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Is modeling sealed sub waste of time in car?

2K views 23 replies 9 participants last post by  sqshoestring 
#1 ·
I was inquiring on a classified ad that when I modeled this particular sub using T/S parameters I got a much larger box volume of like 2.5 cu ft. for a Qtc of about .82. instead of the sellers recommended .88 cu ft. His reply follows.
Like i posted and PMed, you guys can NOT rely on modelling programs for sealed box woofers in a car! They do NOT account for the car's cabin gain, which changes everything! All those programs do is show you the response charachteristics in an anechoic open-air environment.
Trust me....0.88 cube sealed per driver is the KEY!
This has not been my experiance as the relative response of higher vs lower Qtc alignments have been predictable. With the bump in response from typical cabin gain adding the bump from a higher Qtc alignment seems to muddy up the upper bass and makes the transition to the midbass more difficult and requiring lower LP. A big friggin' 'if' is if the T/S parameters are accurate, but a sealed alignment is pretty forgiving for some production variation compared say to ported.
By the way the sub in question is an RF 12 Audiophile w/ Fs=21 hz, Qts=.38, and VAS=9.182 cu ft.
What do y'all think?
 
#2 ·
How much eq power do you have?

If you have a decent eq you can tailor the response of a sealed sub to either dial in a little more low end or dial out some upper bass bloat. It's true that the modeling programs don't tell you what the in car response will be because the software doesn't know unless you measure and input it.

If you don't have much in the way of eq, then you'll want to tailor the box for the desired frequency response.
 
#5 ·
So if I had adequate EQ ability I can pretty much chuck a sub into any size box that would accomodate my space requirements? Makes me wonder why I have spent so much time lately comparing several of the lauded subs here on the forum when I just could have EQ'd my way to good performance. Still seems to make more sense to choose a sub carefully and optimize w/ install first before tuning w/ EQ.
However, maybe my less is more attitude is holding me back as I do have some EQs in the garage, just never used them. I've been even loathe to use my tone controls, until recently.:rolleyes:
 
#3 ·
Sure there is cabin gain, but the sub itself needs to be aligned with the "air spring" created by the box. I'm a believer in .707 as a golden ratio, so I use it.

I have found better sound quality by using these size boxes compared with the manufacturer's ratings. I've tried this and proved it to myself in three different situations:

Eclipse SW8000, the infamous LMT sub. A 10" sub that is recommended in a .8 to 1.0 cubic foot box according to Eclipse, but it sounds REALLY boomy and muddy in that box. Changing to a 1.8 cubic foot box smooths it out to .707ish and it sounds flawless.

JL Audio 10W3, according to JL Audio it should be in a .75 cubic foot box, or as low as .60 cubic feet (if I recall..this is the v1 version so its old!) Anyway, I build this first and sure enough it was too boomy and very muddy. I went to double that size, and the response smoothed out and sounded much more like music.

Fi Audio SSD 10's, again put them in a small box, again got the muddiness. Switched to a bigger box (about .7 cubes per sub), and my response was near perfect.

Truth is, cabin gain is tricky but you can generalize it as a boost like a hump centered around ~80hz. You can always EQ it out if you need to, but most like a little extra in that range.


The deeper truth is that .707 has a strong meaning to me, since it is the base number to virtually everything that occurs in nature, from the spiral of the galaxy to the harmonic scale of music. I don't feel as if it is a coincidence that .707 works as a box Q, given the way it prevails through everything.
 
#6 ·
Sure there is cabin gain, but the sub itself needs to be aligned with the "air spring" created by the box. I'm a believer in .707 as a golden ratio, so I use it...
The deeper truth is that .707 has a strong meaning to me, since it is the base number to virtually everything that occurs in nature, from the spiral of the galaxy to the harmonic scale of music. I don't feel as if it is a coincidence that .707 works as a box Q, given the way it prevails through everything.
Your first statement is what I have experianced w/ regard to good sealed sub performance in car. Qtc of .707 or a somewhat lower or a bit above has sounded optimal for me for any given sub.
However, I have been playing around w/ the DIYMA12 for the past four days and in alignments from .62 to .7 it still sounds a bit bloated and not having good impact when crossed over higher at say 100hz. A lot of folk say they have good performance at a .55 Qtc in a 1 cuft net box. So I dunno.
Your last statement kinda 'mystifies' me re. nature and so forth. Fill us in as it sounds interesting.
 
#4 ·
Like i posted and PMed, you guys can NOT rely on modelling programs for sealed box woofers in a car! They do NOT account for the car's cabin gain, which changes everything! All those programs do is show you the response charachteristics in an anechoic open-air environment.
Trust me....0.88 cube sealed per driver is the KEY!
By that logic it doesn't make sense to model any alignment (let alone sealed).

Yes you have to take cabin gain into consideration for overall frequency response, but that doesn't make modeling useless.
 
#10 ·
I did some calculations based on boxes I've had over the last 10 years and it appears that whether I made them or not, most were in the high .7s to mid .8s so there has to be something to it. All sounded good to me, although I'm no pro.
 
#11 ·
It may not offer the absolute results you are looking for, but it does offer an interesting point of comparison does it not? Especially given that vehicle cabin gain is a constant.

The one caveat, and one that I often encounter... is that many drivers have a wide variance in small signal parameters from the stated manuf. spec. You also have to consider how well small signal modeling correlates with large signal performance.
 
#12 ·
I was inquiring on a classified ad that when I modeled this particular sub using T/S parameters I got a much larger box volume of like 2.5 cu ft. for a Qtc of about .82. instead of the sellers recommended .88 cu ft. His reply follows.
Like i posted and PMed, you guys can NOT rely on modelling programs for sealed box woofers in a car! They do NOT account for the car's cabin gain, which changes everything! All those programs do is show you the response charachteristics in an anechoic open-air environment.
Trust me....0.88 cube sealed per driver is the KEY!
This has not been my experiance as the relative response of higher vs lower Qtc alignments have been predictable. With the bump in response from typical cabin gain adding the bump from a higher Qtc alignment seems to muddy up the upper bass and makes the transition to the midbass more difficult and requiring lower LP. A big friggin' 'if' is if the T/S parameters are accurate, but a sealed alignment is pretty forgiving for some production variation compared say to ported.
By the way the sub in question is an RF 12 Audiophile w/ Fs=21 hz, Qts=.38, and VAS=9.182 cu ft.
What do y'all think?
I'm going to guess that something's wrong with the quoted t/s parameters. What's the rated efficiency or sensitivity of that driver? The t/s parameters suggest an efficiency of just under 90dB/1W/1M, which is quite efficient for a car audio subwoofer.

On design via t/s parameters, NPDang's comments should be given serious consideration. Not only do some manufacturers appear to engage in a bit of "specsmanship", it should be noted that t/s parameters are SMALL SIGNAL parameters (when last have you driven a car audio sub at 1W? :)), and will shift as (1) the cone moves noticeably, and (2) if the coil gets warmer, both of which can be expected to occur at higher power levels. Coming up with a good car audio subwoofer alignment therefore requires not only considering the ACTUAL t/s parameters, but how you expect the driver to perform at higher power levels, and of course box size limitations as it's got to fit somewhere in the car of course. With that in mind, if I'm considering a sealed alignment for a particular driver, I usually go for a Q between 0.8 and 1.0 (higher Q for drivers with lower Xmax or observed excursion limitations), and fine-tune the results with polyester fiberfill stuffing.
 
#14 ·
I'm going to guess that something's wrong with the quoted t/s parameters. What's the rated efficiency or sensitivity of that driver? The t/s parameters suggest an efficiency of just under 90dB/1W/1M, which is quite efficient for a car audio subwoofer.

On design via t/s parameters, NPDang's comments should be given serious consideration. Not only do some manufacturers appear to engage in a bit of "specsmanship", it should be noted that t/s parameters are SMALL SIGNAL parameters (when last have you driven a car audio sub at 1W? :)), and will shift as (1) the cone moves noticeably, and (2) if the coil gets warmer, both of which can be expected to occur at higher power levels. Coming up with a good car audio subwoofer alignment therefore requires not only considering the ACTUAL t/s parameters, but how you expect the driver to perform at higher power levels, and of course box size limitations as it's got to fit somewhere in the car of course. With that in mind, if I'm considering a sealed alignment for a particular driver, I usually go for a Q between 0.8 and 1.0 (higher Q for drivers with lower Xmax or observed excursion limitations), and fine-tune the results with polyester fiberfill stuffing.
Here are the RF specs. http://www.rockfordfosgate.com/library/datasheets/rfa-412_812-ts.pdf It is the 4 ohm version on the left.
 
#16 ·
.707 and 1.618 are directly related to each other. I can't remember exactly how (but I will if you want me to.)

I do know they are one and the same when it comes to what they mean in a mathematical sense.
 
#18 ·
...Its been a while since I studied this in massive depth, but if I remember correctly that .707 associates with the golden mean on a 3D level, meaning for all intents and purposes on a spherical level. I'll have to keep digging but it is 100% connected, this much I know. Which is why when you model at .707, you are showing the most area under the curve in output.
 
#22 ·
I have been told that as the sub plays louder and heats up more, the Q comes up a bit. So, building on the low side of the Qtc might give better performance at high output.
 
#23 ·
That makes so much sense and I can get my head around that! Especially if a sub is to be run hard toward the end of its RMS rating maybe aim for a bit lower Qtc. I guess I must be okay since I tend to run lower power than what my current/past subs are max rated for...and also tend to run lower Qtc anyway.
 
#24 ·
I would never put a sub in my car without modeling, but not much for Qtc as what kind of response I get out of that sub. I'll vary the Qtc to make it closer to what I want. If the sub is in a box, then it is easier to toss it in the car and that is only time I would not model first. All .707 gives you is ideal tuning for the driver/box, it has little to do with your car and what you need unless you picked the perfect sub. But everyone has EQs anyway so not like it matters that much, I mean it does and it does not. I like lows, so I tune a sealed on the large cf side when I have to run a sealed because they never have enough bottom. No, I've never run an IDMax/etc.

I have this sony P5 that is the worse I've ever heard with changing with increased power. I had a 300rms on it that was messed up and not making much power, a PPI with the outputs falling off the board. Then I stuck an old school SS on it and it went way louder...and the louder it went the worse it sounded. Max power it would make 50Hz peak and I swear that was all it made. Low volume it sounded ok but cranking it was a one note wonder. It is in a spec sealed box and I stuffed it. So I just test amps on it now, once I got my IB in. It would be a great SPL sub, and maybe that is what it is.
 
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