Mastering is the last and probably the least understood step in the audio recording process.

Mastering engineer Bob Ludwig is one of the true living legends of the music business. In addition to being a Grammy winning engineer, he has received many TEC Awards for excellence and was the first winner of the Les Paul Award from the Mix Foundation for setting the highest standards of excellence in the creative application of recording technology.

Bob is a classical musician by training, having obtained his bachelor's and master's degrees from the Eastman School of Music. Inspired by Phil Ramone, Bob ended up working with Phil at the legendary A&R Recording Studios in New York. After a few years at A&R Recording, Bob moved to Sterling Sound and then to Masterdisk as Chief Engineer. In 1993, Bob and his wife Gail built Gateway Mastering in Portland, Me., a state-of-the-art record-mastering facility where he still masters records by top artists. This interview is in two parts.

Given how many people listen to music on portable digital players, do you find that producers are mixing for earbuds? Is it common to find an "iPod mix" that you master separately?

Bob Ludwig: No it isn't. Dr. Floyd Toole (of Harman International, makers of JBL speakers) showed that averaging all the different consumer speakers (some bright, some with too much bass or midrange etc.) one ends up with a very flat curve which is empirical proof that mastering with an extremely accurate and flat playback system yields a product that sounds correct on more systems. Like speakers, earbuds run the gamut from the old stock Apple earbuds that sounded tinny and lacking warmth to top-of-the-line Shure earbuds that are extremely accurate, to "hip-hop" earbuds that are overly bass heavy. One must master to sound as good as possible on all systems.

Almost all pop mixes are mixed with the bass and kick drum panned to the center which is proper as many people will be listening on boom boxes which have limited power and having a powerful center channel bass available to both speakers is ideal. Very early recordings of the Rolling Stones and The Beatles (to name two groups) were totally intended for mono and were recorded on 2-channel or 3-channel tape decks solely for creating a mono-only mix. When stereo became popular these early multi-track tapes were re-purposed for stereo and the bass and kick drum were typically locked into either the right or left channel.

With earbuds and headphones this is very unnatural sounding and sometimes it is decided to filter the low bass into the center by mono-ing the signal somewhat. This sounds much better. This is definitely a decision based on current widespread use of earbuds, and it remains an important philosophical question when doing re-issues of old recordings with this problem.

Can you explain how the "loudness" of a mix becomes a factor in mastering? Can you explain compression and how it affects you at the mastering stage?

Bob Ludwig: Compression uses a piece of hardware or software plug in which either enhances or most often limits the dynamic range of the music being fed into it. Compression is crucial to pop music. Live pop music is almost always performed at hearing damaging levels, way above the 85dBspl OSHA threshold for start of possible hearing loss. In order for this immense power to be even somewhat realistically reproduced on consumer systems the pop sound pipeline must be compressed so that musically the performance has the extra energy that the live performance had. For pop music, this translates as a very musical thing. ("The Loudness Wars" video illustrates.)

This problem starts from the fact that human beings, when hearing two examples of the exact same musical program but with one turned up only +0.5 or 1dB, almost all listeners who don't know exactly what they are hearing choose the louder one as "sounding best." Fair enough.

So through the years, the louder example is eclipsed by a yet louder example winning the hearts and minds of the artist, the engineer and the A&R person. At some point, the music is so loud and unnaturally compressed that the aural assault on the ear, while very impressively loud, has sucked the life out of the music and makes the listener subconsciously not want to hear the music again.

At an Audio Engineering Society workshop I was recently in about loudness, Susan Rogers from Berklee College talked about the hair cells in our ears that receive music and she pointed out that loud compressed music does not "change" as much as dynamic music and notes that "we habituate to a stimulus if it stops changing. Change 'wakes up' certain cells that have stopped firing. This is cognitively efficient and therefore automatic." In other words, there are very physical reasons why too much compression turns off our music receptors. Every playback system ever manufactured comes with a playback level control. If one is listening to an album, one should be able to turn that control anywhere you want and the absolute level on the CD should not make a difference. Another place level on a CD does not make the difference one would think is on radio broadcast. It can be shown that in general, loud CDs sound worse and less powerful on commercial FM radio than a CD with a moderate level that lets the radio station compressors handle the loudness problem. Non-classical radio station compressors make soft things loud and loud things soft.

Two areas where producers get upset about not having enough level is the iTunes Shuffle, or even comparing songs on the iTunes software itself, and that moment at the radio station where the PD is going through the week's new releases and deciding which two or three songs will be added to his playlist. Here, sometimes having a little extra level can make a lesser song seem a little more impressive, at least at first listen.

A great example of a contemporary recording that has full dynamic range is the Guns N' Roses Chinese Democracy CD where Axl Rose wanted all the textures of the original mixes to come through and he got his wish! A good example of one of the loudest most distorted CDs is the Metallica Death Magnetic CD where apparently 10,000 fans signed a web petition to have the album remixed because they got to hear how good it sounded on "Guitar Hero" which did not have all the digital limiters the final CD mix had.

In Part 2, Bob gives his top three sonic "don'ts" for producers to avoid.