$300 Ported Sub *updated* - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #11 of 31 Old 08-20-12, 12:39 AM
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re: $300 Ported Sub *updated*

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ggxtreme wrote: View Post
That's what I thought, but wouldn't you just divide the wattage by 2 for 3dB of boost, 4 for 6dB, etc. and use that in your calculations? So effectively a 200W with a 9dB boost would result in a system power of ~33W? I read that was the correct way to deal with boost (ie. amps that have built-in boosts) in WinISD, but I might be thinking of something else.?
That's spot on, as long as you are reducing your System input power in WinISD accordingly. You will notice your output SPL drop drastically as you do that. The actual load of the driver in a box varies with frequency such that you don't always have to follow that rule to size the amp, I'd go off of the Apparent load power plot in WinISD to be more exact given a specific build.

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On another note, I noticed that the 8 ohm version of that 10" driver is available for only $100. I followed the same general idea but upped the box size to 3.5ft^3 tuned at 20Hz, and the f3 is 18Hz. I know the 200W amp will only deliver about half the power with an 8 ohm driver, but this is only a loss of about 3dB in the SPL graph. Is there any real reason not to use that driver instead, assuming I can accomodate the larger box?
I'd stick with the 4 ohm, 3dB is a noticeable difference and it's only $30... and you don't need to increase box size...


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post #12 of 31 Old 08-20-12, 12:47 AM Thread Starter
 
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re: $300 Ported Sub *updated*

I updated my last post before I saw your response. I can step up to a 300W version of the same amp for only $15. The loss in SPL is now ~1.5dB for the overlap range above 23Hz (output SPL is 105dB for most of the range down to 21Hz, which seems like a fine peak level for my listening room and should give me enough dynamic range for movies). Does this change anything? I'm pretty sure the 4 ohm one is fine, but the 3Hz lower f3 seems tempting.

And thanks again for all the replies, you've been very helpful.

Last edited by ggxtreme; 08-20-12 at 01:29 AM.
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post #13 of 31 Old 08-20-12, 08:36 PM
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re: $300 Ported Sub *updated*

Interesting idea, if it comes out cheaper maybe give it a shot? You will also have a uniqueness factor since most people don't run 8 ohm subs on 4 ohm capable amps. AFIK that will not effect performance, it's about the juice the amp delivers.

What I like the most about this project, and considering it's relatively small size, you can always build it a twin in the future


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post #14 of 31 Old 08-20-12, 09:44 PM Thread Starter
 
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re: $300 Ported Sub *updated*

The amp and driver together would be about $20 cheaper, but I would need to build a box about 2" longer in one dimension and 3.5" longer in another. This isn't a problem for the area it would be in (and if this turns out well I'll definitely consider a second one later down the line).

I'm currently facing a dilemma about the porting though. Boxnotes is telling me that my current port idea will have resonance just below 200Hz, which I'm hearing will color the sound. I'm also a bit nervous about chuffing, since I can't find any way of calculating that for a slot port. A solution to both problems would be a round port around 4.8" ID with a small flare: unfortunately, the only piping I can find remotely close to that spec is Schedule 80 5" bore PVC, and NO ONE seems to make or sell that (I can only find 4.3" ID and 6" bore pipes of any kind).

So going back to a slot port, I was considering something like this: http://www.lowes.com/pd_92531-73428-...4&AID=10935405 The internal measurements are 4.38"x4.38" which seem like ideal dimensions for this kind of slot. For chuffing, I was thinking of this kind of flare: http://www.subwoofer-builder.com/flares-rectangular.htm but I wasn't sure about the corners or how big the flare would need to be. Once I factor in the fold in the vent I will need to accomodate it in my enclosure, the volumes I'm dealing with also become more difficult to calculate (Boxnotes only handles round ports).

Last edited by ggxtreme; 08-20-12 at 10:07 PM.
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post #15 of 31 Old 08-20-12, 11:04 PM
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re: $300 Ported Sub *updated*

Can you build a narrower slot port into the box? I can't tell the LPF slope off of the Yung specs, though I would guess even with a 24dB/oct slope you'd want the resonance at least an octave or two up from your LPF point (-48dB or more is usually the target for damping resonances like cone breakup in fullrange speaker design). If you run the usual 80Hz LPF point, I bet it'd be okay... hard to say.

You should look at port velocities in WinISD. With the RSS265HF-8 and 150W, in 3cuft @ 21Hz, a 4.38"x4.38"x29" vent would resonate at 191Hz. That looks like a fine vent option to me by the way, just glue it along the side of the box so that it is braced throughout the whole length of the port. Do you have a router? That's the easiest way to roundover a slot port.


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post #16 of 31 Old 08-20-12, 11:27 PM Thread Starter
 
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re: $300 Ported Sub *updated*

I'd be setting the crossover on the amp to the highest setting, and I'd set the crossover on my receiver to 80Hz. I might just be overthinking the port resonance thing, but I've heard that even resonance well above the used bandwidth can affect the sound. There's no issue if an octave above the crossover is the only gap I need for that resonance.

I came up with two slot designs that keep port velocities below 17m/s and resonance above 200Hz: the square slot you suggested (but 33.16" length, 3.5ft^3 box volume; I can get my hands on a router for the flare if it would be more effective than the PVC flare) and a wide slot (internal width of the enclosure) that runs along the bottom edge and up the back edge. I'm not sure if this second design is suitable for a subwoofer, but it looks pretty cool and the large flare might be effective at preventing chuffing. The square slot you suggested seems most sensible.

I updated the OP with the current design.

Edit: Throwing out a new idea, has anyone tried 3D-printing for parts like a subwoofer port? I'm guessing this will be well out of my budget, but that really seems like an extremely easy solution: model a folded port with a flare, upload the model, receive the part in the mail a few days later.

Last edited by ggxtreme; 08-20-12 at 11:56 PM.
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post #17 of 31 Old 08-21-12, 12:15 AM
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re: $300 Ported Sub *updated*

I think most people just build their own stuff, that's why slot ports made out of MDF are most common. I have also heard that oblong slot port of a 1:12 ratio are preferable for some reason, though I can't say why that might be. I do know that bends in ports will start to make the port less efficient, but bends also will also start to reject higher frequency noise from passing through.

You can always dial down the LPF on your plate amp in addition to your AVRs LPF if you think you are hearing port resonance, the only thing is that those dials are inaccurate so you'd be going by ear. I don't think you will have a problem anyways, if your AVR uses 24dB/oct filters, that'd put a 191Hz port resonance would be at -30dB... I'm guessing it wouldn't be noticeable. If you put your plate amp's 12dB/oct filter at 80 and the AVR at 80, that'd attenuate it to -45dB. Such a hefty LPF would start to effect how the sub integrates with the mains though.

I don't want to start over analyzing this! I don't think you'll have an issue and don't want to scare you away from an otherwise awesome looking build


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post #18 of 31 Old 08-21-12, 12:22 AM Thread Starter
 
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re: $300 Ported Sub *updated*

I think I'll be fine then. The other question I have about the port is the distance from the sides of the enclosure: am I supposed to maintain a distance of diameter/width of the port from the edge of the enclosure all the way through or just at the exit(s)? I'm asking this, because calculating dimensions assuming the port is bent at the half point, and keeping the port 4.38" away from the sides of the enclosure the entire way through, I'd need a box 29" tall for a 33.16" port (too tall for my space). Even a 29" port would still require an enclosure height of about 26.5" (which is just within my limits). I'm kind of hoping I'm wrong about this. Actually, how did you get the 29" length figure? Using a 3ft^3 box tuned to 21Hz, I ended up with a 4.1"x4.1"x30.71" port.

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post #19 of 31 Old 08-21-12, 01:24 AM
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re: $300 Ported Sub *updated*

Ya that's correct, you can't run the port right into a wall, it needs at least one diameter of space.

Oops, I guess I was figuring a 4"x4" vent. My bad. Port velocity looks fine even at that smaller size... so what about building in a ~16sq.in. slotted port with mdf, then?


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post #20 of 31 Old 08-21-12, 01:32 AM
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re: $300 Ported Sub *updated*

Oh I forgot to mention, you can adjust for port flare in WinISD also by adjusting the Qp (port Q) parameter under advanced in the box tab. I don't have a formula or calculator for it, but I think the guidelines are that a port with two flush ends is in the 80-90 range and two flares is 120+. Qa is what you would change for box stuffing/lining.


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