[miss]

hello all
the frequency of the enclosure on the curve of impedance and fixed on 35hz.  it was said to me that the vent of the ellis was fixed at 45hz F3-3db.  I found the volume of 22 liters one can large you do not find?  I thought has an optimal litrage of 18 liters, which think?  
thank you
Fred

[hubert]

fred wrote

hello all

Hi you, welcome

the frequency of the enclosure on the curve of impedance and fixed on 35hz.

it's the fr where the port acts the most efficiently, very close to the driver resonance fr in free air (without any loading)

it was said to me that the vent of the ellis was fixed at 45hz F3-3db

-3db is the standardized measured point at the begining of the roll-of curve in bass range; it is allways slightly higher than fb.

I found the volume of 22 liters one can large you do not find? I thought has an optimal litrage of 18 liters, which think?

from my personal experience, 22 liters give a tight and deep bass; you can easily keep this volume except if you want to build the tower version which couples the bass range to the floor; in this case you could try 18 liters.

[David Ellis]

This post may become slightly long.  Please query further if my commentary doesn't make sense.  My comments aren't designed to FULLY address all the issues present, but to briefly touch many of them.

There were some fabulous individuals that managed to measure loudspeakers then mathematically model their response in various ported enclosures.  These models follow traditional alignments (i.e. SBB4) that folks often discuss.  

In the realm of dampening, these ported enclosures/systems vary, but hover around a system Q of 1 .  This is a ratio of spring versus dampening.  A "normal" sealed enclosure will hover around a system Q of .707.  This alignment offers better dampening than ported system Q of 1.  But... the point of this is to address the change in ported cabinets.

It's my belief that a ported cabinet volume doesn't need to be addressed in extreme detail.  This is because a small variation in cabinet volume will be addressed by changing th port length.  Bigger cabinets will require a shorter port for proper bass balance and smaller cabinets will require a longer port for proper bass balance.

When making these changes, the Q of the cabinet will also change.  Making the cabinet bigger will cause the Q to rise.  The bass will sound deeper, but since the Q is higher it will sound looser.  This is because the woofer system has more spring and less dampening (relatively speaking).

Making the cabinet smaller will cause the Q to drop.  The bass won't be as deep, but the Q will drop.  The result will be tighter sounding bass.  This is because the woofer system has less spring and more dampening (relatively speaking).

Additionally, adding resistance in the woofer circuit will cause the Qes of the woofer to rise.  This happens when inductors are installed in series circuit behind the woofer.  This will subsequently cause the Qts to rise and the "ideal" cabinet should be slightly larger.

I have found the measured T/S parameters of the W18 woofer to predicate a slightly larger cabinet than the factory specs.  The VAS is slightly lower, but the Qts generally hovers around .39 in the woofers I measure.

Given the 1801, it's possible to use a range of cabinet sizes and very safely tune the port by ear.  I think anything from 18liters to 25 liters is very viable.  I think at 30 liters (I haven't tested this), the bass from the W18 woofer would become quite loose and "flabby".  22 liters seems to work very well, and the F3 is a solid 40hz - the lower end of the bass guitar range.  Commercial folks might advertise a more "marketing friendly" lower bass reach, but I won't.  The 1801 will produce effective bass at 40hz.

So, there are factors that preciptate a slightly larger cabinet.  There is also some very safe room for experimentation.  The cabinet size can vary.

Again, my comments aren't all-encompassing.  There are chapters and books written on each of the subjects I briefly addressed above.

I think... that I addressed all the pertinent issues.

Dave

[miss]

thank you has to you two for your comments, I includes/understands better now;)
Miss

[Ron Stewart]

Given the 1801, it's possible to use a range of cabinet sizes and very safely tune the port by ear. I think anything from 18liters to 25 liters is very viable. I think at 30 liters (I haven't tested this), the bass from the W18 woofer would become quite loose and "flabby". 22 liters seems to work very well, and the F3 is a solid 40hz - the lower end of the bass guitar range. Commercial folks might advertise a more "marketing friendly" lower bass reach, but I won't. The 1801 will produce effective bass at 40hz

Hi Dave,

This talk about cabinet volumes prompted me to make a few calculations. I'd like to walk through them, then ask some questions. Maybe somebody can double check my math. Given that I'm posting this on a public forum, I'm sure I made several silly mistakes.

Here's how I compute the volume of an unstuffed 1801 cabinet (regular version). The dimensions below come from the cabinet plans on your web site (the pictures in Steps 3 and 4 in particular).

Let's start by computing the gross volume of the cabinet (ignoring the volume of the two braces).

gross volume = 17.5" x 7.5" x 12" = 1575.0 cubic in.

Now let's compute the volume of one brace. Each brace is 17.5" x 7.5" (x 0.75"), with two 5" diameter holes and one 3" diameter hole.

gross brace volume = 17.5" x 7.5" x 0.75" = 98.4 cubic in.

volume of 5" hole = pi x (2.5" squared) x 0.75"  = 14.7 cubic in.

volume of 3" hole = pi x (1.5" squared) x 0.75" = 5.3 cubic in.

brace volume = 98.4 - (2 x 14.7) - 5.3 = 63.7 cubic in.

With all of those values, we compute the actual cabinet volume:

actual cabinet volume = gross volume - (2 x brace volume)
actual cabinet volume = 1575.0 - (2 x 63.7) = 1447.6 cubic in.
actual cabinet volume (liters) = 1447.6 / 61 cubic in. per liter = 23.7 liters

Now I'll finally ask some questions. Am I right in thinking that the 22 liters you quote reflects the volume of the foam stuffing you recommend? I've read that polyfill effectively increases the cabinet volume. I've always wondered about the effects of acoustic foam, mattress/carpet pad, and Black Hole. Am I right in thinking that those materials do not act like polyfill; instead, they actually reduce the effective cabinet volume?

I also computed the volume of the floorstanding cabinet. Given outside dimensions of 40" x 7.5" x 10" (with single-walled construction), and assuming a center brace with six 5" holes, I calculate the volume like this:

gross volume = 38.5" x 6" x 8.5" = 1963.5 cubic in.

brace volume = (38.5" x 6" x 0.75") - 5 x (pi * 2.5" squared x 0.75)
brace volume = 173.25  - (5 x 14.7) = 99.75 cubic in.

actual volume = gross volume - brace volume = 1863.75 cubic in.
actual volume = 1863.75 / 61 = 30.6 liters

That 30.6 liter volume is way out near the upper range of volumes you think will work. Wouldn't this suggest that anyone building the 1801F really needs to add enough foam to lower the volume into a more acceptable range? (I lined the entire insides of my 1801F's with vinyl floor tiles and 5/8" acoustic foam, so I think I ended up doing just that.)

When you built your experimental 1801Fs, you mentioned that you lined some of the cabinet near the woofer. What do you suppose was the final volume of those cabinets? Might it be close to 30 liters? If so, it's interesting that you didn't hear the loose, flabby bass you wondered about.

Ron

[David Ellis]

Your calculutations are accurate.

You must also remove some cabint volume for the crossover, the port, and the drivers.  These objects affirmatively remove volume from the cabinet.  Black Hole 5 will also remove some volume.  

Next, IMO/IME there really isn't a solid-line where the bass of a ported cabinet becomes "flabby" or "tight".  Rather, there is a slippery-slope of change with regard to cabinet volume and it's correlating bass character.  This is easily modeled using software.

Years ago I built a speaker (not an 1801) using a variable volume cabinet. I filled the bottom of the cabinet with sand increasingly and found the ideal balance bass character and depth resulted at about 22 liters when the Q was approximately 1.

I do hear some change when using the slightly larger 1801F floorstanding cabinet, but this is is not a "fair" a/b isolating cabinet volume since the 1801f port is very close to the floor and relatively distant from the driver.