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Discussion Starter #1
Hi all,
Long time lurker, first time poster! I've recently got into Home theater, and now have a pretty good (for me at least) setup... except my sub! I have spent the past few months reading up on design, and learning how to use WinISD.
After going through a lot of posts and a lot of drivers (the choice here in the UK is a lot different to the common suggestions made for US buyers) I think I have found something that fits the bill.

My brief is:

-Sub HAS to fit under the sofa. 30mm legs, and 3/4" (18mm) MDF leaves me an internal height of just 13.4mm to play with - a mounting depth of 152mm. Taking this into consideration I can get a maximum of 108l net internal volume, ports and all.

-Low extension, with an F3 of less than 20Hz, as the sub is used 90% for movies. Big SPL doesn't worry me as we never watch movies at anything approaching reference levels. Maybe -30dB MAX on my Yamaha RX-V777.

-Should work well with 200W RMS into 4ohms, (my current amp.) In the future I would like to upgrade to an iNuke3000DSP, which would give me 600W @ 4 ohms - so excursion and porting are planned for this to be the maximum power it will ever see.

So - the sub. I have provisionally decided on the Alpine SWR-10D2.
Here's the params:

Sensitivity = 82dB
Re = 1.95 + 1.95
Le = 1.89mH
Fs = 31Hz
Vas = 18L
Qms = 7.84
Qes = 0.54
Qts = 0.51
Sd = 333cm2
Xmax = 20mm (one-way)
Xmech = 70mm (peak to peak)

According to WinISD:
in a 90.5L box, with 3 x 68mm ports (uk guttering downpipe size :ponder:) tuned to 19Hz, I can get down 10 18Hz at +/- 2dB, with an f3 of 16.8Hz and an f10 of 14.8Hz.
The vents are 10.25L, and the driver displacement is 2.25L, giving a total volume of 103L.
There is a dip of -1.8dB centered around 29Hz, but I figure room modes will give me more variation than this, so shouldn't be a problem.
Driver is within excursion down to 5Hz at 200W. The 600W amp has DSP so I can add a HPF later to keep things in check with the bigger amp.

Can anyone think of anything I've missed?
Would someone be kind enough to model this for me and make sure I'm doing things right?

Many thanks,
Tom.
 

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Alpine makes some high quality drivers, the only thing is the design is centered on audio and music and not HT, thus they have higher Fs than "HT" drivers. I still think this is a good project and it will rumble great!

Everything looks fine to me but the port sizing. This is going to be the challenge with a small box tuned so low. Three 68mm ports isn't going to be enough to avoid chuffing, I don't think, especially not at 600W. The next best thing would be to build a slot port into the box, but that would need some things to shift to stay within your size constraint and would bring the port resonance lower into a bigger issue.

If you don't mind the risk of chuffing I'd say go for it.
 

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Good point about port size... BUT, OP might get away with it in this context as it will be buried under a sofa. Port noise can be pretty easily masked by strategic placement if it can't be avoided due to design restrictions.
 

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Discussion Starter #4
Thank you for the replies gents.
I must admit I was slightly worried about the port noise so I've done some tinkering with the box design, and I can get away with a 13.4 x 12cm slot port (equivalent to a 143.1cm round port) which reduces the port velocity a bit, and still allows me to give the driver 69l, with all bracing, driver, and port volumes accounted for.
This seems to be a happy medium (to my limited experience!) between extension (f3 still under 20Hz) and port velocity.

Fusseli - I was originally looking at "HT" drivers, like Dayton and Peerless (I've probably modeled 15 drivers at least), but nothing I've found can even come close to the extension of the Alpine.
 

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Discussion Starter #5
... I just thought about the pole vent! I'm guessing 3mm would not normally be considered enough breathing room for the vent on a sub?
Am I likely to experience problems considering I will never run the sub at max power?
Its not like I sit listening to sine waves at full volume for hours on end (unless I'm EQing, that is ;) )

Cheers,
Tom.
 

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Use as steep of an LPF as you can, as low as you can, like 80 Hz, since with a slot port of that size the port resonance will be in the 100s.

Good luck and let us know how it turns out!
 

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I noticed the very same this morning - first res. is around 113Hz!
I cross over at 80Hz any way, and had no intention of going lower, as my centre's f3 is 70Hz.
My receiver's low pass is 24dB/oct, which only gets me down -11dB (simulated in WinISD) from 80Hz by the time it hits 113Hz.

I've read that you should aim for a f.p.r of at least 1 octave above your low pass (so 160Hz in my case) but this doesn't take into account the low pass slope. How many dB do I need to be down by the time I hit resonance, in order for it to be inaudible?

So as far as I can see it I have 2 options:
1) Go ahead and hope resonance is not a problem. This could mean later having to buy further DSP to increase the rolloff.
2) Reduce my port area again, and lower my wattage aspirations to prevent chuffing.

What would be the sensible option here?

Many thanks for the help so far,
Tom.
 

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How far down you have to be depends on how big a peak you get at resonance... if it's a 10dB peak, then -11dB isn't going to cut it. BUT, if you cross over at 60Hz instead, now you're down more like -21dB which will do you a lot more good. At that point, the sub might have a little trouble blending with the rest of the system... but that's the nature of trade-offs.

The issue you're running into in box dimensions is having enough clearance behind the motor for the vent in the pole piece, correct? I know you're looking hard at that Alpine, but have you considered something like the JL shallow-mount 13, or any other driver in a truncated basket? That would solved your vent clearance problem...
 

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If you are likely to get the inuke you need to model a 1'st order highpass at 10 hz. Do you know what the highpass fequency and slope is on your current amp ? Assuming it is a plate .
 

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Dont tune that driver to low I would not push it much lower than 18Hz and get a couple bass shakers and attach them to the sofa as that will simulate the lower frequencies without wakening the neighbors.
 
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