Dear Hazel,

sound pressure levels (in short SPL) in dB(A) are derived from SPL in dB applying the so-called A-weighting. It is a sort of filtering in the frequency domain, originally developed to take into account the non-linear sensitivity of human hearing.

In practice, in each 1/1 or 1/3 frequency band a "weighting" is added to the measured SPL; most of these weightings are negative, so that the net result is a decreasing of the overall SPL.

The numeric values of A-weightings are specified in table 2 of EN 61672-1, together with "C-weightings" and Z-weightings" (the latter are just a linear weighting, i.e. the weighting are all zeros).

I would like to recommend you a good book on acoustics where you can find a didactic explanation, e.g.: L.L. Beranek, Noise and Vibration Control Engineering: Principles and Applications, John Wiley & Sons (2006) Print ISBN: 9780471449423.

Best regards

Massimo

When you write dB, I assume you mean the sound pressure level referenced to the standard reference amplitude of 20 microPascals = 0 dB. This is purely an amplitude scale and independent of frequency. dB(A) is adjusted by a frequency weighting to account for "perceived loudness". If I recall correctly, at 1000Hz, they will be equal, but at 100Hz, a 100 dB signal would be about 80 dB(A). So noise reduction levels (delta dB)need to be adjusted accordingly. There are other weightings, but A is most common. This wiki page has some graphs and conversions: http://en.wikipedia.org/wiki/A-weighting

Almost all acoustical phenomena are dependent of frequency and sounds consist of a mix of frequencies. A complete "picture" means: expression in a frequency spectrum; in octave bands, third octave bands or even smaller ranges. This applies to sound pressure levels, sound reduction values - both in dB-,sound absorption coefficients,  and others.

For several purposes- like comparing phenomena, setting limits or requirements- it i suseful to express the properties (of a sound, sound reduction etc.) in a single number.

A-weighting is most common for sound pressure levels and the unit is then dB(A). For sound reduction values several weighting procedures are used; some of them are expressed in dB, others in dB(A) related to a specific source spectrum, like traffic noise.

People who are new to acoustics often do not realize that dB measurements represent a comparison or ratio between the quantity being measured (such as sound pressure or intensity) and a reference quantity. Often the reference quantity is implied by the context rather than clearly stated, which can lead to confusion. When measuring sound intensity in air, the reference value is by convention a sound pressure of 20 micropascals (µPa), the textbook value for the average quietest sound that a healthy young human can hear (at a frequency of 1000 Hz). dB measurements referenced to 20 µPa are sometimes written as dB SPL (sound pressure level). dB SPL measurements of audible sounds always have positive values. dB units are also used in contexts unrelated to measurement of absolute sound intensity in the environment, such as signal-to-noise ratio, or signal intensity relative to the maximum that a recording device can record without distortion (sometimes called dB relative to “full-scale” or dBFS; dBFS values are always negative).

dB SPL measurements take no account of how sound power is distributed with respect to frequency. But because the sensitivity of human hearing to sound varies with frequency, an additional scale called dB(A) [where A may be written as a subscript] has been devised that yields measurement values that are more intuitively aligned with human perception of sound loudness. dB(A) measurements assign different weights to the sound in different frequency bands, with the highest weighting being in bands where humans are most sensitive and lower weightings in lower- and higher-frequency bands where human sensitivity diminishes. In general you cannot convert between dB SPL and dB(A) measurements unless you know how the sound power is distributed with respect to frequency. For example a sound might have a very high dB SPL but a very low dB(A) if most of the sound power is below the frequency range that humans hear well.

Thank you for all your responses, they are really helpful at this stage as i am a very new to this field. As far as i understand about the way of using these two different units, I totally agree that it is impossible in concerting dB and dBA. Actually I'm in process of research to achieve knowledge about the effects of brick veneer and structural insulated panel walls in noise reduction for residential building located near to free/highway where the sensitive high road traffic noise level is appearing. Can anyone suggest me data in term of testing or measuring the effect in noise controlling in relation to these two types of facade. I would be very appreciated.

All excellent answers. Unless I missed it in someone's comments, we need to bring out that dB(C)--which includes the lows left out of (A)--is the most common designator for purposes of control for audiometric testing environments, or in setting up for experiments in Speech-in Noise (SiN). Some of us that have been involved in regulatory matters of ambient compliance rules have regarded the A-weighting as the "fudge factor". However, unless there have been changes in recent years (as I've been out of that area for a while) C-weighting is used for measuring compliance of sound control enclosures for OSHA testing. These distinctions can be important for purposes of threshold accuracy. 

Nice answers so far, even I'm not a friend of weighting at all. Why? So, like Max mentioned, there are other curves like dB(C) (don't forget, B, D, E). However, a weighting curve to represent the human auditory perception is only correct for one dB-SPL. The hearing curve is changing with the sound pressure level (generally speaking it's flattening for higher levels). Technically a weighting curve works really good, if you have a sound source which represent all included frequencies with the same level and a receiver (a human being), which represents the average of the (so far) measured mankind - not very natural, isn't it?

Anyway, the question itself is answered.

Best regards,

André

Thank you for your all responses!

Really helpful.