DB Sub for home theatre - need to build a box and have some early questions.

Recusant

Hi All,

I hope this is the right area, it didn't seem to perfectly fit anywhere.

For a loooong time, i've had plans to build a subwoofer enclosure for my home cinema. I have B&W 602 Bookshelf speakers and a Harmon Kardon amplifier (AVR-5000 for memory). These sound nice, but i've never had a subwoofer to go with it so it was always lacking.

Several years ago, i swapped a Phoenix Gold car audio crossover for a Jaycar AP765 sub amplifier (so obscure you can't even Google it!) and then did nothing else. I had no subwoofer as i was waiting for a bargain (and it had to be a good brand, and not ugly, as so many are these days). I kinda put it on the back burner and more or less forgot about it.

I was at a liquidation sale the other day and picked up a Polk 10" DB1040SVC for A$72, the retail was claimed at A$189. Bargain! I couldn't believe it was still there, only crap was left with it. The sales guy said that he hid it out the back, hoping to buy it himself, but decided against it in the end. What luck! I've only ever had Polk once before, in a car audio set up, and they (tweeters) were lovely to listen to.

It was a bit of an impulse buy, as it hasn't been on my radar since the kids came along. Now i have it, i realise i don't know what to do with it! I do not understand how to read the specifications, or how to make it do what is best for the situation. Many years ago, i had a very nice car audio system, but i didn't build it. In fact i've had a few, but i never built a sub box (i did the wiring, i left it to the pros to fabricate mounts/boxes etc). They were always sealed boxes, as i tended to listen to techno and liked the punchy sound. I do realise that home theatre is a very different environment and that it will be asked to be more versatile, with an emphasis on movies rather than techno (in a car!). I see that a lot of home theatre subs have ports.

To that end, i have a couple of initial questions:
- Is my sub best suited for sealed or ported enclosure?
- Will that matter greatly should i choose the opposite?
- Is magnetic shielding necessary these days? (ref http://www.polkaudio.com/education/article.php?id=20)

More questions will follow based on your advice/suggestions etc.

If at all possible, i'd appreciate the discussion to occur in metric units

Forgive my terminology if it's incorrect at any time during this thread.

Recusant

No help?

[Deleted User]

Hello Recusant,
Thanks for posting on Polk's forum. In very general terms woofers that have been designed for use in cars won't work very well when used for home theater applications. The reasons for this have to do with what happens to bass frequencies in a small volume environment (inside a typical passenger car) as opposed to what happens to bass frequency inside a much larger volume environment (inside a typical living room). When Polk designed the DB woofer they made all of the mechanical and electrical characteristics knowing the volume of the car's interior would naturally boost low frequencies adding what is referred to as "room gain". So any enclosure recommendation will have this room gain boost built in and the end result will be a fairly flat response. If you were to take that same woofer/enclosure combination and test it in a typical living room, with a completely different room gain, the frequency response will not have the same results.
Think of putting shock absorbers that were made for a dump truck on a Honda Fit, not going to work very well. I always dislike putting a damper on anyone's desire to experiment in audio, but your efforts might not be what you are looking for. The other difficulty is that when a subwoofer is designed for home audio applications the power amplifier that is used with it has had it's electrical characteristics optimized for that particular woofer and enclosure. There is equalization built into the amp's circuitry to boost certain frequencies and reduce others to give a fairly flat response, sometimes limiters are built in as well.
But, on the positive side, Polk makes available all of the T/S parameters on their car subs and you can probably find an online box building program that would give you some basic modeling for different enclosures.
I hope this is helpful information.
Cheers, Ken

Recusant

Hi Ken.

Thanks for smashing my hopes and dreams into tiny little pieces

Oh well, i don't really want to go to all the effort to build a box that doesn't sound very good. Thanks for being straight-up.

More useless speakers sitting around (i still have all my old car audio after the crash).

What is a T/S parameter?

20hz

I build my own boxes with the help of winisd linerar team ( its a free speaker box program ) that program allows you to enter speaker perameters or it has a huge library built in , the program shows you how low the sub will go how many decibles and how much power it takes , for example if the box is built to small it will take more power and a bass boost on the amp .
When I started I would buy a sheet of wood locally and have them cut the panals there for a few $$ , at home glue it up cut the hole and ready to rock .
My current sub is four 12" mtx t5500 car subs in a isoberic clamshell the box is big and doesnt take a lot of power .
I used to build cabinets trom dimensions from the manufacture but Win Isd goes way past there is no hoping or guessing it shows what it does in the box you choose and the amp you drive it with !
If you choose the correct speaker and run the correct box you wont need any "ciruitry" to get the sound you want , my sub is down 9 db at 20 hz , but for $400 total (thats everything) this thing can break a window .

Recusant wrote: »

Hi Ken.

Thanks for smashing my hopes and dreams into tiny little pieces

Oh well, i don't really want to go to all the effort to build a box that doesn't sound very good. Thanks for being straight-up.

More useless speakers sitting around (i still have all my old car audio after the crash).

What is a T/S parameter?

Jim Shearer

T/S parameters are the performance characteristics of the driver which you need to design a cabinet. See:
http://en.wikipedia.org/wiki/T/S_Parameters

Cheers, Jim

[Deleted User]

Hello,
Here is a link from Polk's web site giving the T/S parameters and enclosure recommendations for use in a typical car:
http://www.polkaudio.com/caraudio/subbox/subbox_plan.php?id=82&mesurements=standard
Best regards, Ken

Recusant

20hz wrote: »

I build my own boxes with the help of winisd linerar team ( its a free speaker box program ) that program allows you to enter speaker perameters or it has a huge library built in , the program shows you how low the sub will go how many decibles and how much power it takes , for example if the box is built to small it will take more power and a bass boost on the amp .
When I started I would buy a sheet of wood locally and have them cut the panals there for a few $$ , at home glue it up cut the hole and ready to rock .

This is interesting, thanks. I have downloaded and played with it, but i'm not sure i'm putting all the right values in (it's not in the existing database).

I'm confident in every input except for "Pe" (which i assumed to be continuous power @ 270watts) and i didn't know what BL was, so left that blank. I'm not sure how to read the results - are they ok? I note that it made the box much bigger than the Polk specs suggest.

Could i possibly ask for your (or anyone elses) input?

Driver            : Polk DB 1040
                    Vas : 28.3
                    Qts : 0.55
                    Fs : 35.00
                    SPL : 86.00
--
Project by        : Recusant
Project for       : you
--
Number of drivers : 1
Box type          : Closed
Box size          : 43.4 l
                                                            
                                                            
             Relative                                       
Freq         Gain         Phase        SPL                  
[Hz]         [dB]         [deg]        [dB]                 
20.00        -14.28       141.17       71.72                
25.00        -10.63       130.80       75.37                
30.00        -7.84        120.10       78.16                
35.00        -5.73        109.44       80.27                
40.00        -4.16        99.22        81.84                
45.00        -3.01        89.77        82.99                
50.00        -2.19        81.30        83.81                
55.00        -1.60        73.86        84.40                
60.00        -1.19        67.39        84.81                
65.00        -0.89        61.81        85.11                
70.00        -0.68        56.98        85.32                
75.00        -0.52        52.80        85.48                
80.00        -0.41        49.16        85.59                
85.00        -0.32        45.96        85.68                
90.00        -0.26        43.15        85.74                
95.00        -0.21        40.66        85.79                
100.00       -0.17        38.44        85.83                

Recusant

Oh nice

I've been playing a bit more, and a ported box of around 83 litres (!) with a 50mm port at 7.06cm long will give a flat response down to under 30Hz

But it gives me a "vent mach" of 0.32 which, according to "BGavin" will introduce port noise.

This is going to take a lot of time!

I wonder if someone who knows this tool inside and out will do the hard work for me :cheesygrin: