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MG MGF Technical - What is a 'cross over'??

Very quick question: -
I read on Rob Bell's rear speaker mod site (I think), that: -

"..There are certain pit falls to avoid.."
".. Most importanty amoung them is that for good stereo effect, the high pitched sounds need to be filtered out from the rear speakers.."
".. you need a cross over.."

What is a 'cross over'?? and how does it work?

Neil H

A cross over is a box of tricks which seperates the high pitched noises from the low pitched noises and sends them off to their respective speakers. Without a simple crossover you will cook your tweeters.
Will Munns

Not sure how it works but it is a little gizmo that can be passive or active (controlable) that filters out part of the sound to give you a pure bass or a pure tweet sound.

In my last car i had a Longmill active one where i could control the sound output, in Scarlet i had a pair of passive 'in-line' ones adjacent to the tweeters i fitted ages ago (now removed).

HTH

SF
Scarlet Fever

err.. okay... so far so good..
So is this a major bit of kit.., a speaker type... or a small plastic thing from Halfords?

Do I buy it in addition to speakers (if I am fitting them in the rear?).

Neil H

A passive crossover can be as simple as a capacitor (bass blocker, put in-line with the tweeter) and/or an inductor (high-frequency blocker, for the bass speaker). It can also be active, with adjustable crossover points (cut-off frequencies). Active ones are better and are boxes (though ideally not from Halfords!). You only need them if the speakers you are installing aren't full range (ie. they can't handle the full 20Hz-20Khz) or if you want to tweak for a better sound.

Tim.
Tim Jenner

The tweeters I fitted some while (years) back came complete with simple capacitors - or at least that is what they looked like - and the tweeters still work OK.

Ted
Ted Newman

Fair enough.. thanks for the input.
My understanding of the problem is that the rear speakers, in general, can only be 4x6's and therefore are probably quite tinny..

Rob suggests a cross-over may reduce the upper end of the spectrum, making the sound a lot better over all since the tinny stuff can come from below..
Is this right?

This cross-over game is beginning to sound like the thing I'll want 'later on' when I get to the ICE, amp and Sub stage.

Neil H

>or a small plastic thing from Halfords?
Kerching!

>Do I buy it in addition to speakers (if I am fitting them in the rear?).

If you're not fitting seperate tweeters then no, fit without.

>Not sure how it works but it is a little gizmo that can
>be passive or active (controlable) that filters out
>part of the sound to give you a pure bass or a pure
>tweet sound.

OK you asked for it!

Generally the term active and passive mean before or after the amplifier respectivly, so if you're using an active filter then you have to amplify it's outputs, and an active filter will take the form of a graphic EQ or some electric gizmo (expensive)

A passive crossover sits after the amplifier and works by using a combination of inductors (coils of wire) and capacitors.

You can imagine a capacitor as two tin cans, end on end with a thin rubber membrane seperating them, now you fill both cans with water thru a hole in each end - got that?

If you attempt to push water thru your 'capacitor' then initially it will move easilly, with resistance building up untill you reach a point where the rubber is pushing back with the force you push in, at this point the flow of water stops, but you can add a little pressure or remove a little pressure and the membrane moves a little, consider the orignal pressure as DC and the additional push/pull as AC, therfor the AC signal is stripped from the DC signal, it passes thru your capacitor, where the DC signal can't. Audio is made up of slow moving signals and of fast moving signals, the fast moving signals pass thru the capacitor easily, the slow moving signals get blocked.
So if you cannect a capacitor in line with your tweeters then the High frequencys got thru, and the low frequencys get stripped.




Will Munns

>Rob suggests a cross-over may reduce the upper end of
>the spectrum, making the sound a lot better over all
>since the tinny stuff can come from below..

Well it would do that, but the real problem is the bass noise distorting the small speakers, rather than the other way around.

>Is this right?
I wouldn't do it, I added the rear speakers to get extra top end, you can't get good bass by stripping the top end, you'll end up dissapointed if you do this.
Will Munns

Neil, despite what I wrote there, I haven't bothered so far to install crossovers for the T-bar speakers in my car. More laziness really.

Stereo imaging is ruined really, but to be totally honest:
1. who honestly cares ;o) [joking, mostly!]
2. very very difficult to achieve good acoustics in a car, even harder in a two seater, and impossible in a drop-top, at 80mph with the roof down!

Priority in a roadster like the F is to actually hear the flippin' stereo when blatting along. T-bar speakers are perfect for this, as the door speaker sound does tend to get 'lost' in the wind roar.
Rob Bell

I fitted a stereo headphone socket to the rear speaker outlets and can now hear at all speeds if I so desire - it also allows me to use the front/rear speaker balance to cut out the car speakers if the wife (or other passenger) does not like my choice or wants to sleep.

I also have a splitter jack so that we can both use headphones.

And before any one brings up the old chestnut - no it is not illegal!

Ted
Ted Newman

Great, settled then..

I will fit them without that cross-over thing..

- Thanks Will.., I feel enlightened / a tenth of the man I thought I was!?!

Hopefully by the end of the w/e I will be ready to agree with you Rob.

Neil H

Will,

I've got a Hienz Beans can and smaller Single Portion Tomato Soup, but I'm not sure where to mount them. Are they best in the left or right door panel?

Tony
Tony Thompson

Neil, a real important thing in sounds, is 'sound staging'. It's really important to get that right.

If you go to a concert, do you listen to the orchestra or the band facing the back? No... you face forward right?

But at the same time, the hall is designed to reflect the sound coming from the instruments from back of your head.

This is where the rear speakers come in. You should only set up the rear speakers loud enough to just be able to realise there is something behind. The vocals and other noise should not be recogniseable. It's called a rear fill. You can fit the cheapest speaker if you wish....won't ruin your sound quailty.
Just put some decent 6x4s or 6x9s (doesn't matter, you shouldn't put the fade back to get more bass out of these)...

and really good front components with a crossover. Separate system with a woofer and a tweeter. The sound is personal taste, but I recommend Infinity speakers / Boston Pros... having a 2ch amplifer to drive these will bring up the sound quality by heaps as well. By the way fronts are 6.5".

Good luck upgrading the sound system.

Regards,
Hanah
Hanah Kim

And also you can always get a 6" or 4" speakers (coaxial) that you adjust the direction of the tweeters. Cerwin Vega had this... and Alpine..

If you want bass, you need a sub. But by sound deadening the T-bar and doors you can get some decent 'midranges' out.
Hanah Kim

This thread was discussed between 02/09/2003 and 09/09/2003

MG MGF Technical index

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