Decibel Level Relative to Audio Volume


I’ve recently discovered Audacity and have been quite impressed by its many functions. Frankly, as a newcomer I’m still learning about how we can improve/adjust audio files from different sources, and as such, I’m a bit confused by something I’ve noticed when looking “behind the scenes” at some of my audio files on iTunes.

I’ve tried their forum with a few of my questions but they’ve gone unanswered. I’m hoping someone here will be able to clarify a few things for me as I continue trying to educate myself.

My question concerns one of the details in the attached image about volume.

After uploading several songs to my iTunes program and inspecting their playback levels, I am confused as to what the category for volume -6.0 dB (for example) actually means.
I know the dB = decibel. But what I’ve discovered is that several of the songs from the same CD or artist can have various dB levels which increases or decreases the volume of the song, sometimes quite a lot.
I have also seen other songs where it shows volume +0.1 or higher.

What is the difference then between the (-) and (+) in the volume?

I don’t mean to sound trivial, but the playback volume differs greatly in songs within the same CD and I’ve noticed that the ones where the (-) dB is greater are much louder than ones where there is a (+) dB.
I’m trying to figure this out to better understand and try to account for this in my future purchases and uploads. I’ve searched the web but cannot find any explanation.
Sound Check: I know about the Sound Check feature, but prefer not to use it because it lowers the output and makes the songs sound unclear.

In addition, What determines the DB level of the song?

Based on the previous details, let’s presume a song’s optimum volume level is -6 to -8 dB.
So for example, if I have a CD or MP3 file and I want to upload a song to my website or phone or whatever, how can I control the volume level to ensure that it is at or close to this optimum level? Is there anything I can do? Or is this strictly determined by the original source recording?
If so, what control do I have over this through the Audacity tools, if any?

Thanks in advance to anyone that can assist!

Please be aware that this is the help forum for Audacity, not iTunes. It should really be Apple that provides technical support for Apple software (given their enormous wealth, they should be able to afford to do so).

it is the “Sound Check” gain level.
“Sound Check” is Apple’s version of “Replay Gain”.
That “volume” tag indicates the amount of gain (the volume adjustment) that will be applied when Sound Check is enabled, so that the song plays at “standard” volume.
The idea is that if the song is very loud, the Sound Check algorithm will add a negative gain tag, so that when the song is played on an Apple device or Apple software with Sound Check enabled, the volume will be automatically reduced to a “standard” loudness. Similarly, a very quiet track will have a positive gain tag added, so that the volume will be automatically boosted (on supported devices / supported software).

Sound Check: I know about the Sound Check feature, but prefer not to use it because it lowers the output and makes the songs sound unclear.

When “testing” Sound Check, be sure to adjust the volumes to match (the best you can) because a volume change can often be perceived as a quality change and you can fool yourself. (Proper scientific, blind, ABX tests have to be level-matched.)

There’s a reason that Sound Check, ReplayGain, or manual volume matching MUST lower the volume of most songs (more below). If you have enough gain on the analog-side, you can turn-up the playback volume to compensate for the digital volume reduction.

Apple does mention “limiting” and that can affect sound quality, but that would only be applied where there is a positive gain change (quiet songs) and where that change pushes the peaks over 0dBFS. Most songs will be reduced in volume and sound quality should not be affected.

ReplayGain, MP3Gain, and WAVGain take a different approach. By default, quiet songs that can’t be volume-matched without [u]clipping[/u] are only boosted to the extent they can be boosted without clipping. (Sometimes there is an option to turn that off and allow clipping.)

In addition, What determines the DB level of the song?

See below.

Based on the previous details, let’s presume a song’s optimum volume level is -6 to -8 dB.

There is no “optimum level” (but there is a target level which Apple doesn’t tell you).

iTunes isn’t showing the volume (although it has scanned and it knows). It’s showing you a change/adjustment required to make all of your songs equally loud.


Decibels are relative and you always need a reference. 0dBFS (zero decibels full scale) is the digital dB reference. It’s defined as the highest you can “count” with a given number of bits. CDs, normal (integer) WAV files, analog-to-digital converters and digital-to-analog converters are all hard-limited to 0dB. Note that a 24- bit file has “bigger numbers” than an 8-bit file, but your drivers automatically scale the data to match your DAC (digital-to-analog converter) so a 0dB 8-bit file plays just as loudly as a 0dB 24-bit file.

This means that digital dB levels are usually negative. Audacity uses floating-point (not integers) internally and it can go over 0dB. MP3s can also go over 0dB. But of course, you can clip your DAC if you play such a file at full-digital volume.

The reference for sound level (the sound that hits your ears) is referenced to 0dB SPL (zero decibels sound pressure level). 0dB SPL is approximately the quietest sound humans can hear, so dB SPL is positive.

There are several different dB references for electrical signals (voltage or wattage).

Since decibels are relative, we can also use them to represent a CHANGE (positive or negative). That’s what you see with -6dB in Sound Check, or if you use Audacity’s Amplify effect. If you amplify by +3dB, that doesn’t tell you anything about the level/volume, only that you’ve changed it by +3dB. Technically, a positive change is gain or amplification and a negative change is attenuation. But, you can use Audacity’s Amplify effect to attenuate/reduce the volume, and sometimes we say “negative gain”.

There is a direct relationship between dBFS and dB SPL. That is, if you reduce the digital (or electronic) volume by 6dB, the SPL is reduced by 6dB. However, there is no calibration between the digital level and the loudness (except in movie theaters). The loudness depends on your volume control, your amplifier, speakers, how close you are to the speakers, etc.

Also, decibels are logarithmic (as is your hearing). That means a doubling of power from 10 Watts to 20 Watts is a +3dB change, and a doubling of power from 100W to 200W is also a +3B change.

Perceived loudness is even more complicated. Your ears are more sensitive to mid-frequencies so a 0dB 1kHz tone sounds louder than 0dB 50Hz or 10kHz tone. And, this effect changes with loudness… If you turn-down the volume it sounds like you’ve turned-down the bass even more. (See [u]Equal Loudness Curves[/u].) Because of this, SPL meters are usually [u]A-Weighted[/u].

Also, your ears don’t respond instantly to the peaks. You can have a quiet-sounding song with normalized/maximized 0dB peaks and a louder song with lower peaks. All of this means that perceived loudness does not correlate well with peak levels. Perceived loudness is more related to the short-term average (or RMS) levels.

Sound Check and ReplayGain take all of this into account and attempt to measure perceived loudness.

Most commercially released songs are normalized for 0dB peaks. That includes many quiet-sounding songs.

So if you want to match volumes, you can’t boost (some of) the quiet songs (without clipping) and the ONLY way to do it is by making most songs quieter. And in the end, some quiet-sounding songs are still too quiet… It’s a compromise between accommodating the quiet songs and reducing the volume of (almost) everything else. MP3Gain (and some implementations of ReplayGain) allow you to change the target volume (which is a positive dB SPL level). But if you increase the target volume you are not giving it as much room to work, and more songs in your library can’t be adjusted (unless you allow clipping).

If you have a limited number of songs (like if you are making a compilation/mix CD) then you don’t have to compromise. You simply normalize/maximize all of the songs, choose the quietest-sounding song as your reference, and adjust the others down to match.


Thank you both very much for your prompt and detailed replies. You’ve helped me greatly in better understanding the relationship between dB and volume that had puzzled me earlier; plus you gave me quite a few new concepts to consider as I get better acquainted with altering and adjusting my music through the Audacity tools. I plan to spend more time researching based on your feedback and going through the FAQs as I come across new questions that come up.

I’d also like to add that of course I had first tried communicating with Apple / iTunes support first by forum, then by phone, and the representatives there knew absolutely nothing about the queries I had. Yes, you would think someone would have some level of experience to at least explain each of the audio categories present in their iTunes system but apparently not. Thus, I hoped seeking out potential audio experts through Audacity would be the next logical step, and clearly it was! And in fact it’s all relative to me, since I’m planning to use Audacity to improve my audio files for playback. I expect I can go quite a lot further using Audacity’s tools as opposed to the simple functions afforded through iTunes such as its equalizer, volume adjust functions, etc.

If I may add one more tack on question – this may be obvious to most, but I’m hoping to determine the difference or if one or another encoding function is better based on two that I’ve seen regularly:


LAME 3.99

I’m sure there are several others but these seem to appear most often. Is there any major difference between the two in terms of audio quality? Is one better than another? If so, how I can I take advantage of one or the other when I download or convert audio files?

Thanks again for those of you who took the time to respond to me earlier, and thanks in advance for any further input based on the above