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What exactly happens, freq.-wise, when you plug one or two ports on a SVS 25-31PC+?


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#1 of 23 John Pine

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Posted January 04 2005 - 03:20 AM

Are you giving up just volume just in the upper bass range (80-120Hz)? If I had the crossover turned down to 60Hz or 40Hz, would it make any difference?

I’m trying to rationalize spending the extra money on a 25-31PC+ as opposed to just just going with a PB12-ISD. The PB12 is more in my price range so why should I pay the difference for the PC+? My system is used primarily (90/10) for 2-channel SACD/DVD-A and concert DVD playback. My listening room is quite large. The room is 23’ x 18’ with a 14’ vaulted ceiling. I’m using full size towers in front and would be only using the PC+ from 60Hz or 40Hz and down. Anyone have a response chart of the 25-31PC+ with one and two ports plugged? For my application will I even notice a difference?


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#2 of 23 ScottCHI

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Posted January 04 2005 - 03:41 AM

very generally, plugging a port lowers the sub's extension at the expense of output. it'll go a little lower, but not as loudly.
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#3 of 23 John Pine

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Posted January 04 2005 - 04:01 AM

Output in just the very lowest range?
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#4 of 23 ScottCHI

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Posted January 04 2005 - 04:24 AM

Quote:
Output in just the very lowest range?
no. it's more complicated than that. it actually changes a sub's entire frequency response curve.

i thought there were some frequency response charts at the svs site of the effects of plugging 1 and 2 ports on one of their subs, but i couldn't find them.

if you can find them, the effect on the curves will be similar on just about any sub.

why are you concerned with this, exactly, though? are you trying to compare the pb12-isd to the 25-31 pci+? because i think you may be trying to over-simplify the comparison. there are some other factors involved when you compare those 2 subs (or any 2 subs). the drivers are different and so are the amps.
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#5 of 23 William_Gravem

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Posted January 04 2005 - 04:55 AM

I bought a PB 12 + and the freq charts were in the manual. I believe they post the manuals as PDF's on their site.

#6 of 23 John Pine

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Posted January 04 2005 - 04:56 AM

Scott: Yeah I also looked on the SVS site for some charts without success.

Scott, I’m trying to justify the additional cost of the PC+ unit over the PB12 in relation to a large room application. I believe the PB12 would easily be able to do the job, but I like the idea of the extra headroom that the PC+ might offer. Also, having the flexibility of tuning the ports is interesting. But, if I have my crossover set down to 60Hz or 40Hz, does having the tuning ports even matter?
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#7 of 23 John Pine

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Posted January 04 2005 - 05:02 AM

Will, I'd like to compare the PB12 chart to a 25-31PC+ with one and two ports plugged. I'm just curious to see how it changes.
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#8 of 23 ScottCHI

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Posted January 04 2005 - 05:13 AM

Quote:
But, if I have my crossover set down to 60Hz or 40Hz, does having the tuning ports even matter?
yes, it matters. but i think you are harping on this port plugging thing, too much. the 25-31+ would be the superior sub, in many ways. if you can afford it, get it. then fiddle with the tuning. don't skimp on your sub.

for $50 more than the 25-31+, the 20-39+ is tuned to 20Hz, btw.
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#9 of 23 John Pine

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Posted January 04 2005 - 05:44 AM

Point taken, didn't realize the 20-39PC+ was only $50 more. But "Damn", it's even taller than my full size towers!
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#10 of 23 ScottCHI

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Posted January 04 2005 - 05:54 AM

have you emailed SVS with your needs, room dimensions, budget, etc. for their recommendations, btw? they'll answer your questions pretty promptly.
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#11 of 23 BrianWoerndle

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Posted January 04 2005 - 09:08 AM

If you are looking for a lot of output in a big room your best bet is probably dual cylinders. I would probably go with dual 20-39CS subs, but I know that is way out of your price range. But, if you got the 20-39CS with Samson S-700 combo now (which is only a little more than you wanted to spend) then you could easily add a 20-39CS (or a 16-46CS to go lower) with the extra amp channel on the Samson if you feel you need more output.
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#12 of 23 John Pine

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Posted January 05 2005 - 02:44 AM

Here's the link I was looking for!

http://svsubwoofers....usportblock.pdf

Brian: Thanks for the feedback but my stand is already full with a TV, DVD player, pre-amp and two amps. "No room at the Inn", for another amp. I'll probably go with the 25-31PC+. I guess I could always add another one later if I needed to. The 20-39PC+ is just too tall, and the "WAF" is an issue. Anyway, since I'm used to a Velo CHT-12, I'm sure the output will be dramatically tigher and stronger.
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#13 of 23 steve nn

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Posted January 05 2005 - 03:25 AM

I think your going to find yourself very pleased with the 25-31PC+ decision John. Considering your application and perimeters, that is the rout I would go. Some times I feel the 25-31+ doesn't get it's just due.

Your last assumption in your prior post rings true with my experience also. I used to run a couple of 15" subs from said company and they did not compare to my first SVS experience/25-31CS+.

Since your 90/10, I wouldn't be surprised if one works out very well. That thing can peel the paint off the walls if called upon and sound great doing it. (My room is 12 X 24 X 10'> 4 openings) Yes not quite the extension but being +/- 3 dB 25 Hz-100 Hz (25Hz mode) you can be confident your not missing much imo.

#14 of 23 John Pine

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Posted January 05 2005 - 04:14 AM

Steve: Good info, thanks for the feedback. Can you tell me what the divided lines stand for at -3 db and -10 db on SVS frequency charts referenced above. Do they show octave changes?
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#15 of 23 ScottCHI

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Posted January 05 2005 - 04:46 AM

Quote:
Can you tell me what the divided lines stand for at -3 db and -10 db on SVS frequency charts referenced above. Do they show octave changes?
they're just there to make it easier to see at which frequency the sub's output is down -3dB and -10dB in each curve.

dBs represent the sub's output or SPL (sound pressure level).

unrelated to that, an octave, in terms of the frequency response (Hz), represents a doubling (or halfing) of the frequency. ie. an octave up from 40Hz is 80Hz, an octave down from 40Hz is 20Hz.

notice how the curves cross between these 2 lines, the -3dB and -10dB points, and the implications therein. in the 25-31+ graphs you can see that with one port block inserted, the sub is down 3dBs at 30Hz whereas with the stock tuning, it goes down to 25Hz or so before it's down 3dBs. on the other hand, however, the stock tuning IS down a full 10dBs around 19Hz whereas with a port blocked it goes down to 16Hz before it's down 10dBs.

SO, the stock tuning has a lower (in terms of frequency) 3dB point but a higher 10dB point. you get more output down really low with the port blocked at the expense of some output a little higher up in frequency. the differences are subtle but can be very noticable.

btw, between the subs you were considering and the others that were mentioned, i too think you made the right decision with the 25-31+. stock-tuned, too, probably.
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#16 of 23 John Pine

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Posted January 05 2005 - 05:17 AM

Scott: Ok, got it. After seeing these graphs I understand now what you meant about changing the entire response curve. Good info, very informative! Thx!
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#17 of 23 steve nn

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Posted January 05 2005 - 05:23 AM

they're just there to make it easier to see at which frequency the sub's output is down -3dB and -10dB in each curve.
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Generally when a sub is rated you will notice a +/- of 3db which is called the F3.(You most likely know that) Anyway Scott explained it very well.

#18 of 23 ScottCHI

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Posted January 05 2005 - 05:53 AM

and to further elaborate, i didn't mention slope, but you can clearly see that it's VERY different in each curve.

that the stock tuning of the sub has a lower -3dB point is both important and informative information regarding the sub's performance. true.

BUT the relative -10dB point becomes important, too, in that it defines the slope of the curve in this region, where the sub's output is rolling off. and that's another important characteristic of the sub's performance.

likewise, and although it's not included in the graph, the -2dB point is probably equally helpful in defining or discussing the curve ABOVE the -3dB point, and yet another important aspect of the sub's comparative performance in the 2 instances, stock versus plugged.

my point being, i suppose, is that saying "more extension at the expense of output" (as i said) oversimplifies what's really going on. interpreting the graphs is more complicated than one might think at first glance.
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#19 of 23 John Pine

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Posted January 05 2005 - 06:54 AM

Scott: Agreed! Ok...I understand personal taste plays a major role but for music applications wouldn't you try to get the flattest response, such as the curve on the stock 25-31PC+?
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#20 of 23 Ralph B

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Posted January 05 2005 - 10:14 AM

room is gonna play a big part and none of us have the same room. we all know this.

I have blocked ports on my 25_31PC+ and one port blocked was best if your gonna go lower and block a port. it was better IMO for music. the bass was tighter but I def lost alot of upper frequencys.

I have liked the stock tune overall. but 20hz tune is good.




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