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Thread: How to rear fill

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    Noob Jdunk54nl's Avatar
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    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.
    2014 F150 Limited -> Kenwood DDX-9907xr -> Helix DSP.2 -> Alpine PDX-V9 -> SI M25 mki in Valicar Stuttgart Pods, Rear SB17's, Sub SI BM MKV's in MTI BOX. Alpine PDX-F6 -> SI Tm65 mkIV, SI M3 mkI in Valicar Stuttgart Pods

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    Senior Member Smitty's Avatar
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    Re: How to rear fill

    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.
    Meh, it'll play.

  3. Back To Top    #3

    Re: How to rear fill

    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
    Last edited by durwood; 02-03-2020 at 09:36 PM.

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    DIYMA Janitor SkizeR's Avatar
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    Re: How to rear fill

    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.
    Last edited by SkizeR; 02-03-2020 at 10:41 PM.

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    DIYMA Janitor SkizeR's Avatar
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    Re: How to rear fill

    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

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    Noob Jdunk54nl's Avatar
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    Re: How to rear fill

    Quote Originally Posted by SkizeR View Post
    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."



    Quote Originally Posted by SkizeR View Post
    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.
    2014 F150 Limited -> Kenwood DDX-9907xr -> Helix DSP.2 -> Alpine PDX-V9 -> SI M25 mki in Valicar Stuttgart Pods, Rear SB17's, Sub SI BM MKV's in MTI BOX. Alpine PDX-F6 -> SI Tm65 mkIV, SI M3 mkI in Valicar Stuttgart Pods

  7. Back To Top    #7

    Re: How to rear fill

    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 ?)

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    Noob Jdunk54nl's Avatar
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    Re: How to 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?
    2014 F150 Limited -> Kenwood DDX-9907xr -> Helix DSP.2 -> Alpine PDX-V9 -> SI M25 mki in Valicar Stuttgart Pods, Rear SB17's, Sub SI BM MKV's in MTI BOX. Alpine PDX-F6 -> SI Tm65 mkIV, SI M3 mkI in Valicar Stuttgart Pods

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    Noob naiku's Avatar
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    Re: How to rear fill

    Quote Originally Posted by Jdunk54nl View Post
    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.
    Another white wagon.
    Instagram - eye_take_pix



  10. Back To Top    #10

    Re: How to rear fill

    Quote Originally Posted by Jdunk54nl View Post
    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.

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