Seeking advice for down firing port length/tuning in a subwoofer enclosure

[unclejunebug]

Hello all,

I'm in the early modeling stages for a 10" ported sub build. Based on my design parameters and port diameter (4") I'm expecting to need to port the bottom of the enclosure. So far I've found some online threads at various sites stating something similar to the following...

1. The distance between the floor and the port should roughly be equal to the port's diameter.
2. The distance between the floor and the port can effectively "lengthen" the port and lower the tuning of the enclosure.

I'd prefer not to have 4" feet on this sub as that seems rather tall. My questions are the following...

1. At what height would the floor no longer affect the port's length? (i.e., if I do make feet that are 4" high would that mean that the tuning frequency would be unaffected?)
2. If the port to floor distance does affect the tuning of the enclosure how can I calculate how to use this to my advantage? Precision Port's calculator tells me that for a 4" port I'd need about 25" of port length for a 2 cu^ft enclosure tuned to 25 Hz. Can I use the distance between the port opening and the floor in a way where I can shorten the length of the port inside the enclosure and still achieve a box tune of 25Hz?

[billfitzmaurice]

The space between the cabinet bottom and floor works more like a flared end than an extension of the port. Your best bet is to model it normally, then use whatever foot height gives the best result.

[DeZZar]

Could you not just port out the front or back and avoid the issue altogether?

Trying to use the air gap between the bottom of the sub and the floor to cover some of the port length is an interesting idea but would require a significant amount of testing and trial and error to reach the correct tuning (probably waste a few ports in the process of trying to balance port length and distance from floor).

I would say two inches is sufficient if its open 360 degrees. A lot of down firing ported commercial offerings are around this or less.

[DeZZar]

Here's an example of a 15" sub with a 6" port. https://www.audioholics.com/subwoofe...se-and-xv30fse - those aren't 6" feet.

[Ugg10]

Would a slot port work, assuming you can keep the height of the slot sufficient to reduce any boundary layer to a mimumum and stop chuffing then these are fairly easy to implement and can be as long as the depth and height of the box combined.

[tomzarbo]

I've done this before on a few speaker designs, and the F3 can end up being a few Hz lower than with the bottom of the speaker facing forward for example.

Take this case on my Classic Tension Headaches, where the bottom of the cabinet with it's down-firing vent was 1.25" from a bottom firing plate. This changed the tuning from 53 Hz with no bottom attached to 38 Hz with the bottom in place. Not a small change... The air under the cabinet became what I would call a 'captured air slug' that adds to the vibrating air slug in the vent.

Here is a discussion on this topic a while back for some more good input: https://techtalk.parts-express.com/f...35#post1316735

I knew this was going to happen and counted on it, but it was a guessing game for me as it was hard to model.

I would think that in your case, a good 2 to 2.5" with four smallish feet should be fine and only lower the F3 a bit if any at all. No need to go to 4" I wouldn't think. John H is correct, run a sweep with DATS to find the final tuning. If too low, you can always cut off some of the port length to adjust to your desires.

[unclejunebug]

I appreciate the responses! I will try and address each one as best I can...

The space between the cabinet bottom and floor works more like a flared end than an extension of the port. Your best bet is to model it normally, then use whatever foot height gives the best result.

Good to know. This being the case I can build the enclosure without the feet and then test some different heights to see what works best.

Could you not just port out the front or back and avoid the issue altogether?

Trying to use the air gap between the bottom of the sub and the floor to cover some of the port length is an interesting idea but would require a significant amount of testing and trial and error to reach the correct tuning (probably waste a few ports in the process of trying to balance port length and distance from floor).

I would say two inches is sufficient if its open 360 degrees. A lot of down firing ported commercial offerings are around this or less.

I'm planning to use a front firing configuration and I prefer not to see the woofer. In order to hide it I'm currently planning to recess the woofer enough to where I can wrap the enclosure with a band of acoustically transparent fabric to create a "grill" of sorts. I didn't think I'd want this fabric to be covering the port opening at all, hence the down-firing port. I'm planning to use a 4" flared Precision Port and the outer flare diameter is 7 1/4" which makes for a rather large thing to try and fit next to a woofer or an amp.

I appreciate that link. I didn't even think about the fact that my current sub has a down-firing port and the feet on it are only about 2" tall.

Would a slot port work, assuming you can keep the height of the slot sufficient to reduce any boundary layer to a mimumum and stop chuffing then these are fairly easy to implement and can be as long as the depth and height of the box combined.

I originally considered a slot port for this sub but ultimately decided against it. I don't recall the specific reason why but I think the Precision Port just seemed easier. I'm planning to cover the enclosure in a walnut veneer so maybe I didn't want to deal with painting the port.

[unclejunebug]

I've done this before on a few speaker designs, and the F3 can end up being a few Hz lower than with the bottom of the speaker facing forward for example.

Take this case on my Classic Tension Headaches, where the bottom of the cabinet with it's down-firing vent was 1.25" from a bottom firing plate. This changed the tuning from 53 Hz with no bottom attached to 38 Hz with the bottom in place. Not a small change... The air under the cabinet became what I would call a 'captured air slug' that adds to the vibrating air slug in the vent.

Here is a discussion on this topic a while back for some more good input: https://techtalk.parts-express.com/f...35#post1316735

I knew this was going to happen and counted on it, but it was a guessing game for me as it was hard to model.

I would think that in your case, a good 2 to 2.5" with four smallish feet should be fine and only lower the F3 a bit if any at all. No need to go to 4" I wouldn't think. John H is correct, run a sweep with DATS to find the final tuning. If too low, you can always cut off some of the port length to adjust to your desires.

Thanks for that link, Tom! I didn't come across that thread in my own search. I don't own DATS or any other testing equipment for that matter so that's not something I can do. Any "testing" of Foot Height vs. Tuning Frequency would have to be done by ear. If 2-2.5" feet won't have much of an effect then I think I'm pretty safe with modeling the port normally and running with it.

EDIT: BTW, those speakers look great!

[tktran]

DATS or an impedance jig to use with REW or ARTA is one of the first and most useful things to building your own speakers.
Or a least a multimeter and some test tones online

Modelling only takes you so far; in fact I always find that the suggested port Length is often longer than is necessary.

Here’s a speaker with a downfiring port. The tuning frequency was anticipated to be 24Hz; but ended up being 20Hz.

Was it my drivers, was it my damping, was it my down-firing port?

too many variables and the only way to know is through measurements

[unclejunebug]

DATS or an impedance jig to use with REW or ARTA is one of the first and most useful things to building your own speakers.
Or a least a multimeter and some test tones online

Modelling only takes you so far; in fact I always find that the suggested port Length is often longer than is necessary.

Here’s a speaker with a downfiring port. The tuning frequency was anticipated to be 24Hz; but ended up being 20Hz.

Was it my drivers, was it my damping, was it my down-firing port?

too many variables and the only way to know is through measurements

I completely understand the benefit of being able to take measurements. As far as my own builds go, I've only ventured as far as building a subwoofer and haven't justified the cost of the equipment. If I were more serious about designing my own speakers and crossovers I'd be more inclined to do so, and maybe someday I'll get there.

You mentioned test tones and a multimeter. Is this some form of testing that can be done without a microphone?

[tktran]

With a multimeter and a website like audiocheck.net

Have a read of Brian Steele’s excellent website
on how to check the tuning frequency of a box:

http://diysubwoofers.org/faq.html

[unclejunebug]

With a multimeter and a website like audiocheck.net

Have a read or Brian Steele’s excellent

http://diysubwoofers.org/faq.html

Thanks for that link! A single 4 ohm resistor is a level of test equipment I can currently get on board with

[unclejunebug]

Perhaps a silly question but is resonance frequency affected by either the amount of wattage driving the woofer or the ohm load of the woofer itself? I assume that it is not since the steps listed at that link suggest connecting a resistor in series with the driver increasing the load which I would then expect to reduce the amp's power output.

[billfitzmaurice]

No, but tread very carefully with series resistors. It takes a surprisingly low power level to toast them. The same applies to using a DMM in current measuring mode.

[unclejunebug]

No, but tread very carefully with series resistors. It takes a surprisingly low power level to toast them. The same applies to using a DMM in current measuring mode.

Thanks for the heads up, Bill! So if that's the case then, I assume volume also doesn't have an effect on resonant frequency? So I should keep the volume level fairly low to avoid toasting my resistor and DMM but still be able to measure?

[Chris Roemer]

The Frequency of resonance (Fs, Radio Shack used to call it Fo <-this being a zero), measured in "free air" (meaning no box, not even a baffle) does not change with (a "reasonable amount of) drive voltage. Many modern "systems" (like DATS, or the WooferTesters) use something below 2.83v (I THINK?). Maybe more like 1 volt? They run a 20-20kHz sweep and it's not very loud.