Home Theater Forum and Systems banner
1 - 5 of 5 Posts

·
Premium Member
Joined
·
26 Posts
Discussion Starter · #1 · (Edited)
I am building my first serious sub, and realized there is some basic "advanced" acoustical theory I need to know to design the enclosure. Spent hours looking for prior answers, but could not find where anyone ever asked my questions, nor could I find a design theory article or book. I have decades of finish carpentry experience (but no "fine" cabinetry work). I am a retired hippy bachelor, so I do not have any WAF factor. I do not care about the size, weight, or convenience of the sub, only that it makes powerful, beautiful music in my dedicated 11.2 multi-channel audio theater (I occasionally bring in the old 46' DLP to watch a movie, but it usually sits in the hobby room where it doesn't interfere with my light show augmented, audio bliss).

I will be placing the new sub in the approximate center of the back wall, firing toward the couch (the couch is set at approximately 2/5 the distance from the back wall to the front wall ( about 8.5' out of 22.5' room length). My "best placement of listening spot and equipment" was found through hours of inch-by-inch experimentation and lots of theoretical input from the Shack and other sources (you wouldn't believe how many filled holes I've created moving speakers, lights, acoustic panels, clouds, shelves, etc... way beyond what any significant other would ever have tolerated...:foottap:

I have purchased a Dayton RSSHO-4 18" Reference HO driver, two TC Sounds 15" passive radiators, and one Dayton SPA1000 plate amplifier, to build the new sub with. I ran the "Bagby" software and got these results: FS 18.80,vas 272 li (9.6 cuft), driver xmax 12.75mm, PR xmax 33mm, added mass 500gm/ea x2, and tuned FB17.67 HZ. I plan to use precut 3/4" MDF pieces, that I glue and screw together to create a laminated case with a minimum wall thickness of 2" (max 5" in places). In this manner I can create a non-rectangular, reinforced cabinet (with asymmetrical interior space) to minimize cabinet resonances. Additionally, I expect the layered assembly to add considerable mass to the enclosure which should help to stabilize the sub (I am guessing an empty cabinet weight of 150 to 200 pounds). My problem is how do I best configure the elements of the sub to maximize the audio performance?

I originally planned to build an old-jukebox-shaped enclosure that mounted the plate amp bottom-center, the driver directly above, and the PR's on R & L opposed sides. I thought that raising the driver up from the floor might enhance the audio experience, and opposed PR's would offset driven forces - then I realized that the linear forces of the driver would be exaggerated by the tall cabinet, and reduce subwoofer performance. So then I considered a front firing driver, with opposed side facing PR's and a rear plate amp... a front firing driver, with front facing PR's, and a rear facing plate amp... then I considered building a stand-alone enclosure for the amp.... etc.... etc...

Then I realized I needed some info before I configured the enclosure...

1 - How does the height of a front-firing sub-driver effect its acoustical performance, in a general sense? Is it better for the driver to be positioned closer to the floor, or nearer to ear level, sonically? To my imagination, the unreflected direct sound of the Dayton driver, might be a preferable to the reflected sound, even if the wavelengths are of the longest sort. However, I am also aware that human perception might be best suited to a diffused reflected input. There is also the issue of the listeners close proximity, in my situation. I do not have the experience to judge, so I am asking for your opinion/experience.

2 - I believe there is some sound output from the PR's as they respond to the driver movement. How should the PR's be aimed to best enhance sonic performance? I read that opposed orientation helped to offset PR movements, but is there a better way for maximum listening pleasure?

3 - It occurs to me that maintaining a low, heavy center-of-gravity, would best resist the enclosures reaction to driver movement, and provide the most efficient support for the driver to actuate, with the most accurate response possible (no lost energy spent moving the cabinet). Is this a "correct" way of thinking? And if so, is there a rule-of-thumb to guide the designer?
---- there is the offsetting issue in question #1. If it is better for the driver to be mounted some distance above the floor, than how to guess-timate the point of balance between desired height and enclosure movement. Not to forgot, the ridiculous weight of the cabinet should help hold things still... :sweat:

4 - I am assuming that a front firing sub is the preferred choice for sub performance. However, having been a derriere way to many times, I would ask if you have a different suggestion?

5 - Last question is about installing feet. My HT is located in basement, 50% below ground level, with foam-padded short carpeting. Walls are finished with wood paneling, over 1/2" sheetrock, over foam insulation, and 2x2 furring. Ceilings joists are packed with insulation, extensively crossbraced, and finished with 1/2" sheetrock and heavy texturing. All finishes and fixtures are tightly screwed, glued, and trimmed so no strange vibrations and minimal sonic coupling. I don't care if the sub sits flat on the carpeted floor or not, and doubt that adding feet to the cabinet would serve any good purpose. Am I mistaken?

I tried to sketch some of my ideas, and I tried to use a CAD program, but I just don't have the skills to get it done.

That's all I can think of. Thanks... Don Mitchell

p.s.... should have mentioned that this is likely the last of the equipment I will buy for "this" remodel (its #3 major). The remainder of the HT upgrade will focus on polycylindrical diffuser installation on ceilings and walls, incorporating new lighting into the ceiling cloud/diffuser treatment, cleaning out room clutter and useless acoustical absorption, and doing a little testing. I have had the EQ Wizard software for years, however, with no one but me doing the work, and the improvements in integrated electronic acoustical equalization, I find it very hard to justify the hours of work, when I have so many other things I hope to accomplish..... thanks again
 

·
HTS Moderator , Reviewer
Joined
·
3,382 Posts
There's an awful lot to address from that one post, so invariably I'll miss something, but here are a few thoughts...

A 5" thick cabinet would be overkill. If designed with proper bracing and strategically placed damping material you should be fine with walls that are 2" thick. Even that is probably on the excessive side, but not unreasonable.

Given that bass is omni-directional there's little need to "configure the elements for audio performance". Front-firing, down-firing, side firing, it all works well. There are subtle variations due to the driver orientation, but I'm not sure any of them hold a significant advantage. For example, down-firing is purported to mask some of the distortions because the sound isn't directly radiating at your ears. But in a double blind test could you tell which way the driver was firing? Probably not. The only caveat is in order to get long life from a down-firing driver it has to be certified for the orientation. Gravity will pull on it 24/7, so if the manufacturer hasn't account for that it might sag over time.

There are no "center of gravity" benefits that I'm aware of. Direct reflections off the ceiling might come into play if the driver is positioned several feet from the ground, but beyond that I can't think of anything else.

There is definitely a contribution from the PR's. Thinks of it like a bass reflex design without the possibility of port noise, because that's essentially what it is. Just like the port augments the output, so too do the PR's. If you have more than one than dual opposed will probably be the best approach. Cancelling out each others movements will have its benefits for sure.

If the floor in your HT is concrete there's little to be gained by using feet. No matter what you do that material basically sucks the life out of a subwoofer. There's no indication of your room size - which is a critical factor to consider when deciding how much subwoofer will suffice - but unless the space is fairly small you might find the need to add more than one. The rest of the room construction sounds good though, so that shouldn't cause any issues.
 

·
Premium Member
Joined
·
26 Posts
Discussion Starter · #3 · (Edited)
Thanks JMan...

I did not mean to imply that the entire cabinet will be 5" thick. By using different sized pieces of 3/4"MDF, that I laminate to each other, I can create a cabinet that has curves here I choose, and varying wall thickness, as I see fit. I plan to have an elliptical shape on external top, and an asymmetrically curved internal shape, so that resonance frequencies will be broken up internally. Since the external cabinet shape will not be following the internal shape, there will be great variation in wall thickness, from 2", up to 5". By carefully choosing the sizes of the pieces and their placement, I can create thick portions of the cabinet wherever I think it will improve sub performance. Since I am not concerned about the weight, and I have learned the importance of rigidity and mass for resonance control, I am intentionally leaning toward the "overkill" side.

While I appreciate that, generally speaking, bass frequencies are not directional, in my case the driver will be located directly behind, and directly centered on my listening position (and 9' away). This will strongly emphasize the direct sound waves coming from the driver. I spoke with Matthew, at Parts Express, and he agrees that, in my case, there should be a noticeable difference in direct vs. indirect bass sound qualities. Consequently, I will be directly targeting the primary listening position with the driver, and installing the PR's opposed, on the R&L faces of the cabinet, to distribute some of the bass energies along the back wall. My goal with this sub is not about filling my smallish room, but about providing a source of extremely low frequency bass, so that I can feel the tympani and bass drums, and the bass strings, and wind notes, as if the instruments are in the room with me - or as close to this as possible.

The center-of-gravity issue is more of a physics issue, rather than an acoustical one. I was concerned about center-of-gravity because the moving mass of an 18" driver, at high output, will possess a lot of inertia. Imagine a tall cabinet vs. a short one - it takes a lot less force to rock the tall cabinet back and forth, than it does to rock the short one. Hence, mounting the driver in a taller cabinet could result in a higher c.o.g., which would encourage more cabinet movement, which wastes more of the drivers energy. So a heavy cabinet, with a low center-of-gravity, should hold the driver basket much more still, as the voice coil operates. I suspect that a driver will sound much better if the basket is held as perfectly still as possible.

On the room size, I'm sorry the dimensions were not more noticeable, where I included them in my signature. Since I have enjoyed building and trying out different acoustical treatments over the last 15years, I have come to understand that room dimensions are one of the most critical aspects of acoustics, so I put them in my signature where they would be available in any and every post I make. My room is smallish (2100 cuft), but, has generally decent dimensions, with manageable nulls and peaks. Since I can't afford a custom home, this is literally the best HT space available in my home, and I have dedicated it to that purpose. This understanding is also why I detailed some of my acoustical treatment plans in my post. Also you might notice I mention I will be using my old SVS sub in the front, while this new sub will be set in the rear.

Because I am 59 yrs old, this will likely be my last major upgrade, and so I am being especially open to input and criticism. I have learned that I most enjoy hi-def, multi-channel music so I am putting my best efforts into perfecting my home "audio" theater this time around (the movie theater experience will have to take care of itself). Regardless of effort, I am looking for every reasonable way I can improve the audio experience in my basement, regardless of its "significance". Hence I was fishing for whatever input people would share so I could be more confident of my sub design. This truly is the only chance I have to get it right....

Well, I have gone on for far to long. I apologize for being so verbose, but I hoped to encourage others input by including a lot of details. Will be drawing up my plans this week, and hopefully building next week...

Thanks... Don Mitchell

p.s... I appreciate you Jim. You provided support for what I already believed to be the case. Matthew concurs. Others on various forums echo the same.... I think I will be creating a sub which exceeds my hearing capacities and that's all I can ask.

There was another member who said it well, so I will paraphrase... something like... by the time a person owns an audiophile set up, most are no longer able to hear the difference... :ponder:.... this is as close as I'm ever going to get... :D

Dang... last thing I just noticed is I put my room dimensions in my profile, not my signature... error has been repaired.
 

·
HTS Moderator , Reviewer
Joined
·
3,382 Posts
I did not mean to imply that the entire cabinet will be 5" thick. By using different sized pieces of 3/4"MDF, that I laminate to each other, I can create a cabinet that has curves here I choose, and varying wall thickness, as I see fit. I plan to have an elliptical shape on external top, and an asymmetrically curved internal shape, so that resonance frequencies will be broken up internally. Since the external cabinet shape will not be following the internal shape, there will be great variation in wall thickness, from 2", up to 5". By carefully choosing the sizes of the pieces and their placement, I can create thick portions of the cabinet wherever I think it will improve sub performance. Since I am not concerned about the weight, and I have learned the importance of rigidity and mass for resonance control, I am intentionally leaning toward the "overkill" side.

This sounds like it's going to be an extraordinary build. I would strongly encourage you to post pictures as you progress. I'm certainly interested in seeing what you create.


I spoke with Matthew, at Parts Express, and he agrees that, in my case, there should be a noticeable difference in direct vs. indirect bass sound qualities.

I will have to respectfully disagree with Matthew because I believe he knows not of what he speaks. There won't be a "noticeable" difference between direct vs indirect radiation of sound, for you or anyone else I'm afraid.

Case in point; arguably the most respected name in subwoofer measurements and objective data is a guy named Josh Ricci (data-bass.com). When he gets a front-firing sub to evaluate do you know what he does with it? Turns it 180 degree's so the driver faces the wall. IMHO, this man knows more than almost anybody in this field, yet he completely ignores conventional wisdom - and what the manufacturer had intended - and inverts the output alignment with no deleterious affects.


The center-of-gravity issue is more of a physics issue, rather than an acoustical one. I was concerned about center-of-gravity because the moving mass of an 18" driver, at high output, will possess a lot of inertia. Imagine a tall cabinet vs. a short one - it takes a lot less force to rock the tall cabinet back and forth, than it does to rock the short one. Hence, mounting the driver in a taller cabinet could result in a higher c.o.g., which would encourage more cabinet movement, which wastes more of the drivers energy. So a heavy cabinet, with a low center-of-gravity, should hold the driver basket much more still, as the voice coil operates. I suspect that a driver will sound much better if the basket is held as perfectly still as possible.

I can see your logic in that. Unless the driver will be mounted several feet above the floor it probably won't matter, but it certainly isn't a problem to be mindful of structural integrity even if the driver is mounted lower.


Because I am 59 yrs old, this will likely be my last major upgrade, and so I am being especially open to input and criticism.

Last major upgrade? Are we taking bets? :) With your apparent level of planning and detail my guess is you have a discerning ear, and as such you more than likely will be making additional improvements to your system once you have a killer sub in place. It's almost inevitable.
 

·
Premium Member
Joined
·
26 Posts
Discussion Starter · #5 · (Edited)
Thanks Jim... and everyone else...

Nothing is impossible, and HT/AV hobby is one of my favorites. But I have noticed that my hearing is chasing after my declining eyesight... and this build will absolutely produce sound quality which exceeds my hearing capabilities. I will certainly tinker with the system, but I think I'll be focusing on other adventures on my bucket list.

My dogs are getting old (my pets... not my feet) and as soon as they pass on, I will be able to go back to adventure travel and scuba diving. There's about a dozen places I want to see visit around the world, and dozens of overland trips from Northern Canada south to the Panama Canal; there's a couple of total solar eclipses to see and maybe another trip to see the aurora again (N or S); and I still haven't gotten to swim with a Great White shark (no joke and outside the cage... :eek:).

So many things... not enough time... and less than adequate funds. I expect that when this HT upgrade is completely detailed out, that it will be my audio-visual "piste de résistance"... plenty good enough to settle for...:hsd: ... but I've been wrong before... :demon:

Cheers and Best of Luck... Don

p.s... have tried and failed to document past projects... but will try again to post a few snappies. I have learned a lot about photo file size management with my new smartphone, and maybe I will be able to avoid the "file size exceeds *%!!?&! limits" message this time.

p.s.s... as to the direct vs. indirect experience. Matthew and I were only discussing that there would be a "noticeable difference", not whether it would be preferable. If the sound qualities of direct vs. indirect were not "noticeable", than there would have been no reason for Mr. Ricci to face the driver toward the wall... yes?? I "hope" that the direct sound from behind me will lend a positive quality to the music that I have not heard before. Specifically, I hope the accuracy and clarity of the reference speaker reproduction will be enhanced by not being reflected, especially at lower power levels. Truthfully there is no way of knowing until I try it out in my space. Nice thing about my design, however, is that I can always rotate it around (with the help of a couple of hired weight lifters), and use it as you described. In all fairness, Mathew did not suggest a preferred orientation, so no issue to agree or disagree on, with him.... this one is all on me... isn't this just so much fun ??? ... insert "Mad Scientist" smiley here =>
 
1 - 5 of 5 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top