Using 2 or More Mics for the Guitar Amp Speaker(s) - Part 1

Using 2 or More Mics for the Guitar Amp Speaker(s) - Part 1 Brought to you by: guitar.com

Just Press Play Logo Gif 96201Get ready for interesting experimentation! There are so many possibilities when using more than one microphone, such as close miking with two mics on one speaker, close miking with two mics on two speakers, close miking with three mics on three speakers, close miking the front and back of an "open back" speaker cabinet using two mics, using room mics, etc.

When using more than one mic in a close mic amp miking situation, the key is to position the two (or more) mics the exact same distance from the speaker(s) to eliminate comb filtering phase problems. Yes, since the speaker is not flat (most are kind of bowl shaped, right?), even with careful placement, it's hard to measure the exact distance with one mic looking at the cone and the other looking about half way from the speaker center and side (typical positions). I'll get into more on that as things unfold in this multiple mic article series.

In this era of hard disk recorders with many tracks available, if you're using such a format, I highly recommend recording each mic to a separate track since this will allow fixing possible comb filtering problems after recording. This will also save time with mic positioning. If you need to record on one track, pay close attention to mic placement as explained in the next article in this series on "Comb Filtering."

Before you continue, I suggest reviewing the archived article Part 3, Mic'ing The Guitar Amp" since so much important physical set up information will not be covered here. You may also want to review the article that mentions mic choices per application, Part 1 Microphones Explained.

 

Mic'ing an Open Back Guitar Amp Using Two Mics, Front and Back

Miking an open back guitar amp or speaker cabinet using one mic in front of the speaker and one in back is rarely used, but may create unique sound possibilities. (Hey, music is all about experimentation. Try the concept and, if you score a hit with your "new" sound, don't forget to tell people you learned about it from Jay Graydon and Guitar.com!) But because it is unlike all of the other multiple miking set ups, I think it's best to explain this setup first as to not mess with the flow of other set ups in which the mics will all be positioned in front of the amp, which I'll get into in the following articles on this subject.

So what we have is an open back speaker cabinet. One mic is positioned in front of the amp and the other is positioned in the rear. If the amp has two or more speakers, it's best to use just one speaker (the same) since we are dealing with a major phase issue. If you're using two different speakers, one miked in the front and another miked in the rear, the air movement may not have the same travel time, not allowing full control of the phase fix. Hey, there are no rules as usual so if you want to try two different speakers, no problem - if the phase test proves full cancellation.

The phase issue: In this case, since we have one mic in front of the speaker and the other in the back of the speaker, the combined mic signals will have an inverted phase, typically called "out of phase" by most recording engineers. Note that the term "out of phase" is not accurate but as mentioned, the term has stuck over the years to mean one of the two (or more) signal paths with a common sound source is reversed in phase.

The Mic Diaphragm/Air Movement

A microphone hears a source signal in one of two basic directions - the sound source air moves towards the diaphragm, or away. If the sound source signal air moves towards the mic diaphragm (such as miking a guitar amp in front of the speaker and the guitar amp speaker is wired to correct phase coherency), the mic translates the initial attack into a positive up-sloping electronic waveform. If the recording studio electronic path is phase coherent, when the mic's electronic energy routes through the mic's output, through the mixer input module, through the recorder, back through the mixer for monitoring the signal, and finally through the studio monitor speakers, the studio monitor speakers pushes air forward at the initial waveform attack. That is what we want.

If the mic sees the sound source air moving away from the diaphragm, i.e., miking an open back guitar amp speaker cabinet, the initial attack would generate a negative electronic waveform since the speaker is pulling air away from the mic diaphragm. In this case, the waveform would "slope down" at initial attack causing the studio monitor speakers to "suck in" instead of pushing air forward. (Note, if you're using just one mic on the guitar amp - miked from the rear - you may not be aware an inverse phase problem is in play since after the initial attack, wave form shapes alternate pushing and pulling the monitor speaker in and out. In that case, there is a slight loss of low frequencies and "punch" at initial attack. If the sound source does not have much low frequency information, it's not such a big deal but it's still best to have the waveform rise at initial attack.)

If the electronic path is phase coherent for both the amp and mixer/recorder chain, the mic positioned in front of the speaker is seeing air movement at initial attack causing a positive waveform. The mic positioned in back of the speaker is seeing air movement causing a negative waveform at initial attack. As the waveform on the front mic changes shape from positive to negative, the rear mic is seeing the opposite waveform shape. If both mic levels on the mixer are the same level, the guitar signal would basically cancel out leaving a very thin sound hardly heard. Even if the mic levels are not the same, phase cancellation will be in play per the level relationship between both signals.

The basic fix is simple - reverse the polarity of the rear amp mic using a phase switch on that mixer module. This will allow the rear mic to show a positive waveform at attack and both the front and rear mic will be an additive signal instead of canceling out. If your mixer does not have phase switches on each module, the fix is to wire up or alter a mic cable inverting the phase.

If you're using a two wire mic cable, on one end of the cable at the connector, reverse the hot and ground wire. If you're using a balanced signal (3 wires) on one end of the cable at the connector, swap pins 2 and 3. Make sure to label the cable inverse phase!!!

Note if you're using a hard disk digital mixer/recorder combination, and if recording both mics on two separate tracks, you could easily fix the phase cancellation after recording by inverting the waveform shape on the rear speaker mic track.

Mic Placement

OK, it's time to set the mic positions and get started. The speaker cage (in the rear) has major metal so there's not much room for experimenting with mic placement. Try about half way between the speaker cone and the sides to start. If the amp is a combo amp, it's best to place the mic on the low side of the speaker (near the floor) to get away from amp electronics (old amps throw off major RF noise). The front mic position will depend upon the rear speaker mic position. If needing more upper-mids and treble, you want to position the mic in the center of the cone or slightly off center.

In any case, for both the front or rear mic, it's best to position the mike around an inch back from the speaker grill (front mic) and the same relative distance for the rear mic. Point the mikes directly at the speaker instead of at an angle (as in the one mic setup) to take full advantage of phase coherency regarding both mics. OK, maybe you like the sound of angling the front mic like 20 degrees (as explained in the article on using one mic, Part 3 Miking the Guitar Amp). If so, try angling the rear mic at 20 degrees in the opposite direction so both mics diaphragms are looking straight at each other so to speak.

As mentioned above, when using more than one mic it's best to record to two separate tracks so we can deal with comb filtering problems after recording. Before this era of nearly unlimited tracks with hard disk recorders, engineers dealt with mic placement manually, checking for comb filtering, and they moved mics ever so slightly to find the best placement. This tedious preparation work helped them avoid frequency comb filtering problems. If you're recording both mics to one track, mic distance placement (from the speaker paper) is very important. We will deal with that after the two mic tracks setup.

Signal Path Routing For Two Mics To Be Recorded to Two Recorder Tracks

Note this is different from the routing we have been using in previous articles.

Important! For all of the following, do not use any effects that alter pitch and tone, meaning pitch shifters, wah-wah, etc. If you will use such effects when recording your track, plug them in after reading the next article in this series, on comb filtering.

(As usual, if you are the guitarist and engineer, if the amp is in a separate room (or closet or the like), you will be monitoring over the control room speakers. If you have no guitar amp isolation, meaning the amp is in the same room, you will be using headphones to monitor. In any case, if you'll be playing chords for the guitar part, play all the open strings when adjusting the level with your other hand. If you'll be playing single notes for the part, simply play one string for now, such as the "D" or "G" string, and adjust the level with the other hand.)

For this example, we are recording the guitar on recorder tracks #7 and #8 and using mixer module's #9 and #10 for the two guitar mic inputs. Let's use mixer module #9 for the front mic and mixer module #10 for the rear mic. If you're using a digital recorder with a built-in mixer, simply adapt with the same layout.

Plug one guitar amp mic cable into mixer module #9's mic input. Plug the other guitar amp mic cable into mixer module #10's mic input. On mixer module's #9 and #10, to start, set the mic pre-amp trim to -20 dB. (If you have only one input gain trim pot per mixer module, it is used for both source line input gain and mic input gain. If you're using a computer hard disk recorder with outboard analog to digital inputs, use the same setting on the input level control to start). A ssign mixer module #9 to bus #7 (bus #7 routes to the recorder track we are using for the front mic). Note that if mixer module #9 has a direct output, instead of using bus #7, patch the direct output into recorder track #7's input. Make sure that module #9 is not sent through the monitor chain and is only routed (bussed) or patched directly into record track #7. Mute this module for now. Assign mixer module #10 to bus #8 (buss #8 routes to the recorder track we are using for the rear mic). Note that if mixer module #10 has a direct output, instead of using bus #8, patch the direct output into recorder track #7's input. Make sure that module #10 is not sent through the monitor chain and is only routed (bussed) or patched directly into record track #8. Mute this module for now. Set recorder tracks #7 and #8 into "input mode" so we can monitor the guitar signal through the recorder, which routes to mixer modules #7and #8. (In digital land, you may want to monitor the mic-input signal on module's #9 and #10 BUT, only do so if you notice a delay when monitoring through the digital mixer and or recorder path. I will get into this subject in future articles). Bring up module #7 (recorder track return) about half way up on the fader throw. Bring up the studio control room monitor level up to a normal listening level. Mixer module #9 level settings: Ask the guitarist to play the part for the song. While the guitarist is playing, un-mute mixer module #9 and slowly bring up the fader to zero (unity gain). This level setting is typically around 3/4ths up on the fader throw - Look at the etching next to the fader as to find the zero mark. 9a. If using an analog mixer and recorder (hardly used these days) adjust the mic pre-amp trim level on the mixer to average zero dB on the recorder track meter for now. For now, mute mixer module #9. If using a digital recorder format, adjust the mic pre amp trim level to -4 dB on the recorder track meter for now (-4 dB is safe in case the guitarist plays louder when recording - the odds are good that will happen!) Always remember that going "into the red" (past zero) on a digital format meter will definitely "clip" the A to D converter (analog to digital converter) which is not advisable. Digital distortion sounds terrible! For now, mute mixer module #9. Mixer module #10 level settings: Ask the guitarist to play the part for the song. While the guitarist is playing, un-mute mixer module #10 and slowly bring up the fader to zero (unity gain). This level setting is typically around 3/4ths up on the fader throw - Look at the etching next to the fader as to find the zero mark. If using an analog mixer and recorder (hardly used these days) adjust the mic pre-amp trim level on the mixer to average zero dB on the recorder track meter for now. For now, mute mixer module #10. If using a digital recorder format, adjust the mic pre-amp trim level to -4 dB on the recorder track meter for now (-4 dB is safe in case the guitarist plays louder when recording.) Always remember that going "into the red" (past zero) on a digital format meter will definitely "clip" the A to D converter (analog to digital converter) which is not advisable. For now, mute mixer module #10.

Note that if you want to use a 3rd mic (or more), simply follow all of the above instructions using another mixer module and another recorder track.

EQ Settings: OK, the path and basic levels are set so it's time to EQ both mics separately. Refer to the previous articles in this series on EQ per the application (clean guitar, distorted guitar, etc.). Mute mixer module #10 and un-mute mixer module #9. EQ the signal on mixer module #9 to taste. When you are happy with the EQ settings, mute mixer module #9 and un-mute mixer module #10, then EQ to taste. In all of the following steps, you may want to tweak the EQ on both mixer modules #9 and #10.

Adding the Compressor: Refer to the previous articles in this series on compressors. Note that since you are using two mics, it's best to use two compressors. You may also want to use a stereo compressor in this case to have either mic control the overall compression. In any case, work with one mic path at a time, meaning if you're starting with mixer module #9's path, mute mixer module #10 for now.

The Blend: Keep in mind that we are recording to two separate recorder tracks so the blend is a simple monitor blend that is not permanent.

To set the blend; pull down both mixer module #7and #8 faders to the bottom of their throws. Make sure both mixer module #9 and #10 are un-muted. While the guitarist is playing (as always when dealing with electronic settings), slowly bring up both faders and experiment with different level settings to achieve a good blend. When needing to change the overall guitar monitor level while listening to the full band, simply move mixer module #9 and #10 faders together, keeping the same relative levels. If you have mixer automation, sub group these two channels.

By the way, you may want to "pan" mixer modules #7 and #8 slightly off center, or pan in any position in the stereo spectrum (if using the 5.1 format for monitoring and mixing, there are so many pan possibilities!) In any case, it's always best to pan both in the "center" to start so as to be able to check for phase when we get into comb filtering problems.

The above takes care of the routing for two mics to two recorder tracks.

We have not gotten around to comb filtering problems caused by mic speaker placement distances as of yet, since we need to deal with combining the two mics to one recorder track to keep a flow. Comb filtering will be covered in the next article in full.

Signal Path Routing For Two Mics To Be Recorded on One Recorder Track

Note this is different than the routing we have been using in previous articles.

Important! For all of the following, do not use any effects that alter pitch and tone, meaning pitch shifters, wah-wah, etc. If you will use such effects when recording your track, plug them in after reading the next article in this series, on comb filtering.

For this example, we are recording the guitar on recorder track #7 and using mixer module #9 and #10 for the two guitar mic inputs. If you're using a digital recorder with a built-in mixer, simply adapt with the same layout.

Plug one guitar amp mic cable into mixer module #9's mic input. Plug the other guitar amp mic cable into mixer module #10's mic input. On mixer module's #9 and #10, to start, set the mic pre-amp trim to -20 dB. (If you have only one input gain trim pot per mixer module, it is used for both source line input gain and mic input gain. If you're using a computer hard disk recorder with outboard analog to digital inputs, use the same setting on the input level control to start). Assign mixer module #9 to bus #7 (bus #7 routes to the recorder track we are using for the front mic). Make sure that module #9 is not sent through the monitor chain and is only routed (bussed) to record track #7. Mute this module for now. Assign mixer module #10 to bus #7 (bus #7 routes to the recorder track we are using for the rear mic). Make sure that module #10 is not sent through the monitor chain and is only routed (bussed) to record track #7. Mute this module for now. Set recorder track #7 into "input mode" so we can monitor the guitar signal through the recorder, which routes to mixer module #7. (In digital land, you may want to monitor the mic-input signal on module's #9 and #10 BUT only do so if you notice a delay when monitoring through the digital mixer and or recorder path. I will get into this subject in future articles). Bring up module #7 (recorder track return) about half way up on the fader throw. Bring up the studio control room monitor level up to a normal listening level. Mixer module #9 level settings: Ask the guitarist to play the part for the song. While the guitarist is playing, un-mute mixer module #9 and slowly bring up the fader to zero (unity gain). This level setting is typically around 3/4ths up on the fader throw - Look at the etching next to the fader as to find the zero mark. If you're using an analog mixer and recorder, adjust the mic pre-amp trim level on the mixer to average zero dB on the recorder track meter for now. For now, mute mixer module #9. If using a digital recorder format, adjust the mic pre amp trim level to -4 dB on the recorder track meter for now (-4 dB is safe in case the guitarist players louder when recording - the odds are good that will happen!) Always remember that going "into the red" (past zero) on a digital format meter will definitely "clip" the A to D converter (analog to digital converter) which is not advisable! Digital distortion sounds terrible. For now, mute mixer module #9. Mixer module #10 level settings: Ask the guitarist to play the part for the song. While the guitarist is playing, un-mute mixer module #10 and slowly bring up the fader to zero (unity gain). This level setting is typically around 3/4ths up on the fader throw - Look at the etching next to the fader as to find the zero mark. If using an analog mixer and recorder, adjust the mic pre-amp trim level on the mixer to average zero dB on the recorder track meter for now. If using a digital recorder format, adjust the mic pre-amp trim level to -4 dB on the recorder track meter for now (-4 dB is safe in case the guitarist players louder when recording).

Note that if you want to use a 3rd mic (or more), simply follow all of the above and just add in another mic and mixer module assigned to bus #7.

EQ Settings: OK, the path and basic levels are set so it's time to EQ both mics separately. Refer to the articles on EQ per the application. Mute mixer module #10 and un-mute mixer module #9. EQ the signal on mixer module #9 to taste. When you are happy with the EQ settings, mute mixer module #9 and un-mute mixer module #10, then EQ to taste. Now it's time to blend the two mic signals. When doing so, you will surely want to revisit the EQ on both mixer modules.

The Blend: To set the blend, pull down both mixer module #9 and #10 faders to the bottom of their throws. Make sure both mixer module #9 and #10 are un-muted. While the guitarist is playing (as always when dealing with electronic settings), slowly bring up both faders and experiment with different level settings to achieve a good blend. Make sure to look at recorder track #7's meter as to achieve proper level settings. More on blending per application.

Adding the Compressor: Refer to the previous articles in this series on compressors. Note that since you are using two mics, if you need to change the overall levels of the two mics, and if the compressor does not have an input level control, the odds are good that your mixer has a bus sub master volume control. We are using bus #7 in our example so use it to set the necessary level to the compressor.

That takes care of the path and settings for both mics to one recorder track.

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