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Old 6th December 2011, 04:01 PM   #1
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Arrow Which type of box would you build based on this frequency graph?

Having discovered that my Home Theatre Processor isn't very good at re-routing LFE to to my main speakers (if at all), I've decided I need a sub of sorts.

My mains are floorstanding transmission line's and their bass response is pretty flat down to about 80Hz, and they still keep going down to 25Hz, albeit quietly (see graph below).

I have 2 spare 12 inch HT300G0 Audax sub drivers that have been sitting in the loft for years, and a couple of spare monoblocs I can use to power them.

My first idea was the set the processor to to output purely LFE below about 80Hz to the sub channel. Then as the sound was to be more for 'effects' than for actual sub bass, I was thinking of mounting a sub driver within each of my sofa's, underneath. There is enough clearance, and my idea was to basically screw a 3ft x 3ft sheet of 18mm MDF to the sofa's frame, cut out a 12 inch hole, and mount the driver pointing downwards. Then I'd just connect up the monoblock and I've got a free-air LFE generating sofa!

But then as always seems to happen, my ideas start developing. Unsatisfied with the potential effort vs. result, I looked into building a proper box.

A sealed box doesn't go low enough.

A vented box has to be very large at 191 litres but produces the deepest bass - flat down to around 30Hz.

A 4th order bandpass is still large at 138 litres, but seems to produce a perfect roll off right at the point where my main speakers start to come alive. However, this box is only flat down to around 40Hz (10Hz higher than a vented box).

Which one do you think is best... And why?

(Pls ignore the white traces)
Click the image to open in full size.
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Old 6th December 2011, 05:05 PM   #2
badman is offline badman  United States
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You can adjust alignment with the bandpass, which I'm assuming you've adjusted here.

Vented alignments in subs are so 1995! Passive radiators work better with modern, lower efficiency subs in realistic boxes. If I were you and wanted reasonable box sizes and had a solid budget, I'd probably build a couple of these:

Creative Sound - Product Details

Probably fair to disclose- I've been using CSS stuff for some years and have a light professional relationship, but that's driven by his excellent products. Trios with passive radiators are what I use.
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Old 6th December 2011, 06:06 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

You should be able to rely on the LFE output rolling off around 80Hz,
so I can't see any real advantage to the bandpass arrangement.

specs are here : http://www.cadaudio.dk/ht300g0.pdf

For subs they are high efficiency, 91dB/W/1m, 94dB/2.83V/1m.

With a high Qts fro vented you end up with boxes a lot bigger than Vas.
something like 120L tuned to 28Hz looks about right to me. Another
option is to fit passive line level low bass boost at the end of the
monobloc leads and use around 60L sealed with low bass boost.
(Or mod the monoblocs feedback loop for a peaking high pass,
say Q=2 (+6dB) at 30Hz for near the same response as vented.)

Vented will go louder at 30Hz than sealed though.

Whilst your sofa's will reperesent a very lossy cabinet, what sort
sort of volume ? Vented still just might work in some fashion.

rgds, sreten.
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Old 6th December 2011, 06:20 PM   #4
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Quote:
Originally Posted by badman View Post
Vented alignments in subs are so 1995! Passive radiators work better with modern, lower efficiency subs in realistic boxes.
By the late 1960s passive radiators were in use, while by that time bass reflex was fairly well understood and had been in use for many, many decades, I don’t get the 1995 reference.

A PR occupies less physical space than a large port needed to tune low, but a
BR still provides more output per $$ than PR , has better transient response, how do you define “work better”?
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Old 6th December 2011, 06:23 PM   #5
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i have 2 of these , for the price you can get better.

ES250PWCH - JBL

but i only paid 350€ each
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Old 6th December 2011, 07:10 PM   #6
bjorno is offline bjorno  Sweden
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Quote:
Originally Posted by judderod View Post
...Having discovered that my Home Theatre Processor isn't very good at re-routing LFE to to my main speakers (if at all), I've decided I need a sub of sorts...My mains are floorstanding transmission line's and their bass response is pretty flat down to about 80Hz,..
...I have 2 spare 12 inch HT300G0 Audax sub drivers...I was thinking of mounting a sub driver within each of my sofa's, underneath. There is enough clearance, and my idea was to basically screw a 3ft x 3ft sheet of 18mm MDF to the sofa's frame, cut out a 12 inch hole, and mount the driver pointing downwards. Then I'd just connect up the monoblock and I've got a free-air LFE generating sofa!...

...Which one do you think is best... And why?...
Hi,

Good idea..LFE-sofa...Why?...IMO,The submitted picture talks for itself:

b
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File Type: jpg AudaxHT300G0_T-TQWT.JPG (230.6 KB, 195 views)
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Old 6th December 2011, 07:12 PM   #7
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I'd go for the big box, but power is nearly free so you could also choose an alignment that lends itself to equalization and correct it electronically. That's what they did with the old Swan project in Speaker Builder, and it's also the idea behind the Bassis(tm) that Marchand sells.

subwoofer equalizer, bass correction equalizer, bass boost
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Old 6th December 2011, 07:13 PM   #8
Zero D is offline Zero D  United Kingdom
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@ judderod

Hi, your 4th order bandpass looks fine on paper, but you will need a LP filter @ the higher Xover f to filter out any garbage. Plus in reality with that in place, that nice looking roll off point @ 80Hz will be affected by the LPF.

Similar to sreten's 120L tuned to 28Hz design, i did this one 150L fb = 30Hz -1db @ 30Hz

tff.gif

Max Power is limited to 118W @ 43Hz

maxp.gif

Ave Max SPL = 113dB
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Old 6th December 2011, 07:34 PM   #9
badman is offline badman  United States
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Quote:
Originally Posted by weltersys View Post
By the late 1960s passive radiators were in use, while by that time bass reflex was fairly well understood and had been in use for many, many decades, I don’t get the 1995 reference.

A PR occupies less physical space than a large port needed to tune low, but a
BR still provides more output per $$ than PR , has better transient response, how do you define “work better”?
Well, the '95 thing was kidding around, guess you're immune to my verbal charms

PRs are far more practical in the smaller boxes favored by most current, high Xmax subs. Additionally, they don't have port resonance issues that ports can have, and ports at subwoofer frequencies are often necessarily compromised in terms of cross sectional area by the limited length available. This means that you can often get more output from a PR vs. a vented alignment with a port that had to be undersized to make length manageable.

This is, as I say, particularly an issue with the lower Vas subs that most of us are using in smaller boxes. The more widespread acceptance of multisub setups makes box size more of an issue than ever before, meaning the balance is shifted towards PRs.

The subs I use, for example, use the creative sound trio 12 with an AE speakers 15" passive. To get the same tuning, a 4 ft long 4" dia port would be required- not only impossible to fit in realistic boxes, it'll have significantly less output available before bad port compression and chuffing. More port area? Sure, we'll go to a 10ft long 6" vent!

With higher Vas subs, in 6-8 ft boxes, the balance changes and you have to evaluate each in standalone.
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Old 6th December 2011, 10:08 PM   #10
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Quote:
Originally Posted by badman View Post
Well, the '95 thing was kidding around, guess you're immune to my verbal charms

PRs are far more practical in the smaller boxes favored by most current, high Xmax subs. Additionally, they don't have port resonance issues that ports can have, and ports at subwoofer frequencies are often necessarily compromised in terms of cross sectional area by the limited length available. This means that you can often get more output from a PR vs. a vented alignment with a port that had to be undersized to make length manageable.

This is, as I say, particularly an issue with the lower Vas subs that most of us are using in smaller boxes. The more widespread acceptance of multisub setups makes box size more of an issue than ever before, meaning the balance is shifted towards PRs.

The subs I use, for example, use the creative sound trio 12 with an AE speakers 15" passive. To get the same tuning, a 4 ft long 4" dia port would be required- not only impossible to fit in realistic boxes, it'll have significantly less output available before bad port compression and chuffing. More port area? Sure, we'll go to a 10ft long 6" vent!

With higher Vas subs, in 6-8 ft boxes, the balance changes and you have to evaluate each in standalone.
Your Trio 12 has more than double the Xmax and an Fs almost an octave lower than the OP’s Ht300g0 . PR make sense for a super low Fb as you may like with your Trio 12, where a proper vent would be as large as the cabinet, but little sense (cost or SQ wise) for the OPs speaker.

As it is, the OP’s sim requires such a large box because the Fb at 30 Hz is already too low for the speakers he has, the big box will exceed Xmax at low power.
Even with that 30 Hz tuning however, a decent size port won’t require anything like the port lengths you describe.

Audax Ht300g0 12"
Le: 1.63 mH
Re: 3.2 ohms
Fs: 36.8 Hz
Vas: 2.75 cu. ft.(78 liter)
Qms: 10.39
Qes: .56
Qts: .53
Xmax: 8mm.


CSS TRIO12
Le: 1.63 mH
Re: 3.2 ohms
Fs: 22Hz
Vas: 97 liter
Qms: 6.4
Qes: .46
Qts: .43
Xmax: 20mm.

Your suggestion of PR make sense for your application, not much for the OPs, regardless of the decade .

Art
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