Posted May 12, 04 - Updated May 11, 05

I. Close Miking - Pritchard Amp Cabinets   

The Tunnel Back™ Cabinets have a bass character that goes beyond what would be capable in their small size from an open back cabinet. This great bass character is directly attributable to the patented Tunnel Back™ cabinet design. The tunnel acts like an acoustic crossover. Above the cross over frequency the sound comes from the front and very little comes through the tunnel and out the rear. Below the cross over frequency the sound comes predominately from the rear. This nature produces good room characteristics because at substantial distances the bass back wave easily overpowers the front wave to provide a great listening experience. This is quite different from open back operation in which the back wave almost cancels the front wave and the cabinet response is somewhat compensated by speaker resonance and that leads to problems in producing good dirty tones.

   The initial recording of demo clips of Pritchard Amps were inadvertently done incorrectly. The miking of the cabinets were done in the standard manner, but the unique character of the Tunnel Back™ cabinets, as later discovered, require special miking approaches. The usual miking approaches produced recordings that lack bass - nearly two of the bottom guitar octaves were significantly attenuated. A completely different second effort rapidly produced the same results and demanded an investigation.

   The typical guitar speaker cabinet, particularly a vintage one, is either closed back or open back. The closed back cabinet only produces a front sound wave, or nominally similar front sound waves. Although there are many complex miking schemes for closed back cabinet, they do not have to deal with back wave effects. An open back cabinet has a back wave that is very much like the front wave, although reversed in phase. Consequently, it can be miked with a single mike, although there are multiple microphone schemes here also. Unlike these cabinets the Tunnel Back™ cabinet produces a back wave which is quite unlike the front wave.

Note: Bass reflex ports potentially present a similar issue, however they are often in the front and sometimes close to the speaker so that miking with some standoff will probably work. Since we did not test any bass reflex cabinets, we can not make any definitive statements.

   The reason why the back sound wave is different than the front wave is simple; it must travel through the tunnel. The tunnel air presents the back of the speaker with an air mass load. This loading lowers the resonant frequency of the speaker as well as reducing the front wave output in the bass region. The tunnel passes predominately low frequencies and produces a back wave bass with greater intensity than the front wave bass. As you can now imagine this cabinet sounds great at a distance but is prone to close miking problems.

1-12 Tunnel Back™ with one U67 Mic

   Upon realizing the close miking problem possibility, frequency response tests were run on cabinet and mike combinations. The typical SM57 approach showed the lack of bass as experienced in the clip recordings. A U67 in the omnidirectional pattern showed the same lack of bass. However, the U67 in the cardioid (unidirectional) pattern found a fairly flat response when close miked. The U67 experience prompted an investigation of the SM57, which nominally has a cardioid pattern. While it does have that unidirectional pattern at middle and treble frequencies, the low frequency pattern is nearly omnidirectional. Consequently, it is subject to the larger but attenuated back wave. The net effect was the significantly attenuated bass.

1-12 Tunnel Back™ With two SM57 Mics

   The SM57 solution is simple – mike both the speaker and the tunnel and mix them. The phase reversal can be handled in three ways: use the phase reversal switch, use a phase reversing cable, or reversing the direction of the microphone. Single miking approaches can be realized with a true cardioid microphone. A single channel miking technique can be done by wiring two SM57's in parallel. This is the way that the tests run below were done. Since this requires a special “Y” cable, they can also be paralleled with a phase reversal as well.

   Although double miking requires an extra microphone, the second mixer channel also provides a benefit beyond good tone – a unique post distortion tone control that is created by giving the rear mike more or less gain than the front mike.

Each type of cabinet was tested, the following are the results:

1-12 Tunnel Back™

1. SM57 - one close and centered on front of speaker and the second placed in a port backward. This was verified in a studio.
2. U67 - use the cardioid unidirectional pattern in the front, close and centered.

1-15 Tunnel Back™

1. SM57 - one close and centered on front of speaker and the second placed in a port backward. This was verified in a studio. Additionally, the rear port was tested with the microphone facing into the port and this worked as well too due to the larger volume of the cabinet.
2. U67 - use the cardioid unidirectional pattern in the front, close and centered.

1-12 / 2-5 Tunnel Back™

1. The U67 does not work on both types of speakers because it must be near both speakers or suffer from the back wave.
2. Two SM57's placed about 18 inches away from the front and back has worked in the lab. This has not been verified in the studio yet.

4-10 Tunnel Back™

1. Close miking on either low speaker works as expected by either the U67 or SM57 microphones.
2. Miking front and back should be done using one of the upper speakers.

4-12Tunnel Back™

1. Not tested yet but front and back miking expected
2. Hint: Room miking should be tried since the Tunnel Back™ is so unique and new

1-12 Closed Back

Standard techniques apply.

 

II. Room Miking - Another Approach

Close miking, even front and back with the standard SM57's, is wanting. The first problem is the SM57 frequency response which drops significantly at low frequencies. While using SM57's may have been reasonable with resonant peaks of closed back cabinets, they do not do the smooth, deep bass of Tunnel Back™ cabinets justice. The SM57's frequency response starts dropping off at 180 Hz and is 5 dB down at 80 Hz (approximately low E) and 10 dB down at 50 Hz (approximately G below low E). The Tunnel Back™ has a better bass response than closed back cabinets because it reduces the resonant frequency from the closed back frequency of approximately 110 Hz (A above Low E) to 50-60 Hz (A to B below low E).

The second reason for room miking is to combine the front and back speaker waves in a natural way.