acme bass

Port Placement: Front, Rear, or Side?


An Oversimplification

(Terry Buddingh mentioned in his generally careful review on the Low B-1 in "Bass Player" magazine, that I had said that "Port placement is irrelevant, because port output is omnidirectional. That is an oversimplification of what I said. For those of your to whom this is a matter of great concern, I have posted here a short discussion of the matter, which is in the owner's manual, and is updated whenever I can think of better words to explain it.)

The Merits of Front or Rear Port Placement

Bass players seem to enjoy discussing the merits of front or rear placement of ports. Based on their experiences, many players and manufacturers alike are convinced that front placement has distinct advantages over rear placement, or vice-versa. When asked to justify their opinions, these partisans will invariably refer to vague impressions and gut feelings. They will never talk about wave propagation or Helmholtz resonation.

Pressure Devices and Their Behaviour

In fact, a bass-reflex enclosure, at the frequencies where the port contributes to the system’s output, is a pressure device, as opposed to a wave device. So the interaction of the enclosure and the port, and the port's position with respect to the woofer, itself, are unaffected by any directional activity. Direction is utterly unimportant. By way of illustration, examine another pressure-related model, a balloon. As you inflate a balloon, does it expand on one direction only, or in all directions at once?

The Omnidirectionality of Low Frequencies

Only after a port’s output leaves the cabinet, and enters the environment, does it assume the characteristics of a wave, one of which is direction of propagation. Direction does assume importance when speaking of waves. Only, however, at frequencies where the size of the source is large compared to the wavelength under consideration. The highest frequencies at which the port in our Low-B systems contribute to system output have wavelengths of 14 feet or so. The speakers are obviously quite small compared to these wavelengths, and the port output is omnidirectional. Again, port placement is irrelevant to system performance.

Putting The Ports Where They Fit!

Why does the B-2 have ports in the back? Because that’s where they fit. Why does the B-4 have its third port in the front? Same reason.

The B-1 system could have been correctly tuned with a shorter, smaller diameter port located in either the front or the back, if location mattered. But it doesn’t. The drawback would have been that at higher power levels, excessive port friction would have resulted in diminished power handling capability. This larger diameter, longer port fits when installed on the side of the box. Plain and simple.

Tell your friends that the reason your B-1 system sounds so good is that the tremendous innovation of a side-mounted port results in some type of magical quantum-leap in performance. In fact, the truth is much less exciting: a high-quality woofer in a solid and correctly-tuned enclosure. Feel free to let other people lose sleep over where their ports are located. Now you know the truth.

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