ISOBARIC 16 ohms load powered by 8 ohms Amp

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eddi0

Member
Joined 2006
Hello,

This is my first post. I would like to know if I run dual 8 ohms 18s in an isobaric configuration it will produce 16 ohms load and if I want to run my QSC RMX 1450 mono bridged @ 8ohms it produces 800 watts is this possible, or will it produce distortion.

If this does not work then I would have to buy (2) 4 ohms 18s and connect it in an isobaric configuration to produce 8 ohms load, then I can use my amp.

Please let me know your input.

Thanks,
Ed

:xeye:
 
Lothar34

Lothar34

Member
Joined 2006
You have two drivers, right?

You can wire them in series or parallel for either a 16- or 4-ohm load. The fact that it's an isobaric setup doesn't enter into it.

If you want an 8ohm load for your amp, then your best bet is to get two 4ohm drivers and wire them in series. Unless you come across a 16ohm driver that you really like. They're fairly rare though.
 
forr

forr

Member
Joined 2004
Be aware that an isobarik configuration using two 18' implies an important moving mass. Some people on this forum have complained of moving subs enclosures. The ratio between the moving mass and the mass of the enclosure was the first reason.
 
A

audioPT

Member
Joined 2002
If you have possibilities, try this:

Screw the sub to a base board and put only 3 feets or spikes.

If the sub stand still...
 

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D

dangus

Member
Joined 2004
Isobaric also requires a big tunnel to join the two drivers, unless you're going clamshell style, which means the volume saving is never as good as 50%, and even halving the volume of a box doesn't really make it look 1/2 the size.
I just don't see the point of isobarics. If you have two drivers, build two boxes and and you'll have higher efficiency and higher max SPL from the same amp power, and probably lower distortion at a given SPL compared to the isobaric box (since the two drivers won't be working as hard). If reducing box volume is so important, use a more appropriate driver, or mass-load the one you have.
 
E

eddi0

Member
Joined 2006
I see your point. Here are the spec of the sub woofers and the box size:

Please let me know if these would work or you recommendation is helpful, b/c I have not bought the speakers yet.

Box:

31.5" x 23.5" x 23.5"

8.4 cubic air space (4.2 per side)

Dual 2" x 22" x 6" slot vents (.15 cubic feet per vent)

7 ply 3/4" Birch construction




Speakers (18" Madison Executioners):

RMS Power Handling: 500 watts

TS Parameters
FS 40 hz
VAS 7.56
SD 213.82
Qts 0.927
Qes 0.969
Qms 21.45
BL 16.91
Re 5.5
Nominal Ohms 8
Le (mH) 1.117
Mms 201.39
X max (inches) 0.375
X mech (inches) 0.75
Peak Power Handling: 1000 watts
Ohms: 8
Frequency Response: 38 to 3100hz
Sensitivity: 100 dB
Magnet: 100 oz.
Basket: Cast Aluminum
Voice Coil: 4" Copper Clad Aluminum
Cone: Treated Paper
Surround: Accordion
Binding Posts: Clip Type
Pole Piece: .
Overall Diameter: 18 13/16"
Overall Depth: 7 5/8"
Mounting Diameter: 16 13/16"
Mounting Depth: 7 1/16"

Thanks, Edward
 
D

dangus

Member
Joined 2004
100 dB sensitivity and "accordian surround" sound like PA speakers to me. That 38 Hz cutoff is probably 10 dB down, which is not much of a subwoofer by diyaudio standards.

For audio and home theater subs you should be looking at drivers with lower efficiency and more Xmax (12 mm and up). If you really want PA subs, check out other threads on horn-loaded subs.
 
Cloth Ears

Cloth Ears

Member
Joined 2001
dangus said:
I just don't see the point of isobarics. If you have two drivers, build two boxes and and you'll have higher efficiency and higher max SPL from the same amp power, and probably lower distortion at a given SPL compared to the isobaric box (since the two drivers won't be working as hard). If reducing box volume is so important, use a more appropriate driver, or mass-load the one you have.
Click to expand...

For the same power input, 2 drivers in Isobaric gives 3dB down on a similar single driver in a box with internal volume twice the size, and is 6dB down on a 2 driver box (or 2 single driver boxes) with 4 times the internal volume. Distortion is improved slightly, depending on the enclosed internal volume between the 2 drivers.

If you have the room, and no spouse to complain about it, the the 2 box solution is the most efficient. For myself, a single 200l box (including drivers) is about all I am allowed in the house. 2x335l boxes was just not going to cut it - and the extra expense of another amplifer sealed the bargain.

In a world without compromise, I know what I would have done...:)
 
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