ultimax 18 box modeling help needed.

[RoboCam]

With a 6" port, the velocity at 16Hz would be 40 m/s, peaking at 52 m/s. This seems to exceed what most people consider acceptable. Two 6" ports might work, but they'd each have to be 45" long (I'm going to have to buy so many of those 12" extensions, lol, but it will be worth it). With this design, the port resonance drops down to 154Hz, but I'm hoping this won't be an issue because these subs will be crossed over at around 40Hz.

Downfiring the port is a great idea. Thanks for that idea. But now that you've made me want to use PSP ports (I've used these on my car subwoofers too), I might have to put them on the front because they look so good, haha.

I also toyed around with the idea of using two 18" passive radiators but the rolloff is just too steep. It would be useless at 10Hz, a frequency of value to me.

Wow, 6 subs in different locations. I'd like to hear something like that someday. In my home theater, I put all 6 of my subs in front, side-by-side, right under the TV, partly because I like the way it looks but there are some nulls in the listening position I wish I could get rid of. Maybe I can add additional subs in other locations. How did you go about selecting the locations for your subs? Did you put a sub at the listening position and crawl around?

The reason I'm not worried about driver damage is because I've seen so many people feed high voltages to their drivers in free air, and I've tried this myself too on my Alpine Type R 15" subwoofer (70mm peak-to-peak). But here's a recipe for permanent damage. 7000 watt amp on a UM10, lol.

https://www.youtube.com/watch?v=06Rjk1IaJ-s

Also, if I put a subsonic filter in the signal path, I'm concerned that there's a chance it could subtract bass at frequencies I want to keep because as I understand it, if you set the filter at a certain setting, that means it's a certain dB down at the selected filter frequency. Subsonic filters might be necessary for protecting relatively fragile drivers, but I don't really consider the UM18 to be fragile. But you might be right, and if I somehow accidentally damage it, I'll consider incorporating some type of protection.

Now I'm wondering if I should make the enclosure 20 cubic feet instead.

You've given me many good things to think about. I appreciate that!

I'd strongly consider using the precision port specifically- that roundover on both the inside and outside helps, and is far bigger than any router bit I'm comfortable using. I wouldn't be so confident you'll hear the chuffing in that case except perhaps in the most extreme edge case. I spent much of my early builds chasing academic perfection and designing for those edge case until I actually tried the compromises and found which ones are ok and which ones aren't -again to me. Those isn't one I cases but it won't cause issues if you do.

171 for the resonance is borderline to me, I'd rather it be 2 octaves away, but I'm also making a similar compromise.

Yes, I acknowledge that moving your head or the sub can activate/engage different room modes. Below a few hundred hz, response is dominated by the room. That's why I've got 6 (4 open baffle) drivers in 4 locations covering that range. To me, that works far better and is more important than sweating port location details.

I can tell you from experience that having the port a few feet from the driver isn't an issue. As far as the wavefronts are considered, those distances remain acoustically small. I've got a speaker that's tuned a little higher with the port almost 4 feet from one of the drivers and I promise you its not an issue.

I'd down fire the port only, not the driver. Give it a try. You might like it. Plywood is cheap.

I can't think of a good reason to go without a subsonic filter. At worst it's neutral ( if you don't have material playing to unload the sub, it filters nothing) and if you do, it protects the sub, but to each their own. You'll no doubt see the sub move. I'd personally rather sacrifice deep subsonics instead of shelling out for a new driver damaged by mechanical overexcursion.

[djg]

You ever visit AVSforum, DIY speaker and subwoofer subforum? They're into really really really big subs over there.

[billfitzmaurice]

Ah, yes, this is why you have to boost the fundamental. However, the song in the link I provided has a strong fundamental with little harmonics.

Perhaps, but I'd need to see an RTA to confirm it. I've never seen an RTA of any low frequency instrument where the 2nd and 3rd harmonics were not louder than the fundamental below 50Hz. As to the audibility of frequencies below 25Hz or so, the only way one can be sure of it is to have a sub capable of high output at very low THD in that range. It just so happens that I do. I've run 20Hz sine waves at over 100dB where my sound meter told me it was there, my RTA confirmed it was there, and that THD and Doppler were -30dB. I couldn't hear it. I could feel it, but until I swept the tone up to about 30Hz could not hear it.

[RoboCam]

Just play a 20Hz test tone. It is very audible. Also, earphones have no problem reproducing those frequencies and below.

The harmonics of instruments may be louder but that's why you have to EQ the lower frequencies to bring out the fundamentals. A lot of music has "hidden" bass, bass most people won't hear when played on their systems flat, even if they are capable of reproducing those frequencies. You have to boost the bass a lot to hear it, sometimes over 24 dB. I have an app on my iPhone that lets me stack filters, so I just make several 20Hz low Q filters. You can also do this with the Dayton DSP LF through the app on your phone.

https://www.parts-express.com/Dayton...roller-230-520

In my home theater, I use an Alesis PEQ-450. It has 10 parametric bands that can all be set from 20Hz to 20kHz, +/- 18dB, Q from lo shelf to hi shelf, so I stack them (set 2 or more at the same frequency and Q). Every song requires different settings, but when adjusted "right," the result sounds so good.

Perhaps, but I'd need to see an RTA to confirm it. I've never seen an RTA of any low frequency instrument where the 2nd and 3rd harmonics were not louder than the fundamental below 50Hz. As to the audibility of frequencies below 25Hz or so, the only way one can be sure of it is to have a sub capable of high output at very low THD in that range. It just so happens that I do. I've run 20Hz sine waves at over 100dB where my sound meter told me it was there, my RTA confirmed it was there, and that THD and Doppler were -30dB. I couldn't hear it. I could feel it, but until I swept the tone up to about 30Hz could not hear it.

[djg]

https://www.avsforum.com/threads/lil...ecker.1451519/

https://www.avsforum.com/threads/lil...-f-20.1329971/

[RoboCam]

I'm after an enclosure for the UM18. Though since you brought this up, and because people are discussing secondary topics, I feel like I have to explain myself in case anyone is wondering. This will also address the notion that there's no content below X frequency worth reproducing for whatever reason.

People like things like the Audiocontrol Epicenter, a subharmonic generator (it takes the bass and reproduces it an octave lower, so if the tone was at 40Hz, it will add another one at 20Hz). The Dayton DSP-LF even has that feature. Now that is what's inaccurate because it generates frequencies that weren't in the original source material, which can sometimes be very enjoyable. But I'm just boosting what's already there. I mean, the original artist put it there, and I greatly appreciate their doing so, as it greatly adds to the enjoyment.

There's actually an entire database of bass boost settings for movies, BEQ. They provide EQ settings for several movies to bring them up to date.

https://data-bass.ipbhost.com/topic/...movies-thread/

What I'm doing is similar to what they're doing but for music.

Thanks for the links. I'll have to check them out.

[RoboCam]

Here's a great example of a song with very low bass, bass an octave lower than what you'd find in most music, even today. This one doesn't require much boost (if any) to enjoy. When I first heard this song in my home theater, I couldn't believe it. It sounded so good! I just love it when the sound engineers and artists use bass this low. Most people watching the movie probably never knew it was there. My couches were literally shaking, and it didn't sound boomy at all, since this isn't boosted midbass. The lower tones weren't unnaturally louder than other notes. Our ears don't detect low frequencies as well, so boosting it makes it more accurate in my opinion. One note shouldn't sound louder than another.

https://www.youtube.com/watch?v=tmOBiUPUBKk

[Chris Roemer]

Sorry Robo, but we all just seem to be wasting your time ...

[RoboCam]

On the contrary. Being able to bounce ideas back and forth here has given me many things to think about. The direction my project is going has definitely changed. I really appreciate the input from everyone.

[RoboCam]

Now I'm experimenting with a different idea, and I'm curious about your feedback. How would two side-firing UM15s in a 23 cu ft enclosure ported at 16Hz compare to a single front-firing UM18 in a 17 cu ft ported enclosure tuned also at 16Hz? Two drivers should give me a 3 dB efficiency boost right? Couple that with a doubling of power, I should see a 6dB gain. Here is the scenario modeled in WinISD (the UM18 is at 1 watt and the 15s are at 1 watt each). The green line represents the two 15s. Would it be better to go with 4 UM15s instead of 2 UM18s? The enclosure is only a little bit bigger (23 cu ft vs 17).

It looks like the UM18 will have a flatter response. The difference at 16Hz is 3.5dB. Is there any benefit to firing one port on either side vs. putting them both in the front? The reason I ask is because the Acoustic Research AR9 has two side-firing woofers, and in their literature, they claim benefits for this configuration though I'm not sure if they apply to the frequencies in question. At least vibrations would cancel in the side firing driver configuration.

[billfitzmaurice]

At least vibrations would cancel in the side firing driver configuration.

They will not. That would only happen if the drivers were wired reverse polarity. In that case panel vibrations would cancel but so would acoustic output. It doesn't matter if two drivers share the same baffle, if they're on opposite sides, if they're on adjacent sides or if they're on the top and bottom. Both cones must move in and out in unison with respect to the box interior. The same applies to ports. As for such massive cabs, if you can go that big you should go horn loaded. You'd get about 10dB more sensitivity from drivers that cost half as much.

[RoboCam]

If the drivers are mounted on opposite sides, they would move in opposite directions when wired in phase. As a result, the box would not vibrate.

I wish I could hear a horn-loaded driver designed to reproduce 16Hz. Are you referring to something like this Danley? They claim output down to 16Hz but that's at -10dB. This UM18 design would output 16Hz at +0.043dB, 11.5Hz at -10dB. I feel that a horn with an F3 of 14Hz would be huge. Sounds like an interesting project though.

https://www.danleysoundlabs.com/prod...ty-subs/dts20/

Look at how steep the rolloff is though. It's -3dB at 18Hz but -10dB at 16Hz, just 2Hz difference! It probably won't give me anything at 10Hz.

They will not. That would only happen if the drivers were wired reverse polarity. In that case panel vibrations would cancel but so would acoustic output. It doesn't matter if two drivers share the same baffle, if they're on opposite sides, if they're on adjacent sides or if they're on the top and bottom. Both cones must move in and out in unison with respect to the box interior. The same applies to ports. As for such massive cabs, if you can go that big you should go horn loaded. You'd get about 10dB more sensitivity from drivers that cost half as much.

[billfitzmaurice]

If the drivers are mounted on opposite sides, they would move in opposite directions when wired in phase. As a result, the box would not vibrate.

The only force cancellation that occurs is the motive force that otherwise might cause the speaker to 'dance' at high levels. The speaker panels will vibrate to exactly the same extent as with both drivers on the same side.

[RoboCam]

I agree with that. I was wondering why you said they wouldn't. I would hope that the enclosure is braced well enough so that vibrations from internal pressure would not be significant.