volume question and critiques appreciated - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 13 Old 05-15-08, 11:59 AM Thread Starter
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volume question and critiques appreciated

I will get going shortly on my ported (16-17 Hz) Tempest X project
as the offspring are finally clearing their stuff out of the garage

Looking at internal dimensions 22.5" H x 24" D x 47.5" W using 3/4 MDF
(the box shape is fixed as it fits in a specific space)

Planning on double thickness MDF on ends of sub and 3 internal "window pane" braces also 3/4 MDF
Planning on using 8" sono port around 30-32" in length
Planning on putting the driver centered on one end and the port centred on the other end (this leaves about 9" from the back of the Tempest X to the port opening)

When calculating volume of the port, I get 100 liters! Is the port figured as "solid" or does one take the outer diameter minus the internal diameter to claculate volume
At what spacing should the 8" sono port be supported?
Is attaching it to bracing with wood glue OK? Need to wrap it with anything?

Might I be better to rethink and centre the driver on the length of the box and direct the port out one of the sides?

I appreciate the help given in the past--I guess I am a bit of a "Nervous Nellie" with respect to getting going, but I guess I am trying to get it as correct as possible my first time out.
Of course that is why I am asking questions of the great folk on this forum
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post #2 of 13 Old 05-15-08, 12:38 PM
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Re: volume question and critiques appreciated

That port is about 26 liters. The volume of the port is pi*r^2*L. r = 4 inches, L = 30 - 32 inches.


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post #3 of 13 Old 05-15-08, 02:24 PM
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Re: volume question and critiques appreciated

If it's possible, you don'y want to line up the port inlet with the rear of the driver-mechanical noise from the driver may resonate and be heard from the port at higher volumes. As far as bracing, i've been building my cabinets with biscuit jointery lately to reduce the internal bracing as much as possible. I would use a window cross brace every 8" withe port running through off center. When it gets close to the driver, do not use a hole in front of the port entrance on that first brace to deflect any mechanical driver noise. Wrapping the port in insulation is your call-whether it helps or not i don't know. I'm curious as to what freq your 1st port resonance is at-Could you use 2 4" ports ?
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post #4 of 13 Old 05-15-08, 03:43 PM
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Re: volume question and critiques appreciated

Use boxnotes to play around with volumes.

Here's yours.....

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post #5 of 13 Old 05-15-08, 05:17 PM Thread Starter
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Re: volume question and critiques appreciated

Quote:
BoomieMCT wrote: View Post
That port is about 26 liters. The volume of the port is pi*r^2*L. r = 4 inches, L = 30 - 32 inches.
I guess the radius on an 8" port is 4" not 8!
What was I thinking--I guess I wasn't!
Thanks BoomieMCT
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post #6 of 13 Old 05-15-08, 05:30 PM Thread Starter
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Re: volume question and critiques appreciated

Quote:
mayhem13 wrote: View Post
If it's possible, you don'y want to line up the port inlet with the rear of the driver-mechanical noise from the driver may resonate and be heard from the port at higher volumes. As far as bracing, i've been building my cabinets with biscuit jointery lately to reduce the internal bracing as much as possible. I would use a window cross brace every 8" withe port running through off center. When it gets close to the driver, do not use a hole in front of the port entrance on that first brace to deflect any mechanical driver noise. Wrapping the port in insulation is your call-whether it helps or not i don't know. I'm curious as to what freq your 1st port resonance is at-Could you use 2 4" ports ?
Thanks Collo,

Its seems that if I put the driver on the long panel facing my seating position, rather than at the one end, the driver to rear wall resonance is quite a bit better (higher). Would I need some stuffing or lining on the rear wall then? Other walls?
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post #7 of 13 Old 05-15-08, 05:37 PM Thread Starter
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Re: volume question and critiques appreciated

Quote:
collo wrote: View Post
Use boxnotes to play around with volumes.

Here's yours.....

Thanks Collo, I now have the boxnotes
It appears the driver on the long face of the box instead of where I had planned on the one end gives a better (higher) driver to rear wall resonance. Would you suggest lining on the wall behind the driver? Lining or stuffing on any other walls of the box?
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post #8 of 13 Old 05-15-08, 08:45 PM
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Re: volume question and critiques appreciated

The driver to rear wall resonance is often the lowest in frequency.

I would line any wall that is involved in a resonance below 200hz (shown as red in boxnotes). Some closed cell foam works OK.

Bear in mind that most damping materials decrease in effectiveness as the frequency goes down.
I wouldn't recommend stuffing a ported box.
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post #9 of 13 Old 05-15-08, 09:45 PM
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Re: volume question and critiques appreciated

Quote:
collo wrote: View Post
I wouldn't recommend stuffing a ported box.
Why do you say this? I've had good experience lightly stuffing ported boxes.


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post #10 of 13 Old 05-16-08, 02:35 AM
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Re: volume question and critiques appreciated

My main concern would be a possible obstruction between the driver and the port intake. I would like to see maximum coupling between the two.

For addressing resonances, I would go for lining as opposed to stuffing.

For a more technical reason, have a look at the comment by John Janowitz
from Acoustic Elegance in this discussion
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