Aluminium vs Polypropylene

[Jack]

Whats the difference between an Aluminium cone and Polypropylene cone? Just looking at these two:
Polypropylene (http://www.incarexpress.co.uk/view_product.php?partno=XSL121P5)
Aluminium (http://www.incarexpress.co.uk/view_product.php?partno=XSLD125P5)
I've already got two of the Polypropylene ones, but they'll be sold with my hatch so I'm looking to get one or two for my new car. Just wondering if its worth paying the extra ?40 to get the Aluminium cone?

Also... is there any difference in sound quality with 4ohm subs over 2ohm ones?

[Nick]

i'd say at a guess...

that aluminuim cone would provide a more Punchier and solid note. Better for Dance/fast paced/sharp music.

The polypropylene cone would provide a softer and more tonefull sound, better suited to R&B slower funkier music.

It's a complete guess, but im pretty sure it'l be the difference.

Also, the 2ohm and 4ohm is just the input impedance.i don't know what difference it makes to sound quality, but your meant to impedance match.

i.e if your stereo has an output impedance of 4ohm, get a 4 ohm sub. if it's a output impedance of 2ohms, get a 2 ohm sub.

or the closest to match.

if your bothered as to why you should impedance match, then ask me and i' might be bothered to explain it. lol

[Jack]

if your bothered as to why you should impedance match, then ask me and i' might be bothered to explain it. lol

Go on then :)

[Nick]

riiiiiiiiiiiiiiiiiiiiiiiiiiight. you asked for it.

Impedance Matching


The job of a transmission line in a telecommunications application is to transport the maximum amount of power from one end to the other. (This differs from power distribution systems where the emphasis is on efficiency.)

http://www.trendcomms.com/multimedia/training/broadband%20networks/web/main/Copper/Theme/Chapter2/Images/Max%20power%20circuit.gif





To find out under what circumstances the maximum power transfer occurs, let us consider this simple dc circuit, which shows a voltage source with an output impedance of RO feeding power to a load RL. It is possible to show mathematically that the maximum power is dissipated in the load when its resistance equals the source resistance.

Here the function relating the power transfer to the ratio of the two resistors is plotted graphically, confirming the same result.
http://www.trendcomms.com/multimedia/training/broadband%20networks/web/main/Copper/Theme/Chapter2/Images/Max%20power%20plot.gif



Although this example uses dc for simplicity, the maximum power transfer theorem applies equally to ac.
http://www.trendcomms.com/multimedia/training/broadband%20networks/web/main/Copper/Theme/Chapter2/Images/Max%20power%20line.gif


If we now consider the input end of our transmission line and represent it by a fixed impedance Z0, then because of the maximum power transfer theorem, the maximum power will be input into the line when it is driven from a source whose impedance is Z0. Similarly at the output end, we can represent the line by a voltage source whose source impedance is Z0, and the maximum power will be supplied to the load when the load impedance is also Z0.

Maximum power is transmitted through a transmission line when it is driven from a source of impedance Z0 and is also terminated in Z0.

Another way of looking at what happens in transmission lines when voltage waves travel along them it to consider the analogy of a flexible rope.
http://www.trendcomms.com/multimedia/training/broadband%20networks/web/main/Copper/Theme/Chapter2/Images/Rope%20men.gif



If the man on the right holds the rope tight, but lets his arms move and allows the end to move freely up and down, any movement made by the man on the left will travel down the rope and be absorbed by his twin on the right. Note that for this to happen the "impedances" of the two men must be equal, ie their arms must be the same weight and flexibility as the rope. The equivalent transmission line is also shown, terminated at both ends with the characteristic impedance.

Now consider what happens if we replace the terminating man by a fixed object.

http://www.trendcomms.com/multimedia/training/broadband%20networks/web/main/Copper/Theme/Chapter2/Images/Rope%20wall.gif




This time the wave travels down the rope but is reflected by the wall and returns to the sending end. This is equivalent to a transmission line which is terminated in a short circuit. All of the transmitted power is reflected back from the shorted end, because none of it is absorbed by the load.

Of course because the rope is not perfectly flexible, the pulse received back at the sending end will be smaller than that sent. Similarly some of the power is absorbed by the transmission line itself and the reflected pulse is smaller than the transmitted pulse.

The same thing happens when the end of the rope is left completely free to move, although it is not so obvious because in this condition it tends to fall on the floor! (Imagine instead the rope hanging vertically with nothing on the end.)

http://www.trendcomms.com/multimedia/training/broadband%20networks/web/main/Copper/Theme/Chapter2/Images/Rope%20open.gif


The equivalent to the open rope is the unterminated transmission line, and again, all of the power is reflected because none can be absorbed by the load.

The use of the correct terminating impedances at both ends of transmission lines is called "Impedance Matching".

lol, stolen from a website, but it shows a very good analogy, which i couldn't think of.

if you'd like to learn more, buy Electronics Explained by Michael Brimicombe. Even a chump with 1/2 a brain cell could learn alot by reading it.
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[Nick]

reading through it again, it explains what is meant, but not the techinicalities of impedance Matching..

If you really want to know what is going in technically, i'l explain that in my own words. But at least that shows you what is meant by impedance matching.

basically, it's to ensure optimum power delivery to the load.

i.e optimum power delivery from your amp to the sub. So macth your sub with the output impedance of the amp !

[Jack]

Soo... if an amp outputs at, say, 4ohms and is connected to a 2ohm sub, the maximum power is not being transferred and some of this power reflects back down the wire to the amp?

[Nick]

well no, the power doesn't reflect, as such

but as an analogy, that's right.

so impedance matching helps. i wouldn't think about it alll toooo seriously tho, you'd probably not notice the difference, this is just pro-hi-fi geek stuff.

[Nick]

if you work through some calculations using IRV, PIV you'l see that the power is simply not being transferred.

[Nick]

buy the brimicombe textbook

it explains electronics soooo well. it's a really really handy subject to know.

[burgo]

why didnt you put the last three posts in one post ??

[Nick]

i really don't know.

[Jack]

Post monkey! :D

Thanks, I'll see if I can get a hold of a copy of that book.

[Nick]

yes it's very good (may be ?20) but if your a bit of a secret geek and into a bit of leccy, it's all good !

Michael Brimicombe - electronics explained

is an A-level book, but starts from the basics.