posted 20 April 2003 03:29            

consolecalibrator.kym

(It has recently occured to me that a squarewave generator would also be useful, as my VU meters have a dot painted at the +3 VU level. No doubt that is used when you have a square wave input instead of a sinewave signal...)

Using this sound, you can generate a sinewave of known peak amplitude in dBFS of the Capybara scale. This signal is fed to a mixing console and the input gains on those channels are adjusted so that the VU reading at -10 dBFS is zero VU. That leaves 10 dB of headroom. (You might choose more or less...)

My board expects nominal +4 dBu voltage levels for 0 VU (VU is a decibel scale too!, it's just that a standard VU meter has a 300 ms ballistic characteristic). So when I have my Capy put out -10 dBFS sinewaves peak-to-peak amplitude, I adjust my board to read 0 VU pre-fader.

This sound also accepts inputs on Capy channels 1,2,3, and 4, so that you can adjust the output level from the console to match back in the Capy. There are 3 meters on the Sound showing dBFS Peak and RMS in the Capy, and dBVU as measured by Capy.

Rows of toggles at the bottom allow you to select the various Capy output and input channels. So if you have more than two outputs going into your board, you can adjust those channels too.

Once I have my board all calibrated and self consistent between it and Capy, I adjust my other sound source channels to match up. Finally, I set a -10 dBFS tone at 1 KHz in the Capy to produce a +75 dBSPL signal coming out of my headphones, using a sound pressure meter from Radio Shack. I use the C-weighted, slow average readings.

I used to ponder whether to use A weighting or C weighting. A weighting is more strongly attenuated at low and high frequencies, and it was originally meant for adjusting telephone systems for relatively low volume levels. C weighting was intended to mimmick the hearing at loud sound levels. At 1 KHz both are essentially the same. But since most music rides at 75 dBSPL and higher, it seems that C weighting is more appropriate.

With a Capy setting of -10 dBFS, no attenuation on the Capy output from the Preferences dialog, and this corresponding to 0 dBVU on the console, and 75 dBSPL in the headphones, it means you won't ever be able to generate more than +85 dBSPL from the Capy. Some other sound source might, but not the Capy. Hence, the sound levels are all at entirely safe levels.

My understanding is that nominal 0 dBVU in a Theatre corresponds to +85 dBSPL. I can believe that! And I don't like to run my system that loud for very long. I find it much easier to listen when the average level is around +75 dBSPL.

By way of comparison, normal adult conversation at 1 meter spacing is approximately 60-65 dBSPL. Soft passages in symphonic recordings are typically no softer than 40 dBSPL. At 85 dBSPL you begin generating noticeable spurious signals in your ears due to the intense sound interacting with your own hearing nonlinearities.

Anyway, I thought this might be useful to others too... so here it is...