Units in Waveform/Waveform(dB) PLEASE HELP!!!

[kadejan]

Hi,

So I've been working with Audacity for a research project in Entomology. We're collecting insect vibratory signal using piezoelectric film and recording interactions in Audacity. We've taken some of the signal and plotted them using Matlab and then done a FFT to find peak frequency, amplitude at the peak frequency and area underneath the Fourier Transform. I've been trying to figure out what the y-axis units when you have your recording displayed in waveform, NOT waveform (dB). From what I understand it's a relative amplitude based off the loudest sound in your recording. Is that correct? And if so does that mean there are no units associated with the amplitude; and even more importantly, does that mean that you cannot compare amplitudes across different recorded files? And if not, what exactly does the waveform (dB) plot do that is different. How do you interpret the units there. I read that those units are relative dB units, does anyone know what that means? Please help, I'm very confused!!!!

Sincerely,
Thoroughly confused

[kozikowski]

The default measurements along the left edge are percent. 1 = 100%. 0.5 = 50%.

100% is where the sound channel runs out of numbers and stops dead. No further increase is possible. This can cause clipping and severe crunchy sound damage if you try to put a very loud show up there. Audacity throws mud in the game by defaulting to 32-bit floating for a sound specification which can actually go above 100%, but let's stay in non-magic measurements.

Percents are very convenient for editing since the majority of audible work is on the screen and with magnification, edit points are a snap to find. However, that's not how your ear works. Your ear works in dB. For example, half-volume to your ears is about -18dB or so which works out to 12%. The show is only half as loud and most of the blue wave is already gone.

The sound signal (not the loudness) goes up or down 6dB for double and half signal level.

"This wave is 6dB down from what it was a second ago."

That means the wave went from 1 to 0.5 (for example), or 100% to 50%. You need three of those to fool your ears into hearing a soft volume, so that's 100% > 50% > 25% >12%.

Your ear is still working at -60dB which is half and half and half, ten times. You clearly can't see that on a percent timeline, but you can in Waveforem dB.

Percents work out really well for video waveforms and theatrical editing, but you really want to be in dB for "real world" sound measurements and analysis.

Koz

[steve]

Same answer as Koz, but different description:

The normal "waveform" view uses a linear vertical scale (-1 to +1)

The "waveform (dB)" view uses a logarithmic scale. The bottom end (minimum level) in this view is set in Preferences (Edit menu > Preferences > Interface) to a value between -36 dB and -145 dB. 0 dB (maximum) is "full scale" - that is, the waveform touches the top or bottom of the track (+/- 1).

When representing a waveform digitally as a signed 32-bit floating point number, the "full scale" range is from -1.0 to +1.0.