Why rear speakers (and center channels, for that matter) typically don’t make sense if imaging is a priority:
If you want to create the impression of a stage in front of you, you shouldn’t have sound coming from behind you… often the argument is that simple.
Think about this:
Stereo recordings are 2 channel things, just left and right.
Many people strive to duplicate this studio-accurate sound. Doing this is difficult enough in a 2-speaker install, in a car, because your seating positions are not exactly centered.. and like we discussed in the phasing document, this will cause problems!.
If you were centrally located exactly between the two speakers, you wouldn’t have a phase difference, and your image would be perfect (in theory).
Unfortunately, seating in a car isn’t close to centered between the two speakers, so forget about instant “perfect imaging”!
And when you add rear speakers, playing the same frequencies as the front, you add additional pathlengths, each of which will be different than the other pathlengths in the car, and now you have 4 speakers causing phasing anomalies, an easy means to imaging disaster. Some frequencies will sound diffused, others will sound as thought they are coming from the rear, bad stuff…
Why does my car have rear speakers?
Understand that car manufacturers don’t care about auto sound… they care about impressing people into buying a car, by saying things like “28 speaker Delco Bose Typhoon Master Premium Stereo!” (and “stereo” would seem to be a misnomer here, with so many speakers! ). I believe solidly that the marketing department originally specified rear speakers for a car, just because it sounds to the average Joe as if you are getting more for your money if you have 4 speakers! Maybe it seems to make more sense.. four seating positions, four speakers.. it seems very intuitive… And honestly, any effort to justify having only 2 speakers in a car could easily be perceived by the public as “oh, look at how that company trying to justify being cheap so they can increase profit margins”…
That doesn’t mean YOU need to perpetuate this marketing mistake – the car companies might be sort of politically “stuck”.. but you aren’t, right?
If you run rear speakers “because I need the midbass the big back speakers provide” you run the risk of creating the illusion that the bass player is sitting in your back seat, which is not how it was recorded, and can be pretty unnatural and disturbing to listen to. The ideal strategy would be to upgrade the front speakers – either quality or size – which would add bass with no penalty. Also, taking the door panel off probably reveals a pretty inferior speaker “baffle”.. if you seal all those holes, you might help your bass response as well!
So why is it bad, though?:
Taking what we now know about absolute phasing, and how speakers of varying pathlengths can interfere destructively with each other, it becomes easy to see how having multiple speakers playing the same frequency can wreak havoc on the phasing response of the signals as they arrive at your ear…
Things are bad enough given the staggered seating position in a car, which virtually ensures that the left and right speakers will never be equal, but adding in two more speakers playing the same frequencies complicates (and most often degrades) things further.
So if you can do without it, don’t include them in your design!
Simple is better… (Remember the K.I.S.S. rule?)
Both from a sound, and cost standpoint! The less you spend ($0) on rear speakers is the more you can spend on nice, high-end front speakers!
The science behind the sound:
The following are actual phasing analysis simulations I performed late last year, using the measurements of my car’s factory locations, a 1995 Honda Civic coupe.
The following plots show the phasing differences due to the factory speaker locations. On the plots, the center line marked “0”, indicates no phasing difference at all, both signals combine in an absolute manner at your head constructively.
The 90 and -90 degree marks indicate points where the speakers are slightly out of phase, by exactly 1/4 wavelength (a 90 degree shift in one direction or the other exists). At this point the speakers no longer combine constructively, but do not combine destructively either, in theory two speakers that are 90 degrees out of phase would be exactly as loud as a single speaker, no better, no worse.
The 180 and -180 degree marks indicate points where the speakers are completely out of phase, by exactly 1/2 wavelength, at which point they would cancel completely. At this point, in theory, the two speakers would cancel to the degree where you would hear nothing.. or, because each ear is a different distance from the speaker, cancellation may be complete at one ear, and not quite at the other, one cue to your subconscious that this sound is not “real”.
Front speakers only:
Note that there are quite a few phasing issues due to nothing other than the left and right speakers having different pathlengths:
Rear Speakers only, running full-range:
Again, note that there are phasing issues with the rear speakers, just due to their left and right speakers having different pathlengths:
Overall combination of both front and rear speakers, all speakers running full range:
Most important to note here is how bad things get when you have all 4 speakers running.. there are many more points where the speakers enter that area above the 90 degree line and below the -90 line, where cancellation will occur. Interesting to note that things actually improved slightly in the 9,000 Hz to 12,000 Hz range due to the rear speakers being present, although this offers little consolation for the dramatic increase in phasing issues that are caused across the rest of the spectrum:
Rear speakers filtered to 80Hz – 640Hz (bear in mind, crossover slope is not only not considered here, in fact in this example would be a perfect cutoff, which will not exist in real installations):
This illustrates one strategy listed above, if you or your customer insists on retaining the rear speakers, imaging damage can be minimized by constraining the rear speakers to a small range. In this example, the rear speakers are constrained to play 80Hz to 640Hz, a full two octaves, which is actually a broader pass band than I typically would run, but I felt that it would better represent the somewhat narrower range I would use, given that my calculations did not take into account crossover slope:
Overall combination of both sets of speakers, rear speakers filtered to 80Hz – 640Hz:
Here, you can see that while this is worse (from a phase perspective) than running only front speakers, that things are dramatically improved over running both front and rear speakers full-range. The plot above 640Hz, in fact, looks identical to the “front speakers only” plot.
verall plot representing all speakers in install playing, given no rear speakers installed:
Obviously, this looks just like front speaker only plot because rears are not present. Personally, this is the ideal that I shoot for in my installs:
Note that these plots were made using measurements to my stock door locations.
If you wanted to further improve on things, equalizing your pathlengths via means like kick-panels and/or HLCD compression horn drivers under the dash can effectively be used, which would result in a front speaker plot that would resemble more of the rear speaker plot shown above, if not even better, depending on the pathlength difference you end up with!
Potentially legitimate reasons for running rear speakers:
…first question you should as yourself is “why do I want rear fill?” If you don’t have a good answer (and rear passengers are not a good answer! ), you should start by NOT running rear fill…
…I consider rear fill an advanced topic, an advanced element, that has an additional expense associated with it to boot…
I believe you shouldn’t spend money on something that has a greater chance of degrading your sound and imaging than it does of enhancing your sound.
After all, if you don’t have a good reason for needing rear fill… I believe you are better off skipping it entirely.
But let’s say you decide you do want rear fill..
There are some properties of sound to consider, such as the simulation of a live listening environment (personally, I strive not to color the music artificially, I strive for accurate reproduction, studio reproduction, although admittedly live tracks won’t sound as “live”, however studio tracks will sound very “studio” realistic, and 99.9% of what I listen to are albums, not “live” recordings)…
In a live listening environment, you might be inside a concert facility, with a rear wall, which will reflect sound back at you. Maybe you want your rear fill to simulate this.
Bear in mind that short wavelengths don’t travel as far, they degrade quicker… and long wavelengths travel very far..
So accurate reflections would simulate this, reproducing just the frequencies that would ‘live” after a long travel and reflection. Another good argument for the low-pass filter on the rear fill!
And bear in mind, it is still difficult convincingly simulating these reflections, as with a real reflection there would be quite a delay before arrival at your head, while in a car there won’t be any delay… maybe those of us who are Electrical Engineers out there wouldn’t find it much of a challenge to build a custom delay circuit for the rear speakers, but for the rest of us, unfortunately there aren’t many commercial products on the market to provide this delay!
My thought is, in a largely glass and plastic reflective car interior, if you only have front speakers, there will be sound that travels backwards, reflects off rear glass and travels forward… which is a better approximation of those live reflections than direct, non-delayed sound from rear speakers, but admittedly falls short of creating an ambiance of an interior larger than a car.
NOW… The biggest reason probably to run rear fill might be if your front speakers don’t provide enough midbass…. this can be tricky because you don’t want it to sound like the bass player is in your rear seat, but sometimes you need some midbass support… either a last resort, or the install can’t budget an upgrade that would improve the situation up front.
Personally I bandpass rear fill in the 100-200Hz region, just that narrow bandpass to provide a little ambiance… maybe to 300Hz if I am feeling generous. But bear in mind that although it doesn’t sound like much, 100-200Hz is one entire octave of the musical spectrum!
Bandpassing the rear fill around this range minimizes the frequencies that cause phasing anomalies, provides midbass support, and covers the range of frequencies that would most survive a reflection in a live listening environment.
Freebies via not using rear fill!
The best benefit to me, is making your subwoofer more efficient! In a coupe or sedan, removing your rear speakers and leaving the holes vacant allows your subwoofer to exist in the same airspace as your interior, like a hatchback… the bass can flow into the interior unconstrained. This can make for quite an improvement…
Sorry, BMW owners, your Sherman-tank-solid sealed rear deck construction won’t gain you anything here…
However, for those of us not owning BMW’s yet, there are cost savings to not running rear fill… With all the money we spend on high end speakers, head units and processors with low distortion numbers and high voltage values, powerful, high quality amplifiers… Isn’t it nice to improve sound quality by NOT buying something?
You don’t need to purchase speakers for the back.. you don’t need to purchase amplification for the back… the reduction in amplification might mean lower costs in power wire, RCA cabling, speaker wiring, etc… it’s a win-win situation!