| 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.
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