Test tones into reflex cab
 

Stewart Pinkerton wrote:

Unfortunately, the original builder of the cabs put a bunch of
narrow slots in the back panels (to try and increase the apparent
volume?). I tried a sock last night, but can't say I noticed a great
deal of difference. I'll have another play...

Aaaah, that's an old Gilbert Briggs trick from Wharfedale in the
'fifties. It's known as a 'distributed port', and is a variation on
the reflex theme. The idea was that the port area was distributed over
a large part of the cabinet, and made the port tuning less sharp.

Oh right, it actually has a more technical name than 'a bunch of slots'. I
think I read years ago that it has the effect of making the apparent volume
of the cab a little bigger.

Been playing with more speaker design software, modelling my existing cab
and driver. Don't know if I have all the parameters in correctly (but it
knows about the B139). It shows two distinct peaks in the impedance at 16Hz
and 40Hz (about 50 ohms), with the dip between them centred on 26Hz (15
ohms). Although the software's numbers are hypothetical, these seem pretty
consistent with what I got with the test tones.

An 80L isobaric cab (sealed box) produces a single peak with a shallower
profile at about 34Hz (15 ohms compared to 4 ohms at 100Hz). Unfortunately,
to get the software to put the impedance peak at the driver's resonance (is
that the right thing to do?), I need something like a 300L cab. Even so, the
curve for the 80L looks better than that for the 130L reflex. In both
designs, lots of damping inside the cabinet made the impedance peaks much
smoother.


--
Wally
www.art-gallery.myby.co.uk
Latest addition: Early Works gallery

Test tones into reflex cab
 

"Stewart Pinkerton" wrote in message


Aaaah, that's an old Gilbert Briggs trick from Wharfedale in the
'fifties. It's known as a 'distributed port', and is a variation on
the reflex theme. The idea was that the port area was distributed over
a large part of the cabinet, and made the port tuning less sharp.

This was probably a good idea in days before Theil-Small, when people
really didn't have a lot of technical guidance about how to properly tune
ports.

Test tones into reflex cab
 

"Stewart Pinkerton" wrote in message


Aaaah, that's an old Gilbert Briggs trick from Wharfedale in the
'fifties. It's known as a 'distributed port', and is a variation on
the reflex theme. The idea was that the port area was distributed over
a large part of the cabinet, and made the port tuning less sharp.

This was probably a good idea in days before Theil-Small, when people
really didn't have a lot of technical guidance about how to properly tune
ports.

Test tones into reflex cab
 

In both designs, lots of damping inside the cabinet made the impedance
peaks much
smoother.


--
Wally

My preference was always for long haired lambs wool as a damping material -
vary the amount by ear with the speakers in the room until it sounds right -
then measure it! I always found it more "forgiving" than the acoustic foams
etc.

Never tried Felicity Kendall's knickers though.........

Test tones into reflex cab
 

In both designs, lots of damping inside the cabinet made the impedance
peaks much
smoother.


--
Wally

My preference was always for long haired lambs wool as a damping material -
vary the amount by ear with the speakers in the room until it sounds right -
then measure it! I always found it more "forgiving" than the acoustic foams
etc.

Never tried Felicity Kendall's knickers though.........