I have recorded some banging noises on a Zoom H1N recorder and downloaded them Audacity. I am now trying to correlate the negative decibel reading I am seeing on the meter reading toolbar in Audacity to a positive decibel reading to indicate the loudness of the bangs. I am very new to Audacity and not a sound expert by any stretch of the imagination, and the information I am reading on the Audacity Manual is too technical for my level of expertise. Can anyone help me understand how to interpret the negative readings in Audacity into positive decibels? Thanks
There’s no direct correlation between the signal level in Audacity, and the loudness (sound pressure level) that was recorded.
By analogy, if you have a photograph of a box, you can measure the size of the box in the picture, but there is no way you can tell how big the real box was unless there is something else in the picture to give an idea of the scale, but even then it isn’t foolproof.
The problem is, nothing is calibrated.
The digital level you get when recording depends on the recording level control and the sensitivity of the microphone, etc. (as well as the acoustic level).
The acoustic playback level depends on the playback volume control, the particular soundcard/DAC, the efficiency of the speakers, how close you are to the speakers, how much room reflection you get (and of course on the digital level). Actually, movie theaters are calibrated, but then it depends on where you are sitting.
But, it can be calibrated and there is a direct correlation. For example, a -3dB reduction in the digital level makes a -3dB change in acoustic level.
…I gotta’ go now. I can get “more technical” when I come back.
…I’m back -
Digital levels are dBFS (decibels full-scale) where the 0dBFS reference is the “digital maximum” so digital dB levels are normally negative. For example, it’s highest you can “count to” with a given number of bits. Analog-to-digital converters (recording), digital-to-analog converters (playback) and regular wave files are all hard-limited 0dB.
Acoustic levels dB SPL (sound pressure level) where the 0dB reference is approximately the quietest sound that can be heard so SPL levels are normally positive.
There are other references/standards for electrical signals, etc. For example, 0dBV is 1 Volt.
Actually, there may be several problems.
As above posts, there is no fixed relationship between negative audio levels and positive sound pressure levels. The silly solution is compare your H1n to a sound pressure level meter.
And from there, you can more or less compute the readings.
Pounding and knocking don’t lend themselves to readings either audio level or SPL. Note the meter has a setting for Fast and Slow? Even at Fast, it won’t read single bangs or knocks.
This is your neighbor banging on the walls or ceiling, right?
Audacity can’t be used for surveillance, law enforcement or conflict resolution.
There may be another way out. I’ve seen more than one reference to a Sound Pressure Level APP for your phone.
Another suggestion would be to record a reference level sound at the same time making sure that any automatic level adjustments are turned off. This could be as simple as stating in a normal voice your distance from the microphone and the current settings for the input device level. Provided that it is repeatable you have a valid basis for comparison with other recordings made using identical equipment and settings. Of course you could lose accuracy by speaking more loudly or more quietly but the changes in timbre would probably make this rather obvious and you could always supplement this with another reference such as a ticking clock.
And that brings us to the A—C switch on that sound meter. Not all sound pressure readings are plain and flat as a music system. Flat is “C” (more or less). Read most things at equal volume.
The actual flat reading is “Z”, but that’s not as handy as you think. That measures everything from low pitch earthquakes to stuff so high only cats can hear it. This meter won’t do that.
“A”, however is not flat. It’s designed to hear the same way you do with bumps and lumps in response to different pitch tones. “A” is the one burned into health and safety laws. You need to be able to speak in “A” to do any direct loudness comparisons with anybody else.
“Unprotected exposure to a 120 dBSPL Jack Hammer at 10 feet shall not exceed two hours…etc.” That’s “A”.
“A” sensitivity peaks at about 2000Hz and is quieter for both higher and lower pitch tones (if you’re counting). For reference, 3000Hz is baby screaming on a jet. Good ear sensitivity right there. Right in the middle of the “A” curve.