulation: Interpreting Metrics
From the above
discussion it can be understood that a noise regulation that quotes
just a dB number is essentially meaningless. The time averaging
and frequency filtering must also be specified. Experience shows
that the same music may be measured to have sound levels over a 25 dB
range depending on the metric that is used. So a clear
understanding of any quoted metric is essential.
