Subsonic Shiva, an unusual design... feasible? (1 Viewer)

[Michael R Price]

A friend of mine is looking to extend the frequency response of his old Advent Loudspeakers well into the subsonic region. He has a very small room, about 7x14x8 feet, and is looking to do this without adding a crossover to the Advents, which extend to around 40Hz before rolling off. Extremely high output (over 105dB or so) is not a concern. He also has a very limited budget. To do this we have envisioned a very unusual design, which uses a single Adire Shiva driver in a very large, 265-liter, box, tuned to an extremely low (11 Hz) frequency. After some tinkering with the LspCAD demo, we have been able to get an extremely flat (predicted) response from around 10 to 35 Hz, which is exactly what he needs. (The reason I believe that this works is that his small room has very high low-freqency gain which compensates for the rolloff of the driver between its peak and the tuning point.) What I am asking is if this design really can work and how it would perform. I find it hard to believe that a value driver like the Shiva can be applied in such an unusual way and still perform as well as other, more conventional (Adire, EBS, etc) Shiva applications.
Here is a description of the planned design.
Adire Shiva driver
Parts Express 110W plate amp
2 lb. polyfill
20x20x54in. MDF box, with two main braces extending vertically through the box, along with smaller braces on the horizontal plane to support the main braces. 4" diameter, non-flared, 24" long port tube for a tuning of around 11Hz. The box volume, with driver, amp, port, and bracing taken into account, is about 265 liters.
I have entered the parameters into LspCAD (QI=10, Qa=30, fill=25%, 24" with 4" ID port) and this is the result. The graph assumes a 2nd-order low pass filter at 40Hz, which is the lowest setting on the cheap Parts Express sub amp (and should provide for the best blend with the Advents, which already have good bass extension in such a small room).

[Large summary graphic link]
[LspCAD demo boxfile link]
Are there any opinions on what can be done to improve the performance of this planned subwoofer, or on whether the design is feasible at all? Again, this is a very unusual design, at least from what I have read so far. Thanks for your input.
[This is my first post at the Home Theater Forum, though I have been reading the forums once in a while for a few months now]

 

[Patrick Sun]

Have you considered the Tempest driver? With that much internal volume, I think you could squeeze some more low end performance and have it be more robust in handling the subsonics.
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[Scott Simonian]

I was going to say the same thing. It shouldnt be that much more expensive. So it is do-able.

 

[Kyle Richardson]

Exactly. In a 265L enclosure the Tempest will feel right at home.
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[Philip Ries]

I'm Michael's friend, the one intending (or certainly hoping) to build this monster of a sub. I decided to register and reply since he had to go somewhere.
We were just discussing using the Tempest instead of the Shiva (I found a way to make more use of the boards of MDF to make the box about 340L)...plugging the Tempest into LPSCad shows an increase in output but not much change in response. How big (well, noticeable) will this increase be? Or would it actually be better than the Shiva in response, like if LPSCad is not so accurate? Now that I look at the graph some more, the Shiva's looks flatter and even a little louder around 10Hz...
By the way, sorry if I have any misconceptions...Michael is more technically knowledgeable than me about this.

 

[ThomasW]

I'm sorry but regardless of what the software "sims" say, you're not going to be able to get a any useable 10Hz output from a single Tempest or Shiva driven with a 110 watt amp.
My big IB has "reasonable" output down to 15Hz = -3db at 20Hz before adding EQ. It uses 12 Shiva's and the output is -5.7db lower at 10Hz. So for all intents and purposes the 10Hz output isn't usable. If you look at the IB output data on the Link Removed website. Those measurements were made nearfield with a $3000 calibrated B&K 4133 mic. If you look Link Removed you'll see CLIO plots of both the 2 IBs before and after applying parametric EQ. These plots were made with a less expensive calibrated mic.
So if I don't get usable 10Hz output with the big IB driven with a large pro amp, you certainly aren't going to get any with a single driver driven with a 110 watt amp.
Also there's almost no software that has information this low.
[Edited last by ThomasW on July 25, 2001 at 06:01 PM]

 

[Mark Seaton]

I think there is even a more important factor to consider here... That 110W plate amp from Parts Express will never put out much of a signal at 10Hz! It's response is boosted 5dB at 35Hz, and then will begin to roll off below that point unless the amp is modified. Finally, you really can't percieve bass anywhere near that low in frequency unless it is at quite high levels where you can at least "feel" it.
If you have the space to go to 340L, go with a Tempest and tune it to maybe 14-18Hz.
Also, with the room-gain issue, you do have to consider the construction of the room you are in. If it's an old dorm room with cinder block walls, then you will indeed get a lot of room gain, but realize that in general, most rooms begin to leak heavily at very low frequencies.
In reality, I would suggest just going with a Tempest in a large sealed box. It should match up nicely to the small room. Otherwise, consider backing down the box volume to about 200L and tune to maybe 14-16Hz which could match up nicely to your room. I would tune to 16Hz and go to a 6" diam. port.
Mark Seaton
[Edited last by Mark Seaton on July 25, 2001 at 05:36 PM]

 

[Scott Simonian]

Shouldnt he make an EBS system? Especially with the room gain and all.
Just a suggestion.

 

[Michael R Price]

It seems as though the Tempest would be a better choice because it has more excursion capability and thus can be driven harder at those subsonic frequencies.
According to LspCAD the response is flat all the way to 10Hz. I would assume this has to be because of the port tuning (sealed box doesn't look like it will cut it for Phil's purposes).
I was under the assumption that the 5db boost at 35Hz could be switched off, allowing the amp to deliver a pretty flat response. So this feature can't be defeated? Seems pretty dumb to have something like that permanently enabled.
Tuning to a higher frequency seems to create a huge peak in response especially with a box of that size... so is it worth trading this for more output capability in the more 'normal' 15-25Hz range?
I read somewhere that perceived sound quality improves when you have better frequency extension, because of phase shifting at higher frequency ranges (which frankly I don't understand at all). That article which mentioned this can be found athttp://www.theperfectvision.com/howto/howto_tech_3.htm and I think that this sub design would not be quite as useful if that claim was not the case. We have also considered using an EBS (or similar) alignment and placing a crossover on the Advents to lighten their low-bass load; would this be an overall better idea than trying out the subsonic design? (The Advents have surprising bass response for a sealed box, however because of his room, the speakers or both there is a serious boominess problem that that would probably solve.)
Again, thanks for the help, we are looking to make this an enjoyable and somewhat succesful DIY project.

 

[Greg Monfort]

Since plaster cracking isn't an issue and the room is small, why not a small sealed Shiva or Tempest with an LRT to adjust its slope to the room?
If you want big for the bragging rights, then a 340L/15Hz EBS maximizes bottom end bang for organ music/99.9% of the special effects on CD/DVD with only 110W and gives spot on transient response for music.
GM
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Loud is beautiful, if it's clean

 

[ThomasW]

quote: According to LspCAD the response is flat all the way to 10Hz.[/quote]
No, please remember the computer program creates a simulation. It doesn't necessarily represent real world performance at 10Hz.
quote: so is it worth trading this for more output capability in the more 'normal' 15-25Hz range?[/quote]
There isn't going to be any usable output at 10Hz, so yes it would be a very good idea to target a more reasonable frequency response.
quote: ...Advents to lighten their low-bass load; would this be an overall better idea than trying out the subsonic design?[/quote]
Yes that's a very good idea. The Advents will last longer and you'll get better quality bass.
quote: 5db boost at 35Hz could be switched off, allowing the amp to deliver a pretty flat response. So this feature can't be defeated?[/quote]
I'm sure Dan Wiggins (Mr Adire ) has information regarding how to do that. It does require doing some surgery to the amp. Checkout this thread Dan discusses performance issues with this amp. If you've had it less than 30 days, I'd suggest returning it and looking for a suitable replacement.
Also please give serious consideration to Mark's suggestion regarding the use of a 6" diameter port. If you use a single straight 4" port there's going to be a ton of port "chuffing". If 6" is too big then atleast use dual flares with 4" material
[Edited last by ThomasW on July 25, 2001 at 09:50 PM]

 

[Philip Ries]

Thanks for addressing the main issue, the practicality...it just didn't seem possible for a driver to hit 10Hz accurately in a project so cheap as this one. I'm glad we came here to discuss it before going ahead...now we'll have to fix the problems as much as possible.
Issue #1: Amplifier
We'll have to look into the capabilities of the amplifier...if it's really a large problem that can't be avoided, we'll need to find another. Does anyone know more about it?
Issue #2: Subsonic Reproduction
I must say, I doubt the sound at 10Hz is really necessary. I agree that there's little to no information there...and like Michael, I don't claim to understand why EQ'ing a subwoofer to be flat to 5Hz (see the link he posted) could really improve the sound above 40, if I recall correctly. But in any case, I think that rather than deciding that there's no way a Shiva or Tempest with this amplifier could touch x Hz, we need to figure out what frequency it will touch. If the simulation program is inaccurate, which I certainly don't doubt for so unconventional a project, then...well, the only superior resource I have is your experience (I can't say I have too much of my own anyway). So, how effective will this design actually be, and what can be done to improve it besides changing the amplifier?
FYI, I live in the basement of the house, if that makes any difference (concrete underneath a rug). And I haven't bought any of the components yet.
Thanks again, guys.
PS: Just read ThomasW's last post. Thanks for the link to that thread, it should be interesting. I'm beginning to doubt this amplifier more and more...I think I'll wait and see how much the mod to the amp helps - clearly it's not a defeatable feature.

 

[ThomasW]

Philip
LspCAD is a very good program, much better than most.
I'll double check your model vs LspCAD Pro and see what happens
[Edited last by ThomasW on July 26, 2001 at 12:24 AM]

 

[Philip Ries]

Well that was certainly fast. But about the graphs...they're pretty close to being flat to 10Hz. I have a crossover at 40Hz so that gradual climb is a faster rolloff. The other difference is probably the room specs. In my room, as frequency increases, the sound actually decreases even without a crossover (at least with my settings - maybe yours are better...in fact they probably are, as I'm sure you're much more familiar with the program than I am). I'm fairly certain that the extension to 10Hz in my model is also due to the bass gain in the room, but again, yours could well be more accurate.
Still...that graph doesn't look too bad to me. I'm thinking it's flat down beyond what I need. So the big question is, do you think it's an accurate simulation?
I'm pleasantly surprised that you took the time to do this. Many thanks!
[Edit]Just so you know, this post is referring to the graphs in the one below it...I originally replied to the blurry graphs he put in the post above this one.[/Edit]
[Edited last by Philip Ries on July 26, 2001 at 11:03 AM]

 

[ThomasW]

Hi
Here ya go, sorry I was trying to get better (less fuzzy)copies, but ran out of time.
And no the computer sims simply aren't accurate at subsonic frequencies.
Link Removed
For comparison to the sim, here's a sine-swept realtime plot from my 4 Shiva IB. 4 shiva's are approximately equal to 2 Tempests. This is a nearfield sweep so minimial room gain is seen. The measurement mic used for these is about -1.5db at 20Hz. Notice there is no usable output at 10Hz
Link Removed
Here's the same system again nearfield but after adding 3 bands of parametric EQ. Again no usable 10Hz output
Link Removed
[Edited last by ThomasW on July 26, 2001 at 08:58 AM]

 

[Nathan Bower]

ThomasW,
I think that you have problems if you cant feel the effects of 10hz with 16 shivas.
I have one NHT 1259 in a sealed box f3 at 35 ish, driven by a modified (no boost) KG5150. I play a 10 hz test tone - I feel the air wobble.
12 shivas (of greater VD) driven by a better amp, and possibly in a better configuration should be able to do almost 5 times as much "damage".
Also remember - the driver is NOT the limiting factor at 10 hz, as the port tuning frequency is about 11hz. The PORT will be performing all the pistonic motions.
Projecteers,
If you want me to show you how to defeat the bass boost in the 5150 - I can probably remember.
The course of action that I would reccomend: get a tempest... stick it in the 360ish litre box, external amp, and "tune" the port by ear. you cant go wrong!
Want less volume? Stuff some bricks in the box.
Want to change the port tuning? Get the hacksaw out. Start low though!
LSPCad has given encouraging predictions, now you should make it your goal to see if these are accurate or not! Sod what other people think, they are only giving predictions too. Tempest = good driver, once youve made that descision I really think you would have a job making a bad sub.
Nathan
PS. One last thing. Because of verrrrrrry low tuning frequency I would reccomend lots of surface area for the ports... dual / quad 6 inchers? You may find that these will fill most of your 360 litre cabinet up!

 

[ThomasW]

Nathan
Believe me the entire 2500 sq ft brick house literally quakes, when the 12 Shiva IB is fed a 10 Hz sine wave

. But for real world applications (meaning a DVD soundtrack) there's not much there. And any output level at 10Hz is so comparitively low to the levels at other frequencies, it's drowned out by higher octaves, 20Hz, 40Hz etc.
[Edited last by ThomasW on July 26, 2001 at 09:12 AM]

 

[Nathan Bower]

Tom,
Ok I understand your drive of thaught now... Fair enough.
Still there are _some_ cd's with 5hz information - im sure dvds will get there in the end.
Roll on the sub-harmonic EQ!
I still say the boys have a go at their proposed design. Ive always wondered what happens if you drive a sub to x-max at the port tuning frequency. Presumibly its pretty loud... smacks of an explosion?
N

 

[Patrick Sun]

I keep asking myself, "do people just pop in a test tone CD and just listen to 10Hz tones all day long?" Really, how many DVDs/CDs have usable 10Hz material?
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[DanWiggins]

Hi all,
I've experimented with this type of bass before. I built an "interesting" 4th order bandpass box that was 3 dB down at 7 Hz (flat to around 10 Hz). IB for the sealed section, 2 cubic feet tuned to 12 Hz for the front section. Bandwidth was ~7 Hz to around 40 Hz. Used a Shiva for the driver, and a pair of 10 foot long 4" diameter vents (all mounted in my crawlspace, vents exited through a small hole in into my listening room).
Anyway, it took about 100W to drive the Shiva to its limits, and I measured output of ~88 dB SPL from 10 Hz and up. So I was getting the results very similar to what you're predicting, Michael and Philip! (BTW, the predictions from LspCAD we're nearly spot-on, versus measured in-room performance. I think I measured about 1 dB more at 12 Hz and about 1 dB less at 30 Hz than LspCAD predicted, but still it was well within +/- 1 dB of real world).
The problem was I could feel it sometimes (on certain tracks), but could never hear it. Blame our ears for that - a quick look at the old Fletcher Munson curve will tell you why. We need a good stout 80+ dB down around 20 Hz to just be audible. You'll need considerably more than that to be audible lower! Figure around 95-98 dB SPL at 10 Hz to just be audible.
AND, it will be completely out of proportion with the rest of the music, too. You'd JUST be able to hear the lowest notes at full-on output from this sub-subwoofer. But the rest of the system would have to be barely running, to match it. I think this is what Thomas is alluding to - the output will be there, it will just be unusable. Too little output (now, with 8 of these going, you should have some usable output down to the low teens!).
Yes, you could feel some interesting things/vibrations down there, but all in all, I wasn't really that impressed. It's been yanked and taken apart now. If I want that "air-shaking" or "couch-shaking" effect, I'll just build some shakers.
Anyway, if you build with some cheap materials, I'd say give it a shot, and try it out. Can't take too much effort and cost if you just crank out a demo/test box. And it will be quite instructive to see just what 88-90 dB SPL at 10 Hz feels like.
Dan Wiggins
Adire Audio