[patsf1]

I’m looking to amp the HK underseat woofers and wondered what kind of power they would need?? Also can a 4 channel amp be bridged to power the woofers. Anyone know how many ohms the woofers are and rms handling? Thanks

[NealfromNZ]

HK are 7 ohm and the factory amp puts 125 peak watts into them.

Bridging a 4 channel amp would work fine as to increase power. 7 ohms easy to drive.

I tried a set of HKs for a while and ran them for a week without a trunk sub just to see how they would articulate bass. Had roughly 145 watts RMS into 8 available. Ran a 35hz 24db HPF filter to see how they would perform near the amps maximum power.

They seem to take it with no smoke occurring or hearing mechanical limits. But I didn’t run any form of bass boost other than running a JBL house curve and not over driving the amp at the highest part of the curve.

However, they sounded muddy with higher power levels so nothing gained when pushed harder. They also acoustically seem to drop quite rapidly below 50 hz.
SPL / volume wise not a lot of difference between say 125 watts and 200 plus peaks. Maybe a DB in it.

The HKs are at their best in that 65 - 125 hz range and without adding too much power.

Swapped them out in the end.

[patsf1]

Thank you that gives me something to go on! I might just leave those woofers as is and literally add a big ass sub in the boot. I think the system is loud and clear enough. There’s a decent sound from the HK system just needs a thump. Thinking of a custom enclosure with an Alpine R2 12” 2ohm sub. The website states only a 24 litre enclosure for the recommended size which is quite small. I guess that is the way things are going.

[Billfitz]

Car subs can use small enclosures thanks to cabin gain, which increases sensitivity on average by 10dB/octave below 80Hz. Cabin gain is how dB competitions realize the silly high numbers that they do.

[NealfromNZ]

Originally Posted by patsf1

Thank you that gives me something to go on! I might just leave those woofers as is and literally add a big ass sub in the boot. I think the system is loud and clear enough. There’s a decent sound from the HK system just needs a thump. Thinking of a custom enclosure with an Alpine R2 12” 2ohm sub. The website states only a 24 litre enclosure for the recommended size which is quite small. I guess that is the way things are going.

Sounding like a good way to go ! Had great results using an old 10 inch in trunk for a while. Was a great compromise between getting low end extension at a reasonable volume and not chasing rattles.

Went to a high output 12 and had to put a reasonable amount of sound damping in after. Trunk lid, number plate and trunk bottom panels buzzing a bit. And as Bill mentioned , you get cabin gain. I find with a modest 400 rms on sub bass can get to hearing damaging levels if turned up.

[patsf1]

Ok, thanks for all the responses. Gathering up all the equipment needed and using up some old stuff. Going with this great ‘new to me’ info with cabin gain. Thinking of using a focal fp 1.800 amp along with a focal 27 v2 sub. There is three options for the enclosure, 1 cu ft seems to be best for the lows. I’ve read a few posts on amping the under seats with decent results. Trying to figure out if there would be much noticeable gain if I’m already adding a trunk sub. Any thoughts guys?

[Billfitz]

None, especially if you high pass them at 70-80Hz. There's no point running them lower than that with a trunk sub handling the lows.

[NealfromNZ]

Also have a look at something like the new SQL 10 from stereo integrity

Will go deeper in a smaller box than the focal and power rating considerably higher so wouldn’t have issues taking the full transit power of the focal amp.

https://stereointegrity.com/product/sql-series/

[NealfromNZ]

Originally Posted by patsf1

Ok, thanks for all the responses. Gathering up all the equipment needed and using up some old stuff. Going with this great ‘new to me’ info with cabin gain. Thinking of using a focal fp 1.800 amp along with a focal 27 v2 sub. There is three options for the enclosure, 1 cu ft seems to be best for the lows. I’ve read a few posts on amping the under seats with decent results. Trying to figure out if there would be much noticeable gain if I’m already adding a trunk sub. Any thoughts guys?

I might think with underseats and amping them the good results are more from DSP control. With mine ( Morels) they have 150 rms/ 300 on tap. With no eq they a quite good and have nicely articulated bass to high volume levels.

But when eq”d for a house curve and time shifted for phase alignment with door mids and sub is where the biggest gains come from.

Also the ability to shift the door mid basses up from 120 ish to 175hz means you can get plenty of punch out of the underseats. My underseats are covering an acoustic 60hz to about 180 hz.

The sub whilst crossed over at 80hz / 24db is rolling of at 60hz.

[patsf1]

Ok cool info guys!! I’m learning as you all are posting. So my understanding would be that if I amp the underseats with the Fly FL800.4 amp, I can only use the frequencies between 50hz to 250hz. The amp says it allows 50hz to 500hz with the lpf. The HK underseat speaker wire outputs should be 250hz down to whatever they finish at 30hz or so?? (also the mids frequency crossover from 250hz). So I could use the Fly amp which in theory should cut the low end frequency from say 30hz to 50hz coming from the HK speaker outputs up to 250hz, where the mids kick in and then crossover the Focal amp at 60hz?? Would this cover all the frequencies? Is this logical or am I way off? Also can the whole system be then eq’d from the 7 band eq on the I drive? All seems quite confusing. Ps I’ll be using a wavtech link2. Cheers

[NealfromNZ]

Not quite with those frequencies.

The HK amp has built in crossovers and looks like the woofer channels play somewhere from 20 - 30 hz and rolls off around the 100 -120hz range so this is your limitation when daisy chaining amplifiers of the HK .

You’ll still achieve good sub bass but the area where the underseat woofer becomes more limited ( eg from 60hz to 120 hz ) is will remain as per stock crossover points between woofer and door mid.

[patsf1]

So big question is there any point amping the under seats with that amp is the million dollar question? I thought it was a good shout using the fly amp as a band pass filter/amp as it plays signal from 50hz and above? Then using the Focal amp to play the lows? Will I hear an improvement using the sub with amped woofers or will it be a whole lot of work for nothing? . I’m thinking now:

HK amp- powers woofers (strong bass and loud) has to be eq’d heavily in settings

Add an aftermarket amp- powers woofers (untested but hopefully same/better outcome) probably have to change eq to flat
Will this hinder the current set up?

Add a sub (more bass and deeper) will I have to change settings in eq to flat so sub isn’t getting too much bass?

Head’s pickled at this point!! Lol

[Arthurrs]

I think you're trying to find easy answers for a few challenges here:
1. Cabin gain can be a benefit and a curse, you need to know how to harness it to your advantage.
2. A good portion of your road noise is in the low frequencies, so factor that in your measurements and adjustments. Measuring the system when the car is parked vs driving can be radically different.
3. You deal with the the cards you're dealt with when it comes to the output of the HK audio system, especially if you're trying to add onto the system. Within the logic eq settings on the BMW head unit, you have a low shelf and a control at 125 Hz, which are "optimized" by the technicians from HK to work with their hardware. I gotta admit the HK system sounds pretty good for what components come with the car, but like you, I find it lacking in the very low end response.
4. Adding an external power amp speaker sense inputs off the floor woofer outputs from the HK amp is certainly a step in the right direction, be sure to find an amp that at the very least will allow you to do additional processing so that both the floor woofers, and trunk sub play nice together. In multiway speaker systems, their interactions are greatest where they overlap each other, so setting the crossovers carefully so that each performs best in their passband is as desirable as making sure that in the overlap region you don't get any undesirable summation or cancellation. The choice of speakers and their placement makes it difficult to offer an easy answer here.

Bottom line: Get a three channel subwoofer amplifier that not only give you the right amount of power for the selected speaker components, but at a very basic level provides some kind of level, crossover, and polarity control for those speaker components. Having some low frequency filters would be icing on the cake. Ideally the floor woofers should have a high pass filter engaged so that your trunk sub's response compliments it by reproducing the frequencies below what the floor woofers are set to reproduce. If there is any overlap, carefully adjust the frequency and slope of these high and low pass filters so that you minimize any cancellation. And yes, I'm oversimplifying things here, getting an optimized low end response from a car audio system is a balance of objective and subjective analysis and decision making.

At some point, you may find yourself getting the wallet out and buying an excellent DSP/Amplifier that would elevate the entire audio system, not just the lower frequencies. That mObridge K2 is a beautiful thing...

[patsf1]

Thanks for your response I was thinking that also, it’s either make the best of what I’ve got and make the right decisions or get the wallet out. I suppose I could tune the bass on the eq so the under seats were doing less work and use the settings on the amp and tune the sub from there I think this may get the best results. I will probably do that. I have a test though. I have an old mtx road thunder 12” active base tube. I’m going to test that for a little bit. Also thinking of making a fibreglass enclosure or just get get a pre fab box for convenience.

[NealfromNZ]

Sorry for the radio silence, got a bit busy with work.

I think Arthur’s post is spot on rather than adding a 4 channel focal bridged amp. It would give more sub bass , but the channels driving the underseat woofers will produce similar power levels to what you have now due to 7 ohm load of factory woofers. But it would give you the ability to cut the bass from below 60hz from them.

If you go dsp /amp then you’ll take the system to the next level rather than piggybacking the HK woofer channels.

I have quite a long post on the F80 forum using a Mobridge solution where I slowly rebuilt my car audio system and centred the audio on a good 3 way front stage with a trunk sub filling in the sub bass region.

DSP was key in terms of time alignment of mids and tweeters and then phase alignment of mids to woofers and woofers to sub. Then individual speakers where tuned to respect house curves.

I think you could achieve 80-85 % of my system running with factory speakers and adding a trunk sub and Bill’s cross over design to mids and tweeters without spending $5.5k that I’ve put into mine.

What you’d end up with is great sub bass that underseat woofers can’t achieve and the ability to have the underseat woofers play to 150-180hz giving more punch and presence to bass. It also allows door speakers to play louder from 150-180hz as they won’t run out of cone travel like they can when playing lower.

Changing the tweeter crossover from a basic capictor to Bill’s design will also lower the distortion from the tweeter and reduce the overlap between tweeter and mid further improving clarity of the system.

[patsf1]

Yes I’m getting my head round it all now, I think your right though it’s a lot of money to achieve noticeable improvements. In fairness, I’ve set the system up through the eq and it seems like I’m getting a decent amount of base from the under seats. Everything is crystal clear. I’ve owned the car for 3 months and my ears are adapting to the different loudness levels from my previous car. I had a e92 coupe with base system which I upgraded to a semi active system. This was done with the MATCH 3 way components amped separately, I had the components run off a 2 channel Alpine amp and underseats bridged to a mono JBL amp however only recently was told that the woofers were meant to be wired in stereo however this never bothered me as the car rocked. The amount of base I got from those woofers was tremendous and never skipped a beat. Go figure….