What does dB(A) or “A-weighted” mean ?

Noise was not of particular concern at the beginning of the century.
The first electrical sound meter was reported by George W Pierce in
Proceedings of the American Academy of Arts and Sciences, v 43 (1907-8)
A couple of decades later the switch from horse-drawn vehicles to
automobiles in cities led to large changes in the background noise
climate. The advent of “talkies” – film sound – was a big stimulus to
sound meter patents of the time, but there was still no standard method
of sound measurement.

The first tentative standard for sound level meters (Z24.3) was
published by the American Standards Association in 1936, sponsored by
the Acoustical Society of America. The tentative standard shows two
frequency weighting curves “A” and “B” which were modelled on the ear’s
response to low and high levels of sound respectively.

The most common weighting today is “A-weighting” dB(A), which is very
similar to that originally defined as Curve “A” in the 1936 standard.
“C-weighting” dB(C), which is used occasionally, has a relatively flat
response. “U-weighting” is a recent weighting which is used for
measuring audible sound in the presence of ultrasound, and can be
combined with A-weighting to give AU-weighting. The A-weighting formula
is given in section 8 of the FAQ.

In addition to frequency weighting, sound pressure can be weighted in
time with fast, slow or impulse response. Measurements of sound
pressure level with A-weighting and fast response are also known as the
“sound level”.

Some sound level meters can measure the average sound level of a noise
over a given time. It is called the equivalent continuous sound level
(L sub eq) and is A-weighted but not time weighted.