same size box, 2xsealed VS isobaric BP ? - Page 2 - diyAudio
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Old 17th January 2008, 10:44 PM   #26
GM is offline GM  United States
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Quote:
Originally posted by RobWells
Hi GM,

Thanks for the info. One small snag is the basket and magnet of the driver will not fit in that pipe.........
Greets!

You're welcome!

Yeah, these low Vas drivers make small TP/THs, but the trade-off is making it fit without compromising its wave action. I figured just putting a blister over the driver with enough area to account for its Vb would work fine.

GM
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Old 17th January 2008, 11:23 PM   #27
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Thought I'd sold my old JBL2020H but I still have it. It looks like a good driver in the same pipe, with a bit more of a 'cabin gain friendly' response.

Rob.
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Old 18th January 2008, 05:15 AM   #28
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Hi GM,


Quote:
Originally posted by GM
Huh! Apparently yesterday's fever did me in more than I thought as I don't remember posting on this thread.
Sounds like you must have been really "done-in", then :-).


Quote:
Originally posted by GM
FWIW, though my doctor says my vision is average for my age I can barely read the HELP files since the script isn't either a higher contrast or larger type and I had to get a younger neighbor to read the green sections to me as they have too little contrast.
Have you tried experimenting with a lower resolution on your display? The Help File should be a bit easier to read at say, 800 x 600 pixels. Hornresp will actually work down to 640 x 480 pixels. The setting I normally use myself is 1024 x 768 pixels.


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Originally posted by GM
Anyway, thanks for the correction.
You're welcome :-).

Kind regards,

David
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Old 19th January 2008, 02:47 PM   #29
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Ok I tested the in car response of the infinity 12.1 in 1 cubic foot enclosure (probably .8 once driver vol removed)

Also tested with my wifes old car sub, a cheapy 10" in a small sealed box.

I couldn't measure them outside as it's been raining all day so the winISD plots will have to suffice for comparison.

There's some interesting modes there, and it seems a need for a flat response from ~40Hz down. The 10" used to measure flat from 80Hz down in my wifes small hatchback.

Rob.
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Old 19th January 2008, 02:50 PM   #30
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And the measured response in the vehicle.

Rob.
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Old 19th January 2008, 02:57 PM   #31
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Quote:
Originally posted by Eva


Anyway, be aware that in the car you are going to get a lot of boost in the 40Hz to 60Hz range and very little boost above 80Hz. A flat box tends to sound like a 50Hz resonator inside a car, and personally I find this rumble quite unpleasant.

You hit the nail on the head there Eva

Rob.
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Old 20th January 2008, 12:06 PM   #32
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Measured both sealed boxes outdoors today. Gives a better idea of their response than the winISD charts.
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Old 20th January 2008, 12:31 PM   #33
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and a pic of groundplane vs in cabin to give an idea of the cars response.
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Old 20th January 2008, 01:34 PM   #34
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So.....

looking at the cabin gain, I'd need a speaker that is flat from 45Hz down, and from 45Hz rises up to 80Hz or higher by about 18dB, depending on how I interpret the graph.

I've had a go and can muster a rise of around 10dB which should help a lot.

I've had a go at modelling a tapped pipe of similar size with my JBL 2020h driver. It seems to resemble the BP curve. Is there anything wrong with having such a sharp peak on the TP ?

Cheers,
Rob
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Old 20th January 2008, 07:03 PM   #35
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Quote:
Originally posted by RobWells


You hit the nail on the head there Eva

Rob.
Now you understand why I design my bandpass enclosures in such a strange way Most people think that I'm crazy when they simulate one of my designs and see the tilted SPL plot. The dip around 50Hz due to the distant tuning frequencies is actually quite useful when you get it.

The roll-off below 40Hz happens because the cabin of most cars is not that rigid and leaks the lowest LF to the outer world resulting in not enough gain. You can easily compensate this with a parametric EQ or by applying some treatment to the large frame sheets and panels, but be aware that cabin gain interacts with such a low tuned rear-chamber resulting in more 30Hz port gain than predicted by software. I don't usually aim for much output below 40Hz, it is around 100Hz where most car systems suffer a (depressing) lack of SPL.
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Old 20th January 2008, 07:29 PM   #36
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Thanks Eva,

I've treated the roof, rear doors, and tailgate so far. Have to order more to do the rear quarters and the front doors. Done the floor too.

It's a shame my comps aren't fitted yet as I'd love to see what horrors await me there.

I also own a pair of peerless 6" 850518 drivers which look good in the BP, but I'm not sure if they'd be loud enough? They hit xmax at around 98dB @ 30Hz - if the 10mm xmax is to be believed.

Cheers,
Rob
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Old 20th January 2008, 07:58 PM   #37
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Peerless 830518 specs
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Old 20th January 2008, 10:39 PM   #38
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Thinking out loud here....

What if you make the sealed box so small that it has a sloping response from ~90Hz down. Then add a bit of bass boost at 35 - 40Hz to flatten the response below 45Hz. ??

Takes up less room than all other options, even if it means buying 2 more subs....

plot done with 4 perfect 12.1's (should have enough displacement for a bit of eq)
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Old 20th January 2008, 11:06 PM   #39
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If space is an issue and the driver/amp can handle the boost without excessive distortion, then why not? Otherwise, I prefer to design for all the acoustic efficiency I can get and attenuate as required since distortion goes down, not up.

GM
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Old 20th January 2008, 11:22 PM   #40
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Hi GM,

Here's the thing...

I want to use my suv for the reasons it was built for, hence huge enclosures impact on the cargo space I| need for a fridge, camping gear, spare parts / wheel etc....

That doesn't mean I don't want great sounds when I'm out 'on the trail'

I can dedicate more volume if it is no more than 6" high as I'm making a set of storage enclosures to hold tools etc and the back half of the enclosure is spare.

I have an amplifier that can do :

425 @ 4ohms
800@ 2 ohms
1100@ 1 ohm

RMS


I already have 2 infinity perfect 12.1's
I have 1 JBL 2020H
I have 2 peerless 6" 830518 drivers.

I don't mind an enclosure approx 3' x 1' x1' , and external ports can be accomodated.

What did you think of the TP I posted with the big gain at 80Hz ?



Edit: Should have said that the small box was not due to space concerns but due to trying to counteract the cabin gain.

Also doing 'selective eq' in car audio means buying more stuff. Right now I have various amps with xo's, plus an 'ok' alpine with adjustable freq's

Rob
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Old 20th January 2008, 11:34 PM   #41
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Quote:
Originally posted by RobWells
Thinking out loud here....

What if you make the sealed box so small that it has a sloping response from ~90Hz down. Then add a bit of bass boost at 35 - 40Hz to flatten the response below 45Hz. ??

Takes up less room than all other options, even if it means buying 2 more subs....

plot done with 4 perfect 12.1's (should have enough displacement for a bit of eq)
This would work, but at the expense of reduced efficiency.

BTW: Don't trust the sharp peaks predicted by hornresp.
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Old 21st January 2008, 12:21 AM   #42
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Quote:
Originally posted by RobWells

What did you think of the TP I posted with the big gain at 80Hz ?

Like I said, you decide on the trade-offs and build what works best overall for your app. I have no experience with such an extreme BP alignment, but it's not one I'd even consider. The few I've done for others were low Q 4th order with a rising response and could be EQ'd flat enough with just the receiver's bass control. Not the last word in HIFI, but hardly one note boomboxes either that seem to pervade car audio.

Note that TPs/THs can be EQ'd somewhat with damping.

GM
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Old 21st January 2008, 01:23 AM   #43
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Haven't tried to draw it out, so don't know how much effort it will take to make the drivers fit, but the Peerless in a clamshell isobaric can be tuned low with obvious rising response and a big dip around the cabin gain peak with some much needed gain above it in ~81.33 L plus all the internal baffling. Since isobaric doubles Xmax, the SPL is more than enough if you have a 200 W amp into 2 ohms. The peaking at Fp can be damped if it's obvious/objectionable:

GM
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Old 21st January 2008, 01:25 AM   #44
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plot:
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Old 21st January 2008, 02:28 AM   #45
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Excuse me, but how can isobaric double Xmax?
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Old 22nd January 2008, 04:01 PM   #46
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hi Eva - ya ever build Karlson boxes? - heres a forum to discuss K's
http://gainclone.com/Karlson/index.php?board=1.0
Freddy
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Old 22nd January 2008, 04:27 PM   #47
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Hi GM,

That peerless horn does look like it would fit the cabin gain well.

I'm going to build a BP with the perfect 12.1's, and also have a look at building that horn aswell.

It will be interesting to see how they measure up to the sims.

Thanks for all the help so far GM and Eva.

Really appreciated.

Rob,
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Old 29th January 2008, 01:55 AM   #48
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Quote:
Originally posted by Eva
Excuse me, but how can isobaric double Xmax?
Hmm, in my haste/whatever I see I over simplified to the point of misleading, sorry!

Down low, the increased box stiffness of isobaric yields a higher acoustic efficiency, i.e. greater gain for a given excursion, but as compliance increases with increasing frequency the gain rolls off, so the sim's rising response will flatten out on the top end. Hopefully not so much as to cause a major broadband dip in the in-SUV response.

GM
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Old 7th February 2008, 07:14 PM   #49
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Ok I've finished up on a quick test box for the bandpass box.

Following are ground plane vs winISD model, a couple of pics of the box (yes it's ugly but made from off cuts etc..) and a groundplane vs in car plot. Looks like the huge dip at 60Hz is an acoustic problem, not something eq will fix. Hopefully my mains will work down to 60Hz or so and fill in the gap.

Will do a test box for the tapped horn next and then take a break until my main speakers are fitted and measured.

Cheers,

Rob.
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Old 7th February 2008, 07:16 PM   #50
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