First time subwoofer design questions

[unclejunebug]

Yeah, that's typical. In real life the amount of content down there is minimal at best and there's nothing to worry about.

Excellent, thanks for the confirmation!

Without any design experience of my own I have no reference point for how the graphs/models translate to real world performance so I appreciate everyone's help and patience. As you may have noticed I have a tendency to over analyze things

I think its high time I stop attempting to change up the design and just get started though. I'm making some saw-dust tomorrow!

[wogg]

Yeah, that's typical. In real life the amount of content down there is minimal at best and there's nothing to worry about.

[unclejunebug]

I realized that I had applied a high and low pass filter to my WinISD project. After turning them both off the excursion graph looks far worse under 27Hz or so. Though I'm not really sure there's any way around this without a high pass filter of some kind which the SA70 doesn't have. Is this just a typical result for a subwoofer design?

[LOUT]

17mm! Don't blow anything on my account haha. I'm using the SA70 but I haven't come across anything in the documentation that states there's any low frequency protection.

No worries, that was all a while ago shortly after I bought the first one.

Don't lean on my rough estimate too much though...probably safer to assume somewhere between 12-16mm Xmech and either leave the amp about 3db down from full if you'll be blasting movie sound-effects or think about adding a small 0.5uF-0.75uF capacitor/s to the line level inputs (they're listed as 10.4Kohm in the manual, so I think a cap value around 0.5-0.75uF should give a nice bit of protection for bass under 30hz-20hz). Because a Y splitter/combiner RCA (or stereo sub line-level output from your reciever) is recommended for the SA70, I'm not sure if you'd use a pair of 0.5-0.75uF caps or if you'd need to use half value if capping both inputs....or you could just cap the single output from the reciever if you're only using a single mono output I suppose.

Probably not really a problem worth worrying about in the first place, but it's an option if you're really nervous about damage and plan to crank it with really deep movie-sound-effect lows.
I think bass from music this low and this loud is so rare that it's not even a concern at 70watts or less.

[unclejunebug]

I tried to do a little testing with an SD215a-88 using low frequency sine waves, a mixer for more visible level setting, and a modest ProAudio amplifier with low-frequency protection disabled. I think I got roughly around 17mm of excursion without any super terrible noises or obvious breaking, but I was too nervous to keep pushing my luck farther.

17mm! Don't blow anything on my account haha. I'm using the SA70 but I haven't come across anything in the documentation that states there's any low frequency protection.

[LOUT]

Thanks for the responses, so distortion will go up a little at high volume but no damage to the woofer will result.

Is Xmech a calculated value? I don't see it listed on the spec sheet for the SD215A.

Xmech seems to rarely be listed by many brands.
I've often heard Xmech tends to be roughly 2X the XMAX value, but I'm guessing that's not a hard+fast rule or anything.

I tried to do a little testing with an SD215a-88 using low frequency sine waves, a mixer for more visible level setting, and a modest ProAudio amplifier with low-frequency protection disabled. I think I got roughly around 17mm of excursion without any super terrible noises or obvious breaking, but I was too nervous to keep pushing my luck farther.
I should probably find a more accurate way of testing and then just go for it. They're cheap enough to replace if I screw up, but I've grown too attached, lol.

If the amp has low-frequency protection for frequencies below 25hz I think you'll be totally safe. If there's no protection, then I think super loud bass below 20-25hz might be able to hurt the sub at over 40-50watts.

[unclejunebug]

Thanks for the responses, so distortion will go up a little at high volume but no damage to the woofer will result.

Is Xmech a calculated value? I don't see it listed on the spec sheet for the SD215A.

[wogg]

Yup, you'll be able to go over Xmax a bit when cranked, but you'll be short of Xmech. That means distortion will increase a bit when cranked, but the woofer will be just fine.

[Chris Roemer]

The excursion is for 70w (if that's what you plug into the "Signal" tab).
My SD215 specs might not be the MOST current, but I see it just bumping the Xmax line at 60wRMS.
I've used that driver in a vented box using just PE's SA25 amp, and it keeps up OK w/a pair of 6" 2-ways (on the treadmill).
The SA70 is a better choice. Don't use all the gain and you should be okay.

[unclejunebug]

I originally did some modeling for this project with the DCS205 but now that I have the SD215A-88 I decided to throw the specs into WinISD and take a look at the performance. The main thing that jumped out to me was the cone excursion graph. The white line in the graph below is the DCS205, the purple is the SD215A, and the red is the SD215's Xmax spec (6mm). With the system input power set to 70watts the excursion of the SD215 is greater than the sub's Xmax spec from about 40-60Hz. Xmax for the DCS205 is 8.8mm so it's under the spec for all of the sub's usable range.

If I reduce the input power the excursion obviously is reduced and at 50watts it's right on the Xmax line. Are these graphs showing excursion at full volume? I can't imagine that this sub would be cranked up all the way all the time (or ever for that matter).

[unclejunebug]

so many good options here.

I did something similar with very cheap components -- all in $112

now being used every day in my buddy's living room system

http://techtalk.parts-express.com/fo...ubwoofer-build

I came across your thread while searching around for small enclosure designs, looks really nice!

[LOUT]

I love the color you made them, it manages black/grey in a warm/"alive" way without looking like grey-fading barnwood. With the nice woodgrain stripes it's like a grey tabby cat..in speaker form.
Okay, that sounds weirder reading it back than it did in my head.

[ceiol]

and the $112 includes the enclosure, driver, and even the amplifier!!!

[ceiol]

so many good options here.

I did something similar with very cheap components -- all in $112

now being used every day in my buddy's living room system

http://techtalk.parts-express.com/fo...ubwoofer-build

[unclejunebug]

Seems like the SD215A will be sufficient unless I'm willing to move up to a higher wattage amp (which I'm not). I will stick with what I have a try not to change it! :D