Regular InCar tech contributor Stephen Dawson takes us on a voyage of discovery through the expansive and transformative terrains of in-car time alignment and EQ.
Tech writer/reviewer Stephen Dawson takes us on a journey across time… alignment that is. Stephen discusses the importance of coherent frequency arrival at the listening position for best sound quality in the car.
There is an inherent problem in car audio. One quite apart from power supply limitations and having to compete with the noise in a car. at problem is the location of the speakers. In a proper home stereo system the position of the listener and the two loudspeakers are somewhere close to the three points of an equilateral triangle. In a car, that is simply not possible. In a right hand drive car the driver will be closer to the right hand speaker than to the left. Adding more speakers complicates matters further.
As a result, without certain facilities in the car’s head unit, e ective stereo and therefore acceptable soundstage and imaging properties are essentially impossible in the car.
Before getting into what these corrective electronics do to x the problem, let’s understand the problem a little better.
Stereo is fake
Two channel stereo sound is actually widely misunderstood. Well done, with ne loudspeakers, quality electronics and careful loudspeaker placement and an appropriate environment it can sound thrillingly realistic and three dimen- sional. But this is at best a highly realistic fake, just like 3D video.
Both stereo sound and 3D video systems work by delivering separate left and right signals to your left and right sensory organs: ears and eyes respectively. One way that you see three dimensionality in real life is by your eyes seeing things at slightly di erent angles. So two cameras are used to capture views from di erent angles, and those are duly delivered to your eyes via, for example, a Blu-ray 3D system and the special eyewear which makes sure each eye sees only what it’s supposed to.
But that di erent angle thing really only works for relatively close objects. To make it work for distant objects, special 3D cameras are used with their left and right lenses much further apart than your eyes are. Animated lms show much greater di erences in the di erent eye views than you would see with an equivalent scene in real life.
We detect the direction of sound in a number of ways, but perhaps the most important one is in timing. Let’s say that you are standing in an open eld and looking straight ahead. Someone is standing ve metres in front of you, but a metre to your left. Even if your eyes are closed, when he speaks you will instantly know his position fairly accurately to the left or right, although you will only have a rough idea of his distance. You can tell largely because his voice will reach your left ear very slightly before it gets to your right ear.
His voice will send sound waves more or less directly to your left ear. But his voice has further to travel to your right ear, both thanks to the angle, and also because the sound then has to curl around part of your face before reaching your ear.
e time di erence is not much. If the air temperature is 20-degrees Celcius, sound travels at 343.5 metres per second. Using Mr Pythagoras (and allowing 20cm for the width of your head), that time di erence works out to be just 400 microseconds. But you don’t hear the voice twice, once from each ear slightly out of sync with the other. Your brain fuses the two sounds into one, and merely notes that because the left ear’s sound is slightly in advance of the other ear’s, the source of the sound must be, well, just about there.
Stereo music kind of replicates that. Instead of your ears, the sound is captured by two micro-
phones (keeping it simple for these purposes). is sound is then conveyed through the recording and playback chain to two loudspeakers, one to your front left and the other to your front right. e sound from the left speaker will reach you rst. If your head is pointed right between the two loudspeakers, your left ear is going to get the sound 400 microseconds before your right ear does.
So, of course, it’s just like you were hearing the sound directly in the rst place, right?
Well, not exactly. Now there are two sources of sound instead of one. e left hand speaker emits its sound and it gets to your left ear. But the left hand speaker’s sound will also go to your right ear, delayed by that same 400us. In fact, at just about the same time as the sound from the right speaker reaches your right ear. And then the right speaker’s sound will, 400us later, hit the left ear.
With all this going on, it’s astonishing that the stereo e ect works at all. Fortunately, the brain does a lot of signal processing. Your ancestors didn’t live long enough to reproduce if they couldn’t spin around and ing a spear in the right direction when they heard a carnivore approach. A process called binaural fusion gathers all the delayed sounds – which in real life would merely be re ections and echoes from various environmental surfaces – and packages them up with the original sound to avoid confusion, adding only a sense of air or reverberant space from which one gets a feel of things like the environment and distance of the sound.
ose sorts of confusions can actually be eliminated using binaural recording techniques and listening through headphones, but that’s another story entirely.
Time is of the essence Amazing as our brain’s signal processing is, it can be tricked by timing di erences, especially arti cial ones. If the same sound is being produced by two di erent sources at slightly di erent times – up to 40 milliseconds di erence – the two sounds will fuse into one, subjectively, and that one sound will seem to be coming exclusively from the source of the rst sound to arrive. It will typically do this even if the second source is up to 10 decibels louder.
Sound reinforcement professionals use this trick in large concert venues – loudspeakers further back in the auditorium o er a slightly delayed sound which, as a result, seems to be coming from the stage instead.
In the car So you’re driving along in your right hand drive Australian car. Let’s keep it simple: you have just front speakers, with the tweeters installed where the
top of the dashboard meets the A-column. Representative distances might be 860mm from your head to the right speaker, 1260mm to the left. The difference is 400mm. Your favourite singer’s voice is coming out of both speakers equally, but it’s reaching you first from the right speaker, which is closer, by a bit over one millisecond in time.
That’s enough so that the singer’s voice doesn’t sound like it’s coming from the centre of the dashboard, nor from directly in front of you, but from the right speaker. That kind of spoils the experience. Add rear speakers as well and the complications pile up.
It’s not just your sense of direction that can be mucked up by the timing of the sound from the speakers in your car. Got a subwoofer? Its output may interfere with the output of your main speakers around the crossover frequency. If the timing is wrong, the subwoofer might be momentarily pressurising the air in the car’s cabin just as the main speakers are depressurising it. Power into the subwoofer being spent on counteracting more power from the head unit! Both subwoofer and main speakers should be pushing at the same time at those critical crossover frequencies.
All these things are easily fixed ... if your head unit has the right processing built in. This is vital for the best performance.
It’s usually called ‘Time Correction’ or ‘Time Alignment’, or something similar, and it allows you to delay the sound individually to each speaker and the subwoofer to bring them all into coherent alignment.
Set it correctly and the left front speaker will produce your favourite singer’s voice while the right speaker waits a millisecond and then issues its contribution. Because of that delay, the sound from both speakers reaches your left and right ears at the same time. Now the voice sounds like it is right in front of you, rather than dragged off to the right.
Likewise, both front speakers need to be delayed by a few milliseconds to allow the bass from the subwoofer to reach your ears at the same time as the bass from the front speakers. Rather than subtracting as the other speakers are adding, the speakers and sub will be cooperating in pumping air at those crossover frequencies.
The Time Correction setup will be somewhere in the audio settings of your head unit and will typically allow you to set the distance each speaker is from the driving position. Just deploy a tape measure.
Subwoofer 2nd step
Ah, a tape measure. If only that’s all it took! In general, that will do the trick in the time correction of the main speakers, but the subwoofer may be a different matter. The sub may be from a different manufacturer.
If an active subwoofer then it will have its own amplifier, or you could have a dedicated amplifier with possibly different characteristics to the other amplification used within the system. And in many cases the acoustic path isn’t straightforward (such as when it’s installed in the car boot). Dial in a naive distance measurement, and the results may be unimpressive, with weaker than expected bass around the crossover frequency.
Just winding up the EQ probably won’t fix that. If the weakness is due to the subwoofer being out of phase with the other speakers, boosting the effected region will just waste power as the speakers will continue to work against each other.
So a final step for subwoofer time alignment is to do it the old fashioned way. Put on some music with plenty of mid-bass frequencies – say the higher strings on a bass guitar. (Really deep bass won’t help here because that’s being produced by the subwoofer only. What we want is content handled around equally by both the sub and the main speakers.)
So put on this style of music, go to the setup panel, and adjust the time delay on the subwoofer, step by step. Try both directions. What you want is the position where that mid-bass is loudest. That will mean that all the speakers are working in phase at those frequencies.
Have fun with the entire time alignment process but don’t be completely selfish. If your head unit provides memories or presets, then it can be worth setting up several different arrangements. One would be optimised for the driver, one for the front passenger, and perhaps have one optimised for the centre of the car, in case you want to share a somewhat compromised sound with a car full of people.
Time alignment, and indeed EQ in general as well as other soundtailoring functions are now available on smartphone platforms — this one is from Alpine.
A misaligned 3D image can serve as a good illustration of what happens with incoherent sound sources. Time alignment allows the sound from different sources to fuse into one.
Audison’s bit Ten (and the bit One too) offers substantial EQ, filter/crossover settings and time alignment capabilities. Rockford Fosgate’s 3sixty.3 interactive signal processor’s extensive on-screen EQ controls.
This recently reviewed Alpine head unit features time alignmnet adjustable at the ‘Front Left’, ‘Front Right’, ‘Rear Left’, ‘Rear Right’ and left and right for the subwoofers too.