same size box, 2xsealed VS isobaric BP ? - diyAudio
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Old 15th January 2008, 07:52 PM   #1
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Default same size box, 2xsealed VS isobaric BP ?

Hi all,

I've been modelling my 2 infinity perfect 12.1 drivers for an install in my SUV. The logical / no brainer thing is to go for a sealed box for them. However I felt a BP could protect the drivers better so played around with a 6th order BP and came up with something of similar size but a lot more efficient. I tried to match it to the sealed box response so it plays flatter in vehicle.

I made it's upper cutoffs -6dB point at the frequency I intend to xo at. I'm hoping the upper slope will shift nearer to a 12dB slope once cabin gain is involved. Then I can add a 12dB slope to it to get me back to 24dB/oct.

I have around 400W per driver so power is not a concern. Also my amp has a 30Hz high pass /24dB filter so I would be able to protect the BP below its lower tuning frequency.

Any thoughts ? - will a 4" port be enough for the 30Hz tuning? Is there anything fundamentally wrong with what I've modelled?

Cheers,

Rob.

The pic below shows 2x infinity perfects sealed against 2 isobaric loaded in a BP.
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Old 15th January 2008, 07:55 PM   #2
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TS specs for the Infinity perfect 12.1 's
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Old 15th January 2008, 07:59 PM   #3
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What do you mean by protect the drivers better? The 6th-order box will unload the cone below resonance just like a normal vented box. However it's increased efficiency will mean you don't put in as much power.

By Small's theory the port is too little area, but there are plenty of people happily living with that kind of vent speed in car audio land.

Cabin gain won't help with the steepness of the upper roll-off, in fact it will make it even worse. Also it looks quite peaky, probably a punchy sound some will like a lot.
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Old 15th January 2008, 08:14 PM   #4
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Protect as in it's going in the back of a 4x4 that's going to have camping gear, tools, whatever thrown in the back - easy to put a hole in a driver / get driver wet from having tailgate open.

I have a 30Hz high pass on my amp that would protect it below cutoff btw)

Cabin gain yep I got it wrong - was thinking of gaining as freq increased, not decreased. (As it does on the low end.)

If I increase the port area the port gets pretty long.

Maybe just stick to sealed and fit some heavy duty grilles.

I thought it might be fun to build something different for a change though.

Rob.
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Old 15th January 2008, 10:53 PM   #5
GM is offline GM  United States
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Greets!

Since a tapped pipe is a 6th order BP, I figured a single driver in ~the same size cab as your isobaric BP (~113.5 L) would perform adequately in a vehicle even without the second driver's extra Xmax/power requirements, especially after cabin gain is factored in. Anyway, this one at 35 V (306.25 W) doesn't reach Xmax in half space till ~16 Hz, so even with the windows down it can in theory make you deaf over time if you choose and you have a spare driver in case you break one.

GM
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Old 16th January 2008, 03:06 AM   #6
Eva is offline Eva  Spain
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This post describes my way of aligning 6th order bandpass enclosures in cars. Unfortunately it seems that I posted it in the wrong place...

http://www.diyaudio.com/forums/showt...83#post1392883
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Old 16th January 2008, 04:41 AM   #7
GM is offline GM  United States
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So it would seem. FYI, the tapped pipe I posted has plenty of Xmax available to EQ its ~flat response to a rising one without losing any of its bottom end output potential, so at normal levels it will require little power compared to a typical car audio BP. Anyway, nice designs, you obviously put a lot of effort in them.

GM
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Old 16th January 2008, 05:52 PM   #8
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Hi Eva,

Yes I saw that post when I searched. The only thing is when I place the tuning points that far apart I am getting a bit of droop between them and losing some of the lower output.

Thats why I tried to copy the response of a sealed box, at least from 30 Hz up...

Rob
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Old 16th January 2008, 05:56 PM   #9
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That looks like a good tapped horn GM - I have the driver inserted into hornresp but did not achieve anything like that.

EQ becomes a bit more complicated in a simple car setup, but I can use my ultradrive for test purposes.

Cheers,

Rob.

I'm thinking I could knock up one of each type of box and have a measure / listen.
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Old 16th January 2008, 09:55 PM   #10
Eva is offline Eva  Spain
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You attempted to use the tuning frequencies that I recommended with a too small rear chamber and a too big front one. Try to play a bit with the chamber volumes and see if you can get a better response (but some drivers just don't allow to tune the box that way).

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.

On the other hand, a 6th-order bandpass with tilted down response and some clean output up to 150Hz integrates very well with front speakers. Remember that even 24dB/oct LR crossovers require over one octave of overlap (and phase matching has to be considered too). If you use a wide front port and place it in front of the driver, the frequency response above port resonance will not fall as predicted by software, and phase will not rotate as dramatically.
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