How to rear fill

1st rule:

This isn't a thread to argue against rear fill. This is for those of us that want to try rear fill, how to do it.

2nd rule:

See rule 1.


Okay now that we got that out of the way, lets talk how to properly implement rear fill.

I have read nearly everything I could find on DIYMA and here are the basics that I have come up with, but it is fairly general and some of you may have better "how to's" based on more recent stuff.

1) Must be L-R
2) Attenuated somewhere between 6-10db
3) Delayed somewhere around 15-20ms.
4) Bandpassed somewhere around 300hz to like 3500hz (to match your midrange seems about right and based in energy dissipation/reflection stuff)

I've been playing around with this and trying to get it dialed in. Here are my issues/thoughts so far.

1) I actually really like it so far. It brings something that I can't quite explain to the listening. It is also kind of fun to listen to just the rear speakers and how weird the music becomes with no common center information. Things get cut off weirdly.

2) I still need to fine tune the attenuation and delay. I originally had it around 17.5ms but that sounded like it was too much of an echo effect. I now have it around 11ms but that doesn't quite seem like enough.

3) I only have one channel on my dsp for rear fill so I have channel 7 split for both rear speakers that are located in the rear door of my crew cab. This makes the left inherently louder than right due to distance. I am thinking about how to fix this with some resistor or something.

4) It is hard to tune due to the L-R and the channels being tied together. Definitely need to use uncorrelated pink noise if you set it up like me. Make sure if you do pink noise gain setting on your amp, you also play uncorrelated for these speakers...otherwise the volume gets to basically nothing.


So who here has tried it and what did you do to find the best success with rear fill? I know Skizer just did this in Doug's Audi so maybe he can share what he did with that.
 
I haven't messed with rear fill in 20 years. The last time I did it, I used some MB Quart 4" and reversed the polarity on one of my RCA connectors. I did not have a DSP so I only liked it when they were barely audible. I have been fronts only since then but I have toyed with the idea now that I have a DSP.

On a side note. Mine doesn't really qualify because it was R-L. Sorry but I didn't like the L-R.
 
1) Must be L-R
2) Attenuated somewhere between 6-10db
3) Delayed somewhere around 15-20ms.
4) Bandpassed somewhere around 300hz to like 3500hz (to match your midrange seems about right and based in energy dissipation/reflection stuff)

That is one option. There are other ways to use rearfill.

I present Option 2 (fix room modes AND fix azimuth angle mismatch).
1) <250hz (ballpark starting point, can go up or down depending on how you like it)
2) attenuate only what is needed to blend with front midbass
3) delay if you want to further blend with front midbass (optional)
4) add a high pass >60-80Hz if using whimpy woofers. 6.5's cannot play subbass with authority, (at lower volumes like in a parking lot maybe).

Why? Because the car "room" is small and this is where room modes live in car audio. In home audio it exists in sub-bass because the room dimensions are larger. It will smooth out nulls in the listening position that cannot be fixed with EQ. Plus with another woofer located behind your listening spot, you can fix the azimuth angle mis-match issue of offset seating between left and right woofers. This is called vector summation.

Try this, room length may or may not include trunk depending on how solid your backseat is or if you drive a hatchback/suv/wagon/van,etc. Green is ok, anything else is bad news.
https://www.bobgolds.com/Mode/RoomModes.htm
 
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doesnt need to be delayed that much. Between 8ms and 12ms is enough to "disassociate" the rear from the fronts. Could also play all the way up if you want. depends on preference.

also, set your amp up so that you have gains avaiable for both channels


As far as Dougs Audi, he has 3 way active front, 2 way active center, 2 way coaxials in the rear doors, and 3" widebands in the D pillars, and a single sub. For rears after they were tuned as normal, the rear doors recieved an extra 8ms of delay, and the d pillars recieved an extra 12ms of delay. The rear doors were i think 55% Left minus 45% Right, then regular L-R for the d pillars. I think the rear doors had a bit more high frequency in them too. As you can see i tried to stagger them apart from each other. First time ever doing that, along with the full blown center channel, but that thing turned out sweet. If anyone is in Phoenix, AZ go take a listen to it. He actually got the new BLAM Multix speakers in it. Those 6" drivers easy rival the GB60's, maybe even surpass them in midbass output. insane.
 
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Also, rear fill is easy. get your speakers as high, wide, and deep as possible, flatten response as usual with eq, apply additional delay of 8-12ms, attenuate a bit, bandpass maybe, do left minus right on the signal, and done
 
also, set your amp up so that you have gains avaiable for both channels

I wish I could do this, but that would require buying a better dsp at this point. My amps don't have individual gains for each channel. One day I will buy a dsp with more channel outs.....until then this is my "patch."



doesnt need to be delayed that much. Between 8ms and 12ms is enough to "disassociate" the rear from the fronts. Could also play all the way up if you want. depends on preference.


As far as Dougs Audi, he has 3 way active front, 2 way active center, 2 way coaxials in the rear doors, and 3" widebands in the D pillars, and a single sub. For rears after they were tuned as normal, the rear doors recieved an extra 8ms of delay, and the d pillars recieved an extra 12ms of delay. The rear doors were i think 55% Left minus 45% Right, then regular L-R for the d pillars. I think the rear doors had a bit more high frequency in them too. As you can see i tried to stagger them apart from each other. First time ever doing that, along with the full blown center channel, but that thing turned out sweet. If anyone is in Phoenix, AZ go take a listen to it. He actually got the new BLAM Multix speakers in it. Those 6" drivers easy rival the GB60's, maybe even surpass them in midbass output. insane.

So the farther physically back the speakers are, the less electrical delay then need as they will have physical delay. So if the speakers are the closest speaker to me (rear door speakers are in the door right next to the B pillar), then they would need more delay correct? But not too much delay that it sounds like an echo, or maybe I need to attenuate more?


I would love to eventually get speakers in the top outside of the C-pillars (D Pillars??? whatever the ones by the rear window of a truck are...) for the rear fill, but other things first.
 
L-R ?

Don't we want L-R for the left and R-L for the right ?

I recently changed my rear fill from mono to L-R/R-l and I liked it. A little bit clearer image. Delay is about 11ms as that's the most I could dial in while keeping everything else in sync. Rear speakers are unfortunately down in the bottom of the passenger doors (4 door truck).

With mono, as I raised the volume it added to the "room" but in a diffuse way. With the L-R/R-L, raising the rear volume seemed to more directly "widen and open" the stage while remaining a bit more clear up front.
Sorry for the audiophile speak but that's all we have.

I implemented L-R using the helix (50% each input channel, with the right channel inverted), and then wired the right speaker with reverse polarity. I am also running a single signal out of my dsp to the amp with y-cable RCA's, fortunately the amp has individual channel gain.

I will say I much prefer the sound with rear fill, it really changes it from "all up front" like I'm listening to monitors in a small room to a much more immersive experience. I mean I"ve spent a lot of time raising the rear volume up and down right at the threshold of noticeability and I really prefer the rear fill quite a bit.
My truck already has a very wide expansive stage without rear fill so it really sounds nice. My other car may have slightly better tonality and "sound" but its not even close on stage. I'm planning to add rear fill to this car to help improve that, should be nice with a 4 door sedan with a rear deck (or possibly c pillars ?)
 
I don’t think it really matters ultimately. If you wired one speaker backwards then you have reversed its polarity anyway to accomplish the same thing.

L-R just keeps the non common stuff so you get +L and -R still out of the speakers (-L and +R for R-L) so by reversing speaker wires on +L and -R, you’d get -L and +R.....I think anyway...

if you only have one dsp channel coming out...how are you getting both L-R and R-L?
 
So if the speakers are the closest speaker to me (rear door speakers are in the door right next to the B pillar), then they would need more delay correct? But not too much delay that it sounds like an echo, or maybe I need to attenuate more?


I would love to eventually get speakers in the top outside of the C-pillars (D Pillars??? whatever the ones by the rear window of a truck are...) for the rear fill, but other things first.

In my experience, using those door speakers near the B pillar, located low in the door, is not that good for rear fill. They're too close to the front, in order to have them not interfere with the front stage they're (again, this was in my car) dialed back so much it's not worth it. Moving them back to the sides of the trunk, between C and D pillars made them much more effective.

In my new vehicle I'm hoping to figure out a way to mount rear speakers in the D pillar, otherwise I'll just skip running rear fill.
 
I don’t think it really matters ultimately. If you wired one speaker backwards then you have reversed its polarity anyway to accomplish the same thing.

L-R just keeps the non common stuff so you get +L and -R still out of the speakers (-L and +R for R-L) so by reversing speaker wires on +L and -R, you’d get -L and +R.....I think anyway...

if you only have one dsp channel coming out...how are you getting both L-R and R-L?

You just explained it. I'm sending L-R to the amplifier, and then wiring the right speaker reverse polarity, so it is seeing -(L-R)=R-L.

The theory I've read indicates it does make a difference, you are getting only left info on the left and only getting right info on the right. This helps the "large space" illusion with less dilution of the front image.
In my own experience this past weekend, there was a nice improvement over mono.
 
In my experience, using those door speakers near the B pillar, located low in the door, is not that good for rear fill. They're too close to the front, in order to have them not interfere with the front stage they're (again, this was in my car) dialed back so much it's not worth it. Moving them back to the sides of the trunk, between C and D pillars made them much more effective.

In my new vehicle I'm hoping to figure out a way to mount rear speakers in the D pillar, otherwise I'll just skip running rear fill.

Yup, not ideal placement, but so far it does add to the sound and isn't really taking anything away. My center is still centered and my right/left are still right/left. The right left are being pulled back a little, so I need to adjust something, but I need to play with time and volume some more.
 
i've had rear-fill installed in my car, ready to use for over a year now. however, it's been off most of this time. why? it's tough to get right and ruins staging cues when it's wrong. :P

copied some of my garbage comments over from diyma from a thread about rear-fill:

i tried 300-3000/6db. tried at 7k as well, also tried high-pass only. not much of a difference in my book. i might try it again this year as i get the front more dialed in.


my rearfill are in the c-pillars facing the opposite rear passenger-assist (oh shit) handles. i don't need any additional width so depth is what i was after. it also keeps the speaker pods subtle and non-obstructing.

in my car: if the right rearfill is too hot, right of center pulls UP and right. if the left rearfill is too hot, the left side pulls towards the listener creating the 'horseshoe' effect.

net effect in my car? better sense of room and stage. sense of stage/room was more impacted by my tweeters and high frequency extension. getting the front-stage tune correct meant getting stage depth/layering for free.

if you can get the trumpet on sara k's "if i could sing your blues" correct, you're onto something. you can find it on various chesky discs.
https://www.youtube.com/watch?v=_OItKgDwctg


some other tracks i use for depth/sense of space:

sunny sumter's nick of time (meca bonus tracks): should have a good 3 layers of depth. (some systems only have two layers) vocals center, upright bass left of center and behind, piano in the back/left.

rebecca pidgeon's auld lang syn / bring it on home: should have a good sense of large stage and large room. (some only have sense of stage, only sense of room, or none)

wycliffe gordon's won't you come home bill bailey: should sound like a large stage and large room. trumpet should be nearly all the way left. trombone should be further left and behind the trumpet. drums should be in the back and right of center. tuba left of center in the back. woodwinds right and front. banjo right of center and front. brass rip about 3:30 should rip should reverberate right and about half way back across the stage.

with the rest of your system together, try some bookshelf or surround speakers in the vehicle and play with aiming, time alignment, delay, etc :)
 
and a little more related to my experience tuning rear fill:

I start out with a stereo signal and a bandpass crossover. Then eq the sound flat’ish like i would front sound, from the listening position. Then center the rear fill with relative levels and time alignment. Then l-r/r-l mix the signal, add delay and reduce levels.

Too much delay, it sounds echoed. Not enough delay, it pulls back. Too much level pulls back or echoed. Too low, no effect.
 
I finally tried true differential rear fill for the first time with my Helix DSP.3 - wasn't my cup of tea (added so little that I didn't find it very beneficial). Although, I was using the highest delay allowed by the DSP.3 (about 20ms) since everything I read said 20ms+. Maybe I should try lower delay amounts.

Ended up going back to attenuated, delayed and bandpassed "rear speakers" instead. :-)

I have the rears (in rear deck on Dodge Challenger) playing 75hz-4000hz, approx 8-10dB lower than the fronts with about a 10ms delay. I like the extra "fullness" it gives without noticeably pulling the front stage back at all. I tried fronts only for a while and just prefer the rear speakers being on.

Will probably experiment more with differential rear fill at some point, but was not fond of the initial results.
 
I finally tried true differential rear fill for the first time with my Helix DSP.3 - wasn't my cup of tea (added so little that I didn't find it very beneficial). Although, I was using the highest delay allowed by the DSP.3 (about 20ms) since everything I read said 20ms+. Maybe I should try lower delay amounts.

Ended up going back to attenuated, delayed and bandpassed "rear speakers" instead. :-)

I have the rears (in rear deck on Dodge Challenger) playing 75hz-4000hz, approx 8-10dB lower than the fronts with about a 10ms delay. I like the extra "fullness" it gives without noticeably pulling the front stage back at all. I tried fronts only for a while and just prefer the rear speakers being on.

Will probably experiment more with differential rear fill at some point, but was not fond of the initial results.

Your "regular" rear settings is about what I have for my rear fill except I have mine L-R. Try leaving everything the same and just do L-R (To get R-L just reverse the right side speaker wires at the amp).
 
Jtrosky are you saying that "true rear differential rear fill" is processing that the Helix does ? Not just (L-R) (R-L) as mentioned above by Jdun54nl ?
The fact that you like rear speakers playing but not "true rear fill" is interesting.
I'm confused about what specifically you tried that you didn't like.
Are you rear speakers playing in stereo or mono now ?
I actually think 20+ms delay is probably too much, did you try "true rear fill" with less delay like the 10ms you are using now ?
 
Jtrosky are you saying that "true rear differential rear fill" is processing that the Helix does ? Not just (L-R) (R-L) as mentioned above by Jdun54nl ?
The fact that you like rear speakers playing but not "true rear fill" is interesting.
I'm confused about what specifically you tried that you didn't like.
Are you rear speakers playing in stereo or mono now ?
I actually think 20+ms delay is probably too much, did you try "true rear fill" with less delay like the 10ms you are using now ?
Yes, the Helix allows you to set up true differential rear fill. Here is a post by Nick over on DIYMA that explains the procedure (it's actually really quick and easy to setup). You don't need to change any physical wiring or anything.

https://www.diymobileaudio.com/threads/how-to-rear-fill-with-helix-dsp.307730/

I know I tried it with ~20ms of delay for a few drives and just found that I didn't like it as much as the setup I normally use (stereo, attenuated, bandpassed "rear speakers"). I get a much "fuller" sound (with same levels) this was vs. real differential rear fill.

I do think I very briefly tried ~10ms delay as well, but didn't keep it that was very long as I always read that 20ms+ of delay was normally needed for "proper" rear fill. So I left it at that. Either way, it just didn't add enough to the overall sound for me compared to what I do now. Maybe I need to increase the levels a little too though.

EDIT: OR... Maybe what I'm doing on the Helix really isn't *true* differential rear fill? I honestly don't know for sure... I do know that I got absolutely no sound from the rear speakers while playing my tuning pink noise though (which I think confirms that it was setup correctly)! :-)

EDIT2: Here is another cool site talking about different types of rear fill - found it interesting and informative: https://glasswolf.net/papers/rearfill.html
 
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If you’ve got your rear outputs on another set of inputs, you could add input delay to add another 5-10ms on the helix
 
Hi. Apologies for necromancy, but this looks like the right place to post.

I've been reading over how to do some kind of differential rear fill and am uncertain about wiring. The message above says, "reverse L- and R+". Okay, here's a picture of that: https://i.imgur.com/IrjujcZ.png

Is that intended? Send positive signals from both LR to L, and negatives only to R? If "reverse means swap with one another," that's the diagram.

I didn't even know speakers could work at all in a config like this, haha.
 
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