This article was written on December 1, 2012 and is the copy written intellectual property of the trustees of

Should I Send MP3 Files Out for Mastering?

In a word: no.

The MP3 format is a data compression scheme used to remove the data from an audio file that scientists have decided that you can’t hear. Data compression comes from computer science is not the same kind of compression that we use to reduce dynamic range in audio.

When we listen to sounds as humans we experience what is called masking. Masking is a phenomenon that when one sound is significantly louder than another, you cannot hear nor focus on the quieter sound. Scientists have latched on to this perception quality of the human audio interface and found ways to reduce the amount of information stored in an audio file by removing information that describes sounds that shouldn’t be able to be heard because of the masking effect.

Of course science isn’t always perfect. This data compression scheme has enabled consumers to store thousands and thousands of songs on very small audio devices. But the truth is, these files do not contain all of the information originally encoded in the final mixes and masters.

Although at higher bit rates MP3s can actually sound very good, they still do not contain all the accuracy of a lossless format like a Wave or AIFF file.

Another limitation of the MP3 file is the frequency response. Some MP3 formats do not contain any information above 15 kHz. Wave and a IFF files, depending on the sample rate, will run all the way up to 22 kHz or higher. Many people can hear above 15KHz, so it is important to preserve and reproduce the information in that frequency range.

It is troubling to us here at that even some audio engineers don’t understand that delivering an MP3 to their customer is like not delivering the whole project.

Please be sure when working with an engineer to get the high-resolution, lossless, files of your tracks and mixes. Make sure you have files that are Wave files, Broadcast Wave files, or AIFF files.

Several times a month we receive mixes that are in MP3 format. We are often surprised to hear that the engineer does not have lossless formats of the projects. Although we can work on MP3 files, when we are force to work with them, we already starting the mastering project at a disadvantage. This disadvantage is brought about simply because of the lack of knowledge on the engineer’s part.

Once a file is converted to an MP3 or compressed format or converted to a lower bit depth or sampling rate than the original, that resolution (on those files) can never be resurrected. The information is lost forever.

It is true that the final mixes will likely be distributed on MP3. The audio processing during the capture or tracking process, the mixing process, and the mastering process should be done at high-resolution audio formats. Typically these are done at higher sample rates than the final product will be. Almost always will these also be done at higher bit depths than which the final project will be delivered.

It is very important to do the audio processing at higher bit depths and sample rates because digital processes do math upon the audio files. When we use higher sample rates and higher bit depths, we are allowing the rounding errors from the mathematics being performed on the audio to be much more accurate. Also using lossless formats we are maintaining all of the quality of the original recordings.

The truth is, an audio project should stay at the sample rate and bit depth of the original recording out to the very last steps of the mastering process.

That means if you start a recording project at 24 bits 48 kHz, then the project should stay at that bit depth and sample rate all the way out through overdubs and mixing and into mastering. The mastering engineer may up-sample this to a higher bit depths and sample rates simply because of the rounding errors involved with the digital processes they may use.  They may also upsample to gain a higher resolution recording when using real analog outboard mastering equipment. The mastering engineer knows that they are not going to gain quality or resolution by up sampling and upping the bit depth.

In the very last step of mastering: the very last step; a sample rate conversion will occur to the final sample rate and bit depth that the project will be delivered on. If the customer prefers to have their files in MP3 format as well, this will be the stage when the mastering engineer creates high quality MP3 files as well.

A reputable mastering studio like will have thoroughly researched and developed tools for converting wave files into MP3 files. These tools are of much higher quality than MP3 converters shipped with programs like iTunes.

As we have said before, the business of audio and mastering is the sum of very small gains. One of the reasons you may choose to hire an outside professional mastering facility is that they have the tools and equipment and personnel in place to exploit these advantages.

In other words, we at are recommending that customers not perform sample rate conversions, not perform bit depth conversions, and not convert Wave or AIFF files to MP3 files. Let the mastering studio you choose do that.

This article was written on December 1, 2012 and is the copy written intellectual property of the trustees of