Port obstruction

[Helge A. Bentsen]

How far away from a port can a piece of bracing be before it's effect on airflow is considered irrelevant?
I'm playing around with a old box and noticed that two of the four ports have bracing and stuff in their way.


This is a picture of the worst offender, at the lower right you can se a piece of dampening material, I can cut that away easily.
Below that is the handle, can't do much about that.
But on the left side there is a piece of bracing. It's less than half the port diameter away. Is this considered too close?
I can cut it away and replace it, it used to support the old horn in this box. Right now the box is tuned slightly low, so I'm working on raising it from 50 to around 60hz.

 

[Max Warasila]

The rule of thumb I use is the equivalent diameter of the port between the mouth and obstructions.

 

[Uwe Riemer]

How far away from a port can a piece of bracing be before it's effect on airflow is considered irrelevant?
I'm playing around with a old box and noticed that two of the four ports have bracing and stuff in their way.

View attachment 209416
This is a picture of the worst offender, at the lower right you can se a piece of dampening material, I can cut that away easily.
Below that is the handle, can't do much about that.
But on the left side there is a piece of bracing. It's less than half the port diameter away. Is this considered too close?
I can cut it away and replace it, it used to support the old horn in this box. Right now the box is tuned slightly low, so I'm working on raising it from 50 to around 60hz.

Why do you need a "rule of thumb" when you have the enclosure at hand.
Some high power ( up to the maximum intended power level ) long sine sweeps with the High Pass, you would like to use in the system, in place should reveal problems in the pass band by adding noise.
Just low pass the signal around 150Hz to avoid hurting your ears

 

[Carl Klinkenborg]

There are more likely to be issues with the port design itself (turbulence and rectification) rather than its closeness to bracing. Further, what is the port and driver size?

 

[Helge A. Bentsen]

Finally got around to measure the box dimensions.
Internally its 348mm wide at the rear, 467mm at the front and 928mm tall.
Calculated roughly 69L internally (I measured handles and bracing, 69L is left when I subtract these).
Four ports, 67mm in diameter, 70mm long.
Around 15L goes away for drivers (2x 12NDL76+TPL-200H.
Leaves me with 52L. I measured the box impedance with drivers using DATS, looks like it's tuned to 54hz right now.
I would like to raise this to around 70hz, the box has a pretty steep rolloff below 125hz measured full-range, I'm targeting a 80hz crossover.

 

[Max Warasila]

Good news! That should be an easy fix.

I would recommend trying for around 65 Hz rather than 70 Hz after the difficulties I had with a tune too close to my x-over recently.

 

[Uwe Riemer]

Finally got around to measure the box dimensions.
Internally its 348mm wide at the rear, 467mm at the front and 928mm tall.
Calculated roughly 69L internally (I measured handles and bracing, 69L is left when I subtract these).
Four ports, 67mm in diameter, 70mm long.
Around 15L goes away for drivers (2x 12NDL76+TPL-200H.
Leaves me with 52L. I measured the box impedance with drivers using DATS, looks like it's tuned to 54hz right now.
I would like to raise this to around 70hz, the box has a pretty steep rolloff below 125hz measured full-range, I'm targeting a 80hz crossover.

Something does not add up
With net volume of 52l the upper resonance peak should be above 100Hz, not slightly below 90Hz. This suggest that your actual cabinet is bigger, around 85l.
And the lower resonance peak looks weird also, it should be way lower.

What drive levels does this DATS provide ?

On the positive side: If you could make the internal volume close to 52l, you can leave the ports alone or shorten them to 50mm, this would be something to work out in the real world ( for me )

And one last thing: 80Hz acoustical HP is asking too much IMO, I would try something between 90 and 100Hz

 

[Helge A. Bentsen]

I agree, something doesn't seem right with this cabinet.
And I'll be the first to admit that my knowledge of ports is limited....

I have a couple of things I wish to try, first is cutting off a bit of all the ports and remeasure.
Did that tonight, i cut them down by 12mm and the response changed a bit.

Both measurements are with the shorter ports, one is at 0dBU, the other at +10dBU.

 

[Carl Klinkenborg]

The ports are impossibly small for two 12" drivers and cannot work well at higher levels. You will be chasing your tail with this port area so perhaps look at increasing it and starting from there.

 

[Helge A. Bentsen]

Finally got around to measure the box dimensions.
Internally its 348mm wide at the rear, 467mm at the front and 928mm tall.
Calculated roughly 69L internally (I measured handles and bracing, 69L is left when I subtract these).
Four ports, 67mm in diameter, 70mm long.
Around 15L goes away for drivers (2x 12NDL76+TPL-200H.
Leaves me with 52L. I measured the box impedance with drivers using DATS, looks like it's tuned to 54hz right now.
I would like to raise this to around 70hz, the box has a pretty steep rolloff below 125hz measured full-range, I'm targeting a 80hz crossover.

View attachment 209419

Ok, redid my calculations, my math is off.
Way off.

The box is 128L (Bracing & handles excluded)
The drivers occupy 7.5L, that leaves me with around 120L.

Now the numbers in WinISD and what I measure correlates.
This make me think I can shorten the ports furter for a total lengt of 34mm for a 65hz tuning.

 

[Uwe Riemer]

Ok, redid my calculations, my math is off.
Way off.

The box is 128L (Bracing & handles excluded)
The drivers occupy 7.5L, that leaves me with around 120L.

Now the numbers in WinISD and what I measure correlates.
This make me think I can shorten the ports furter for a total lengt of 34mm for a 65hz tuning.

Higher tuning by shortening the ports will help a little bit, but reducing the box volume would be way more effective in the frequency range from 80 to 150 Hz.
But hey, if you only do light Jazz with the speaker....
Might be a good idea with this HF driver anyways

 

[Helge A. Bentsen]

Higher tuning by shortening the ports will help a little bit, but reducing the box volume would be way more effective in the frequency range from 80 to 150 Hz.
But hey, if you only do light Jazz with the speaker....
Might be a good idea with this HF driver anyways

Reducing the volume is not a bad suggestion.
I shortened the ports to 46mm today, slight response change i the impulse response. Swept the ports with a reasonable SPL, there's a lot of air moving through them.
I'm going to break out Smaart tomorrow and have a look at the frequency response. Might try to fill the cabinet with something temporarily to reduce the internal volume and see what happens as well.

 

[Uwe Riemer]

The ports are impossibly small for two 12" drivers and cannot work well at higher levels. You will be chasing your tail with this port area so perhaps look at increasing it and starting from there.

In general general statements are wrong

 

[Carl Klinkenborg]

In general general statements are wrong

Not a general statement, a proven fact!

 

[Max Warasila]

Not a general statement, a proven fact!

It was a generalized statement for sure. If you're tuning below (or even at) the -6 dB point of the crossover, the necessary effective port area goes way down. While I agree with you, if the x-over was at 80Hz and the tune was at 54 Hz, this would be plenty of port area.

 

[Helge A. Bentsen]

Made two cabinet dividers, ended up with around 42L per driver.
By listening to the cabinet I hear more low-end, haven't measured it yet.
Impedance plot:

 

[Carl Klinkenborg]

It was a generalized statement for sure. If you're tuning below (or even at) the -6 dB point of the crossover, the necessary effective port area goes way down. While I agree with you, if the x-over was at 80Hz and the tune was at 54 Hz, this would be plenty of port area.

My apologies - I had not realised you were tuning below crossover!

 

[Max Warasila]

Made two cabinet dividers, ended up with around 42L per driver.
By listening to the cabinet I hear more low-end, haven't measured it yet.
Impedance plot:

View attachment 209423

This is about what I ended up doing with a pair of 12"+1" boxes I did with the leftover 12NDL76 from my PM69 experiences. It sounded pretty good to me - I ended up adding a bit of insulation/foam to help with the reflections in the cabinet. I tried tuning it higher to about 70 Hz and it never really felt right IMO. YMMV.

 

[Helge A. Bentsen]

This is about what I ended up doing with a pair of 12"+1" boxes I did with the leftover 12NDL76 from my PM69 experiences. It sounded pretty good to me - I ended up adding a bit of insulation/foam to help with the reflections in the cabinet. I tried tuning it higher to about 70 Hz and it never really felt right IMO. YMMV.

Did a tuning session today in my warehouse, here is an average of 3 mic positions, the speaker is on top of a 2x18".

 

[Max Warasila]

Did a tuning session today in my warehouse, here is an average of 3 mic positions, the speaker is on top of a 2x18".
View attachment 209424

If it were me (and it's not so feel free to do as you please), I would give 12dB/oct a try for your crossover, or even mixing slopes just for more consistent time alignment along the coverage pattern. Whatever you've got on top of it is impressively flat.