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Old 10th February 2006, 02:19 PM   #1
fazman is offline fazman  Canada
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Join Date: May 2005
Location: Windsor Ontario
Default QTC question?

Hi, Is a QTC of .65 too low for a sealed sub?
I'm looking to make my sub more musical instead of boomy.
The sub is ported now but it is 5 cbic ft.
If I occupy 1 cubic ft of space inside it will bring it to 4 and a QTC of .65 (Also I will plug the port)

Thanks
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Old 10th February 2006, 03:37 PM   #2
sreten is offline sreten  United Kingdom
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hi,

Q1. no and neither is 0.6 in certain cases.

Its fairly pointless filling space inside a sub to get a Q.

Use all the space to get the lowest box resonant frequency.

If they are currently vented try reducing port frequency by 0.7.
(halve the area or double the length)

If still not tight enough try it sealed.

If sealed is too tight adding some series resistance will lift Q.

/sreten.
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Old 10th February 2006, 10:10 PM   #3
simon5 is offline simon5  Canada
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Try plugging the port first. Then fill space.

Some people use Qtc of 0.5 to be more musical.

Yeah you could also lower the tuning frequency if you can.
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Old 10th February 2006, 11:17 PM   #4
fazman is offline fazman  Canada
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Thanks guys,
The tuning frequency is already at fs. (19hz) with a Precision port.
It was (is) very impressive for low frequency extension- everything in my basement rattles, I'm just getting tired of the "slow" bass. I find I never use it for music just movies.
Even my 31 band EQ will do only so much.
So I'm hoping your suggestions will help for sealing it up. I will try plugging just the port then go from there.
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Old 11th February 2006, 03:02 AM   #5
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fazman,

I'm a little bit confused if you are talking about driver Q which is usually called Qtc or total system Q which usually referes to sealed designs and refers to the whole speaker system with cabinet and driver.

How large is the driver. A driver Qtc of .65 is very high for a vented cabinet and usually requires a huge cabinet.

For a vented cabinet drivers with a Qtc of between about .32 and .38 get the best combination of extension and realistic cabinet size. Drivers with lower Qtc are common and are designed to fit into a smaller cabinet but they don't go as low.

IF your driver is large like 12 inches a five cubic foot cabinet could actually be too small for it with a vented alignment.

I would put a driver with that high of a Q into an aperiodic enclosure. Or a large sealed enclosure.


How old is this speaker. Rarely is a vented speaker tuned to the Fs anymore as this was really old technology. Speakers in the low Q range of .2 to .28 are usually tuned somewhat above the Fs of the driver. Speakers in the .30 to .38 range get tuning below the Fs which is why they reach down lower.

Speakers above Qtc of say .40 are not usually used in a vented alignment anymore because they require such a huge cabinet.

Your driver maybe is probably underdamped and so it's slow to respond to music signals and yet becuase the port is tuned to the Fs is has lots of bass at the speakers natural resonant frequency.

I would try and rebuild the cabinet to modern standard by building up and bracing the insides and putting in a more suitable driver.

If you are talking about total system Q for a sealed design then Q of .65 is OK. In fact it would be better than most commercial subs which are a lot higher and hence can sound boomy. A critically damped sealed subwoofer would have a Q of around .505. Typical commercial is around .7 to 1.0 so .65 is doing pretty good for a sealed sub.


Based on what you have described it sound as if you have a vented system that by todays standards would be considered improperly designed. It may rattle the house but it isn't musically accurate.

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Old 11th February 2006, 03:16 PM   #6
fazman is offline fazman  Canada
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Quote:
Originally posted by Hezz
fazman,



If you are talking about total system Q for a sealed design then Q of .65 is OK. In fact it would be better than most commercial subs which are a lot higher and hence can sound boomy. A critically damped sealed subwoofer would have a Q of around .505. Typical commercial is around .7 to 1.0 so .65 is doing pretty good for a sealed sub.


Based on what you have described it sound as if you have a vented system that by todays standards would be considered improperly designed. It may rattle the house but it isn't musically accurate.

hezz, thanks for the clarification between qtc and total system q.
In my post I meant total system Q.
The system I'm talking about is a sub (custom).
The driver is a Titanic MKIII 15" subwoofer.
So it is not old by any means.
I was originally worried that if I use my existing vented cab which is 5 cubic ft. tuned to 19 hz. and turn it into a sealed design that the cab might be too large and over damped.
With this it would yield a total system Q of about .65 which by everyones response seems to be OK.

Since the post I temporarily sealed it and although I lost some bottom end (35hz and down) it does seem to blend better with the mains albeit still a little slow. I guess this is the nature of that particular driver
I'm just wondering now if I should be adding more stuffing.
As it is now it has some fiberglass pink insulation along the walls of the cab.
What exactly does the stuffing do to the total system Q?

Thanks again guys.
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Old 11th February 2006, 07:26 PM   #7
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if i'm not mistaken, it lowers system Q somewhat! i've got an Atlas 12 i'm running in a 4.75 cu. ft. well braced enclosure in which i have a 12" x4" from flare to flare precision port i use for HT. but plug the port for music which i mainly listen to. it seems to sound much better for music when plugged. i'm not really sure what the responses are plugged, or open due to the past controversey on the T/S Paramaters of the Atlas drivers as i don't think they were ever corrected so, i just plug the port when i want more accuracy.
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Old 12th February 2006, 05:04 PM   #8
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fazman,

If you could build up the old larger cabinet and brace it really good so it doesn't rattle and you have room for it, it would be better sounding than the small titanic enclosure.

Total system Q of .65 would in fact be a good target. Your subwoofer will be better sounding than the small box titanic subwoofers because it will have a more fast and relaxed and less boomy sound. But you will have to built the enclosure to be strong and resonant free.

With the 15 inch titanic in a large rigid enclosure you will have an exellent subwoofer. However it may be that the driver will be even better in a large vented enclosure. I would download WinISD and compare the two alignments in the same size vented and sealed cabinets.

Five cubic feet may be large enough for a good vented alignment for this driver.

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Old 12th February 2006, 11:15 PM   #9
fazman is offline fazman  Canada
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Quote:
Originally posted by Hezz
fazman,

If you could build up the old larger cabinet and brace it really good so it doesn't rattle and you have room for it, it would be better sounding than the small titanic enclosure.

Total system Q of .65 would in fact be a good target. Your subwoofer will be better sounding than the small box titanic subwoofers because it will have a more fast and relaxed and less boomy sound. But you will have to built the enclosure to be strong and resonant free.

With the 15 inch titanic in a large rigid enclosure you will have an exellent subwoofer. However it may be that the driver will be even better in a large vented enclosure. I would download WinISD and compare the two alignments in the same size vented and sealed cabinets.

Five cubic feet may be large enough for a good vented alignment for this driver.

Hezz,
This is the whole point of why I want to change it.
It is in a 5 cubic ft enclosure vented with a flared precision port right now. All I'm doing is stuffing the port real tight to make it act as a sealed alignment. I'll be playing with the amount of stuffing to sort of tune it to my liking.
The sealed alignment sounds much better in my room.
The front baffle and back baffles are 1.5 inches thick and there are 3 2x4's for bracing, so I'd say that it is fairly inert.
No matter how much I try to get the vented to sound good for music I always end up feeling that it lagged behind the mains (too slow). But the thing does go scary deep.
I'm running a QSC in bridged mode for power (800 watts in a 4 ohm load) and I'm using a crappy old Peavy 31 band EQ to notch out some peaks. I end up getting under 20 hz at my listening position with a Rat Shack meter registering over 100db! Ya it's fairly impressive but what's the point of all of it if music is important to ya?

The dilema for me is that I want my cake and I want to eat it too.
I want bone rattling bass for HT and quick response for music listening. I'll just have to work at it a bit so I get a nice blend and I'm happy for both. I'm getting there.
...And if it doesn't work out I might just sell the Titanic and get one of the 12 inch Dayton RS subs instead we'll see.
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Old 13th February 2006, 03:19 AM   #10
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fazmman,

Sounds like you are on the right track. If the room that you are listening in is not large there will be a lot of room gain at lower frequencies and the vented alignment may be too much bass.

IF you want the best of both worlds try aperiodic dampening in the large cabinet. You will have less deep bass response but improved transient response. You can approximate an infinite baffle enclosure which is the best sounding.

For a large driver you can make an aperiodic vent out of a six inch metal round speaker frame. Foam does not work very well but compressed fiberglass or dacron polyester seems to work. Try compressing about 1 inch thick fiberglass into a .25 thickness for the vent.

However, a driver as large as fifteen inches will probably never be as fast as the smaller drivers in the main system. Make sure the subwoofer is crossed over at or below 80 Hz and the speed issues will not be as great.

It sounds like the speaker is really too much for your room so this would be the best approach. It will tame the bass a little and speed up the speakers tracking of the signal.

It looks like the driver has a Qtc of .38 so it should work well in either a closed, vented or aperiodic enclosure.
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