[Hallicrafters] BACK OF PM-23 SPEAKERS?

[Barry Hauser]

Don wrote:
> I am pretty sure that the PM-23 speaker cabinet did not have a back 
> cover as there are no holes in mine to which a cover could be attached.
That 
> is common  of speakers and radios of those days not to have a back
cover. 
> I have thought of adding a back cover to mine to see if it would 
> improve the audio quality. Does anybody know if it would?
> 

Don, Tim, & gang:

I have some experience from way back homebrewing speaker systems -- and
IMHO, a back would not improve the audio quality.

Very generally, closed box speaker enclosures fall into two popular
categories -- sealed, acoustic suspension types and base reflex types.

This is all about what to do with the "back wave".  A conventional cone
speaker in free air pushes air forward, and simultaneously pulls air from
the back.  A widerange driver or woofer will have very poor bass output
without some kind of enclosure, because the back wave cancels out the
front wave, if there is nothing to keep the two separate.  

With an acoustic suspension system, the approach is to bottle up the back
wave -- keep it from getting out.  However, this reduces the efficiency
of the driver because it's alternately compressing and expanding the air
in the sealed enclosure.  This resistance is turned to an advantage in
the design of high quality audio speakers to level out the frequency
response, but it's a complex affair which involves the design of the
driver, volume and dimensions of the enclosure, etc.  The drivers used
generally have more loosely sprung rubbery surrounds and "spiders" -- at
the neck of the voice coil.  The air in the box provides most of the
"tension".  The cone and surround have to be more airtight than usual and
the enclosure generally has to be more dense and vibration free than
otherwise.  It is also sealed airtight to avoid air leaks that might
cause odd sounds, even whistling.

The other major approach is the bass reflex system.  These take all forms
and shapes, but the idea is to make use of the backwave by reversing its
phase.  A common form has a tuned port at the front of the cabinet. 
Theoretically, when the cone moves backward, there is a delay before it
causes air to be pushed forward through the port and the reverse when the
cone moves forward.  Volume and shape of the enclosure are critical and
the resonant frequency of the driver is supposed to be taken into
consideration with the resonance of the enclosure.  Some ports are just
holes, but can be tuned by means of the diameter and the addition of
tubes.  Some base reflex designs have the port on the back.

That's a rather poor description, but the best I can remember.  Now for
the open back speaker:  It is closer to the base reflex in design in that
the driver is working against less resistance.  It will also
reflect/change phase of the lower frequencies -- depending on how it's
positioned.  So, the best thing you can do is to experiment by moving the
speaker back and forth, closer and farther from a back wall.  You should
notice a difference in the bass and deeper voices -- depending on what's
playing.

If you close up the back, it will probably reduce the low end efficiency
and may cause rattles,.uneven frequency response and possibly some odd
noises.  The enclosure and driver weren't designed for it.

Many speakers, of various types, sound better when positioned in a clear
corner.  The walls act like extensions and the principles are like those
of a horn speaker -- which is another story.

Barry

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