[sdiy] Adding audio channels, to increase S/N

[Glen]

Please, help refresh my memory. 

If the same signal is fed to multiple audio channels, and their outputs are
later summed, the S/N ratio is higher than if only using a single channel.
For example, some people would send the same signal to several channels of
audio tape, in order to increase the S/N ratio of the tape recording. The
idea being, the signal would reinforce itself when the various copies were
added back together, but the random background hiss would not directly add
up, and be somewhat muted in comparison to the signal.

I just can't remember what the improvement is, measured in dB, when you
double the number of channels used. Is it 3dB? I know it wasn't much for a
simple doubling, but if you had several channels available, the improvement
was supposed to be dramatic.

I should remember the answer to this, but my mind must be distracted at the
moment.

Thanks for any info,
Glen Berry