The Pritchard Amps Tunnel Back™ Speaker Systems is a new, patented loudspeaker concept that amazingly creates a sound bigger than the cabinet. Tunnel Back™ speakers are artistic, smaller, and lighter. Players love'm! They have many truly great advantages and only one small requirement.
They have the artistic sound of the open back, but produce more bass. They have smoother bottom since the speaker resonant frequency is lowered - not raised as closed back and ported cabinets. Tunnel Back™ cabinets are compatible with artistic amplifiers while ported cabinets suffer in size or response from the inherent low damping factors. The only special requirement is minor - they need to be double miked - front and back or front and room - but many artists do this anyway.

   The Tunnel Back™ Speaker System differs from other systems in the way it deals with the natural resonance of the speaker or driver. Here they are:

Cabinet Type	Resonant Condition
Open Back	Uses resonance to counter the natural bass roll off of the cabinet. The excessive travel at or near resonance makes the bass muddy from excessive cone travel
Closed Back	The resonance produces an unpleasant, booming bass response. The resonance is also higher and in the range of musical instruments. This is a part of the rock sound.
Ported Cabinets	The resonance is higher and canceled if the port is properly designed. Frequencies below resonance are attenuated more. Small cabinet designs require amplifiers with substantial feedback - feedback that becomes quite ugly when over driven. The speaker is likely to over travel since it is then unloaded.
Tunnel Back™	The resonance is lowered, out of the range of the instrument so while present, it is not excited by instrument notes. And it enhances the percussive character.

Tunnel Back™ Cabinet Theory

   The accompanying drawing is the cross section of the 1-12 cabinet. It shows a front-mounted speaker connected to a speaker board. A separate speaker grill board covers and protects the speaker. The cabinet also has a top, a bottom and two sides. The air waves from the speaker are constrained by the shelf and the back which forces the air to flow through the tunnel in the back. This column of air as shown by the dashed line AIR FLOW lowers the resonant frequency of the speaker or driver and does so sufficiently to move the resonant frequency below the range of the instrument. When the resonant frequency is below the range of the instrument, only transients in the playing of the instruments will excite the resonance. This leads to a pleasant percussive quality and excellent bass character.

   The column of air lowers the resonant frequency by effectively adding weight to the speaker cone. This differs from an open back cabinet in that the column is not so concentrated and consequently does not have an effect. It also differs from a closed back cabinet where the air behind the speaker acts more like a spring and consequently drives the resonant frequency higher.

   In addition to lowering the resonant frequency of the speaker, the column of air in the tunnel acts as an acoustic crossover between the front and rear sonic waves. This loading by the tunnel reduces the bass from the front of the speaker and redirects it out the rear. Simultaneously the air in the tunnel reduces the mid and the treble of the rear sound wave. Since the bass is not cancelled the cabinet amazingly sounds bigger than it is. The cancellation of front and back waves only occurs in the bass region in a small area directly in front of the cabinet - hence this can pose a miking problem if the mics are not properly positioned (See Miking a Tunnel-Back).

Since the bass is not cancelled the cabinet amazingly sounds bigger than it is.

   Although this particular concept is used often in the Pritchard Amps combos, there are other variations on this theme more applicable to other speaker forms, such as found in the single 15 extension cabinets. These and other concepts are found in Pritchard’s US Patent 6,411,720 that was issued June 25, 2002 and named “Speaker Systems with Lower Frequency of Resonance.”

Ported and Transmission Line Cabinet Problems

   The ported cabinet loudspeaker synthesis design rules are quite strict and depend heavily on an amplifier that has low output impedance or is highly damped and a speaker (driver) with a low Q and little resonance. Unfortunately these conditions do not apply to musical instrument amplifiers or speakers. Highly damped amplifiers produce their low output impedance with a significant amount of feedback. While this feedback reduces the output impedance and reduces harmonic distortion, it also tries to overcome power supply limitations. Consequently, the response to being over driven is bad: substantial, rapidly rising, and high-order harmonics that produce unpleasant listening. Conversely, the Tunnel Back™ Speaker works with amplifiers designed to be over driven. They have little feedback, and consequently, low damping factors. Musical instrument speaker (driver) designs value efficiency over low Q and consequently does not have a Q factor as low as the modern high-fidelity drivers.

   The transmission line cabinets load the driver, usually the back of the driver, with an acoustic transmission line which is a quarter wave length at the driver's resonant frequency. L.J.S. Bradbury applied wave theory to earlier observations that fibrous material slowed the speed of sound. Thus, a fiber-filled tube could be a shorter, smaller acoustic transmission line. Unfortunately, these lines fail to provide enough damping for the high Q speakers common to the guitar and musical instrument arts.