Porthole Orientation

I built my initial cabinets with the porthole exiting the front of the cabinet.  I figured that this would do a better job of reinforcing the bass.  Later I learned that front portholes were inferior.  Rear portholes are better.  This was first obvious to me through hearing, and then through very simple theory.  I performed an a/b test using two almost identical pairs of speakers.  There was only one difference between the pairs of speakers – the porthole orientation.  One pair had the port exiting the front of the cabinet.  The other had the port exiting the rear of the cabinet.

The character of the bass was superior in the rear ported speaker.  The increased clarity was immediately obvious in Andre Bocelli’s voice at about 150hz. The bass guitar also sounded much tighter and leaner, with no loss of extension.  The bass drum followed suit.  The improvement was true with Bocelli’s voice, bass notes, and everything in the lower region of frequency response.  I believe the muddy bass in the front ported speaker was due to some cancellations above the port tuning due to the very close proximity of the port exit and the driver cone.  I believe the cleaner bass in the rear ported speaker was due to the increased distance between the port exit and the driver cone.

The other superior quality the rear porthole speaker possessed was cleaner midrange.  The porthole will setup a pipe resonance corresponding to the port length.  This phenomena is very similar to pipe organ resonance.  The front ported speaker had a slightly smeared midrange around 800hz.  This wasn’t evident in the rear ported speaker.  The rear ported speaker midrange resonance exits away from the listener and gets absorbed/diffracted.  

Both speakers sounded subjectively “good”, but the only reason to put the port in the front is for marketing. Only very low frequencies emit from the port.  In the case of the 1801, the longest frequency emitting from the port is about 40hz.  A 40hz sine wave is 25 1/2 feet long.  This is very non directional in most rooms.  Most subwoofer guys consider 80hz to be an approximation of audibly locatable bass.  The 1801 port reinforcement is well below this.

There is a very good article describing the operation of a port here: http://www.hometheaterhifi.com/volume_5_2/cmilleressayporting.html

A front porthole will spit midrange port noise directly into the listeners face while the rear porthole will diffuse this midrange noise.  Some of this can be attenuated via stuffing material, but there will still be some port resonance.  This is the greater sin of the front ported speaker.   

Most good speakers have their porthole’s exiting the rear of the cabinet.  This makes sense.