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Mixing plays a central role whether producing music in a studio, or playing at a live gig. This post is a clear explanation and helpful overview of the topic from Mark Garrison's book Encyclopedia of Home Recording.
Mixing, or mixdown, is the act of taking multiple discrete tracks and turning them into one stereo, mono, or surround-sound track. The resulting track is then ready for mastering and/or reproduction.
Mixing is a very important part of the recording process because many irreversible decisions need to be made. Some engineers will pass their recordings on to be mixed by another engineer who specializes in mixing. This can be of particular benefit for those who record their own music, as it brings a fresh, objective set of ears to the project.
There are four basic areas which must be considered when mixing: spatial, tonal, dynamic, and attentional characteristics. A successful mix takes advantage of all four of these aspects.
A good mix provides a feeling of space, both in the stereo field (left to right) and depth (front to back). Instruments should feel as if they are originating from different places, some closer, some farther away, and in a variety of locations within the stereo field. The primary tools for manipulating spatial perception during mixing are the pan knob, reverb, and compression.
If the goal of the mix is to recreate a natural listening environment, such as a live performance, then spatial decisions should reflect that goal. Instruments should be placed in logical positions and reverb should reflect natural environments. Alternatively, recording is often used as a venue to create listening situations that could never exist in a live performance. With this approach the engineer has great creative freedom and can create fantastical or “larger than life” mixes.
The tonal range and balance of a mix affects its intelligibility and its fullness. A great mix feels like the entire range of the audible spectrum is being represented somewhat evenly. This is mainly controlled with arrangement, choice of instruments/microphones, and equalization.
There are two problems in the area of tone that should be avoided: gaps and buildup. Gaps occur when part of the frequency spectrum is not being used. When gaps occur, the engineer needs to asses why and whether it is acceptable. In some cases, gaps are necessary and expected due to the content. A children’s choir, for example, will be only in the upper part of the frequency spectrum. In some traditional musical styles, gaps are also common and expected. Awareness of cultural and traditional expectations of the musical style’s tone is important in these situations.
Buildup is a more common problem in modern music. Buildup occurs when too much is happening in a given frequency range, usually because more than one instrument is fighting for attention. The most problematic area for buildup is in the low-mid range where multiple instruments’ ranges overlap.
To avoid frequency buildup it is best to plan what instrument needs to be most prominent in each frequency range and to cut that range in other instruments that overlap it. For example, if the thump of the bass guitar needs to be prominent, the kick drum should be cut in that range. The slap of the beater hitting the skin on the kick can, in turn, be made more prominent, while cutting the bass in that range to make room.
The term dynamics is mainly used in recording to refer to changes in volume, but other dynamic variations are important as well. Because people are naturally drawn to contrast, dynamic changes make songs more engaging and keep them from becoming monotonous. Changes in volume, tone, key, tempo, and instrumentation are examples of the ways a mix can be made more dynamic.
A great mix not only sounds good but draws the attention of the listener and pulls it through the song. When a song has a strong melodic hook, getting attention is much easier. A dynamic change, unique tonality, or distinct effect can draw attention to this hook. An example of this is the warble of over-done pitch correction in Cher’s “Believe”.
The mix should also draw the listener through the song as it progresses. When the lead instrument (often the vocals) pauses, something needs to take the listener’s attention in the interim. This may mean bringing in another instrument or bringing the focus to one that was previously in the background. It may be helpful to think of the mix as the stage of a TV show or play. When the focal characters leave the stage, others need to enter right away. Even a few moments without a focal point would cause the pace of the show to seem slow and cause the viewer to lose interest. The same is true in music. Carefully guiding the listener’s attention through the song results in an interesting and engaging mix.
Ideally, mixing should be done using studio reference monitors. These are speakers that are designed to reproduce the sound accurately, with as little tonal coloration as possible. Studio monitors are said to have a “flat” frequency response, meaning they don’t boost or cut in any frequency range. An important distinction here is that while home stereo speakers are designed to sound good, studio monitors are designed to sound accurate.
Near-field monitors are by far the most common speakers for mixing. These are reasonably small, accurate speakers that are designed for a close listening distance (usually about 3–6') at moderate volumes. There are many different brands and models of near-field monitors available, and the prices vary greatly. It is advisable to buy the best quality monitors your budget will allow because all decisions about your mix will be based on their reproduction. The most important thing, however, is to know the monitors you have. It is a good idea to frequently listen to well produced music though your monitors to know how a good mix sounds through them.
In addition to near-field monitors, most studios will have at least two other listening devices, usually a pair of large speakers with good bass response and a small, cheap stereo. While the engineer will spend most of the time mixing on the near-field monitors, he or she will check the mix on the alternate speakers periodically during the mix. The mix should also be checked on headphones, but it is inadvisable to use headphones as a primary monitoring source because the proximity of the speaker to the ear strongly affects perception of tonal and spatial characteristics.
The ideal mix position creates an equilateral triangle, with the engineer’s head and the two monitors making up the three points (see Fig. 58). Vertically, the engineer’s ears should be even with the midpoint between the woofer and the tweeter of the monitors.
Because the human ear responds differently to frequencies at different volumes, a mix can sound quite different when the volume is changed. The ideal mixing volume is said to be 80 dB SPL, as this is a common listening level and will yield the least change as the volume is raised or lowered. Most engineers will check their mixes at different volumes throughout the mixing process to ensure that the mix does not change drastically as the volume changes.
The room that mixing takes place in should also be considered when optimizing the mixing environment. Care should be taken to control the amount of reflected sound within the room. The amount of reflections being created in the room will affect decisions when adding reverb and EQ to elements of a mix. A highly reflective room will sway decisions towards less reverb and less brightness in a mix, while a very dead room will often result in adding too much reverb and too much brightness. An appropriate balance must be found that fits the specific room. Much like knowing your monitors, being highly familiar with how your room sounds will improve the quality of your mixes.
Once a mix has been completed, it is generally advisable to listen to it on a variety of sound systems before sending it for reproduction. This will help ensure that there will be no surprises in the final product. One of the most important places to check the mix is the car. The car is a very unique listening environment and can strongly color the tone of the mix. The car is also one of the places where people listen to music the most, making it an important consideration.