dBA - Calculating weighting filter frequency to dBA and dBC DIN IEC 651 calculator equal-loudness contours

Sound measuring (Noise measuring)

Calculation: Frequency f → dBA and dBC − The difference
Weighting filter after DIN EN 61672-1 2003-10 (DIN-IEC 651)

● Notice: Sound level meters measure the sound pressure level in dBSPL ●
Sound pressure is a sound field quantity.

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At sound level analyzers the display (attack time t_in) is time weighted. There are different settings:

Slow (S):	t_in	=	1000 ms
Fast (F):	t_in	=	125 ms
Pulse (I):	t_in	=	35 ms	t_out	=	1500 ms

Formulas to calculate the weighting filter curves A, B und C

Note - Comparing dBSPL and dBA:
             There is no conversion formula for measured dBA
             values to sound pressure level dBSPL or vice versa.
             That is only possible measuring one single frequency.

There is no "dBA" curve given as threshold of human hearing.

Readings of a pure 1 kHz tone should be identical, whether weighted or not.

The A-weighting filter curve is defined from 20 Hz to 20 kHz.
This is intended as a rough replica of the 40 dB loudness of the
curve of "equal-loudness contours" for low frequencies.
That means, the cutoff low frequencies are really not measured.

Because of the complexity of the equal loudness contour curves one was looking for an easy
simple evaluation scale and chose the A weighted curve, which reflects the course of the
hearing curve for bass frequencies of the low loudness level L_N = 40 phon approximately.
From the noise spectrum one gets a single number, which corresponds to the real loudness at
higher levels lesser and lesser. Because of the easy measuring possibility and the ease of use
of this single number the industry marketing took the dBA sound pressure level measurement.

dBA: The decibel A filter is widely used. dBA roughly corresponds to the inverse of the
40 dB curve at 1 kHz equal-loudness curve for the human hearing.
An A curve always provides for "nice" values when low frequency noise signals are included.
An A filter of a measured motorcycle noise must show untrue values. You should know that.
From a dBA measurement no accurate description of the expected volume is possible.

The weighted sound level is neither a physiological nor a physical parameter.

The weighted sound pressure level is less accurate in telling the perceived
loudness as a true loudness measuring, but it is much easier to handle.

Pro audio equipment often lists an A-weighted noise spec
– not because it correlates well with our hearing – but because it
can "hide" nasty hum components that make for bad noise specs.
Words to bright minds: Always wonder what a manufacturer
is hiding when they use A-weighting. *)

*) http://www.google.com/search?q=Always+wonder+what+a+manufacturer+Rane&filter=0

dBC: The decibel C filter is practically linear over several octaves and is
suitable for subjective measurements at higher sound pressure levels.

Notice:
The sound pressure p as a sound field quantity
moves the diaphragms of the microphones
and the ear drums of our sense of hearing.
And what operates the sound level meters?

Why should our ears need the sound intensity?

Comparing dBA and dBC

Relative Response (dB)	Frequency f in Hz
Relative Response (dB)	31.5	63	125	250	500	1000	2000	4000	8000	16000
dB(A)	−39.4	−26.2	−16.1	−8.6	−3.2	0	+1.2	+1.0	−1.1	−6.6
dB(C)	−3.0	−0.8	−0.2	0	0	0	−0.2	−0.8	−3.0	−8.5

The B-weighting curves and the D-weighting curves disappeared
ages ago from the standards. Don't use that anymore.

In the English-speaking countries they were working with rating curves, which were
independent of sound level freely selected and were designated A, B and C. The
weighting scale is based on a curve at 40 phon equal loudness and is the level and
frequency dependence of hearing for pure tones for quiet sounds useful. The rating
curves B and C were based on higher volume levels. Weighting filter with the three
curves can be easily realized using passive RC filters. 1967, agreement was
reached in the International Standards Organization (ISO) on the future instead of
three rating curves worldwide to only use measuring devices with A-weighting. This
allowed a single international registration of noise emissions (equipment out) and
pollution (arriving at the ear noise), even if the values obtained corresponded with
sine tones and narrow band noises about the actual volume level. For broadband
noise - this is in practice the most common - we get too low values. Because of this
deviation of the results it was decided that simple devices are no longer sound
meter, but only be described as sound level meter. Trade and industry were initially
satisfied, the A-weighted sound level was quickly incorporated into the legislation
and noise regulations.

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