first diy project, help needed

[darren700]

i am building my first diy subwoofer. i have a Stereo Integrity Magnum 15" subwoofer in my car presently in a 3.5 cuft @ 28hz box in my car. im considering selling all my car stereo equipment except my sub which i want to use in my home theater (axiom m60's, vp150, qs-8's) im not sure however if the parameters for this sub make it good for home theater.. i was thinkin of building a 8.0 cu ft at 21hz box for it, based on the parameters would it make a good sub for ht, and would that box size be good? i was plannin on using a single 6" flared precision port, and firing it frontward along with the sub, it will be powered with this: dayton 1000watt amp
and help and suggestions are greatly appreciated

[ThomasW]

Hi,

I saw your post on AVS.....

IMO, 20Hz tuning is a bit high unless this is a music only design.

For HT you probably should consider a tuning around 15-18Hz. That will protect the driver from unloading with most of the special effects recorded in DVD soundtracks.

[darren700]

well i was plannin on using 18hz but the manufacturer recommened against that, as well as a 8ft box.. what about 5 cu ft @ 18hz?

[WillyD]

ThomasW is right, of course.

10ft^3 tuned to 15hz actually looks great, as far as being an "LLT".

[darren700]

i dont have the room for a 10 cu ft box...
im torn between 8cu ft and 4 cu ft right now, both with 18hz tuning probably, would i get somewhat flat response with 18hz? or would i have to heavy eq it? (getting behringer feedback destroyer as eq)

[ThomasW]

There's an issue with the driver in a ported box 'unloading' (flapping) when frequencies below the Fb are played. As a result it's best to use the lowest 'reasonable' tuning point. There's a ton of sound effects in the 15Hz range in movie sound tracks.

Don't fixated on seeing a flat response with a computer sim. Just like cabin-gain in car subs, home subs gain output at lower frequencies from room gain.

[WillyD]

Definitely go with the bigger box (8ft^3) and 17-18Hz tuning.

[darren700]

half the people say 4 ft^3 the other say 8 ft^3, half say 22 hz, other half say 15 hz, im really lost here as what to go with
and since nobody has used this sub for ht yet, theres nothing to go by. i dont want to end up having to build two boxes. if i tuned to 18 hz in a 5 ft^3 box, the 6" port would have to be 42" long.. too long, as i plan on using these for ports: http://www.thrilleraudio.com/detail.aspx?ID=536 i would just buy extensions, but i want to keep my box height at 39.5" to match my mains height, so 42" of port simply cannot be done.

5 @ 18 = 42" port too long
5 @ 20 = 33.54" port tuning too high?
8 @ 17 = 28.43" port models well and is perfect length for port + extension
8 @ 20 = 19.32" port models well, but tuning too high?

as you can see im very unsure.. the manufacturer recomemded staying within 4-5 ft^3 and around 20hz, but is there any reason why i cant go bigger / tuned lower? would it be because of the driver parameters that cant handle it?

thanks alot

[darren700]

this is what nick from stereo integrity wrote to me:

"8 ft^3 is totally un-necessary for a Mag 15. Going with an enclosure that size is only going to get you a lower F3 on paper (i.e., WinISD). In your listening room the differences in 4-5 ft^3 tuned to
25-23 Hz and 8 ft^3 tuned to 18 Hz is going to be negligible. You might get a few cycles lower with the 8 ft^3 box but you will sacrifice the upper bandwidth of your subwoofer system in order to do so. If you plan on blending the Mag 15 with tower speakers I highly suggest staying within the 4 to 5 ft^3 range. WinISD is a moderate enclosure modeling program for anechoic chambers - even in a large living room in a house there is transfer function, which aids in the extension of a subwoofer system. That, and unless you have a stack of Telarc albums/CD's that you routinely listen to, the 18 Hz tuning won't help you out for 99.9999% of music and/or movies."

[WillyD]

Nick obviously hasn't pulled up WinISD or Unibox to see how 8ft @ 17Hz really models.

Technically, the F3 is higher than the smaller 5ft box tuned to 20Hz (or higher but that doesn't matter because room gain would help fill in the low-end to acheive a more naturally flat FR.

The 8ft^3 tuned to 17hz would basically be an EBS sub.

http://www.diysubwoofers.org/prt/ported5.htm

Adrian D on soundsolutions forum is wrong. Transient response wouldn't be worse with the lower tuned/larger sub, if anything it would be better. You also have to remember that if you plan on going with that Dayton 1000W amp, you'd be better off with the extend bass shelf as oppose to on that slowly rolls off because that Dayton amp has a HP built in.

I forgot where, but somewhere in the 18-20Hz range.

[ThomasW]

I'm sure Nick's a nice guy, I'd be willing to bet I've been designing and building subs longer than he's been walking the planet....:wink:

We're not talking about a few isolated music CD's. We're talking about subsonic sound effects in movies.

For example Blackhawk down....



War of the Worlds......



At high SPL either of these would destroy a sub tuned to the low 20Hz range.....

[darren700]

the dayton amp includes an adjustable parametric eq, but i plan on pairing that with a behringer feedback destroyer and my trusty radioshack spl meter to get as close to flat response as possible

[darren700]

Quote:
Originally Posted by mrogowski at AVS
"Darren, If your scope has changed and you plan to use an EQ, then it doesn't matter too much which way you go. You can boost where you want and cut where you want. That's the magic of using an EQ.

If 8 cubes in your room fits ok, then go for it. Whether its 4 or 8 you can always compensate with added electronics like the Behringer. The manufacturer is simply telling you that you won't have to. I think he knows his designs better than anyone here...

Best,
Mark"


so would it be better to go with the manufacturer's recommendation of 4 cu ft @ 20 hz?
would i lose any low end at all as compared to 8 cu ft @ 18hz after i have eq'd it?
or does eq pretty much work magic and allow me to make it flat to 15-16 hz no matter
what? also and i looking at the right type of eq? does the behringer feedback destroyer
pro look like a good option for what im trying to eq?

[Brian Walter]

Darren,

There are two different aspects of design being discussed here that you need to sort out. One is how much low end you really need, the other is protection of the subwoofer below port tuning frequency. Nick is probably right in that you likely won't hear that much difference between a 23 hz tunning and an 18 hz tunning. But Thomas is also right in that you need to protect the sub from really low frequencies which are prevalent in HT. For example, at high spl, a 15 hz signal played through a box with a 23 hz tunning could destroy the sub, but with a 16-18 hz tunning, the sub would probably be fine.

In either case you really should provide a high pass crossover to remove the really low frequencies below tunning. The problem is that the HP crossover needs to start rolling off before the frequency that it is designed to protect, thereby rolling off your low end response. So the lower you tune, the lower the crossover can be set and the lower your end response.

As always, it's all about trade offs, I suggest building the largest box you can justify and then tune it as low as practical. With that much excursion, you probably should look at no smaller than a 6" port and you should keep the length under 36" or so to minimize port resonance problems. I don't know much about the port resonance stuff, so others can chime in here and help you out.

Brian Walter