In the recording studio, it's inevitable that almost every session we do will include at least one instrument providing the lowest frequencies, usually with a monophonic musical part. From the double bass to the electric bass, by way of the tuba, the pianist's left hand or any kind of synthesizer, we'll always have a bass line that defines the musical harmony and, together with the drums, provides the key rhythmic weight that defines the song being recorded.
While in modern music the low-end instruments are usually electric basses or synthesizers, in other styles such as jazz, folk or classical music we may find double basses or acoustic basses, and in dance music mainly synthesizers. In any case, regardless of the type of instrument, its role is basically the same.

As you can see in the accompanying sidebar, there's a wide range of low-end instruments, each with different characteristics to keep in mind during the recording, mixing or post-production stages.

Mic, line or both?
At the recording stage, in the case of a synthesizer, all we may do is create a MIDI track in our sequencer and record those events. It will be in later stages that we can tweak the corresponding audio to get the best results. However, when an electric bass or double bass comes into play, the process of capturing the signal and recording is usually more involved.

These days, going with a DI line for an electric bass usually gives good results, normally followed by a compressor or one of the relatively new bass-specific modeling preamps, like the Line 6 Bass Pod. If we use one of these, the recording process becomes much simpler, since all we have to do is select a suitable preset, set levels and we're done. But it's not always that simple.

Direct Injection – D.I.
The goal of using a DI is to get an absolutely clean bass signal unaffected by any other element. If we use the amplifier's DI output, we need to make sure the output is Pre-EQ, so the amp's EQ settings don't reach us.

If we use an external DI box, we have to make sure it has a high-impedance input (preferably greater than 1,000) or that the electric bass in question has active pickups. If we also use a compressor, we should set it to affect the signal only slightly, because if we get it wrong it will be impossible to remove that mistake in the mix. We can start with a 3:1 ratio and a release time of about 250 ms. If we have a compressor with automatic attack and release settings (Auto Mode), it may adapt better to the recording if the style or playing changes over the course of the song, something to keep very much in mind. If we can or prefer to adjust these parameters, we can set the attack between 15 and 50 ms, always listening to make sure we don't lose too much tone because of a very fast attack. If the bassist uses classic slap techniques, we'll need to set this parameter even more carefully. In general, a slower attack better preserves the instrument's sound. With conventional playing, beyond 6 or 7 dB of gain reduction the compressor's presence starts to become clearly noticeable, so the reduction should stay below those values. If in doubt about whether we're using the compressor properly, it's better to leave it out and, once the recording is done, compress the signal or do a ReAmp (see below).

Amplifier
Some bassists and engineers prefer to mic the amplifier instead of using a DI line, since they consider the latter somewhat sterile. Just as guitarists consider their amp's sound a vital part of their overall sound, today's bassists have amps with great tone.

We'll face the dilemma of which microphone to use and how to place it. If we choose a dynamic mic, we should check whether it has any low cut and, if so, switch it off. Another option is to choose the mic we use for the kick drum. Both the kick and the bass move in the same range of fundamental frequencies, so it should be a good choice. Pointing the chosen mic at the center of the cone isn't the best option; we'll get more brightness. If we aim it toward the outer part of the speaker we'll get a warmer, fuller tone. It should also be angled slightly relative to the speaker. A distance of between 12 and 30 cm is appropriate. If the speaker cabinet has several cones, we'll pick one of them, avoiding pointing at the tweeter if it has one. We'll keep in mind the difference in sound between choosing a cone close to the floor or one farther away, especially if the floor is reflective, like wood or similar, rather than carpet. If we're after a dub sound, one option is to place the mic at the cabinet's port, being careful with the signal level, since there's a lot of air pressure there.

If we have a contact mic we can place it on the floor, especially if it's wooden, as long as the bass is the only sound source.

As always, small changes in mic position will give us different tonal shades, so it's better to reach the desired sound this way rather than going straight to the EQ without first trying to move the microphone.

If the room we're recording in isn't very large, we'll listen for whether certain notes ring out more than others because of the room modes, trying to move or shift the speaker to minimize this as much as possible.

We'll take into account whether the amplifier is tube or solid-state. With the former, we can probably be somewhat more subtle with the compression we use.

Mic + Amp
The third option is to use both signals, DI and mic, recording both and mixing them "al dente" later as we see fit. We'll need to consider the phase relationship between the two signals, as well as the arrival time of each one at our DAW. By default, the mic will arrive several milliseconds later, which can affect the phase and therefore the tone of the signal that results from summing the two. We'll try flipping the phase on one of them or shifting one by a few milliseconds until they're perfectly aligned.

ReAmp
This technique is widely used with different instruments. Basically it means sending the already recorded signal back to the amplifier, preferably a clean line or DI-type signal. All professional recording equipment operates at a +4 level. This means that if we feed the signal directly into the amplifier, we'll overload its input. So we'll need a device that reduces this signal and properly matches its impedance without adding phase shifts, distortion or noise, and without losing response across its frequency range.

This way, we can change amps or use several, experiment with close or ambient mic placement, all without needing the musician present to play the part over and over.

The company Radial Engineering makes several units of this type, all of them of very high quality.

Double bass
Although the electric bass and the double bass produce the same notes in the same octaves, for the sound engineer recording a double bass is always a bigger challenge. To begin with, the sound is produced by a sizable resonating body but with considerably lower intensity. We can use a microphone, pickups or a combination of both.

If we go with a microphone, a good option is a large-diaphragm condenser, such as a Neumann U87 or TLM 107. It doesn't absolutely have to be this type, since some small-diaphragm mics like Sennheiser's MKH series are also a great option.

The typical mic placement on this instrument is 20 to 30 cm in front, slightly above the bridge. The front plate of the double bass is what mainly radiates this instrument's sound. We can favor a fuller-bodied sound by pointing the mic toward the f-hole on the treble-string side. With cardioid or directional mics we need to watch out for the proximity effect, which can give us an exaggerated low-frequency output.

Resting the double bass on a surface like wood or a riser can significantly color the instrument's sound, so we need to stay alert to that.

Often the musician will be standing while playing the instrument. The instrument will move and, as a result, we can get big level differences depending on the player's movements, if the mic is positioned independently of the instrument. One option is to attach the mic to the instrument, wrapping it in foam rubber and positioning and securing it below the bridge or at the f-hole mentioned above. If we use a large mic like a Neumann TLM, we can choose from different mounts, such as those made by Audix.

If the double bassist comes from the jazz world, it's very likely they'll have a pickup fitted to the instrument. This pickup will give us a more mechanical sound, with greater clarity and definition but less body. While on minimally decent electric instruments the pickups are also decent, in the acoustic realm we can get some truly unpleasant surprises with the quality of these components or their placement.

We should pay special attention to these pickups because their impedance isn't the usual. In fact, some of them come with some kind of preamp, like those from Fishman. As we mentioned earlier, matching the impedance is vital to getting a full, rich tone, all the more so on this instrument.

Also, the way these components are attached to the double bass is usually flimsy most of the time, so we need to watch out for any strange sound or rattling caused by their vibration. If we decide to combine the two signals, mic and pickup, once again we'll need to check the phase between them to get the best sound possible.

Let's remember that bass lines, regardless of how they were produced, are, together with the drums, the engine of today's music, so we should give them maximum time and attention to achieve exceptional rhythm tracks.

Classic microphones for recording electric bass
Sennheiser MD421
Audix D6
Akg 112
Akg D11
Beyerdynamic TGX50
Shure SM58
Shure SM57