First up, Ive done a search and havent been able to find an answer to these questions.

In the search of good SQ (and to try reclaim my boot) Ive been researching the possibility of putting a 12" up front in the passenger footwell of my VT sedan. Using an old jaycar subbox (roughly 30L internal volume) Ive sat a 12" JL sub up front, firing towards the engine bay, and the difference in SQ is enormous. SPL doesnt seem down at all either, with the setup being the same (if not louder) than when I had the 2 subs in a ported box in the boot. Im also surprised that for the most part the bass still seems to be coming from behind (or rather, enveloping me) rather than being obviously to me bottom left. But thats the problem as well, for PART of the time its painfully obvious where its coming from.

At what frequency does bass start being directional? Im thinking that if I LP my sub just a little below this frequency I can avoid the problem altogether, and filter it so my mids (8" drivers) take over from there.

The other idea I had, is when I make a Fibreglass enclosure in the footwell, it to port the box and yet run the port outside the box, underneath the dash and exiting in the drivers footwell. Would this also have the desired effect of balancing the sub? Im also tempted to port the box as even though the sealed box up front is now VERY punchy and destinct, some of that lower frequency response is obviously a bit down.

With a quick measure just now I managed a figure of around 33L in the footwell, which is fine for a sealed box but probably too small to run it ported, but I do know theres a big foam packing block in the footwell under the carpet, I believe if I remove this, I could probably gain around another 10L, which hopefulyl should be JUST enough to port the enclosure.

Another quick(?) question, whilst tuning the setup before (subs running off a JL audio 1000/1 amplifier) I noticed it has an RCA passthrough with its own filter settings. My other amp, a JL 300/4, has the option of being run off 1 or 2 sets of RCA inputs. Im currently using the h/u sub rca into the mono, and the front rca into the 300/4 (set to run 4 channels off only 1 set of inputs), but cause the filters are setup so you can only HP OR LP. Its fine for my tweeters to just have a HP, but Id like to both HP AND LP my mids. Could I run my front RCA into the monoblock, using the monos filters to get me a good sub signal (got an infrasonic and LP), then use the passthrough rca with a HP (say 80hz?) into the 300/4, so I can then use it to LP the mids at say 2khz, and HP the tweeters at whereever I decide? Im not sure if any of that made any sense, but basically I just want to get my mids only playing between say 70hz and 2.5khz, or thereabouts after final tuning).

The subbass should be 63Hz or below up-front, but 70Hz might be ok.

yes, you can use the filters in the 1000/1 then pass-thru to the midbass/tweeters with the filters in the 300/4 - that's what it's designed for

I would recommend not porting the box up-front, but use the eq in the amp set low (say 20Hz or 30Hz) and boost it about 4-6dB to pick up the bottom end.

don't forget the thickness of the sub box walls will decrease the volume of your sub box in the footwell

Yeah I had somewhat accounted for the wall thickness, but but using fibreglass hopefully I can get the most out of the limited space.

I'll play around with my 8" once theyre in and see if they play low enough to filter the sub off at 63hz (theyre apparantely pretty damn good 8"s so I would hope they can)

My main concern with the pass through RCAs is that then I would be running virtually the whole system off a single set of RCAs. I dont mind the loss of tuneability at the H/U (nothing special anyhow), but rather would I be "splitting" the rca voltage. Im not sure how the JL amps passthrough work, whether the passthrough outputs are just parrelleled to the inputs. Ive only got 2v RCA outputs (and I realise that realistically its not even that).

Ill have a play with boosting the lower frequencies of the sealed box in the next few days. I take it by using sealed I can just disable the infransonic filter? Or should I still keep it on but at say 20hz?

RCA Audio signals should suffer voltage loss from being split/passed on, once or twice.

Remember, your LPF isnt a brick wall, meaning even if its set at 24dB/oct, at 63Hz, it will still be playing 95Hz(very directional) at ~-12dB, and once you have your system turned up, that will still be audible, albeit by then it should have blended with the mids....

The best way to find out when it stops being directional, is to play with your xover settings once all is said and done, and both mids and sub are in. That way you get a realistic result, rather than all the theory work that can be put on paper(ie, what is said here)...

I find running my sub at 63Hz @ 24dB/oct, in the rear of my car, it is still directional at times, however i am only running 6.5's up front, so the lowest i can pass them and still be able to turn it up is 63Hz. That said, i have found it a much less directional sound, albeit much less 'oomph'(being that the 'oomph' is now powered by 40wrms and 6.5" rather than 700wrms and 12"), but the sound also blends alot nicer...

Just my 2c.

Alex

if it helps at all, I have my sub behind the passanger in the rear passanger footwell. Ive found that recently I've been low passing it at 50hz, rather than 63. the directionality disappears (although I do have 3m of time alignment) and your mids should be up to the task of playing low enough to cover it.

not quite. Cable with a voltage of 2 and an amperage of say 1 when split in half will result in two cables with a voltage of 2 and an amperage of 0.5. Voltage is constant, amperaged is halved everytime the cable is split

If the sub is in the front, then it shouldn't matter too much how high / low you cross it (within reason, of coruse).

As long as the sub is correctly level matched with your midbass drivers, there is no major peakiness in mid-bas or subbass, and sub and midbass drivers are hitting in time, localisation of subbass frequencies should not be a problem.

My sub at the moment is sitting in my boot crossed at 160hz and my subbass is up front roughly 95% of the time - the other 5% is only because I've been too busy to put the extra tuning time into it to make it 100%.

Only issue I can see is that the sub is offset to one side of the car - this could potentially pose a problem but aside from this you should be able to get it good

Thanks to the concept of frequency / amplitude masking, those remaining higher frequency tones should (I belive) be inaudiable, even at high frequencies.

Our ears have this interesting characteristic whereby if there is a dominant frequency (i.e. high amplitude) close to a submissive frequency (i.e. low amplitude) our ears esentially mask (i.e. block out) the less dominant frequency.

I belive the rule of thumb was that any less powerful tone will be ignored by your ears if it is withing 100hz of the more dominant frequency. This phenomenon occurs even with a difference of 3dB, do by the time the sub-bass frequencies are down by 12dB (i.e. 95hz) or even down by 6dB (78hz) any tones reproduced will be inaudible.

Aside from this, it's my understanding that frequencies below 100hz are already non-directional to human ear (although I may be mistaken)?

All else aside, when it comes to upfront bass in a car, the frequencies being played are indeed only part of the story

Pete is correct on the frequency masking, but without the papers in front of me, I cant quote the actual numbers.

And since when was 95Hz directional? It has a wavelength of 3.61metres, so the distance between your ears (~20cm) will produce a phase difference of around 5%, with a change of ~20degrees. No chance you'll be ever able to directionalise that frequency. Play tones to confirm.

The harmonics produced by vibrations and rattles will though....

A big reason why frequency response can be so much more important then many peolpe realise.

Thank's to the principles of frequency masking, even a relatively modest gap in response (lets say, a 3dB drop from 80hz-120hz) can suddenly make it appear as if those frequencies are completely missing, because you ears ignore those frequencies.

It's not only gaps you need to watch out for either. If you happen to have a peaky bottom end (lets say +3dB from 50hz - 80hz) and a peaky midbass (lets say +4 from 150hz - 250hz) then it can have the same effect - even though your system is perfectly capable of playing that 80hz-150hz range, it's as if it isn't there because it is masked by the dominating subbass and midbas frequencies immediately around it.

It's actually a very interesting principle, with a number of musical artists apparently taking advantage of the principle in order contain hidden lyrics within their music.

That said, frequency response is not the be-all-and-end-all of audio. It's definately one of the most critical however - get your resposne perfect and your sound will improve massively, however actually getting it perfect is another matter altogether... even with very comprehensive EQ and crossover adjustment, it's almost impossible to get it perfect.

the JL passthrough can be filtered (HP, LP or straight thru) - by switching the switch. you can set it to use the LP filter as per the input stage, or use the separate control for HP/LP. it has an internal buffer so no signal loss.

Cheers guys. Putting 8s in the front doors so I'll fade them into the 12" around the 50hz mark and experiment round there.