Compressing music for in-car listening

Have you run across any software for "compressing" MP3 (or WAV) files? I'm looking for something that does "simple" gain riding – raises the volume of soft passages and lowers the volume of loud passages – so that when I'm playing music in the car I'm not alternating between ear-blasting highs and silent lows. It continues to amaze me that some enterprising manufacturer of car stereos hasn't produced something that does this, but I suppose since most customers listen to music with a dynamic range of a little better than 3dB, the market just isn't there.
George Hand

When most music was still in analog formats, radios often had a "loudness" control that did what you want. The effects varied widely, but the general idea was to boost the volume of both low and high frequencies when you were listening at low levels. This worked well in the sense that you could push the button and see if the music sounded "better".

Today, cheap DSP (digital signal processing) chips make it easy to do, and a modern "loudness" control should be able to make far more accurate changes to compensate for changes in sound pressure levels at different volumes. DSPs can also change the sound to give different effects, so you could have a "concert hall" or a "car radio" type of sound. If you can't find a car radio with similar options (or if readers can't suggest any) then you can find them in portable MP3 players. Samsung models, for example, have included settings such as 3D Sound, Bass Boost, and Concert Hall. If you can plug an MP3 player into your car radio for playback, that might solve the problem.

There's probably not much need for a "loudness" control nowadays because most pop and rock tracks are mastered with extreme loudness built in. This has lead to the so-called loudness wars, where tracks have their dynamic range compressed and the gain increased so they always sound loud. Also, radio stations are well aware that car drivers are listening in a noisy environment, and even classical stations may increase the level of compression on "drive time" shows.

Normally, music is recorded on the assumption that people will listen to it in a quiet environment: quiet passages will be quiet and loud ones loud. If you're driving a car, there's a high level of background noise, so the quiet passages have to be louder than that. Car radios therefore have to operate within a restricted dynamic range. That isn't usually a problem for "loud" genres such as pop, hip-hop and dance music, but can be a problem for classical music and jazz.

If you can't find or don't want to pay for a suitable car radio or MP3 player, you could create some music files that are more suitable for in-car listening. This is more work, and will also mean you will need to keep two different sets of files. Also, the results can be unpredictable. Each of your tracks will have been recorded and mixed at different times for different types of playback (vinyl, cassette, CD etc) by recording engineers with different ideas about suitable compression levels. Each track will therefore require different set of adjustments. If you were a talented audio engineer, you could listen to each track and then reprocess it using a sound program such as Audacity to create an in-car version. However, very few people have the talent, and nobody really has the time.

"Dynamic range compression" (DRC) is different from "normalisation", which usually finds the peak amplitudes of a track then adjusts the volume of the whole track to a predetermined level. However, if you have a lot of MP3 files that have different sound levels, then it might help if you normalised them all using MP3Gain software. I'm mentioning this program for two reasons. First, it does a statistical analysis to work out how loud the file actually sounds, instead of just looking at the peaks. Second, it doesn't decode and then re-encode the MP3 file, so there should be no loss in sound quality. (Note: MP3Gain Pro is an entirely different program written by a different person: it used to be called SuperMp3Normalizer.) For more information on the proposed Replay Gain standard implemented by MP3Gain and variants such as AACGain, see the Hydrogen Audio wiki.

In terms of reprocessing files, SoX, "the Swiss Army knife of sound processing programs", will certainly do what you want. Its volume/level effects include compand (signal level compression/expansion/limiting), loudness (gain control with ISO 226 loudness compensation), mcompand (multi-band compression/expansion/limiting) and norm (normalisation). The key thing is multi-band compression, because you will want to treat the different audio frequency bands differently. However, SoX is a command line utility, which is unlikely to appeal to many ordinary users.

Stereo Tool is a user-friendly graphical Winamp plug-in that is available in free and registered versions, and claims to deliver "professional quality audio processing". The site says: "The registered version is intended for commercial users of Stereo Tool, especially aimed at FM radio stations." The free version includes a 10-band compressor/limiter.

But in practical terms, it's probably better to compromise on sound quality and use a tool that will enable you to reprocess a large folder of files without much effort. One suggestion from the excellent Hydrogen Audio site is to use Foobar 2000 with the VLevel component: this is "a dynamic compressor which amplifies the quiet parts of music", which is what you want.

However, I reckon your best bet is Chris's Dynamic Compressor, which is sometimes called "Chris' compressor". Chris Capel had exactly the same problem as you, so he wrote "a program that makes it easier to listen to classical music, or other music that has a wide range of volumes, at low volumes or in high noise conditions (such as in your car) so that you can still hear the soft parts. I've also written a plug-in version, designed to be used with the free audio editor Audacity."

The standalone version works either as a command line tool or as a graphical program. It probably works best with WAV files but the site says it does allow you to "compress to/from MP3s (LAME required)".

Chris' compressor seems to work well with most music files, and if not, you can always delete the tracks you don't like. It's free so you don't have much to lose, and (excuse the pun) everything to gain.