[kaigoss69]

For those with trunk subs, you may want to try something very easy to improve the performance of the subwoofer and the midbass drivers, as well as improve the imaging of the subwoofer.

First, a little background:

The midbass drivers (underseats), as well as the trunk sub, play the same frequencies at the crossover point. So if you have them crossed over at 80Hz, they both play that tone. If the sub and midbass are the same distance from your head, then the sound waves will arrive "in phase", which is good because the sound becomes louder, and your brain cannot distinguish between the two sources, making the sound appear to come from one location.



Unfortunately it's not a perfect world, and in our cars the subwoofers are farther away from our heads than the midbass drivers. Therefore, the sound waves are often offset to each other, which creates a "phase shift". In the worst case, the shift is 180 degrees, which results in massive cancellation of the two waves. This drastically reduces the sound pressure level at the crossover frequency, and also enables our brain to locate the sub. When this happens, you can hear the sub play in the trunk, which is very annoying to say the least.



So what can we do to re-align the two waves (and make Scooby happy again)? - There are several ways to do this, but the easiest, cheapest, and fastest way takes literally seconds. All you need to do is to reverse the speaker wires going to your sub. So (+) goes to (-) and vice versa. What this does it flips the sub's sound wave and it brings it back in synch with the wave from the midbass drivers.



Try it, it may very well work! In case you cannot hear the difference between the two, there is an easy way to measure it:

• Download an "SPL" measurement app for your smartphone (JL Audio Tools is a good one, and free). UPDATE: JL Audio Tools does not properly measure tones under 100Hz. I have tested the app SPLnFFT, and it works great at 60Hz and up.
• Download a "test tone" from the internet, at the exact same frequency as your crossover setting.
• Position the phone in the head position (you can just wedge it between the seat and the headrest, with the mic pointing forward)
• Disconnect your subwoofer (so only the midbass drivers are playing) and play the tone through your MP3 player (you are sitting in the passenger seat). Increase the HU volume until you get around 70 - 80dB (it doesn't have to be exact). Record the max. dB reading.
• Now do the opposite and disconnect the midbass (so only the sub is playing). Don't change the HU volume. Adjust the sub amp gain until you get the same reading as before.
• Reconnect the midbass, remeasure, and record the max. dB reading
• Now reverse the sub's speaker wires and remeasure

Whichever connection gives you the higher reading is the one with better phase alignment! In my car, it was clear:

• SPL with sub in phase: 72dB
• SPL with sub out of phase: 87dB

[mob17]

Nice work.

Someone really should create a sticky on the MS-8.

[makkan00]

Informative Kaigoss69.

I am very lucky to have phase switch on my amp to play with.

[afsar]

Shouldn't MS-8 autotune take care of phase, also introduce delay in the nearer driver to match it with the farther driver?

[kaigoss69]

Originally Posted by afsar

Shouldn't MS-8 autotune take care of phase, also introduce delay in the nearer driver to match it with the farther driver?

To be clear, my post is not specific to MS-8 installations, it is for ALL installations.

Now to answer your questions, the answer is NO and NO. MS-8 does not correct phase, and it does not time-align to the sub. While the need for time alignment to the sub (farthest driver) is debatable, I believe phase correction is not. JBL/Harman believe that issues with phase are best resolved through EQ. I beg to differ however, as do many others. As you can see above, the processor would need to apply 15dB of boost just to correct the phase issue at the crossover frequency. That eats up valuable EQ headroom, and it may therefore very well end up running out of steam before it can finish EQ'ing the whole system. That's the reason why level setting is so important, as well as correcting subwoofer phase.

It's a shame, because I think it would have been very easy to incorporate a phase check/correction step into the auto-tune algorithm, but sadly, it was left out.

[808MGuy]

I've tried the phasing thing and what I get is either a lot of midbass or none at all. I haven't tried the time alignment to try to line up the midbasses to the subs though. The only time alignment I've done is with the front stage speakers (midbass, mids and tweeters). Maybe I'll try to play with this a bit more and see if I can get everything to play nicely.

[WilliamS]

Time alignment is the true answer. Taking the Woofer out of phase can have other acoustic effects that you may or may not hear. The proper way is to either have proper time alignment, or remove the underseats from the equation.

Again this may work for some frequencies but not all.

[kaigoss69]

Yes, if you have the processing available to delay the cabin speakers to "wait" for the sub, that would be the preferred way to do it. Many people here do not have that option though!

[afsar]

hmm interesting. Agree could be needed if MS-8 isn't present. As per JBL website they claim to do time alignment, see here.

And if that weren’t enough for a great sounding system already, the MS-8’s DSP takes the measurement input and applies and advanced time correction algorithm. By applying this time correction, the MS-8 ensures that sound from all speakers will hit your ears simultaneously. This greatly improves the system’s imaging, clarity and realism.

[Technic]

Originally Posted by Goofyguy

Time alignment is the true answer. Taking the Woofer out of phase can have other acoustic effects that you may or may not hear. The proper way is to either have proper time alignment, or remove the underseats from the equation.

Again this may work for some frequencies but not all.

Now that I have a PXA-H800 (replacing an MS-8) I can change phase and T/A on the fly, and my results supports your post: changing phase will give an apparent bump in SPL but with a detriment in SQ. The sub impact is simply gone, midbass is up and some audio artifacts -for lack of a better word- are heard all over the place. Like something is... out of phase.

T/A is much more effective, although too much of it will make the sound to be like in a tunnel.

[kaigoss69]

To be clear, changing the polarity of a single speaker does not create any weird or "off" sounds. In fact, the sound coming from the speaker alone is indistinguishable between normal and reversed polarity.

To achieve seamless integration between speakers in adjacent passbands, they MUST be phase aligned at the crossover frequency. The easiest, and most accurate way of achieving this is through time alignment. You are delaying the closer speaker to you, such that the wave from the farther speaker arrives at your head at the same angle (phase). If you do not have the luxury of T/A, then phase is your next best option. If the sound waves from the midbass and sub speakers are 180 degrees (or close) apart, then you swapping the phase on the sub is basically the same thing as delaying the speaker closer to you by the time that is equivalent to half the wavelength.

People may hear different effects (outside the crossover frequency range) when the polarity of one speaker is reversed, because phase is frequency dependent. So at 80Hz the waves may be perfectly aligned, but at 60 and 100 Hz they will be off again, due to changing wavelengths. That's why many people prefer steeper crossover slopes, because it reduces such interference.

[kaigoss69]

Originally Posted by afsar

hmm interesting. Agree could be needed if MS-8 isn't present. As per JBL website they claim to do time alignment, see here.

And if that weren’t enough for a great sounding system already, the MS-8’s DSP takes the measurement input and applies and advanced time correction algorithm. By applying this time correction, the MS-8 ensures that sound from all speakers will hit your ears simultaneously. This greatly improves the system’s imaging, clarity and realism.

Nope, the sub is not included in the "time correction algorithm".

[808MGuy]

Finally got a chance to mess around with T/A with the midbasses. Previously, I only had delays set for the tweeters and mids and for the L midbass in relation to the R but not the midbasses in relation to the sub. Today I added in some delay for the midbasses in relation to the sub and thus had to delay everything else accordingly. The transition between the midbass and sub is a lot smoother now. I had to adjust the crossover slope on the midbasses and even dial down the sub a little as the bass really picked up once the T/A was set. After I did that, it sounded much better. It might put off my urge for more power for a little while

[kaigoss69]

Just to update this thread, I have since upgraded to a ski-pass mounted subwoofer, and I recently installed a new amp that allows for incremental phase adjustment between 0 and 180 degrees. The sub - when measured to the head position - is within a few inches of the same distance as the midbass drivers, so naturally I assumed that they would be in phase with each other...but having all that adjustability at my fingertips, I went ahead and measured the SPL at the crossover point between 0 and 180 degrees adjustment. Well, turns out that the highest SPL at the crossover point is achieved between 150 and 180 degrees out of phase. It shows that in the car environment, one should never make any assumptions based on physics alone, better get out the SPL meter and measure for yourself.

[Tim603]

My head unit has a reverse mode, as easy as clicking a button! And it does sound better in reverse

[taibanl]

Using a mid bass slope of 12dB/octave and a sub slope of 24dB/octave will accomplish the same phasing. I have used this with good results.

[808MGuy]

The last post I made in this thread was when I was still using the Zapco DC amps. Since then I've replaced it with new amps and I'm using the Arc PS8 for processing. Here is a very helpful online tool that helps with T/A for anyone who is new to setting T/A.

http://tracerite.com/calc.html

[kaigoss69]

The JL Audio Tools app has a cool polarity checker and I found out that my underseats were connected with the wrong polarity. So in the end logic still prevails, but nevertheless it never hurts to check.

[Wongway]

Thanks for this thread... more tools to help me out with!

By chance, anyone recommend a good SPL measurement tool for Android?
SPLnFFT doesn't seem to be available for Android that I see.

[tdgesq]

Get the AudioTool app off of play store. It's around $10. It measures spl and much much more. You can also get the dayton microphone from Parts Express for around $25 that plugs directly into your phone. It will make your measurements much more accurate.

[Wongway]

Thanks for that tdgesq!