dhmcfadin

Let me preface this post by stating that this is my opinion based on evidence I have gathered, people I have spoken with, and my own personal system builds. I see a lot of people asking questions about upgrading the rear speakers. I thought I would create a post that explains rear fill and why I believe rear fill in our trucks, without proper speaker placement and processing, is a total waste of money, effort, and time.

Here's the deal with rear fill. Take this from a person who has morel tempo ultra integra's in my rear doors and have had them turned off in my dsp for the last 5 months. This is what I have learned.

There is no question that rear fill is something that can improve the overall listening experience in a vehicle. The caveat to this is that rear fill only improves the listening experience when properly implemented. The even bigger caveat to rear fill is, if not properly implemented, it can wreck a sound stage and image. It's also very important to understand the rear fill is NOTHING LIKE surround sound speakers in your home. Completely different. Music is recorded in stereo left and right not surround. That being said, I spent $300.00 on speakers I don’t use. Why is that? Couple reasons:

1. On-axis vs off-axis frequency response. This is commonly forgotten when talking about speakers. We look at specs, power handling, xmax, etc but we very rarely look at the specs from an analytical standpoint. AKA, what speaker will work best in my particular speaker configuration/ speaker positioning? All speakers perform differently at different axis. On axis would be having the speaker sit directly in-front of you. As you move the speaker up, down, left right, front, back, the axis of the speaker face from your ears starts changing. This is called off-axis. As you move the speaker, the frequency can start to degrade. The degradation of frequency response as you move the speaker further off axis is called Beaming. Beaming is essentially when the speaker stops radiating sound in an omni-directional manner (i.e., sprays sound everywhere) and starts radiating sound in a narrow cone, like a flashlight beam, hence beaming. This eliminates the wider sound stage one would hope rear fill would add. The higher the frequency of the sound, the worse the beaming will get until you no longer have usable/enjoyable output from your speaker. Take a look at these two graphs.
   1. This first graph is the frequency response of a speaker and how different off axis positioning affects the frequency response or listening experience. You can see that as the speaker is moved further off axis from the listening position, the frequency response becomes pretty ugly after about 2000 hz.
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   2. This next chart represents at what frequency does beaming begin to take effect on a particular size of speaker. For this discussion, because we are talking about rear fill in our trucks, this graph models a 6.5" driver which is what you would install in the stock location in the rear door. It also represents at what degree off axis beaming begins to occur and what axis a speaker essentially becomes unusable from an SQ perspective. As you can see with a 6.5" speaker, beaming can occur as low as 1000hz even 30 degrees off axis.
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2. What is real fill really? Rear fill, in its simplest form, is for creating the auditory illusion that the sound stage is bigger than the boundaries of the vehicle. That our image is wider than our dash board and deeper than our windshield. As I have said before, a vehicle is already a terrible, if not the worst, environment for a speaker to play in. There are a multitude of different surfaces the reflect, deflect, absorb sound waves. This is one of the reasons why we use DSP's, to attempt to curb all of these negative effects. We use a DSP to attempt to fix the frequency response of a speaker from our listening position. That's why, without an RTA, tuning a dsp is a shot in the dark. The best you can do is time alignment. This poor environment in conjunction with the negative affects of off axis speaker positioning in our vehicles is why we tune our DSP taking measurements from the listening position. We don’t take measurement from different parts around the vehicle and then average them together. Only from the listening position. Now that rear fill is defined, let’s take another look at the beginning of this post where I talk about rear fill only being useful when properly implemented. Based on the diagrams above, can you tell if the positioning of our rear speakers in their stock location is a good position for rear fill? Can you tell based on the diagrams above the position of our rear speaker will improve or worsen the overall image we are fighting so hard to obtain?
3. So, we know what off axis frequency response looks like and what we use rear fill to achieve. Now let’s talk about what frequencies we want rear fill to reproduce to increase the size of our sound stage. First of all, rear fill is not about improving bass or lower frequencies. It's also not about improving higher frequency sounds. It's about widening and deepening the sound stage. Rear fill should have a highpass crossover of about 200 hz and a lowpass crossover of about 4000 hz. There are varying opinions on this, but those numbers are about the average depending on who you talk to. First, let’s talk about at what degree off axis the rear speakers are currently positioned in our trucks. Based on the graph below and assuming we are just talking about one axis here (I'm not evening going to bring up the fact the rear speakers are off axis in multiple directions) we could say the left rear speaker is about 240 degrees off axis and the right rear speaker is about 120 degrees off axis from our listening position. Or if looking at the graph above, we could the rear speakers are each 120 degrees off axis respectively. Knowing that we have the rears crossed between 200hz and 4000hz and knowing at what degree beaming occurs on a 6.5" speaker between these frequencies, do you think adding rear fill to our trucks, in the stock location, will benefit our listening experience or hurt our listening experience? My answer is that it will only hurt our listening experience. And I came to this conclusion based on the information I presented above. In my opinion, the rear speakers provide no benefit and only hurt your listening experience. Based on the evidence above, not only are you subjected to beaming but the frequency response of your rear speakers at 120 degrees off axis is going to very rough.
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4. Lastly, let’s look at what sounds fall within that 200-4000 hz range. As you will notice, the large majority of these musical sounds are what give you your sound stage. It's the sounds that tells you that the drums are on the right, the bass is on the left, the electric guitar is to the right of center, the piano is rear left of center, that the shakers are far far right, etc. It's the sounds that give musical tracks depth, reach, the illusion that you are sitting in-front of the stage listening and visually seeing the artist play their various instruments across the stage. When you consider the extreme importance of this frequency range, you begin to see that beaming and poor frequency response through this range is detrimental to your system because no longer will your songs have their unique signature and wide, individual set of instruments. Instead your stage become narrower and you create a "wall of sound" effect instead of a stereo image. With the front stage speaker placement, this can be controlled and fixed. With the OEM rear stage speaker placement so far off axis, you are forever fighting an uphill battle.
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Now, don’t get me wrong, rear fill properly implemented is a totally different discussion. Properly implemented with custom speaker placement and a DSP that is capable of processing rear fill is a different ball game. In those case, rear fill can be an invaluable addition to a serious SQ oriented system. There are tons of world class SQ competitors who run 16 channel dsp's, Dirac Live, left and right rear fill with rear center channel processing, the list goes on. But it is only in these cases and with those components and design that I would ever recommend rear fill because without proper positioning and processing, you are only hurting your listening experience. Maybe I don’t have enough people ride in the back seat of truck or maybe I only care about my listening experience, but these are the realities of rear fill from a factual and mathematical standpoint based on the speaker positioning in our trucks. Save your money. There are better places to spend it on your system.

doug97gxe

add my Hertz hsk 165 to the list of rear fill that has been turned off

Ajb1205

It's crazy how much more detail there is without the rear speakers. Before I really understood how a front stage should sound, I just thought that the music should be all around you (kind of like a surround sound system to an extent).

Great write up as usual dhmcfadin!

planetix

Well this has certainly gotten me re-thinking my rear speakers I just ordered a UMIK-1 for REW too. Going to be busy this upcoming weekend.

doyall

Excellent analysis. Thanks!

But knowing this major takeaway beforehand:

"...a vehicle is already a terrible, if not the worst, environment for a speaker to play in,"

is why I only do modest speaker upgrades (even the rears) to my vehicles under the premise that practically anything has to be better than the factory OEM speakers.

dhmcfadin

Keep in mind, the front stage can be fixed with EQ. I singled out the rears because they +120 degrees off axis from the listening position in the stock form. The front speaker placement in our trucks even from oem isn't bad and can provide an excellent sounding system.

jdunk54nl

Just to add what I shared in the 2017 v8 audio upgrade thread:

I agree with DHM,

Rear fill in reality (as Andy Weheymer, the owner of audio frog, has so eloquently said it) is to produce the ambience of the room. At a concert or at a recording studio, all of the music is coming from in front of you. You do hear things that are behind you, but those are long range reflections of the audio and ambience of the room that the recording is from.

This is my biggest reason for not running rear fill. I don't want to play the all of the music out of my rear speakers, that is not how the artists intended the sound to be played. If I had a processor that could decode (or properly add) the ambience and the long range reflections of the actual room that the recording was performed in, then rear fill would be beneficial. But most do not have this ability and therefore can't actually add anything useful with the rear fill. There are some processors that have the logic functions (Dolby pro logic 2, DTS NEO, Logic 7) capable of doing this. The JBL MS8 (Andy Weheymer was one of the leads on this) was one.

If we are currently using rear speakers, those speakers are playing the entire music that we have assigned them. We can, at best, limit the frequencies so we can limit the beaming issue, but that is about it.

Here is a link with Andy talking about it.
https://www.diymobileaudio.com/forum...32-post31.html

and here is an entire thread devoted to on-axis vs off-axis (much already explained by DHM).
https://www.diymobileaudio.com/forum...-off-axis.html__________________

HushCarAudio

Great write up but I have a different opinion on the subject. If most people use the fader and put all the sound to the front, most people will say the sound system sound worse than when the fader is put in the middle. If I gave my clients a setup where all the soundstage is in the front similar to a concert I'd be out of business quickly. You'd have to wonder why all 2 seater sports cars still have rear speakers following this logic. You could argue that the rear seat passengers want sound in a 4 passenger vehicle but what about the manufacturers that make 2 seaters. Why would they waste money on rear speakers if they didn't improve on the overall sound? Correctly balancing the rear sound in my F150 front to back sounds amazing. I would hate it if only sound were coming out of my front speakers. Every single car at any audio show/convention makes use of rear speakers as well. I think each of us have differing opinions on this and that's fine. I don't think there is an end all statement that is true about only running front speakers period. That's my 2 cents.

jdunk54nl

A couple of questions/topic starters (this is just a discussion not an argument),

• What people want, and how things sound the best can be different and a lot just do not realize. EX. If people knew about parts-express/madisound, they wouldn't be wasting money on "cheap" subs from walmart when they could buy better and cheaper subs from Parts-Express/Madisound. I also tried selling Polk Audio MM1242 with a custom box, which is a pretty good sub for the price new. I tried selling it for $50 with box and couldn't because it wasn't JL, Alpine, or Kicker. I turned it into my home theatre sub and it is awesome!
• How many of your clients that want rear speakers also ask you to put in a DSP and do EQ using a calibrated microphone without you suggesting it (I don't even know if you do DSP tuning)? A lot of people just want LOUD, not clarity. Have you actually tried putting a DSP in their vehicles, EQ'ing it, and then showing them the difference?
• Have you ever heard a vehicle that has the proper logic capabilities to produce a room? I have listened to Andy's car (with his one off 16(?) channel amp/DSP) and it was on another level. My truck at the time wasn't even close and rear fill hurt my soundstage a lot, even though I attempted to EQ it the best I could.
• If car manufacturers actually put money/thought into their audio systems, then this aftermarket audio world would be nearly out of business. They are getting better, but still leaves a lot to be desired. So the argument of, why do car manufacturers put rear speakers in is pretty mute.

dhmcfadin

Couple things:

When you fade all the sound the front, you are actually boosting the output of the front channels. This boosting can cause distortion which why when faded all the way front, the sound can be tiny. The distortion is the output signal clipping. That's why it sounds worse. Think about about when you set the gains on your amp. You set them at a predetermined input level. In your example, suddenly you fade all signal to the front on the head unit and boost the input signal to your amp a couple dB and now you are clipping your outputs and listening to pure distortion. Of course that won't sound good.

You might say, "but if I go to a stock vehicle without an amp and fade everything forward, it still sounds like crap" Thats also understandable because while there is no gain clipping, there is also zero eq. It's gonna sound terrible either way.

When you are at concert, there is still sound coming from behind you due to the sound waves reflecting off the venues wall. Obviously the focus of the sound is still in front of you. Why do you think a symphony hall is shaped the way it is? There aren't speakers in the rear. Just instruments in the front. We can use a dsp and eq in a vehicle to create this effect more efficiently vs rear speakers off axis.

2 seater sports cars, in most cases, have rear speakers for the same reason ford installs a blended left/right "center" in our trucks. It's to give the illusion of a full sound stage but all it does is hurt imaging. It's generally only there to compensate for lower power output and again, poor eq. The other reason two seat sports cars have speakers near the head rest is because generally sports car have a removable top and the power output of the factory system is simply not enough to keep up with the wind noise with the top down. So they add more speakers to improve the output.

Not every car at a show uses rear fill. Especially in sq shows. I know this for certain.

Lastly, If your front stage only sounds like it is in front of you only then your tuning is incorrect. And that's not an insult. It took me a LONG time to figure this out myself. You don't need rears to create that auditory illusion. You just need proper speaker placement, time alignment, and eq. Your front stage with no rear fill should feel wider than your dash board and deeper than your windshield. As music fades from left to right, you should almost think the sound started from behind your left shoulder. You should believe there are rear speakers playing when they aren't.

In no way am I saying rear fill is bad. It can be very valuable but only IF it is implemented properly. And to implement it properly takes a lot of work and a lot of processing to do it right. For 99.9% of the builds we talk about on this forum, I would put up quite a bit of money that a front stage only truck properly tuned will sound better than a truck with off axis rears faded in and a lack of proper processing. I only came to conclusion based on factual evidence, it's not a theory. It just took me longer to realize what was hurting my system more than helping. My tune is what makes or breaks the system. End of the day, if someone can't get their system where they want and just feel they need rear speakers, well they can easily add them. All I'm saying is don't waste your time or money from the beginning until you truly know what your system is capable of.