0dB is the “digital maximum”*. If you go over 0dB you can get [u]clipping[/u] (distortion). On the (default) Audacity waveform display 0dB peaks will hit +1 and -1.
Audacity uses floating-point internally so it can go over 0dB without clipping.
Regular (integer) WAV files, digital-to-analog converters (playback), analog-to-digital converters (recording), and CDs are all hard-limited to 0dB.
This means that if your file goes over 0dB in Audacity but you are not playing at “full digital volume”, you may not be clipping your DAC (digital-to-analog converter) so you may not hear the distortion until you export the file.
It also means that if you use a format that can go over 0dB, you should still keep your peak down to 0dB or less because if the file goes over 0dB the listener’s DAC can clip.
The Amplify effect will default to whatever (positive or negative) gain is needed for “maximized” 0dB peaks. (This is also called “normalization”.)
BTW - It’s a good idea to Amplify or Normalize as the last step before exporting because effects/editing/processing can sometimes push the levels into clipping. (You can also run Amplify just to check the peaks, and then cancel the effect if you don’t actually want to make a volume change.)
Peak levels don’t correlate well with perceived loudness… If you normalize all of your recordings for 0dB peaks they won’t all be the same loudness.
Most commercial recordings are compressed and limited to bring-up the overall loudness without clipping. This can be tricky for amateurs because compression & limiting are a kind of distortion and they can sound like distortion or add unwanted side-effects.
but the maximum recording levels achievable did not exceed 12.
I assume you mean -12dB. That’s fine. It’s OK to boost digitally after recording. If you’ve done any analog recording you know that you want a “hot” signal to overcome tape noise, but with digital there is no tape noise.** Also, tape “soft clips” as you go over 0dB (and it can go over 0dB) so it’s OK to go occasionally-slightly “into the red”. Digital hard-clips at 0dB so it’s a good idea to leave plenty of headroom.
On playing back, the voice was low in volume and ‘muffled’,
Is the sound muffled when you just listen to the tape playback? …If the digitized audio is degraded compared to the original analog sound, we need to talk about your setup because that shouldn’t happen.
You can use EQ (Equalization) to bring-up the high frequencies and/or cut the low frequencies and hopefully make it more “clear”, but boosting the highs will also boost any tape hiss.
and I used Amplify to get it to a reasonable level, but that made the applause between gags distort badly.
If you don’t check the “Allow Clipping” box, Amplify won’t allow you to push the peaks into clipping.
Subsequent use of the Limiter helped significantly, but resulted in the introduction of some previously unnoticed background noise.
Yes… Limiting by itself doesn’t increase noise, but make-up gain (or otherwise boosting the levels) will bring-up the noise along with everything else. The same thing will happen if you crank-up the volume at playback time, so there’s no harm in doing it digitally.
You can also use the Envelope Tool to “manually” fade-down the volume during the applause. That’s more work but it will probably work better than limiting. (The trick with the Envelope Tool is to “fade” so you’re not making any sudden up or down jumps in volume.)
But again, if you fade-down the applause and then increase the overall volume, you’ll boost the noise.
You can try the Noise Reduction effect, but this really works best if you have a constant low-level background noise. …It works best when you don’t really need it. If the noise is bad (like the noise you likely have on a “live” recording), “The cure can be worse than the disease.”
- 0dB is the highest you can “count” with a given number of bits. Everything is automatically scaled so a 24-bit file is not louder than an 8-bit file. With more bits, the increased dynamic range comes on the quiet-side. (With floating point audio 0dB has a numeric value of 1.0 and for all practical purposes there is no upper or lower limit.)
** There is something called digital quantization noise, but it’s more than 90dB down at 16-bits so we normally can’t hear it, and it’s usually way-quieter than the analog noise.