volume question and critiques appreciated

[asiquis]

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

[BoomieMCT]

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

[mayhem13]

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 ?

[collo]

Use boxnotes to play around with volumes.

Here's yours.....

[asiquis]

BoomieMCT wrote:

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

[asiquis]

mayhem13 wrote:

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?

[asiquis]

collo wrote:

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?

[collo]

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.

[BoomieMCT]

collo wrote:

I wouldn't recommend stuffing a ported box.

Why do you say this? I've had good experience lightly stuffing ported boxes.

[collo]

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

[asiquis]

collo wrote:

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

Maximun coupling....would that mean as in a sono sub the port is directly in line with the driver? without any stuffing--only lining.

[Blaser]

I agree Collo. Stuffing is better for sealed subs as it makes the box to appear larger. Lining is enough for vented ones.

[collo]

asiquis wrote:

Maximun coupling....would that mean as in a sono sub the port is directly in line with the driver....

Not necessarily.

Consider the port as being "charged" by the springiness supplied by the air in the box. If the port was right on top of the driver, there may not be enough clearance for the air to work its magic.

The recommended clearance of one port diameter should be enough though.

It would be interesting to compare port output at the tuning frequency for different configurations. Maybe someone from the car audio competition crowd would know the answer.