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is a 4 Ohm cab going to be the same volume as two 8 Ohm cabs from the same amp?

TrevorG

By TrevorG,
in Amps and Cabs

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TrevorG Community Regular

TrevorG

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Sorry if it sounds like a stupid question and I know the cab's power rating is as relevant as the Ohmage but I'm still getting the hang of this impedance business. Just read about someone wanting to add a 4 Ohm cab to an 8 Ohm cab and wondered - 1) how much extra volume they might get before the amp blew and 2) is there any point in getting a second 8 Ohm cab if they will be no louder than the single 4 Ohm cab?

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Bill Fitzmaurice Veteran

Bill Fitzmaurice

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There's no such thing as ohmage. Ohms are used to measure resistance, impedance, capacitive reactance and inductive reactance, so if one says' ohmage' it could refer to any or all of those factors.

Adding a 4 ohm cab to an 8 ohm cab won't cause anything to blow, assuming the amp is rated for the resulting 2.67 ohm impedance load.

Adding a second identical 8 ohm cab will result in a 3dB sensitivity increase compared to a single 4 ohm cab, assuming the drivers are identical save for their impedance. The pair of 8 ohm cabs would have twice the driver displacement of the 4 ohm cab, so they would have 6dB higher maximum output capacity.

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TrevorG Community Regular

TrevorG

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51 minutes ago, Bill Fitzmaurice said:

There's no such thing as ohmage. Ohms are used to measure resistance, impedance, capacitive reactance and inductive reactance, so if one says' ohmage' it could refer to any or all of those factors.

Adding a 4 ohm cab to an 8 ohm cab won't cause anything to blow, assuming the amp is rated for the resulting 2.67 ohm impedance load.

Adding a second identical 8 ohm cab will result in a 3dB sensitivity increase compared to a single 4 ohm cab, assuming the drivers are identical save for their impedance. The pair of 8 ohm cabs would have twice the driver displacement of the 4 ohm cab, so they would have 6dB higher maximum output capacity.

Wow! Thanks. I've never used anything but amps rated at 4 ohms (hope that's the correct term)so I've no idea how common amps at 2.67 are.  So is the advantage of two 8 Ohm cabs just dispersion? And also - what's inductive reactance?

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TrevorG Community Regular

TrevorG

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3 minutes ago, discreet said:

3db is a lot! It's a logarithmic scale. :)

OK. Thanks. I'm playing big catch up here. I get that say 18db is twice the "sound pressure" of 15db(I think) but I thought the perceived volume increase was much less. Where am I going wrong?

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discreet Grand Master

discreet

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33 minutes ago, TrevorG said:

Where am I going wrong?

I don't think you are. :) Mixing an 8 and a 4 ohm cab is inadvisable, which leaves the option of 2 x 8 ohm cabs (assuming a 4ohm amp) or one 4 ohm cab. 2 8ohm cabs will be louder and sound better but are more hassle to haul. One 4 ohm cab is easier to haul but you won't be able to push it as much, mainly due to mechanical limitations. There is also the option of using one 8ohm cab depending on how loud you need to get. Using one 8ohm cab will get you approx 350w max from a 500w amp. In practice this is often more than sufficient!

I'll always opt for one 4ohm (or 8ohm) cab because I'm a lazy bugger and in a relatively quiet band. I don't need to get loud. Getting loud is overrated anyway, imho.

Currently I'm using a 350w 2x10 combo which sounds great, gets quite loud enough and weighs 16kg for the win. ;)

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TrevorG Community Regular

TrevorG

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13 minutes ago, discreet said:

I'll always opt for one 4ohm (or 8ohm) cab because I'm a lazy bugger... 

Currently I'm using a 350w 2x10 combo which sounds great, gets quite loud enough and weighs 16kg for the win. ;)

Now weight I understand - I have a 1000watt Markbass combo for the same reason(and since joining a band with the loudest guitarist in England I've added a 102P - which is why I was curious as to whether I'd have been better off with a 4Ohm cab).

Many thanks, gentlemen I feel hugely educated but still have a ton more to learn (please tell me you don't have to know it all to be a good player)!!!  :)

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discreet Grand Master

discreet

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19 minutes ago, TrevorG said:

...please tell me you don't have to know it all to be a good player!!!

I reckon I'm a fair player but am always showing my technical ignorance on this board (especially in threads that Bill contributes to - he is the master). Don't worry about it. ;)

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TrevorG Community Regular

TrevorG

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4 minutes ago, discreet said:

I reckon I'm a fair player but am always showing my technical ignorance on this board (especially in threads that Bill contributes to - he is the master). Don't worry about it. ;)

Well I'm not a bad player but I can keep you company sitting in the corner with the dunce cap. Does Bill do seminars? I'll start saving up!!

Edited by TrevorG
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RandomBass Proficient

RandomBass

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8 minutes ago, discreet said:

Well I'm not shagging that elephant again today - you'll have to have another listen tomorrow when there isn't a plane flying over. :|

I'm off to put The Toy Dolls on....

 

Oooooooooooooooh.......

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