same size box, 2xsealed VS isobaric BP ?

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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?



The pic below shows 2x infinity perfects sealed against 2 isobaric loaded in a BP.


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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.
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.


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.



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Joined 2003
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.

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...



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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.



I'm thinking I could knock up one of each type of box and have a measure / listen.
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.
Hi Eva,

Increasing the rear chamber and decreasing the front chamber just makes the curve more like a camels humps. I lose the 10 dB rise of the top tuning and the curve just looks worse. What you're saying actually makes the curve flatter rather than having a slope ?

Maybe the drivers just don't work in a BP.

I think I'll measure the cabin gain in the car before I continue.

Your driver is difficult due to the high Qts. Drivers with a Qts around 0.3 are much easier.

This is more or less how I would do it... The sealed box plots are included for reference.

Frequency response:

Cone excursion:


In the 6th order bandpass, rear chamber Q is 3 and front chamber Q is 7. A Q of 3 is easily achieved by placing the vent near one corner of the box. Power for cone excursion is 100W in both cases, note the big difference in favour of the bandpass despite the increased output at all useable frequencies. In practice, inside a car, +/-5mm are enough for 500W or so (a long excursion driver is neither required nor desirable).

The frequency response is intentionally shaped to be 12dB down at 30Hz with respect to 60Hz, since cabin gain will tend to exert the opposite effect.
GM, can I be cheeky and ask for a shot of the input screen of hornresp ? I work with ply all day so it's easy to knock up simple test boxes but the tapped horn stuff just doesn't come easy to me. I'm just getting used to the basics of FLH's and then along comes the tapped horns and I'm having trouble keeping up:eek:

OK Eva I'll definitely build the version you've suggested there and post measurements here.

I'd also like to have a go at the tapped horn version from GM.

Would be an interesting comparison.

I promise to build and measure them both btw - am not just dreaming here.



my original BP against my revised BP against Evas BP against twin sealed


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Are you really going to build it? This is great! If you can post plans of internal layout before building the prototype, I can tell you my opinion.

These are some links to pictures of other car bass stuff that I designed...

This one used a 15" RCF driver. The two front vents had to be enlarged considerably in order to get them tuned around 100Hz with the length required for proper connection to the cabin once the enclosure was installed in the car. cordoba/base/PC170006.jpg cordoba/base/PC170007.jpg cordoba/base/PC170009.jpg cordoba/base/PC170008.jpg

Have you ever seen a folded bass horn in a car? This one was never completed because the man doing the woodworking was not up to the job of doing the final expansion part. Anyway, the shape of the car continued the expansion well enough for over 150dB of clean low-cone-excursion 40Hz-200Hz bass :D golf IV/base/P5240004.jpg golf IV/base/P5240006.jpg golf IV/base/P6010001.jpg

This is a 6th order bandpass enclosure with a 10" inch driver in a Honda Shadow 750 motorbike :eek: The enclosure also contains a 4 channel amplifier, two deep-cycle back-up batteries and a head-unit... The vents are in the bottom side, like the amplifier for proper cooling. Crazy... shadow 750/base/honda shadown corregida.jpg especiales/honda shadow 750/base/Imagen 013.jpg especiales/honda shadow 750/base/Imagen 008.jpg especiales/honda shadow 750/base/Imagen 007.jpg especiales/honda shadow 750/base/Imagen 009.jpg especiales/honda shadow 750/base/Imagen 039.jpg especiales/honda shadow 750/base/Imagen 066.jpg especiales/honda shadow 750/base/Imagen 071.jpg

Got some motivation?? :D:D:D
Hi Eva,

Yes I will build it - maybe not in days but it will be done - have not built a box for a while now but love doing stuff for the sake of it. I loved the bike install btw :D

The main reason for me doing this is I have never heard a 6th BP box and have read so many opinions on the net about them... ie: they're rubbish slow crap... through to wow low distortion, like horns etc... For the record I run a pair of labhorns in my listening room for subs so like the 'horn sound'


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