Bass reflex bass bin, advice wanted re acoustic lining

[dstmbgh]

I have a pair of original NHT Superzeros and I'm building a single bass bin to accompany them for a more full range music only setup. I say bass bin and not subwoofer because the Superzeros have little to no real bass (see http://www.stereophile.com/content/n...r-measurements), so the bass bin will play up to just a little over 100 hz. For this reason I'm looking at using the Dayton Audio SPA250 plate amp because it's low pass filter is 24db/octave, thinking that the steeper slope will help with the increasing directionality (i.e., localization) of higher bass frequencies. Living in an apartment, economy of space is a high priority. It occurred to me to build a bass bin that would go under my old, large RCA console tv, exactly matiching the footprint of the oak swivel base. This necessitates a shielded woofer, which, as you may know, are now virtually unavailable. Thankfully Parts Express is again stocking the Dayton Audio RS180S-8, a 7" shielded woofer, which seems to meet my needs very well. I'll use two in parallel in a 65 liter box tuned to 29-32 hz with a 3" i.d. port (of appropriate length) for an extended bass shelf (EBS) alignment. With the two woofers sitting right next to the floor, I'm anticipating some room gain/boundary reinforcement will compliment the slight downward slope of the EBS alignment.

Now to my question, or my request for your advice. Considering a range of approx 30 to 110 hz with a 24db/octave low pass filter and a bass reflex box with sufficient bracing and wall thickness to limit wall vibrations (also, the two largest panels will be "sandwiched" between a carpeted floor and the base of the tv), what, if any, acoustic lining do you recommend for the interior walls. Internet information regarding acoustic lining for bass reflex enclosures limited to bass frequencies (e.g. subwoofers) varies considerably and is contradictory. For example: don't use any acoustic lining that could interfere with the desired resonance(s) of a bass reflex alignment, line half of the interior walls including the back with a layer of fiberglass, line with open cell foam, line with closed cell foam, line with egg-crate foam, don't treat the walls at all but instead situate some fiberglass/polyfill in the interior of the enclosure directly behind the woofer(s), etc., etc. I don't need to make the enclosure "act" like a bigger box -- a reason for sometimes using stuffing -- as I'll be able to comfortably get the 65 liters I need. I simply want the box to work per the design and for the bass to be clean. So... what's your advice?

Thanks in advance for any knowledgeable help offered.

David

[AdelaaR]

I have a pair of Statements here.
they use those exact same drivers, but in a 100 liter box.
They sound excellent, bass-wise speaking.
For bass reflex, I believe you do not want to stuff too much because you actually need the energy to reach the port. The idea is to make sure the cabinet walls can't resonate by making them thick enough and bracing them enough.
You also want some lining on the back of the cabinet, behind the drivers, to reduce direct reflections back to the drivers.
Rockwool is quite cheap and seems to work fine for me.

[fbov]

...what, if any, acoustic lining do you recommend for the interior walls. ...I simply want the box to work per the design and for the bass to be clean.

First off, you've described my mains - 65L NatP's tuned to 31Hz - so I can validate your design approach. Here's my standard box interior acoustic treatment spiel.

Different treatments have different goals, use the one that fits your need.

Surface treatments absorb sound, but better at higher frequencies, defined as wavelengths that are a significant fraction of the treatment thickness. A 100Hz tone with an 11.3' wavelength won't be affected much by a 2" surface treatment, while a 1000Hz tone with a 1.13' (13.5") wavelength will be.
- since subs are 100Hz and less, internal surface absorber may not have an effect, ported or not. It will, however, damp any harmonics from the driver.
- a 2-way speaker that crosses at 2KHz will have significant energy at frequencies where absorbers are very effective, resulting in audible benefit from surface treatment. It also damps standing waves in the box

Volume fill also absorbs sound, but again, with strong wavelength dependence. It is normally much less dense than surface absorbers, which provides a useful degree of flexibility.
- a sealed sub benefits from a high degree of fill due to changes in the effective volume seen by the driver. I leave the details to someone more knowledgable.
- a sealed 2-way benefits the same as a sub, but also from higher frequency absorption. While not as dense as surface absorbers, fill is a lot thicker...
- a vented 2-way benefits from fill at the ends of a tall box, and around the port base. It's detrimental when used to fill the volume (as in sealed) because it absorbs a portion of the box resonance.
- some special cases (MTLT, among others) use fill to damp the upper range while still permitting a degree of resonance for bass reinforcement.

Hopefully, by this point, you realize your bass bin is a hybrid - a mid-woof used like a subwoofer. As long as the box is << shortest wavelength, theory says you can leave it bare. Personally, I find carpet underlayment is very cheap, so I line everything with 1/2 to 1" of the stuff, and go from there, adding fill as- and where-needed. In this case, as I said, I used fill at the top of the 40" towers, and at the bottom up to the level of the bottom port, mainly to eliminate standing waves. The rest is 1" underlayment foam.

Make more sense now?

Too bad you didn't do this a couple years ago; these would have been perfect. I used 2 in 2.1 systems for my kids.

http://www.parts-express.com/pe/showdetl.cfm?partnumber=295-480

Have fun,
Frank

[dstmbgh]

First off, you've described my mains - 65L NatP's tuned to 31Hz - so I can validate your design approach. Here's my standard box interior acoustic treatment spiel.

Different treatments have different goals, use the one that fits your need.

Surface treatments absorb sound, but better at higher frequencies, defined as wavelengths that are a significant fraction of the treatment thickness. A 100Hz tone with an 11.3' wavelength won't be affected much by a 2" surface treatment, while a 1000Hz tone with a 1.13' (13.5") wavelength will be.
- since subs are 100Hz and less, internal surface absorber may not have an effect, ported or not. It will, however, damp any harmonics from the driver.
- a 2-way speaker that crosses at 2KHz will have significant energy at frequencies where absorbers are very effective, resulting in audible benefit from surface treatment. It also damps standing waves in the box

Volume fill also absorbs sound, but again, with strong wavelength dependence. It is normally much less dense than surface absorbers, which provides a useful degree of flexibility.
- a sealed sub benefits from a high degree of fill due to changes in the effective volume seen by the driver. I leave the details to someone more knowledgable.
- a sealed 2-way benefits the same as a sub, but also from higher frequency absorption. While not as dense as surface absorbers, fill is a lot thicker...
- a vented 2-way benefits from fill at the ends of a tall box, and around the port base. It's detrimental when used to fill the volume (as in sealed) because it absorbs a portion of the box resonance.
- some special cases (MTLT, among others) use fill to damp the upper range while still permitting a degree of resonance for bass reinforcement.

Hopefully, by this point, you realize your bass bin is a hybrid - a mid-woof used like a subwoofer. As long as the box is << shortest wavelength, theory says you can leave it bare. Personally, I find carpet underlayment is very cheap, so I line everything with 1/2 to 1" of the stuff, and go from there, adding fill as- and where-needed. In this case, as I said, I used fill at the top of the 40" towers, and at the bottom up to the level of the bottom port, mainly to eliminate standing waves. The rest is 1" underlayment foam.

Make more sense now?

Too bad you didn't do this a couple years ago; these would have been perfect. I used 2 in 2.1 systems for my kids.

http://www.parts-express.com/pe/showdetl.cfm?partnumber=295-480

Have fun,
Frank

Thanks, Frank, for the succinct overview re the use of acoustic lining/stuffing. I'm pleased that you responded, as it was your posts regarding your NatP's with an EBS alignment that seemed to dig pretty deep that inspired me to use the DA RS180S-8 for this application, after realizing that it was essentially the only shielded driver still available in anything close to a woofer. I understand all of what you said above. With my bass bin it would appear that primarily what I want to attempt to do is suppress the higher frequency harmonics that could exit out the front port (or through the drivers). So, it sounds like what you might do if you were me is line all interior walls with carpet underlayment and leave it at that. On the PE forum it was recommended to line all walls with Walmart convoluted open-cell foam mattress topper. Let's say I do that, should I adjust the cabinet volume at all , or target 65 liters as close as I can and tune to 31 Hz. If I can realize your previously published Unibox alignment with clean sounding bass I'll be very happy.

Thanks, Frank, for taking the time to reply.

[dstmbgh]

I have a pair of Statements here.
they use those exact same drivers, but in a 100 liter box.
They sound excellent, bass-wise speaking.
For bass reflex, I believe you do not want to stuff too much because you actually need the energy to reach the port. The idea is to make sure the cabinet walls can't resonate by making them thick enough and bracing them enough.
You also want some lining on the back of the cabinet, behind the drivers, to reduce direct reflections back to the drivers.
Rockwool is quite cheap and seems to work fine for me.

Thanks for your response, but are you sure that we're talking about the exact same driver -- RS180S-8 (7" shielded). I see that the Mini Statements originally used two of these in a 50 liter ported enclosure. I didn't find a design where two of these drivers "see" 100 liters. From what I could find 65 liters tuned to 30 Hz (give or take a Hz) for an EBS alignment was pushing the envelope about as far as one could. I can go bigger if bigger's better, but I know that it's not always. Thanks for any clarification.

[AdelaaR]

Oh yeah ... 180's ... I got confused ... sorry for that.

[fbov]

Thanks, Frank...So, it sounds like what you might do if you were me is line all interior walls with carpet underlayment and leave it at that..

Exactly. My Triskas, being passive radiators, have 1" of foam on all accessible internal surfaces. My Tempest X is in a sealed box full of fiberglass (the sub is in hand-me-down boxes from kgveteran, but he did it right).

...Walmart convoluted open-cell foam mattress topper... should I adjust the cabinet volume at all...

Just finished talking about this here (in the context of structrual vs acoustic damping):

http://www.htguide.com/forum/showpost.php4?p=556619&postcount=4

Glad to hear I was your inspiration. If it works as well as mine, you'll be very happy!

Just remember that "bigger" has it's own issues, primarily loss of cone excursion control at low frequencies. A 65L, 32Hz design has 20% overexcursion at 1/2 rated power, as those are the tradeoff points I chose. I don't anticipate they'll ever see full power sine waves, and in my opinion, this gives me sufficient transient capacity. A smaller box or higher tune will give you back power handling, but at the expense of the EBS alignment.

In contrast, my sealed sub is bulletproof at rated power, but it's got a 27mm Xmax and can handle ~8dB of boost at 16Hz... a different application than any RS180.

Have fun,
Frank

[dstmbgh]

Effects of air permeable damping/lining of bass reflex cab's &amp; their tuning

...line all interior walls with carpet underlayment...[or]...line all walls with Walmart convoluted open-cell foam mattress topper...[then]...should I adjust the cabinet volume at all?

I found some further information/testing relevant to my question. Few diy'ers have more experience building loudspeakers than Troels Gravesen. Here are his test results regarding adjusting cabinet volume and port length calculations based on real world outcomes. Very interesting and pertinent reading.

http://www.troelsgravesen.dk/ports.htm

http://www.troelsgravesen.dk/vent_tuning.htm

His results indicate that it would be inaccurate to say that lining a bass reflex cab with air permeable damping will not change apparent box volume. In his tests merely lining (not filling) the interior walls of a bass reflex cab with either 30mm egg-crate foam or Monacor MDM3 (35mm thick blend of sheep's wool and polyester) increased virtual box volume 15%, thereby lowering the box tuning. In addition, he finds that box tuning port calculators are never in accordance with real world measurements, and thus port length always has to be shortened to actually hit targeted Fb.

Interesting. Hard to argue with real world tests as opposed to theory, math that's supposed to be neat and tidy, or anecdotal experience.

Further comment welcomed.

David

[fbov]

...RS180S-8 ...essentially the only shielded driver still available in anything close to a woofer. ...

Only if you're considering classic ferrite magnets. Neodymium magnets are inherently shielded. Do a search for "subwoofer neo" and you'll find a bunch of options at PE, from the TB W5 through 21" monsters.

And remember that just because you can measure it doesn't mean it matters to your ears. Gravesen's analysis is interesting, until you look his method, and the degree of shift. Find Tom Nousaine's 1995 article for more, similar data. I think we'd all agree that the biggest effect occurs when you first cover bare walls, but after that... Can you hear a 1Hz change in F3?

Have fun,
Frank

[dstmbgh]

Neodymium magnets are inherently shielded.

Didn't know that. So are you saying that I could put a neodymium magnet woofer under my old console tv without stray magnetic fields affecting the picture tube? If so, just as well as I can expect with the RS180S-8's, you think?

While I haven't started making sawdust yet, I'm pretty settled with the design I have in mind -- using your EBS alignment for two RS180S-8 woofers. 65 liters net volume will work well enough under the tv, and, being a music only system paired with NHT SuperZeros, I'm far more concerned about the quality of bass from 40-110 hz than what kind of spl's I get at 30 hz. Therefore, even if a "subwoofer neo" will give me no problems with my tv picture tube, right now I don't feel too motivated to search for other alternatives. That is, of course, Frank, unless you have a specific driver in mind that you think I might ultimately be happier with.

Find Tom Nousaine's 1995 article for more, similar data.

Thanks, Frank, for the suggestion. Sounds familiar -- may have read this before. I'm just recently returning to this hobby after having begun it more than a quarter of a century ago. I was one of the original subscribers to Speaker Builder magazine, and Rick Craig drew up for me a hifi/PA speaker before he graduated high school.

Yes, I know, I'm being too anal retentive regarding making all possible adjustments to hit the targeted alignment as perfectly as I can. Confession: I'm one of those OCD diy'ers who in the past has submerged a driver (protected by cellophane) in water to calculate the air space displaced by the driver in the enclosure under construction. Sad.

Thanks for your help.

David

[fbov]

I've done a sim using the lowest Fs of the neo subs, the TB WT-1427G. At the same 65L volume, you'd tune ~10Hz lower and move F10 from the mid-20's to the high teens.

The bigger difference is in power handling; the TB's are bulletproof at 500W, 2x rated power, while the RS180S's are hitting their limit at 1/2 rated power. And cost... unless you like it low and loud, the sub option is probably not worth it.

Have fun,
Frank

[dstmbgh]

Frank, I really appreciate your helpfulness. I agree with you, for my purposes I don't think there's enough benefit to use the neo 10" instead. Two 7" will move a similar amount of air as a 10", and, mated with the SuperZero's for music only, I'll don't think I would see the benefit of either the extra power handling or the extra low Hz. Also, as you've noticed, I inquired further regarding your hint that neo's could be used instead of "shielded," and no one has suggested that I could expect superior results with a neo when it comes to not interfering with the CRT. Since the bass bin will be situated immediately underneath the CRT, I sure don't want to encounter interference after I've built them. If neo's were superior (regarding stray fields) then that may be a reason to pay a few extra bucks. I'm interpreting your, Wolf's and cjd's remarks -- "shielded" vs "inherently shielded" -- to mean that they are equivalent for my purposes when it comes to not giving me problems with my CRT.

So I think I'm ready to start making sawdust again, after my speaker building sabbatical. I found today that the local Sam Ash store carries acoustical foam, both one foot square sheets (wedge) and 2' x 4' sheets (3 different topographies), that I could use for lining if I want the psychological benefit of feeling like I'm getting superior results to what I'd have with Walmart's convoluted foam mattress topper.

Thanks again, Frank, for your help.

[BOBinGA]

David,
Having built a speaker with an RS180S-8, I can tell you that it's not a sub. It's a nice woofer, don't get me wrong, but a sub it ain't. I would strongly suggest that you listen to Frank's advice. A single Tang Band W8-1363SB 8" or WT-1427G 10" Neo Subwoofer would do a better job, but either will require some horsepower behind it. It's a shame that neo magnet prices shot up to astronomical levels a few years ago, but if you want a shielded sub, its still a good choice.

-Bob

[dstmbgh]

Having built a speaker with an RS180S-8, I can tell you that it's not a sub. It's a nice woofer, don't get me wrong, but a sub it ain't. I would strongly suggest that you listen to Frank's advice. A single Tang Band W8-1363SB 8" or WT-1427G 10" Neo Subwoofer would do a better job, but either will require some horsepower behind it. It's a shame that neo magnet prices shot up to astronomical levels a few years ago, but if you want a shielded sub, its still a good choice.

Bob, thanks for your input and for sharing your experience. At this point I have little interest in the sub-bass frequencies, but rather I'm most interested in the quality of the bass from 40-110 Hz. Remember, I'm pairing this bass bin with old NHT SuperZeros. If I spend too much to add bass -- the driver(s) and the "horsepower" to drive them -- i'd be smarter to instead purchase new more full-range mains. The SuperZeros still work well enough and suit my small apartment. My objective is to improve the music listening experience I have wth the SuperZeros with a monetary thriftiness that doesn't foolishly exceed the SuperZero's value. Plus, I also currently have the constraints of small apartment living: sub frequencies may not be "at home" with the space constraints, and the neighbors beneath me may not "enjoy" my creation of sub frequencies above their heads.

I'm staying open to all input (thanks for yours), but at the same time becoming more settled in my design choices. If I use two RS180S-8's with the Dayton Audio SPA250 plate amp and fbov's 65L/32Hz EBS alignment, then I think I'll achieve my objective without overshooting the mark. I'll have nice woofers in a nice alignment driven by a nice amp that blends nicely with my nice little (and very budget) old SuperZeros. And, hopefully, no less happy neighbors than I have now.

[BOBinGA]

The RS180s should do a nice job of filling in the bottom. In the speaker I built, I only used one and tuned to 40hz. Two drivers and a lower tuning will certainly help get the most out of them. Send pictures.

-Bob