How to Mic a Bass Speaker Cabinet

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1 How to Mic a Bass Speaker Cabinet

How to Mic a Bass Speaker Cabinet

It is useful to explore techniques of mic'ing a bass speaker cabinet for speaker-, microphone- and room-based acoustic textures that compliment the overall feel of a particular song recording. Mic'ing a bass speaker cabinet adds steps to the recording process but the techniques described in this article can really enhance the final mix of a song.

The easiest and most reliable way to record an electric bass guitar signal is direct injection or direct input. The D.I is a separate box or an XLR output jack from an amplifier that feeds bass instrument output into the recording signal chain. Typically, the use of D.I. includes a preamplifier to warm the signal, followed by a compressor to boost the presence of the signal, and then on to the recording device. Other effects may be added before or after the recording. D.I. is practically foolproof, protects a recording from external noise bleed, and is a simple way to lay down bass tracks with minimum fuss. See Direct_Bass_Recording and wikipedia:Direct_Input for more on D.I.

In the old days, direct mic'ing was the only way to record bass. Now, recording engineers often use D.I. and speaker mic'ing by blending the two signals on mixdown for a result that could not be achieved solely with D.I. or mic'ing. Simultaneously using different kinds of microphones and speakers offers an endless range of tone options for mixdown.

Incorrect phasing is the main drawback to mixing D.I. and mic'd signals, which weakens blended sound on the recording. This is easy to fix with proper mic positioning and sound editing software (see below).

There is no one-size-fits-all way to mic a bass speaker cabinet. Whether a recording result is good or bad depends on the judgment of musicians, recording engineers and producers. This article describes several ways to mic a bass speaker cabinet, and presents sound samples showing the effects of different microphones and microphone placement. That information will help you conduct your own experiments to create an optimal bass recording.

What a Mic'd Signal Brings to a Bass Recording

A mic'd signal captures the sonic personality of a particular speaker cabinet, microphone and mic placement, and room presence – none of which occur in a D.I. signal. Geoff Emerick describes how he leveraged this potential as chief engineer for The Beatles during the recording of their album, Sergeant Pepper:

In turn, it was Paul's desire for perfection that enabled me to finally come up with a recording technique that yielded the ultra-smooth bass sound he and I had been pursuing for years. The key was that we would move his bass amp out of the baffles and into the center of the studio; I would then place a microphone about six feet away. With the studio empty; you could actually hear a little bit of the ambience of the room around the bass, which really helped; it gave the sound a certain roundness and put it in its own space. The sound we crafted effectively transformed the bass from a supporting rhythm instrument into a lead instrument. (Quoted from Geoff Emerick and Howard Massey, Here, There and Everywhere: My Life Recording the Music of The Beatles, p. 170 [New York: Gotham Books/Penguin Group, 2006].)

Ideas for Microphone Positioning

Sound samples below demonstrate different recording results based on how a microphone is positioned in front of a speaker and how far the microphone is placed from the speaker. Three typical options for microphone positioning include:

Centered – The microphone is placed perpendicular to the center cone of the speaker. The centered position captures pure, clean sound of the speaker while minimizing room ambience. Close proximity of the mic to the speaker may also emphasize lower frequencies. Loud volume may also create large sound pressure on the mic, so a dynamic microphone is preferred. If you use a condenser mic in close proximity, be sure to switch the preattenuation pad to boost the mic's capability to handle the increased sound pressure.

Off Center – This positioning retains the mic's perpendicular placement to the speaker, but shifts the head away from the speaker's center cone. In theory, this mode minimizes sound pressure on the mic and lets more high frequencies bypass mic input.

Off Axis – Some sound engineers angle a mike 45 degrees from the speaker, particularly when they are double-mic'ing the same speaker. This alternative also is supposed to let high frequencies zip past the mic and accentuate lower frequencies.

Bass Sound Samples Recorded with Dynamic and Condenser Microphones

Determine how to mic your bass speaker cabinet by trying variations of mic positioning and distance placement. Sound samples in the matrix below let you hear 25 variations on this theme. Details about how these were created are below the matrix. You may want to listen on earphones to detect subtle differences. Use your computer’s sound balance control to do A-B comparisons for each example: the left channel presents sound recorded with a Sennheiser MD 421 II dynamic microphone; the right channel used an AKG Perception 400 condenser microphone.

**Sound Samples of Mic'd Bass Speaker Cabinets**
Distance from Grill	Off Center	Off Axis
1/4”
3”
6”
9”
12”
15”
18”
24”	--	--
36”	--	--
48”	--	--
60”	--	--

How the Recordings Were Made

Microphones

Sennheiser MD 421 II dynamic microphone (left channel) and AKG Perception 400 large diaphragm condenser microphone (right channel)

Recording engineers generally use a dynamic microphone to record sound from a bass speaker cabinet due to its ability to clearly reproduce loud, low-frequency signals. A dynamic microphone’s cardioid pick-up pattern usually avoids picking up other nearby sounds, which may be a drawback of using a condenser microphone. The left-channel signal in sound samples above used a Sennheiser MD 421 II dynamic microphone.

A condenser microphone used with preattenuation pad can also be used for distortion-free close-in recording of a bass speaker cabinet. The extra sensitivity of a condenser microphone may convey more warmth to the recording and can capture higher-frequency harmonic details that might be missed by some dynamic microphones. Be sure to set the condenser mic's pick-up pattern to cardioid instead of figure-eight or omnidirectional to focus recording on the bass speaker cabinet's output. The right-channel signal in sound samples above used an AKG Perception 400 large diaphragm condenser microphone.

Amplification

Two David Eden D112XLT bass cabinets where placed about three feet apart for simultaneous mic'd recording

Bass output was channeled through two David Eden D112XLT bass speaker cabinets separated by about two feet (see photo). Tweeters on both cabinets were turned one hash mark from the off position. Amplification was through a David Eden WT330 bass amplifier. All push button controls on the amp were in the out position. Other amp settings were:

Gain 1:00
Enhance off
Bass 1:00
Mid 11:30
Treble 11:30
Master Volume 2:00

Recording Gear

Signals from the two microphones were routed through two ART TubeMP pre-amplifiers, one of which provided phantom power to the condenser microphone. Signals then routed through a dbx266XL Compressor/Gate, which applied compression to both channels at a 3:1 ratio and automatic gain reduction. Signals then routed through a Mackie 808S mixer (no EQ applied) and into a Marantz Superscope CD Recording System PSD300. Recorded format was in stereo WAV files.

Note: for the sound matrix above, recording preamp volume was boosted as mics were moved further away from the speakers to normalize recorded volume levels. You may want to experiment with boosting speaker volume as mic distance increases to exploit unique room ambience characteristics. In general, mic'd recording quality is better with lower bass speaker volume.

Sound File Output Processing

The recorded CD was ripped to a laptop computer. Declipping and auto-amplification for volume normalization was applied to the stereo sound files with Audacity sound processing software. After export to new WAV files, Power MP3 WMA Converter software converted the format to MP3 with Very High Quality, 192kbps Variety Bitrate Encoding. Sound files were then uploaded to WikiMusician.

Music

Music played by DaveB for the sound samples above was a repetitive bass dexterity exercise called X-Drill shown in the chart below.

Fixing Out-of-Phase Recordings of Blended Bass Signals

D.I. signals are recorded at the speed of light because they are completely contained within the recording system. A mic'd signal is slightly delayed as it passes from the speaker, through air, and into the microphone at a rate of 1,100 feet-per-second. The delay is about one millisecond-per-foot, which is just enough to give the blended bass signals that soft, mushy sound you hear when speaker wires are reversed on a stereo system. The left image shows two bass signals in phase; the right image shows them 180 degrees out of phase, which completely cancel each other. The degree to which signals are out of phase depend mainly on the mic's distance from the speaker.

In-Phase bass signals

Out-of-Phase bass signals (180 degrees off)

There are complex ways to solve this problem with hardware before the mic'd signal enters the recording system. Placing the mic in the best recording position is the optimal way, which can require lots of trial-and-error tests. One technique is to crank up the bass guitar volume and listen through headphones plugged into the mixer for a hum caused by an out-of-phase signal. For example, if D.I. goes through channel 1 and the mic'd signal through channel 2 -- and if you hear a hum -- simply move the mic around until the hum disappears. If you're also using a room mic say through channel 3, you can repeat this test between channels 2 and 3. Alternatively, you can also use digital recording software like ProTools, Logic or Cubase to realign the out-of-phase wave forms. Depending on nomenclature of your software, you simply "grab" the out-of-phase wave form and "nudge" it slightly to line up the wave forms.