This phenomenon can be observed in many areas of the music industry, particularly broadcasting and albums released on CD and DVD. In the case of CDs, the war stems from artists' and producers' desires to create CDs that sound as loud as possible or louder than CDs from competing artists or recording labels.
However, as the maximum amplitude of a CD is at a fixed level, the overall loudness can only be increased by reducing the dynamic range. This is done by pushing the lower-level program material higher, while the loudest peak sounds are either destroyed or severely diminished. Certain extreme uses of compression can introduce distortion or clipping to the waveform of the recording.
The following text is printed inside the digipak packaging of metal band Iced Earth's single I Walk Among You: "This is a dynamic metal record! Play it loud!!! (We refuse to ruin our production by compressing the hell out of it so that it's mastered at ridiculous volumes! That kills the vibe and dynamics of the mix. Just turn it up on your stereo!)"
If a CD is broadcast by a radio station, the station will apply its own signal processing, which further reduces the dynamic range of the broadcast material to closely match levels of absolute amplitude, regardless of the original record loudness.
Opponents have also called for immediate changes in the music industry regarding the level of loudness. In August 2006, the vice-president of A&R for One Haven Music, a Sony Music company, in an open letter decrying the loudness war, claimed that mastering engineers are being forced against their will or are preemptively making releases louder in order to get the attention of industry heads. Some bands are being petitioned to re-release CDs with less distortion. This may indicate a general public discontent to this practice and a call to put an end to the loudness war.
Many bands have their records made louder against their will. Several organizations have been founded to attempt to put the choice back in the hands of the bands. The nonprofit organization Turn Me Up! has also been formed to encourage the sale of quieter records by placing a "Turn Me Up!" sticker on albums that have full dynamic range.
Hearing experts, such as a hearing researcher at House Ear Institute in Los Angeles, are also concerned that the loudness of new albums could possibly harm listeners' hearing, particularly in children.
The practice of increasing a CD's loudness to be louder than competing CDs can have two effects. Since there is a maximum loudness level available to recording (as opposed to playback, in which the loudness is limited by the playback speakers and amplifiers), boosting the overall loudness of a song or track eventually creates a piece that is maximally and uniformly loud from beginning to end. The entire song has been flattened against the loudness ceiling, as it were. This creates music with a small dynamic range (i.e., little difference between loud and quiet sections), rendering it fatiguing and robbing it of emotional power, according to Robert Levine of Rolling Stone.
The other possible effect is distortion. In the digital realm, this is usually referred to as clipping. Digital media cannot output signals higher than digital full scale (0 dBFS), so whenever the peak of a signal is pushed past this point, it results in the wave form becoming clipped. When this occurs, it can sometimes produce an audible click. However, certain sounds like drum hits will reach their peak for only a very short time, and if that peak is much louder than the rest of the signal, this click will be heard. In many cases, the peaks of drum hits are clipped but are not noticeable to the casual listener. However, if clipping occurs too much in a recording, it can make the recording sound distorted, a sound which some listeners find harsh and fatiguing to listen to. How much is too much is a matter of taste, but most pop and rock CDs, and many jazz CDs, have some amount of digital clipping.
Analog media, on the other hand, dynamically compress the signal as it exceeds its saturation point. Such distortion can be utilized in the digital realm as well, either by transferring audio processed with tape or valve saturation to a digital recording medium, or by using computer software to emulate the effect (this process is usually referred to as "saturation"). Analog distortion, real or emulated, results in harmonics that can appear to the listener as a slight "crackle" or "fuzz" within the sound. The effect can vary depending on the sound itself, as well as the amount and kind of distortion used. Because analog distortion does not flatline to the extent that digital clipping does, the results are less harsh-sounding and can result in a desirable warmth to the recording, at the cost of slightly less transient response. The amount of distortion increases the more a signal is overdriven, ranging from transparent to highly audible, and just like digital clipping, certain instruments or musical arrangements can better mask distortion than others.
In other cases, compression or limiting is used. While the resulting distortion is lessened from the final product this way, it has the side effect of significantly reducing transient response (most often heard as lessened drum impact), and, when taken to severe levels, can reduce the natural dynamics of other instruments within the recording. Loudness increasing techniques, however, do not always affect macrodynamics (the difference in volume between sections of a song) if used with care and detail. Multi-band compression is commonly used to make a mix more uniform and easier to balance, more compatible with low-end equipment, or to achieve a certain sound or artistic effect. Dynamic range or broadcast-style compression, on the other hand, will be applied to the music to make the volume in different song sections more uniform. This can make the recording more suitable for background listening or noisy environments but can also reduce the dynamic expressiveness of the song as a whole.
The practice of focusing on loudness in mastering can be traced back to the introduction of the compact disc itself but also existed to some extent when vinyl was the primary released recording medium. Many record companies would print compilation records, and when artists and producers found their song was quieter than others on the compilation, they would insist that their song be remastered to be competitive. Also, many Motown records pushed the limits of how loud records could be made, and record labels there were "notorious for cutting some of the hottest 45s in the industry." However, because of the limitations of the vinyl format, loudness and compression on a released recording were restricted in order to make the physical medium playable—restrictions which do not exist on digital media such as CDs—and as a result, increasing loudness levels never reached the significance that they have in the CD era. In addition, modern computer-based digital audio effects processing allows mastering engineers to have greater control over the loudness of a song; for example, it gives them the ability to use a "brick wall" limiter which limits the volume level of an audio signal with no delay (hardware equivalents have a short delay due to processing time).
The stages of CD loudness increase are often split over the two-and-a-half decades of the medium's existence. Since CDs were not the primary medium for popular music until the tail end of the 1980s, there was little motivation for competitive loudness practices then. CD players were also very expensive and thus commonly exclusive to high-end systems that would show the shortcomings of higher recording levels.
As a result, the common practice of mastering CDs involved matching the highest peak of a recording at, or close to, digital full scale, and referring to digital levels along the lines of more familiar analog VU meters. When using VU meters, a certain point (usually -14 dBFS, or about 20% of the disc's amplitude on a linear scale) was used in the same way as the saturation point (signified as 0 dB) of analog recording, with several dB of the CDs recording level reserved for amplitude exceeding the saturation point (often referred to as the "red zone", signified by a red bar in the meter display), because digital media cannot exceed 0 dB. The average level of the average rock song during most of the decade was around -18 dBFS.
At the turn of the decade, CDs louder than this level began to surface, and CD volumes became more and more likely to exceed the digital limit as long as such amplification would not involve clipping more than approximately two to four digital samples, resulting in recordings where the peaks on an average rock or beat-heavy pop CD hovered near (usually in the range of -3 dB) 0 dB but only occasionally reached it. Guns N' Roses's 1987 album Appetite for Destruction is an early example of this, with levels averaging -15 dB for all the tracks.
In the early 1990s, some mastering engineers decided to take this a step further and treat the CD's levels exactly as they would the levels of an analog tape and equate digital full scale with the analog saturation point, with the recording just loud enough so that each (or almost every) beat would peak at or close to 0 dBFS. Though there were some early cases (such as Metallica's self-titled Black Album in 1991), albums mastered in this fashion generally did not appear until 1992. Alice in Chains's Dirt and Faith No More's Angel Dust are some examples from this year. The loudness of CDs during this period varied greatly depending on the philosophies of the engineer and others involved in the mastering process. This style of "hot" mastering became commonplace in 1994, though exceptions, such as the album Superunknown by Soundgarden from the same year, still existed. The most common loudness for a rock CD in terms of average power was around -12 dBFS. Overall, most rock and pop CDs released in the 1990s followed this method to a certain extent.
The concept of making CDs "hotter" began to appeal to people within the industry, due in part to how noticeably louder CDs had become and also in part to the notion that customers preferred louder CDs. Engineers, musicians and labels each developed their own ideas of how CDs could be made louder. In 1994, the digital brickwall limiter with look-ahead (to pull down peak levels before they happened) was first mass-produced. While the increase in CD loudness was gradual throughout the 1990s, some opted to push the format to the limit, such as on Oasis' widely popular album (What's the Story) Morning Glory?, which averaged -8 dBFS on many of its tracks—a rare occurrence, especially in the year it was released (1995). In 1997, Iggy Pop assisted in the remix and remaster of the 1973 album Raw Power by his former band The Stooges, creating an album which, to this day, is arguably the loudest rock CD ever recorded. It has an average of -4 dBFS in places, which is rare even by today's standards, though getting more and more common.
The standards of loudness would reach its limit in the 2000s. -10 dB had been the standard for the past several years, but this was often pushed to -9 dB. However, -6 to -5 dBFS is common in rock, pop and rap music. Quieter exceptions to today's standards are rare. The latest releases as of 2008 have reached average levels as high as -3 dBFS, such as Angels and Airwaves' iEmpire, which yields almost 30 times the loudness of a THX standard recording (-20 dBFS).
Some recent recordings released on vinyl do not undergo the same kind of loudness-based mastering, although many still do. This is partly due to technical limitations of the format and partly due to vinyl now being a product for the niche market favoured by a small number of hi-fi enthusiasts—similar to the CD's role in the mid-1980s.
Some SACD and DVD-Audio releases are affected as well. However, nearly all DVD-Audio discs also contain a Dolby Digital (AC3) or DTS sound track to allow the disc to be played in DVD-Video players without DVD-Audio playback capability. Dolby Digital has a defined and calibrated reference average playback level (-20 dBFS), and the DTS track will also follow this level. It is therefore beneficial that the high-resolution DVD-Audio soundtrack will be produced at the same reference level—and this indeed is normally the case.
As these new high-resolution formats are marketed largely at audiophiles, attempts to master them for loudness would almost certainly be counterproductive, as the target audience is likely to be highly critical of sound quality and dynamics.