Driver Selection With WinISD Data... PLEASE Help A Complete Newb!!

[srosenberg]

good day,

i am attempting to design bass modules - i hesitate to call them subwoofers, as i'm not looking for REALLY low extension - to extend the range of my monitor speakers in my 2 channel audio system. the cabinets will be sealed and need to be pretty compact, a close estimate on volume is around 17 L (~1000 ft3). i'm hoping for strong output from around 35 hz - 80 hz, and based on my reading, a Qtc not in excess of 0.65 (which i'm finding difficult given the cabinet size, even if i factor in polyfill).

i've entered the data for around 40 drivers in the 8" - 10" range into WinISD and have been analyzing the results to determine the best options. there are some speakers like the ScanSpeak 23W/4557T that have extraordinary output (-3db at 35 hz in a 17 L sealed box), however, the Qtc is 0.89, which i've read will be boomy sounding (great for theater, less ideal for music). Then there are drivers like the Acoustic Elegance TD10M which yield a system Q of 0.51, however, have rather limited extension (-18 db at 35 hz).

the way i'm evaluating suitability of drivers is to find one that offers reasonable extension and a low Qtc value. using the WinISD program, i'm recording the Qtc value at 17 L and 22 L (22 accounts for polyfill) and the -3db, -6db, and -10db frequencies, as well as the output at 35 hz.

is this a reasonable approach?

are there other variables i should be looking into, too?

what targets should i be shooting for where the Qtc, -3db, -6db, -10db, and 35 hz results are concerned?

this is my first time at attempting anything like this, and i've got a pile of data - i'm just unsure how to interpret it to make a reasonable decision. HELP!!!

any inputs would be MUCH appreciated.

[srosenberg]

ps, i realize specs and data don't tell the whole story all the time, so i'd also be appreciative of any recommendations for drivers in the 8" - 10" range that you think would work well in this particular application.

THANKS!

[Tytte71]

Could you please add a little info on the complete speaker/amp system, what kind of room you will use it in... Does it have to be a paticular kind of design, such as sealed cabinet, etc?

[srosenberg]

Originally Posted by Tytte71

Could you please add a little info on the complete speaker/amp system, what kind of room you will use it in... Does it have to be a paticular kind of design, such as sealed cabinet, etc?

my main amps are 70 WPC tube monos; speakers are 91 db efficient monitors that play down to 60 hz; room is 16 ft x 18 ft.

my plan is to high pass the mains / low pass the subs at around 80 hz. i am not limited by cabinet type (sealed, etc), but am limited by cabinet size - the width can not be much more than 12", the dept can not be much more than 18", the height can not exceed 10". i'd like to keep them as short as possible, so if a 10" driver is used, they would need to be downfiring (an 8" driver could be down or frontfiring).

thanks for any inputs.

[revboden]

so... maximum working volume of .73 ft^3 huh. super mini sub, i like.

two of these in series would be ok and very inexpensive. 6-1/2" Subwoofer Speaker 299-114

edit: wait... you're not trying to pressurize a room that size with drivers that small are you ?

[Tytte71]

In EU terms your room is 4,9m x 5,5m and the maximum outer dimensions of the sub is roughly 30x48x25cm (wxdxh). To make things simple, we could say you use 20mm MDF sheets for the cabinet building. That would leave internal dimensions of 26x44x21cm which corresponds to 24l. I would say that your 17l might be a tad on the conservative side (including space for bracings and driver). Doesn't matter really - the volume is small, but there is hope - at some cost
You mentioned that you are attempting to design bass modules. I read that as you could allow at least two of these babies?
Also, I read between the lines that you are going to use separate sub/bass amp(s)?
Another thing - you should decide for what maximum sound pressure you are designing for. I assume your main speakers with the tube amp have the capacity of producing some 110dB. My recommendation would be to to design for your maximum expected sound pressure and then add some (minimum) 6dB headroom.

To achieve loud distortion free bass in such small cabinets there are in principle only two practical alternatives:
1. Sealed cabinet... needs a driver that have huge xmax and your would also need massive amount of power. Long throw high power woofers are very demanding to make good and it often costs a lot, but parts-express sells the TC Sounds Epic 10 at a nice price. You could pair this one with a 500-1000W class D amp. Eq-ing the lower octaves and using a subsonic filtre should leave you high quality bass at some 102dB/30Hz in room (two subs and you get 108dB). You even have plenty of headroom within these data. If you get hold on second hand Audiopulse RV10D2 you even have a better driver with a very linear motor suffering little from power compression.
2. Passive radiator... substitutes BR loading. Challenge with BR loading is two-folded. When designing a BR loaded cabinet the port tuning should be set as low as possible to avoid poor impulse response and to leave Group Delay into the none audible region. A nice start would be a port tuning of about 20Hz, but in such small cabinet the port would be extremely long. The solution is to use passive radiator. This is more or less the same as using a BR-loading, but you should keep the tuning frequency of the passive radiator(s) as low as possible. You gain increased sensitivity, lowers xmax and/or can use a smaller power amp. There are several drivers that could be put in a small cabinet for this purpose, but you could try simulating with the brand new Seas L26RO4Y and report the results

I would recommend you to google and download the Excel sheet Unibox made by Kristian Ougaard. It is very easy to use and you can model SC/BR/PR cabinets in paralell. The BR modelling is pretty good. Simple and free Later you could install more advanced software.

[srosenberg]

Originally Posted by revboden

so... maximum working volume of .73 ft^3 huh. super mini sub, i like.

two of these in series would be ok and very inexpensive. 6-1/2" Subwoofer Speaker 299-114

edit: wait... you're not trying to pressurize a room that size with drivers that small are you ?

super mini - YES!
sub - well, as close as i can get given the diminutive cabinets.

i'm just trying to add as much bottom end as i can to my monitors. they are wilson audio watts, and play very nicely down to the mid to upper 50's... i'm hoping by adding a pair of 'bass modules' under each (like a mini puppy) i can improve the low end.

i've checked 40+ drivers in WinISD to see which will give me the best extension and strongest output while maintaining low Qtc. using these variables, i've identified several options - it's just a matter of determining what the right balance is between Qtc, Extension, and Output. are there other variables that i should consider, such as Xmax or Le, that will help narrow in on the best option?

here's my 'short list' so far...

[srosenberg]

@ Tytte71...

thanks so much for the detailed response. yes, the plan is to make two of these 'little' subs - one to pair with each of my monitors. and yes, you have surmised properly that i will be using separate amplification. my initial plan was to mount a compact plate amp onto each cabinet - the 300 W O Audio plate amp is small enough to fit nicely, though it sounds as if more power might be necessary.

"long throw, high power woofers" - does this mean i should be looking for a driver with high Xmax and Pe specifications?

[Tytte71]

Originally Posted by srosenberg

@ Tytte71...

thanks so much for the detailed response. yes, the plan is to make two of these 'little' subs - one to pair with each of my monitors. and yes, you have surmised properly that i will be using separate amplification. my initial plan was to mount a compact plate amp onto each cabinet - the 300 W O Audio plate amp is small enough to fit nicely, though it sounds as if more power might be necessary.

"long throw, high power woofers" - does this mean i should be looking for a driver with high Xmax and Pe specifications?

High xmax/Pe is important, especially for subs in small cabinets- otherwise they will run out of juice pretty early.
It looks like you have got a very nice amp/main speaker setup, so I would recommend making subs that justifies the setup. 300W could be fine when dealing with passive radiator (or BR) systems, but for sealed cabinet it may become a little on the low side.

There are some other factors you should also keep in mind;

The motor system is important for concluding on the capacity. Unfortunately most (hifi driver) manufacturers don't show large signal analysis in their data sheets.
What happens is that as you increase power and consequently the stroke increases, the motor becomes more and more asymetric (behaves different in the +/- direction) and non linear. Have a look at e.g. Dayton Titanic woofer at Parts-express... PE shows some large signal analysis for these drivers.
Also imagine that you have a driver that is 1% efficient (which is fairly high for these kind of drivers). This implies for each 100W that is fed into the driver, 1W is converted to sound effect while 99W have to disipate as heat in the voice coil arrangement. Heat can severly affect the linearity of the motor and lead to thermal compression.
Selecting a subwoofer with high power capacity is a good rule of thumb. Look at typical high power woofers and study voice coil construction, choice of voice coil former, how the gap and shortening rings are arranged, etc.
In these very small chambered cabinets with high stroke drivers, the cabinet pressure will soon be pretty high. Sealed cabinets deals with high pressure but it gets even worse with BR and passive radiator systems. To avoid deformation of the woofer diaphragm, ultra stiff diaphragm is needed. Therefore aluminium diaphragmis is often preferred on these kind of drivers. Alu is also good for distributing heat from the voice coil. High pressurised subs demands very rigid cabinets. Don't save money on glue, srews or other remedies that hold it together
Then, some woofers are quite, others not. With this I mean there are some woofers that makes eigen sound when pushed heavily at low frequencies. This is also information that rarely can be found at manufacturers' sites.

Conclusion to this... little is found in manufacurers data - ask for others experience with specific drivers and you have a much better chance to succeed on first try

[sreten]

Hi,

Wilson Audio WATTs are nearfield monitors. They not accurate farfield
if used on stands away from walls. Placing near boundaries improves
matters bass wise but messes up nearly everything else.

The PUPPY is not just a subwoofer, it works in parallel with the WATT
to turn it into a farfield monitor. Adding a sub here as described isn't
going to address this fundamental issue.



See : A Speaker project it shows the general principle.
and the stereophile measurements section for the various Watt/Puppy
versions, you'll see its not really that simple as the above implies.

What you need first IMO is something like this on the amplifier input :
Baffle Step Compensation


Figure 3 - The Baffle Step Correction Circuit

Yuo can try and make your subs do what the PUPPYs do,
but the above seems a simpler and more flexible solution.

Stereophile indicates the WATTs are tuned to 40Hz.

Unless high bass volumes are an issue REL recommend running the
main speakers full range, and for valve amplifiers running their
subs from the outputs taps of the amplifier, not a line level.

REL Acoustics : Great Britain's most acclaimed Sub Bass Systems click on "pro's".

You really should be using WinISDPro to model excursion and power.

Driver parameters don't really tell the whole story, especially distortion.

See the Dayton 8" here : Zaph|Audio
Dayton Audio RSS210HF-4 8" Reference HF Subwoofer 4 Ohm 295-456

Should suit your case, using a well stuffed sealed box.

Carefully model and choose your the correct sub amplifier, make
sure it can be set low enough if the WATTs are run full range.

Note that its possible to EQ the sub to any response within its limits,
and many real world subs use this approach, read up on Linkwitz
Transforms, loads of power in small boxes is used to extend the
bass by EQ. Note Max SPL sealed depends on driver size and
excursion only, as long as you have enough power.

e.g. see Pluto + subwoofer

rgds, sreten.