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  • Rotary Speaker Cabinet Miking

    By Phil O'Keefe |

    Mic placement and technique can significantly alter the sound of these classic rotary speaker systems


    By Phil O'Keefe


    The Leslie speaker (Leslie is a registered trademark of Hammond Suzuki USA, Inc.) was invented by Don Leslie, and it made its commercial debut way back in 1941. Consisting of a cabinet containing a power amplifier, a pair of speakers and a system of motors and pulleys to "spin" acoustic baffles that redirect the sound from the two speaker elements, it adds considerable complexity, motion and interest to whatever sound is routed through the system. While the Leslie speaker is probably most closely associated with Hammond organs, it can also be used with other sound sources, such as guitars and even vocals or drums and other pre-recorded tracks via re-amping. Capturing the sound of this electro-mechanical beast requires a little knowledge about how it works, so let's dig right in…





    A typical Leslie cabinet, such as the model 142 shown in Figure 1, is divided into three sections. The center of the cabinet houses the crossover and two speakers - a downward firing woofer and an upward firing high frequency compression driver or "horn".  The crossover splits the audio into two frequency bands, and the sound below 800 Hz is routed to the woofer, and the sound above 800 Hz is sent to a compression driver. There is a rotating horn in the upper part of the cabinet, and the high frequencies from the compression driver get sent out through it. Interestingly, while there appears to be two opposing "horns", only one of them is open to the compression driver; the second one is closed off, and serves merely as a counterweight so the horn will remain balanced as it spins around. The bottom section of the cabinet contains a large rotating drum with a curved "ramp" or scoop; this redirects the sound of the downward firing low frequency woofer and "throws" it out of the lower front, sides and rear of the cabinet. This lowest section of the Leslie cabinet also houses the 40W tube power amplifier. While some later models may have a solid state amp, or rely on an external amplifier, the classic models, when viewed from the front (as in Figure 1) have a tube amp in the lower right hand corner. The location of this amp is important to know about, since it can be a source of noise. There is also switching with most Leslie speakers; this allows the musician to go from a slow rotary speed (sometimes called "chorale") to a fast ("tremolo ") speed. The solenoid switches that control the motor speeds are located over near the power amplifier, and will produce a loud "click" or "clunk" sound whenever the speeds are changed; for this reason, I recommend avoiding that "side" of the cabinet when placing your microphones; especially the low frequency / drum mic.




    Figure 1: A Leslie Model 142, as seen from the front.



    The sound changes in very interesting and fun ways when the Leslie's speed is switched. This is in part due to the different mass of the rotary horn and drum elements; when the speed is switched, the larger and heavier drum takes significantly more time to speed up or slow down than the horn does. The interaction of the sound from the two speakers -- the phase shifts and the sound reflections off the cabinetry, Doppler shift pitch changes and amplitude (volume / tremolo) fluctuations make for a very complex and interesting sound, especially when motor speeds are changed and as the drum and horn transition to the new setting. The effect on the upper horn is more FM, or frequency modulation in nature. As the horn spins towards the listener's position, the pitch of the sound rises slightly, and as it spins away from the listener, the pitch drops slightly. The sound of the Doppler shift changes, depending on the horn's speed. The horn rotates at about 50 RPM in Chorale mode, and about 400 RPM on the Tremolo setting. The drum has slightly slower speeds. It tends to rotate at about 40 RPM in Chorale, and 340 RPM in Tremolo mode. The drum can take five or six seconds to transition speeds, and the overall effect of the sound of the spin of the drum is more AM, or amplitude modulation in nature. Due to the lower frequency range, the effect is perceived as more or a up and down volume fluctuation as the drum turns towards and then away from the listener. Of course, there are some elements of AM and FM with both drivers, and the amount of this that you capture on your recording can be affected by how closely you place the microphone(s) to the cabinet, as well as the balance of horn to drum that you use in your mix.





    Ask any ten engineers where to place a mic, and you're bound to get ten different answers. As with most sound sources, where you place the microphones makes a big difference in the way the final recording will sound, but there are some unique characteristics to a Leslie speaker that you need to be aware of. First of all, since a Leslie cabinet has two speakers, and they're each reproducing different parts of the frequency spectrum, ideally you're really going to need at least two microphones in order to do the job properly. What if you absolutely have no choice but to do it with a single microphone? Then I would mic from the side or rear, with the mic about six feet back from the cabinet and three feet above the floor, and aimed straight at the cabinet. Aim it or lower it further towards the drum if you want more low frequencies from the drum, and raise it or aim it more towards the horn if you want more top and brightness.


    You'll find that things get a lot easier when you add a second microphone. This allows you to use one to capture the horn, and the second mic to catch the drum. (Figure 2), resulting in better flexibility in terms of capturing the two. Record each mic to their own track and you will be able to adjust the balance in the mix.




    Figure 2: Using two microphones generally gives better results and provides more flexibility when balancing the sound at mixdown. Here a Cascade Fathead II (top) is aimed towards the horn, and an E/V RE320 (bottom) is aimed at the spinning drum.



    The horn at the top of the cabinet can be single miked, or for an even more dramatic effect, it can be miked in stereo. Because the high frequency range of the Leslie's rotating horn doesn't extend much past 7 kHz, there's no need to use mikes with extended high frequency response, and small diaphragm dynamic microphones or ribbon mikes are commonly used instead of condenser mikes. A few good choices include the Shure SM57, Audix i5, AKG D1000E and the Cascade Fathead II (pictured).


    Miking from the rear of the cabinet can result in a slightly more open and less diffused sound than miking from the front or sides and with the sound passing "though" the cabinet's louvers, but it's also possible to pick up more pulley and mechanical noises this way. Ideally, you'll need to experiment to see what sounds best to you. Sometimes you can get good results by putting the two horn mikes on each of the two opposing sides of the cabinet. Placing one mic at each of the rear corners and angling them in slightly towards the horn is another good approach. Another commonly used technique is to place one at the rear and one on one side (Figure 3).




    Figure 3: A pair of Cascade Fathead II's placed fairly close-in; one at the rear and one on the side, capturing the Leslie horn in stereo. Note the round black metal pop filter placed in front of the rear mic (left) to help protect it from wind gusts.



    The lower rotating drum is normally only captured with a single microphone. I prefer a large diaphragm dynamic for this. Some good candidates would be the Sennheiser MD421, Electro-Voice RE20 and RE320 (pictured in Figure 4), and the Audio Technica ATM250. Even when using fairly distant placement for the high frequency horn, some engineers will keep the low frequency drum mic placed in fairly close; if you try this, you may want to use a high pass filter set somewhere around 800 Hz for the horn mikes to reduce any low frequency bleed from the drum that they picked up due to their more distant placement.




    Figure 4: An Electro-Voice RE320 is aimed at the low frequency drum in this shot, which shows the mike arrangement used in the "close miked" audio clips. Note the use of Auralex Aural Xpanders behind the ribbon mikes to help reduce the amount of "room" picked up by the rear of these bi-directional mikes.





    Be aware of the fact that the power amplifier on many Leslie speakers is located in the lower left side of the enclosure (when viewed from the back side), and it's notorious for making noise. Not only electronic interference and hum, but also mechanical noises, such as "clunks and clicks" when the speed switch is triggered by the player. By positioning the microphone on the opposite side of the drum, you can minimize the amount of noise from the amplifier and switching that the microphone captures. Another concern with the drum is the large amount of wind noise it can generate. To help reduce the sound of wind "blowing into the microphone", I always use a foam windscreen over the drum mic. If it's really bad, you can also try inserting a vocal "pop filter" between the mic and the speaker to further reduce wind gusts that blow directly into the mic, but usually placement alone will make a big difference in the amount of wind noise you get.


    Speaking of wind, the one concern with ribbon mikes is that areas close to the cabinet openings and vents are prone to wind gusts from the spinning horn and drum, and a wind blast can destroy a fragile ribbon element. If you plan on using ribbon mikes, make sure you place them back a bit so they're not getting blasted with air, or use a suitable pop filter to protect your mic from wind damage.


    While minimizing the mechanical pops, clicks, hum and wind noise is generally something I try to do, I've given up on trying to eliminate it completely. First of all, it's pretty much impossible to do, and secondly, as long as it's minimized, it generally isn't going to be too obtrusive -- in fact, I'm convinced that the absence of some of that noise is what some listeners subconsciously listen for to determine whether they're hearing a "real" Leslie or a good simulation. Have a listen to the attached clips - I specifically waited until chords were ringing out and sustaining and fading away before switching the Leslie speeds; this was done to let you hear how much "click" we get with the microphones positioned as indicated in the photos by doing the switching in a quieter, more "exposed" part. I also left in some small sections where I'm not playing so you can listen to the wind and mechanical noise.


    Moving the microphones further away from the cabinet (Figure 5) generally gives a smoother sound with more "room" ambience and less dramatic volume fluctuations in the recorded sound, while close miking accentuates the tremolo effect; especially at high speed. While the microphones have all been moved back about 3 feet, even more distant placements can often work too. Some engineers like to keep the drum mike in close and just move the horn mikes further back; if you do so, don't forget to nudge the tracks back into phase and time alignment if needed.




    Figure 5: The mikes have been moved back about three feet in these pictures and for the "distant miked" clips. Don't be afraid to experiment with even more distant placement of the horn mikes.





    The recordings are of a simple classic Leslie guitar riff. It is played twice at slow rotor speed, then as the last note rings out, the Leslie speed is switched to fast (listen for the clicks), and once the Leslie has ramped up to full speed, the riff is played twice more, then as the last note rings out, the speed switch is again triggered and the Leslie speed ramps down. The signal path was kept straightforward - a Fender Duo Sonic II into a Speakeasy preamp and straight into a Leslie 142, with two Cascade Fathead II ribbon mikes for the horn, and an Electro-Voice RE320 on the drum. API 312 mic preamps were used on all three mikes, with no additional processing or filtering of any kind added… but feel free to experiment with high and low pass filters.


    The attached audio flies are available in two forms -- multitrack and stereo. If you just want to listen to the close and distant mic recordings as pictured in Figure 4 (close) and Figure 5 (distant), the stereo files should be all you need. If you'd like to dig a little deeper, download the multitrack versions. I've kept the clips very short so the downloads should not take too long. Once you've unzipped the multitrack files, drag or import all three files to "0" on your DAW's timeline, with each file on its own separate track. Then experiment. Try soloing just the low frequency drum track and listen to it. Notice the difference in ramp up and down speeds compared to when you solo one of the horn tracks. Also note the sound of the different frequency ranges that each speaker / driver covers. There's plenty to experiment with in terms of the high frequency horn tracks too. Start by muting off one of the horn tracks and listen to just a single horn mic paired up with the drum mike. Then try unmuting both horn tracks and panning them hard left and right, or as wide or as narrow as you prefer, with the drum track panned to the center. Notice how "bandpassed" the horn mike(s) sound when soloed - there's nothing much there in the ultra high frequencies (above 6 kHz), and nothing below 800 Hz. Of course, you can also create a second trio of tracks and drag the second (distant) mic placement version of the tracks into the same DAW "song" or "session"; this will allow you to quickly compare the sound of the close and slightly more distant mic placements -- collectively (use a mix group for each set of three tracks to solo or mute them collectively) and by soloing individual mikes (just disable your groups).

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