Gain of Firing 4th Order BP Against Rear Glass? - Realm of Excursion



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Old 08-14-2012   #1 (permalink)
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Default Gain of Firing 4th Order BP Against Rear Glass?

My main question is if the 4th order ported through the rear deck will sound much louder than the ported box on the rear seat despite having similar anechoic SPL responses (see graphs below)?
Would porting a 4th order up through the rear deck against the rear glass effectively turn a sedan into a hatchback? I'm speculating that by facing the subwoofer against the rear window it is almost like running a Speaker in 1/8th space (+9dB gain) along with pseudo-horn-loading? Correct?

Here are some models (in WinISD Pro 0.57a) of various enclosures for two 12" Rockford Power HX2.

Orange: A ported box, 3.3CF net tuned to 36Hz on 1200W RMS.
Cyan: A bandpass enclosure, 4.5CF (2CF sealed/2.5CF ported @ 55Hz).
Yellow: A 3CF sealed box.



Looking at the graphs, the ported box is the loudest (what I'm running right now). The bandpass box features a similar response curve as the ported enclosure. For comparison I've plotted a middle sized sealed enclosure.

I've heard similar subwoofers (12" Rockford T1s in a small 2CF sealed box getting 1500W RMS) in a Toyota Estima (minivan) and compared to my ported box this system had MUCH more impact. It wasn't even close (made your nose tickle, vision blurry). We are in a similar power range with similar cone area so in theory I should have the sealed box dominated by a decent margin at 35-45Hz by a good 6dB. That certainly didn't sound and feel like that was the case. I reckon having the subwoofer in the rear seat facing forward despite having the cabin sealed from the trunk is still resulting in severe cancellation with minimal cabin gain.
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Old 08-14-2012   #2 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

Take all this with a handful of salt, I'm new to 4ths and am basing my thoughts on observations more than knowledge of how they actually work.

That's a pitiful response from the 4th, which might be the subs themselves, but try dropping the sealed side down to 1.5cuft and bump up the ported side as much as you can, at least to a 2:1 ratio. 55hz is kinda high for a 4th as well, perhaps bring it down in 2-3hz increments until the gain levels out. I would think 45-50hz would be better.
Even the sealed box seems to have a poor response, dropping that a whole cube might give a wider response.

Loading off the window might gain you something, but you'd probably have to angle the mouth so as to not choke it with a lesser distance from the edge of the rear seat to the window than the distance across the depth of the port mouth.
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Old 08-14-2012   #3 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

Quote:
Originally Posted by 666WMD View Post
That all this with a handful of salt, I'm new to 4ths and am basing my thoughts on observations more than knowledge of how they actually work.

Good thing you covered your ass on this one because...

That's a pitiful response from the 4th, which might be the subs themselves, but try dropping the sealed side down to 1.5cuft and bump up the ported side as much as you can, at least to a 2:1 ratio.

wrong. there is no reason to go 2:1 or more unless you are designing it just for spl. Im fairly certain Jud explained this one to you, but the sealed portion controls the lower end of the response, and the ported section controls the upper end of the response. the response he has there is nearly identical to the ported box, far from pitiful.

55hz is kinda high for a 4th as well, perhaps bring it down in 2-3hz increments until the gain levels out. I would think 45-50hz would be better.

wrong. this depends entirely on what the objective is. this tuning freq is the upper cut off. a 4th with a port tuning of 45 will drop off shortly after that, making the mids take up a large portion of the upper bass. on the opposite end, a 4th with a tuning of 80 hz could be used to have the response extend much further, at the cost of some efficiency. remember theres an efficiency/bandwith compromise. as one increases, the other decreases. 4ths are very, very versatile, and the cut off should be determined by where the mids cut off.

Even the sealed box seems to have a poor response, dropping that a whole cube might give a wider response.

wrong. dropping the volume on the sealed box would decrease the low end extension. increasing it would increase the extension.

Loading off the window might gain you something, but you'd probably have to angle the mouth so as to not choke it with a lesser distance from the edge of the rear seat to the window than the distance across the depth of the port mouth.

loading off the window will certainly not gain 9 dbz, but i would expect it to make some improvement. loading off the window should help, as well as having some gain from having the port directly inside the cabin
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Old 08-14-2012   #4 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

These subs need 1.25 feet per sub sealed. So at a minimum you will need 2.5 feet sealed, anything less and you will sacrifice low end output. You could stuff the sealed with poly-fill and gain a little if space is an issue. But I don't think the sealed section is your problem. The ported section needs to be 2:1 of that of the sealed section. So basically your looking at a box which is going to be around 7.5 cubic feet. That is pretty big in your application. I suggest exploring the option of a ported box through the rear deck.
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Old 08-14-2012   #5 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

Keep in mind if you use winISD it does not take into account for cabin gain.
So take the plots with a big ole grain of salt.

Or take a known box, measure it, model it, the difference is the transfer function of the car. Then add that to the model of the 4th order. In real life it isn't that easy, since the response depends on the box position and the mic/ear position. I like to use a linkwitz transform to simulate +12dB/oct gain from 60Hz down. ([email protected], [email protected] [email protected] etc) But there's also about 10dB of static gain too, since the wave is being confined. IE loading off a wall is louder than loading off just the ground (half space). WinISD assumes half space.

Edit. And any loading the 4th sees, it's likely the ported box will see as well.



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Old 08-14-2012   #6 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

That all this with a handful of salt, I'm new to 4ths and am basing my thoughts on observations more than knowledge of how they actually work.

Good thing you covered your ass on this one because...

That's a pitiful response from the 4th, which might be the subs themselves, but try dropping the sealed side down to 1.5cuft and bump up the ported side as much as you can, at least to a 2:1 ratio.

wrong. there is no reason to go 2:1 or more unless you are designing it just for spl. Im fairly certain Jud explained this one to you, but the sealed portion controls the lower end of the response, and the ported section controls the upper end of the response. the response he has there is nearly identical to the ported box, far from pitiful.

First: In his original post he said "if the 4th order ported through the rear deck will sound much louder than the ported box on the rear seat" which kinda implies he does want it louder. The whole point of giving up more volume for a 4th is to make it BETTER than a ported box, not nearly identical. Why bother if you're not going to gain anything? It doesn't even perform in the same width as the ported, so it is pitiful.

55hz is kinda high for a 4th as well, perhaps bring it down in 2-3hz increments until the gain levels out. I would think 45-50hz would be better.

wrong. this depends entirely on what the objective is. this tuning freq is the upper cut off. a 4th with a port tuning of 45 will drop off shortly after that, making the mids take up a large portion of the upper bass. on the opposite end, a 4th with a tuning of 80 hz could be used to have the response extend much further, at the cost of some efficiency. remember theres an efficiency/bandwith compromise. as one increases, the other decreases. 4ths are very, very versatile, and the cut off should be determined by where the mids cut off.

Second: What's the point in having such a high cutoff in a 4th with 2 12" SUBSONIC woofers? He might as well buy some 8/10" midbass and run them IB in the rear deck. The whole point is to get louder remember? If he want's the 45-70hz range louder, he'd be better off running different speakers in a different enclosure.

Even the sealed box seems to have a poor response, dropping that a whole cube might give a wider response.

wrong. dropping the volume on the sealed box would decrease the low end extension. increasing it would increase the extension.

Third: Dropping it would not decrease the extension very far by my understanding, it would simply increase the roll off more steeply. In order to increase the output, the sealed volume would have to be sacrificed to gain ported volume and more output. This is dependent on wanting to keep the volume approximately around the same, or it would end up being a massive box as Andy also said.

Loading off the window might gain you something, but you'd probably have to angle the mouth so as to not choke it with a lesser distance from the edge of the rear seat to the window than the distance across the depth of the port mouth.
Quote:
Originally Posted by cxa0897 View Post
loading off the window will certainly not gain 9 dbz, but i would expect it to make some improvement. loading off the window should help, as well as having some gain from having the port directly inside the cabin
Basically those subs are crap in a 4th if the modeling is anywhere near accurate. Put them in your HT and buy something better.
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Old 08-14-2012   #7 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

It will be louder. It has nearly an identical half space response, and with loading it should exceed the ported response. a hardfast 2:1 idea is the issue I have as every sub differs. I cited an example of how to extend or improve intensity in the upper and lower region. 4ths are very versatile and can be molded to meet his needs. His definition of loud may not be numbers, it may be about getting deep, or maybe even a flatter response to avoid the null typical of ported boxes. That is why you sacrifice space for them. They are "customizable" and can offer large efficiencies.
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Old 08-14-2012   #8 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

666WMD,
Basically I am not to concerned about having the subwoofers strongly produce above ~65Hz. The reason behind the 55Hz tuning is that given the box constraints in terms of size I wanted to have solid output in the 35-50Hz range (I try to peak around 45Hz as this is my favorite frequency). Tuning lower (say 48-50Hz) would reduce the peak and 55-65Hz response would suffer severely. The other reason for picking 1CF each for the sealed chambers is that this seems to be the optimal size vs. low-end extension. I ran tried 1-1.5CF and the low end was compromised too much (essentially the roll-off was much faster that of the ported box). I want a 2.5-3CF only seemed to improve the response in the 20-30Hz region with minimal improvement in the critical 35-50Hz band.

In regards to subwoofers, I have on hand a pair of SWR-1243d's but these don't model as flat as the Rockfords. Additionally I have a single Diamond Audio TDX15 and that woofer has an EBP of like 39 which would make it great for a bandpass but I'm not certain I could fit that in my trunk.

Frankly speaking, I'm none too sure why people claim bandpasses are more efficient. From all the modelling I did, the ported box will dominate a bandpass box if the volumes of the boxes are the equal(even if both are very large). The only reason I'm doing this is to direct the output of the woofer to a location to best exploit boundary/hornloading. I'm also under the impression that the lower frequencies with a wavelength of over 40ft will be enchanced since they will have time to resonate off the rear glass many times to develop up to quarter wavelength before reaching the ear.

If the back seat were to be optimal position I would just build a bigger ported box there. However I have a very strong gut feeling that there is significant cancellation in that location since the subwoofer cones is almost in the middle of the car maybe 4" behind the B-pillar. (Also it sounds stupid but theres also the fact that a sealed setup sounded louder and hit harder (vibrated the mirrors to make them change position)).

smfonic,
The suggestion of using a ported box with the port running through the deck is not feasible (see photo of the worst possible trunk ever; double rear firewall, gasoline tank, and small trunk opening). This is why I'm running the ported box on the rear seat since there was almost 10dB attenuation by having it in the trunk (even with the stock 8" subwoofer open to help it breathe). The 4th order port will have to be external and snake between the the 15cm gap between the gas tank and rear deck. It will also have to be of sufficient length to get to the rear deck (maybe 25-30cm) while not being too long (keep tuning around 50Hz). This is why I'm restricting myself to a ported section of around 3CF since a larger ported section will required shorter runs of port or a wider port area (which is tight as it is (max. 15cm diameter tube will fit). The box also much be removable so it must fit through the trunk opening.


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Old 08-14-2012   #9 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

Bandpass boxes can be more efficient if designed that way, but it's at the sacrifice of bandwith. There is a concept called gain bandwith product. In a nutshell, the product of gain and the bandwith isus a cotant figure. Increase one, the other decreases. We can design a flat fourth from 20-80, but it won't be efficient. Or, we can design a box with massive efficiency, but only from 25-40. Its the versatility that makes them attractive. You can almost fine tune the box to your liking.
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Old 08-14-2012   #10 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

Try modeling them in 4cuft ported with 2 6" ports 24" long (should tune to around 35-36hz). If you have rear speaker holes you aren't using, 24" should be an ideal length to port from the box in the trunk up through the speaker holes and off the glass. Face the subs back, I've seen people in a similar situation to you have good results with that configuration in their Merc's and Audi's.
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Old 08-14-2012   #11 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

Hmmmmm, could you trade this car in for something else? That trunk is something out of a Si-Fi terror movie.

Anyways, where did you take these pics? Looks like AstroTurf in the background.
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Old 08-14-2012   #12 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

I would prefer not to have the subwoofer firing into the trunk since I have a ducktail spoiler molded onto the trunk and I'd worry that the flex would just crack it. I know that at tuning all of the output comes from the port but in other cases the output from the cone would also be severely reduced since the trunk is so restrictive.

I can see how by reducing the sealed portion and cranking up the size of the ported section you will have a gain thats huge in a particular passband. But after youve done this you have an application specific one note wonder. I don't think this would be nice to listen to daily. It just seems to me that the ported will give a bigger gain over a larger band of frequencies using a smaller box and to me this is seems much more efficient and appealing. I'm not trying to make a one note wonder but something that hits fairly strong between 30-60Hz.

Btw, the picture was taken in a parking lot but the lights give the ground a green hue.
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Old 08-14-2012   #13 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

Ported on the rear seat then. Subs/ports up as far back as you can get them.
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Old 08-15-2012   #14 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

Stick with firing a 4th order bandpass through the rear deck. Or if you want to go ported, you could look into series tuned 6th order bandpasses. Either way firing into the trunk is not an option.

As for 4th order bandpasses vs ported efficiency, they're both 4th order boxes. With subs that have a lowish ebp in a 4th order you can get a solid 3dB gain in the passband vs a sealed box. With mid epb subs you net about 3dB across the band vs a sealed box. a 4th order bandpass won't really be more efficient than a ported box. I just prefer having 2 12dB/oct rolloffs, instead of one 24db/oct roll off. It gives you more flexibility.

Anyways if you take that 4th order and instead of aiming for a flat response, you aim for a Q of .7, you should be able to get close to the same response as a sealed box. But with 3dB gain, and of course a 12dB/oct roll off of the high frequency response. So place that high enough that it doesn't adversely affect you. Anyways this is my prefered use of the 4th order bandpass, as it ends up having a fairly flat in cabin response(similar to how sealed boxes do, the 12dB/oct roll off is filled in by the 12dB/oct cabin gain. The trick is to get them to start at the same frequency)

I think somewhere on here I have measurements I did with some 15s in my intrepid.
EDIT: Here it is
http://forum.realmofexcursion.com/en...tml#post755105

Anyways I didn't fire against the rear glass, like you'll be doing, but honestly I don't thing that will gain much over firing back in a typical trunk car. I don't thing the rear glass will have any more loading affect on the port, than the back of a trunk would. If you really want to model this, download akabak, but it's pretty difficult to use.

Also I hit 8000 posts. Awesome.



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Last edited by Fallen; 08-15-2012 at 07:27 AM.
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Old 08-15-2012   #15 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

Explanation is much appreciated! I get it now, two 12db/oct roll-offs vs a single-ended 24db/oct roll-off.

I'm going to have a look at Akabak tomorrow.

Just a couple more questions before I start the build:
-Theoretically, let's say I have equal responses (current ported box vs. 4th order). Will firing it off the rear deck be louder than having the woofers firing from the same plane as the B-pillars (woofers are 10cm away from rear seats with twin slot ports between the seats). That's my main motivation in trying out the 4th order.

-Is there software that can model/predict the spatial response at a particular point in an echoic environment? ie. What I have in mind would to input a simplified geometric model of the vehicle environment (measure the width and height of cabin, length between A,B,C pillars, slope of windshields and depth of dash/rear deck). I'd like to see happens when I convolve the transfer function of this environment with the anechoic response of the speaker box set from a certain point in space.
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Old 08-15-2012   #16 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

Quote:
Originally Posted by recon440 View Post
-Is there software that can model/predict the spatial response at a particular point in an echoic environment? ie. What I have in mind would to input a simplified geometric model of the vehicle environment (measure the width and height of cabin, length between A,B,C pillars, slope of windshields and depth of dash/rear deck). I'd like to see happens when I convolve the transfer function of this environment with the anechoic response of the speaker box set from a certain point in space.
I've tried using LEAP (comercial software), and with it you can do basic geometry like you mentioned. And then you can specify the position of the mic, for the response at a given point.

One of the issues with the geometry approach, is the cabin is leaky. And it's difficult to measure the leakiness. You can make estimates.

Anyways I'm terrible at convolutions to be honest. If I'm not mistaken you just need to add the transfer function of the enclosure with the transfer function of the cabin. You could convolve the impulse response of the enclosure with the input and then convolve that with the cabins impulse response to get the transfer function of the whole system. If I remember right.



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Old 08-15-2012   #17 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

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Originally Posted by Fallen View Post
I've tried using LEAP (comercial software), and with it you can do basic geometry like you mentioned. And then you can specify the position of the mic, for the response at a given point.

One of the issues with the geometry approach, is the cabin is leaky. And it's difficult to measure the leakiness. You can make estimates.

Anyways I'm terrible at convolutions to be honest. If I'm not mistaken you just need to add the transfer function of the enclosure with the transfer function of the cabin. You could convolve the impulse response of the enclosure with the input and then convolve that with the cabins impulse response to get the transfer function of the whole system. If I remember right.

Convolution of the two responses should yield something relatively close if signals taught me anything. Two impulse responses convolved would result in the system response. Problem here is trying to find the cabin gain function to convolve with the enclosure gain. The math in itself would be easy enough in matlab or mathcad, or anything similar. Doing it it by hand would be miserable.
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Old 08-15-2012   #18 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

It's really miserable since we're talking about response in 3 dimensions.

WinISD is just in 1 dimension, at 1m away from the speaker in half space.

I don't even want to know what the math is like for the response vs position. I don't even know how one would plot that. Need like 4D googles on lol. Ok probably wouldn't ever plot that. But anyways sounds like a lot of effort. But the makes of LEAP figured it out somehow.


Wait, would you need to take the impulse response (or acquire the transfer function) with 3 mics to do it? This is really beyond me.



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Old 08-15-2012   #19 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

I would make several simplifying assumptions to keep things sane.. Treat the subs as a point source, and take the freq response of the box with a mic right there In front of it. Then I would just place a mic where the listening point would be using the same sweep and find the difference in responses. That's the vehicles transfer function. This can be done with the existing box or something simpler like a sealed box which should be a lot flatterThat convolved with the half space enclosure response should give a pretty good result, assuming everything is accurately measured. Trying to play around with three dimensions would be horrifying. I just don't think it'd be worthwhile
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Old 08-15-2012   #20 (permalink)
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

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I would make several simplifying assumptions to keep things sane.. Treat the subs as a point source, and take the freq response of the box with a mic right there In front of it. Then I would just place a mic where the listening point would be using the same sweep and find the difference in responses. That's the vehicles transfer function. This can be done with the existing box or something simpler like a sealed box which should be a lot flatterThat convolved with the half space enclosure response should give a pretty good result, assuming everything is accurately measured. Trying to play around with three dimensions would be horrifying. I just don't think it'd be worthwhile
Sacrificing a insanely complete model for sanity? BAH. Ima just build 4th orders with a 3:1 ratio ported chamber volume/sealed chamber and tune it to 40Hz. I CAN HAZ LOUD?

It reminds me of a modeling paradox. I forget the name of it, but it's a long the lines of the more complex the model, the less well it models the system. In all seriousness though, I've had good experience just assuming 10dB static gain and a +12dB/oct from 60Hz down ([email protected], [email protected], +24 @15). While being accurate is important, is it worth the effort? :P yo chuck, you an me should make a device that measures the impulse response of SPL vehicles. Think termlab but for impulse response. Think of the power we'll have muhahaha.



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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

Quote:
Originally Posted by recon440 View Post
-Theoretically, let's say I have equal responses (current ported box vs. 4th order). Will firing it off the rear deck be louder than having the woofers firing from the same plane as the B-pillars (woofers are 10cm away from rear seats with twin slot ports between the seats). That's my main motivation in trying out the 4th order.
Wouldn't firing the subs/port up or toward the window while in the cabin be aiming for the same effect as firing subs back in a trunk? Ie: Giving the wave as much time to form as possible before getting to the listening point?
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Default Re: Gain of Firing 4th Order BP Against Rear Glass?

Hey fallen, link me to your th pics!
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