C-weighted , A-weighted ??? - Axiom Message Boards

I have been looking in the internet, but I havent found a good response, but I know I am not the best web searcher around, so I will take the shortcut and ask this forum:

I am trying to find the definitions of the A & C weights, found in the RatShack SPL meters and some amps specs (for example, the Outlaw 770 I own specifies S/N of 119dB A-weighted).

Also about noise, I got confused in an article that says that the white and pink noise differs in the freq. spectra that each one carries, but did not specified the freq range or any other thing about it. In one of the papers I read, I kind of understood that pink noise is just between 20Hz-20KHz, and white noise is all over the place. Is that right? is noise just a sum of sine waves in all frequencies or there is something else in noise generators?

If anyone can throw an explanation or a link where I can learn this concepts, I'll appreciate it.

Thanks

Audio Glossary

Choosing the 'C' weighting will make the meter respond more-or-less uniformly over the frequency range from 32 - 10,000 Hz, and the 'A' weighting will make the meter more sensitive to frequencies in the range 500-10,000 Hz. The response switch allows for changing the speed of the meter's response from 'SLOW' to 'FAST'. A slow response setting will make the meter less sensitive to rapid changes in sound level and can be used for measuring average pressure levels. The 'fast' setting is more useful when peak sound levels are being measured since in this mode, the meter will respond to very rapid changes in pressure level.

Here is a good link.

http://www.audiophilia.com/hardware/spl.htm

Pink noise and white noise are both random signals containing all frequencies -- you can think of the difference in terms of the two being EQ'ed differently. Let's take two frequency ranges -- 100-200 Hz and 1000-2000 Hz -- for the example.

Pink noise is equal energy per octave, so the 100-200Hz range would have the same total energy as 1000-2000 Hz since both are an octave.

White noise is equal energy per "number of hz", so the 100-200 Hz range would have one-tenth the energy of the 1000-2000 Hz range. In other words, white noise has louder "high notes" and sounds like the treble is turned way up relative to pink noise

From an audio point of view, pink noise is "flat" across the frequency range and makes more sense for us to use as a reference. The main point of interest with white noise is that it "occurs naturally", ie the noise you get from a radio between channels, the noise you get when you point a radio telescope out into space, the thermal noise you get in an electronic circuit etc...

White noise (of the type that John refers to) is quite literally the sound of the big bang. So cool...

And waterfalls, explosions, rubbing a stick against a sidewalk and other naturally occuring sources of noise.

Bren R.

Rubbing a stick on the sidewalk ain't gonna produce static on a TV...

In reply to:
Rubbing a stick on the sidewalk ain't gonna produce static on a TV...

. . .but it sure is fun. For some, I suppose. . .err. . .nevermind.

In reply to:
Rubbing a stick on the sidewalk ain't gonna produce static on a TV...

But.. that... not... the idea behind... statement... no sense....

Ouch. My brain.

Bren R.

Heh heh heh... that was fun.

Naturally occuring noise can be non-white (off-white?). A very important source of noise at low frequencies in electronic components is "1/f" noise, which is "pink" according to the definition below. Above the 1/f corner frequency, the noise becomes flat versus frequency. There are many sources of 1/f noise, depending on the technology, but the important thing is that 1/f noise is also natural.

Not that any of this matters, because your electronics generate so little intrinsic noise compared to the signal amplitude that you'll never notice it unless you do something stupid like crank the receiver to 11 and admire the hiss with no input. If it's white noise, then the noise is coming from thermal sources (current passing through resistors) or you're picking up noise from the big bang. If it's pink, I bet it's the Si transistors in your internal electronics that are to blame.

Audio is at very low frequencies compared to radio, and noise that comes from a radio is noise that was at RF frequencies (where it was flat over the radio band). After being downconverted to audio, the noise stayed flat.

Hope this makes some sense... --Martin

Thanks guys, very nice info.

I think that opening another post might be a good idea, about noise ID and troubleshooting., so under technical questions I am opening a new post following this tread on noise.