Ported enclosure size/vas/qtc questions! - Home Theater Forum and Systems - HomeTheaterShack.com

Old 04-07-11, 06:13 PM Thread Starter
Shackster

Join Date: Feb 2011
Posts: 18
Ported enclosure size/vas/qtc questions!

I've been wondering about this. I have read that the ideal Qtc of a subwoofer is 0.707. Firstly I have found the following info somewhere on the net;

To arrive at the Qtc number you take the Vas and divide it by Vb (volume of box) then take that number and square root it, then add 1 then multiply it by the Qts of the woofer. Qtc = Qts * (sqrt(Vas/Vb) + 1) The "ideal" Qtc said by many is .707

Now Qtc is only applicable for a sealed sub right? It seems pretty straightforward to work out a pretty precise ideal volume for a particular driver in a sealed cab using this formula. So I was wondering what the equivalent for a ported subwoofer is? I am basically wondering how to decide on the optimal size for ported.

I have managed to shoehorn my drivers into the tubes that I have, (300 litre cylinders) fiddled about with sonosub and winisd to try and work out a decent tune and all that and the results are good, or at least they seem good to me, but I have read that these drivers would perform better in a larger enclosure and I'm trying to find out exactly how the optimal size is worked out. Is there a recommended Vas to Vb ratio or something? I basically made them as big as I could get away with and fiddled with the port length etc until it seemed to work out ok, but there must be a proper way of doing it?
Cheers guys
swiftpete is offline

Old 04-07-11, 07:09 PM
Elite Shackster

Join Date: Aug 2008
Posts: 7,142
Re: Ported enclosure size/vas/qtc questions!

The reason most go ported is to utilise the otherwise waisted back wave of the driver to create additional output. As cabinet size increases, the damping effect of the cabinet reduces and you need less power for more cone excursion, which translates into more spl for less power. Therefore, larger cabinets are more efficient, need less power for a given output, and give more output for a given amount of power.

When people speak of performance, its usually ignoring sound quality in terms of system Q. Performance focuses on maximum output, keeping that output clean, and keeping the system efficient. Large low tune cabs do exactly this. They remove cabinet effects on the sound the driver produces, maximise efficiency and allow any given driver to maximise how low it can go while maintaining clean output. They then totally rely on the drivers natural sound quality for their character.

As such, there is no real datum by which to measure a ported sub. You design it on your personal restrictions, preference, or a bit of both.

Sealed subs are a balancing act. To low a Q will rob it of energy, and without a port to boost output the loss of upper bass sound isnt really rewarded by masses of low clean output. High Q is usually the reserve of weak systems using high Q to make it seem like it has ability it doesnt really, and ultimately it just sound coloured, boomy, and sounds like it drones into a one note bass. For this reason there is a balance that has to be struck with sealed subs. The only proper way to get low end performance in a sealed design is to add in more driver area and power, which as we all know can get pretty costly.

Ported subs goals are to maximise output. Ultimately its upto the designer to decide if they like how it sounds, and whether of not they want to have more impact or low end grunt in any given design. There isnt really a fixed methodology in this respect. There is of course a point where by the design may as well be sealed if you tune it too high, but then you wouldnt be considering a ported design anyway so the point is moot really. The only time this differs is when using a pretty cheap driver and you need to force decent output out of it, like in those HiTB systems.

Thats basically how I see things anyway. There will of course be people who consider ported designs to aimed at accuracy. Larger ported cabinets have less effect on the drivers reproduction and final sound quality, so if thats what your aiming for then the design will suit you. I do think there is a point when you can go too far and rob a subwoofer of sound quality in search of pure low clean output, which is why the Q of IB drivers is closer to .7 rather than .5 or lower, but preference can begin to come into that debate, and such debates usually become endless.
Moonfly is offline
Old 04-07-11, 11:02 PM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: Ported enclosure size/vas/qtc questions!

If this is for HT, music, or both, then there is an optimum box size and tuning frequency for your drivers depending on what your goals are and what the subs capabilities are.. Upload the driver file for your sub and we'll see how it models.
Mike P. is offline
Old 04-08-11, 05:05 AM Thread Starter
Shackster

Join Date: Feb 2011
Posts: 18
Re: Ported enclosure size/vas/qtc questions!

Thanks for the replies. The reason I was interested is because I have read that 600-650 litres is a size that is often used for the drivers I have. Mike they're Fi Q18 D2. I have twin cylinders, each has gross volume of 336 and effective 291. Powered by EP4000. Tuned to 14hz with 6 inch ports approx 32 inches long. I don't get any port noise during general viewing at my normal listening volume, which is -20 (sub channel +3db from ref) but if I turn the system up to reference levels I start to hear port noise. As I don't listen at anywhere near that volume the port noise isn't really a massive concern currently, but if there was a good reason to put a larger port in there I would.

They're used exclusively for home theatre as I have a separate system for music. I originally copied a design that I think I found on here and had 320 litre cylinder, 14.5hz tune and 15 inch driver. I decided to upgrade to 18s and keep the cylinder I already had while adding another. I'm still trying to figure out winisd to be honest. When buggering about with port velocities etc, I used winisdonline and the online vent calculator, which are simplified versions of the full thing and I used the sonosub program as well. But I would like to learn a bit more as I am stabbing in the dark a bit really.

I think the sound from them is awesome, but I want to make sure I'm getting the best I can from them really and unfortunately, while I don't 100% know what I'm doing I can't just relax and enjoy..
swiftpete is offline
Old 04-08-11, 05:55 AM
Elite Shackster

Join Date: Aug 2008
Posts: 7,142
Re: Ported enclosure size/vas/qtc questions!

Getting 100% is a relative term. 100% of what. 100% output, pure low end grunt, slam, what?

You can no doubt get more low end output from them by increasing cabinet size. You have already noted you cant use all the grunt on offer as it is so why go to that trouble. We all have to work to our own personal constraints, and if you cant fit larger subs in then whats to worry about.

I know where your coming from, I have been down the road myself which is why I have built numerous subs. If there is something you feel your missing then by all means explore the fix, but if not, and you physically cant move the subs or accommodate them being larger, then I would settle for what you have.

It sounds to me like you just want to explore things a bit more and learn. You need to find someone that wants a DIY sub and build it for them, thats what I keep doing.
Moonfly is offline
Old 04-08-11, 06:02 AM
Elite Shackster
gperkins

Join Date: Aug 2008
Posts: 3,068
Re: Ported enclosure size/vas/qtc questions!

My previous Q18's were in a 11 cu ft box tuned to 13.5hz and my port velocity was around 38 m/s with no hpf. No port noise at all even at near reference.

The new Q18's were re designed to go in a smaller box so a 600 litre box would not be required in my opinion. The old Q's which I have, they are perfect for that size box. You could still put them in a 35 cu ft box each and run 1000 watts through them. But that is all to do with the parameters.

This is why I use a slot port rather than a round port because off more flexibility. A bigger box does help with port velocity and being able to make a bigger port but make a box too big and the alignment goes to pot and out off shape.

A 6" port even for your size sub is still too small. An 8" would be better but then having that in a smallish sub may mean issues with port resonances.

Its a balancing act in which all areas need to be looked at when designing a good sub.
gperkins_1973 is offline
Old 04-08-11, 08:31 AM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: Ported enclosure size/vas/qtc questions!

Quote:
I have twin cylinders, each has gross volume of 336 and effective 291
What is the internal height of the tubes?

Mike P. is offline
Old 04-08-11, 09:13 AM Thread Starter
Shackster

Join Date: Feb 2011
Posts: 18
Re: Ported enclosure size/vas/qtc questions!

Mike, they're 215cm (84.6in) tubes, 45cm (17.75in) diameter. I used 18mm mdf for endcaps, so with that taken off I'm left with just over 211cm, which is just over 83 inches.

Graham, I know that technically 6 inch ports are too small, I realised that when I was modelling, or doing my best. I was originally just going to buy some bigger stuff, but I had the 6 inch port tubing to hand so thought I'd give them a go. I don't get a problem, unless I'm pushing the subs to a level that I don't normally use. If there was a benefit to larger ports then I would install them, but I don't want to go the trouble and expense of doing it if it's not going to benefit me in my current room, where I don't push the subs to their limits. Of course I am open to hearing why they're too small and if they're affecting anything in any other way as if it would benefit the subs to do it, then I can do that.

Dan, you're right I do just want to learn really. The performance I'm getting at the moment is awesome and I'm very happy with the sound quality and the power they have. There are just things about the process that I don't understand, that's why I'm posting really to try and further my knowledge. The crazy thing is, that all of my family and friends have seen, heard and felt my setup and everyone says it's awesome. I have two friends who have dedicated film rooms, with projectors and surround sound, but neither of them have a sub. I offered to help build subs with them. Neither of them are bothered. I don't get it...?! I guess some people just don't care.

Last edited by swiftpete; 04-08-11 at 09:19 AM.
swiftpete is offline
Old 04-08-11, 09:18 AM
Elite Shackster
gperkins

Join Date: Aug 2008
Posts: 3,068
Re: Ported enclosure size/vas/qtc questions!

For me personally I would have re made them again so it satisfied my own mind. I had alot off port noised all all the SVS subs I had and I would not want to go through that again. Even a slight noise I would be able to pick out. I've built my subs 3 times in the pursuit for betterness and I would do it again without a 2nd thought.

Do you have a hpf on them because even applying a slight amount off filtering will reduce the velocity down. it may help.

cheers

Graham
gperkins_1973 is offline
Old 04-08-11, 09:22 AM Thread Starter
Shackster

Join Date: Feb 2011
Posts: 18
Re: Ported enclosure size/vas/qtc questions!

I don't have a hpf. I only notice any port noise when I push them to reference level, which I've done a couple of times just to see what they can do (the house shakes like crazy). I don't listen at that level in my room as it's way too loud and at the volume I listen at (-20) there is no port noise whatsoever, even during something like WOTW with my ear right next to the port. So I didn't really see the point. Although I don't mind doing it as I said if there is going to be some other benefit.
swiftpete is offline

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