Decision Time

[mgrabow]

Well, I reached a point in my build where I need to decide something. I was double checking my math. I am using a Usher 8945A for a mid. The midrange chamber in this design is 12 liters. It was originally designed for a different driver than I am using, but for a variety of reasons I am using this 8945A. I calculate the 12 liters will work fine with a 2.5 X 13 3/4 in port (fb 43hz). This will reach through the bass chamber out the rear of the cabinet. You can look at my build thread to see the drawings...

http://techtalk.parts-express.com/showthread.php?t=218279

Should I have the 8945A vented as well as the two Usher 8955's (vented at 28hz)I am using for Bass? I can go sealed on the 8945 and reduce midbox to 4 liters, thus increasing my bass box by 8 liters. Not a bad thing since I am using heavy fill to get the two 8955's to work in a 3 Cu/Ft enclosure...


I need to decide before I go any further because I need to drill the hole for the port in the rear panel of the midrange enclosure before I start gluing it up...

[davepellegrene]

Re: Decision Time

Well, I reached a point in my build where I need to decide something. I was double checking my math. I am using a Usher 8945A for a mid. The midrange chamber in this design is 12 liters. It was originally designed for a different driver than I am using, but for a variety of reasons I am using this 8945A. I calculate the 12 liters will work fine with a 2.5 X 13 3/4 in port (fb 43hz). This will reach through the bass chamber out the rear of the cabinet. You can look at my build thread to see the drawings...

http://techtalk.parts-express.com/showthread.php?t=218279

Should I have the 8945A vented as well as the two Usher 8955's (vented at 28hz)I am using for Bass? I can go sealed on the 8945 and reduce midbox to 4 liters, thus increasing my bass box by 8 liters. Not a bad thing since I am using heavy fill to get the two 8955's to work in a 3 Cu/Ft enclosure...


I need to decide before I go any further because I need to drill the hole for the port in the rear panel of the midrange enclosure before I start gluing it up...

I am probably the last one to answer this but why would you port the mid to reach to 43 Hz when you will more then likely cross it to the woofer way above that? I ask this because in my Rock It build I thought the same thing, venting the mid, and that was the conclusion I came up with so I went sealed.
Dave

[[email protected]]

Re: Decision Time

i concur with dave.

[Face]

Re: Decision Time

Isn't a sealed mid better for group delay?

[Wolf]

Re: Decision Time

Seal the mid. It's easier to blend with the woofers, one reason of which is why Mike stated.

It's like blending a sealed monitor with a vented sub.

Later,
Wolf

[Pete Schumacher]

Re: Decision Time

What they said.

Venting won't matter due to the fact that you're likely crossing 3 octaves above where a port would cut off anyway. Keep it simple and seal it tight, with a good helping of fiberglass to eliminate any standing waves.

That should make for one excellent mid. I sure wish Usher would crank out some 5" units to compete with the smaller Scans and SEAS mids and midwoofs.

[r-carpenter]

Re: Decision Time

I sure wish Usher would crank out some 5" units to compete with the smaller Scans and SEAS mids and midwoofs.

man! my thoughts exactly!

[mgrabow]

Re: Decision Time

Dur.... Sometimes I get so involved with the construction, I forget the obvious, like the crossover freq being well above the port freq...

As far as the 8945 goes does 4 liters sound about right?

[davepellegrene]

Re: Decision Time

.... with a good helping of fiberglass to eliminate any standing waves.

Not trying to high jack your thread, but I think this question is in line with the discussion.

While taking measurements how do you know the difference between standing waves and adjustments needing made in the crossover? Will it look different in the graph or do you just adjust the damping first? I guess another good question is can you fix a problem in the crossover with damping? Or am I way over thinking this

Dave

[Kapton Planet]

Re: Decision Time

As far as the 8945 goes does 4 liters sound about right?

No, I'd suggest staying with 12 liters, I wouldn't go any smaller.

[r-carpenter]

Re: Decision Time

Not trying to high jack your thread, but I think this question is in line with the discussion.

While taking measurements how do you know the difference between standing waves and adjustments needing made in the crossover? Will it look different in the graph or do you just adjust the damping first? I guess another good question is can you fix a problem in the crossover with damping? Or am I way over thinking this

Dave

Standing waves will show up as distortion picks, impedance picks and water fall artifacts. Strong standing wave may create sharp FR peak. Of cause you need to take all this data free air and compare to the driver in the box. Really shines the lite on wave behavior inside the enclosure.
As far as Usher goes, MG is crossing it high enough where the driver's diaphragm doesn't really move, so a larger volume will provide more room for padding and stuffing and eliminating back radiation of the driver.

[Pete Schumacher]

Re: Decision Time

Not trying to high jack your thread, but I think this question is in line with the discussion.

While taking measurements how do you know the difference between standing waves and adjustments needing made in the crossover? Will it look different in the graph or do you just adjust the damping first? I guess another good question is can you fix a problem in the crossover with damping? Or am I way over thinking this

Dave

Standing waves will show up not only in the response, but in the impedance plot. I know there's some debate on how much to stuff a midrange enclosure, but standing waves need to be eliminated, and only a good amount of stuffing will adequately address that issue, at least 1 pound per cubic foot, maybe more. I wouldn't use stuffing to address crossover issues.

[davepellegrene]

Re: Decision Time

So if I mount the driver in the enclosure with the back off, take measurements, then install the back, remeasure and add damping until I get the same as the open back measurements would this be a good approach?. Pete you say add stuffing. What about lining the enclosure with rigid fiberglass first?

Thanks Dave

[r-carpenter]

Re: Decision Time

mount the driver in the open baffle,,, free air. Measure it.
Then mount the driver in your enclosure and remeasure again. Impedance, FR, distortion and waterfall. Add staffing and measure again. Compare. At this point, it'll become clear. Lining will help as well.
I've gone as far as building anechoic chamber inside open mid chamber.