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Topic: Sound level meter / analysis (stereo MP3s) (Read 579 times) previous topic - next topic
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Sound level meter / analysis (stereo MP3s)

In the old days when putting a selection of songs together you could check the volume level with an analogue needle style meter, if it went into the red you could just reduce the volume a little before recording etc.

I have searched for hours online and cannot find anything that does this, everyone goes on about LUFS, RMS, gain etc when I just want to match the volume level.

I tried foobar2000 it analyses a track to just under 10db whereas Audacity says the same track is +0.7db, so one of them is wrong.

To me this is something so fundamental and simply yet I cannot find anything online to support with this. Does anyone know of some basic software that will do this, either graphical or analogue dial format please?

Re: Sound level meter / analysis (stereo MP3s)

Reply #1
Analog vu-meters ( either with needles or with leds) use to show the power of the signal and they are modeled to show values in dBFS ( deciBell respect of Full Scale ).  (Implementations may vary in the way they do it)
Most probably, what you see in audacity is the peak (i.e. the amplitude) of the signal, also in dBFS.

The other mechanisms you mention ( LUFS, RMS... ) are means to know the loudness of a delimited duration of the sound. RMS use to be about 50milliseconds. LUFS can either be an immediate measure (similar to RMS), or a representative measure of the whole duration. 
Their unit is also dB, but it is not dBFS, rather the estimated power produced when played under a predefined setup at a predefined volume. As such, the raw value isn't too useful, but once a reference value is choosen ( 89dB for Replaygain, 85 dB for LUFS, ... ) , the software can say "this tracks is x dB higher than the reference, so the volume has to be reduced by x in order to be played at the reference loudness).

Since you mention recording, analog clipping is quite different than digital clipping. There used to be no problem to go slightly above it. The consequence was some sort of hard limiter, and on playback clipping would depend on the amplifier and the speakers (both distorting the signal)

Since the days of the "Loudness War" (caused by increased amplitude compression), the peak has not been a good measure at all about loudness, and that's the reason why people searched for ways to compare tracks and automatically change its gain, so that the user doesn't need to change the volume from track to track.
In other words: Two tracks with the same peak value could have a difference of 10dB in their perceived loudness.

Re: Sound level meter / analysis (stereo MP3s)

Reply #2
Quote
when I just want to match the volume level.
If you are using foobar2000 ReplayGain can match the volumes.   There are other players that support ReplayGain too.  ReplayGain pre-scans your files and "tags" them with a adjustment to be applied when it's played.  

Apple has something similar called Sound Check.   There is also MP3Gain and WaveGain which make "permanent" changes to the files so they work with any player software

If you see 10dB for ReplayGain (probably -10dB to turn it down) that's the adjustment required to hit the ReplayGain target loudness.  ReplayGain uses an LUFS target (or something like LUFS) but it's adjusted-calibrated to correspond to a SPL loudness so the ReplayGain target loudness is shown as a positive dB level rather than a negative dB LUFS level.    (Of course it doesn't have any clue how loud you are actually listening.)

LUFS is a kind of (digital) average that takes into account the fact that our ears are most-sensitive to mid frequencies.

RMS is also a kind of average.   It's a "simple" mathematical calculation with no consideration of human perception.   Here in the U.S. we have 120VAC RMS at the power outlet.  It's a 60Hz sine wave with positive & negative peaks of about 170V.    The true-average is zero, since it's negative half of the time.  The average of the absolute values is about 108V.      120VAC RMS will light a (regular incandescent) light bulb to the same brightness and consume the same wattage as 120VDC.

Quote
Audacity says the same track is +0.7db, so one of them is wrong.
That would be the peak.   Peaks don't correlate well with perceived loudness,   Most commercial tracks are normalized ("maximized") for 0dB peaks, but some are still louder than others.   

0dBFS is defined by the highest you can "count to" with a given number of bits.   Regular (integer) WAV files, CDs, ADCs and DACs are hard-limited to 0dB.  When you play the file, everything is scaled to match the bit-depth of the DAC so a 24-bit file isn't louder than an 8-bit file.

MP3 can go over 0dB and the lossy MP3 compression makes some peaks higher and some lower.   It's not uncommon for an MP3 ripped from a CD to go over 0dB.    If you play it at "full digital volume" you'll clip your DAC.  But as far as I know that slight clipping is not audible.

VU meters were a compromise to show the "loudness" while being fast-enough to give a rough indication of the peaks.   (Most home VU meters weren't true VU meters).    A VU meter was an electro-mechanical device and it didn't require any electronics.   A peak meter would have required tubes or transistors, and a power supply, etc.  

In the analog days, tape and electronics tended to "soft clip" and it was OK to go occasionally "into the red".   Digital hard-clips at 0dB so a peak meter is more important (and easy to implement digitally or with modern electronics).

Re: Sound level meter / analysis (stereo MP3s)

Reply #3
MP3 can go over 0dB and the lossy MP3 compression makes some peaks higher and some lower.   It's not uncommon for an MP3 ripped from a CD to go over 0dB.    If you play it at "full digital volume" you'll clip your DAC.  But as far as I know that slight clipping is not audible.
I'm not sure if that's what you meant but DAC will only accept integer format, so the clipping will happen already on the computer side, somewhere between decoding the MP3 and sending PCM to DAC.

Re: Sound level meter / analysis (stereo MP3s)

Reply #4
Quote
'm not sure if that's what you meant but DAC will only accept integer format, so the clipping will happen already on the computer side, somewhere between decoding the MP3 and sending PCM to DAC.
Normally you can turn down the digital volume in in your media player software before it's converted to integer.