[QUOTE=painball]Why exactly would you double the wattage when you halve the resistance? Wouldn't that overdrive the amp and speakers?[/QUOTE]
Total Output power = Amps rated output power x (amps rated ohms / ohms of the load)

That above equation lets you work out the output power of the amp dependant on what load it is being ran through.
If an amp is [B]rated[/B] to put out 200 watts while through 4 ohms.
Yet you are running it through 16 ohms.
Then the output power will be reduced as follows:
200 x (4 / 16) = 50 watts

If you run it through a 2 ohm load:
200 x (4/2) = 400 watts

Amps may have an indicator on them that they cannot be ran at less than a specific amount of ohms. For reasons that if you do then you may cause damage to the amp due to heat.
Many amps cannot be ran at less than 4 ohms. Some others can handle down to 2 ohms, but these usually have large cooling fans and heatsinks built into them.
So, provided your amp can handle it, you can work out the total load of the speakers you can take off the amp in accordance with the output power of the amp.

airborne50caliber: Impedence is the resistance of an alternating current. As apposed to a straight DC resistance.
Audio signals are an alternating current, therefore speakers have an impedence rating.
Some frequencies may be resisted to differnt amounts. The average amount that the entire frequency responce range is resisted by the speaker is known as the impedence.

thank god for this thread

im looking into buying a PA that would have 3 inputs going into it most of the time, a bass, and 2 mics, i went to mothers music recently and found what i think is a pretty good deal but i wanted to run by it with some people taht know what thier doing since i dont always trust the store clerks

i want to buy the Yamaha STAGEPAS it has 4 inputs and a control panel, obviously, and 2 150 watt speakers, the guy tested it out at the store and he filled up the large store just lightly saying test test, im looking for something that can be used for small shows and will last me a long while, small shows like garages and possibly community centers, i know if 3 inputs are running itll take away alot of teh juice, but im just wondering if the price at $840 canadian for the Yamaha STAGEPAS is to good to be true, thanks

That sounds like a pretty good purcahse. If you are happy with it then go for it.

But... When running bass through the PA, you really are cutting down a lot on the capabilities of the PA. It should be fine to run a couple of mics into during practising and some smaller gigs. But once you run bass through it, it will struggle to keep up.

Perhaps you may be better off upgrading the bassists amp so that they do not need to run through the PA.

really? i was to belive you could run anything through there, do you mean itll struggle to keep up because im putting a bass in there and the mics, or because its just a bass and it wont work well with 3 inputs, if thats the case, would it work well with only one mic and the bass?

Bass works a lot with lower frequencies. And to accurately reproduce these frequencies and for them to be loud enough you need a lot of headroom (or watts).
If you dont have heaps of power spare, you could get an unwanted distortion coming into your sound. And the rest of what you are running through the PA will also suffer.

To successfully run bass and vocals through a PA, I think you'd need a minimum of around 600 watts or so. But once agin this depends on the size of the venue you will be playing at.

I'd reccomend you just use that PA for the vocals. And see if you could borrow or save up for a bigger amp for the basist.

alright sounds good, thanks alot for your help

in a more electronics than PA sense, but i'm sure you'll know...

1. would i be ok splitting a signal into the 2 inputs of a stereo amp, and then connecting the outputs together?

2. if each output was designed for a 4 ohm load, would i want an 8, 4 or 2 ohm load?

No. Do not connect the outputs together.
It should be okay to just parrallel split the input signal. But keep each output of the poweramp completely sepearate.
If you want to bridge a poweramp you will need to buy one that can actually be bridged. Otherwise, just stick to using it in stereo or only one channel at a time if needs be.

ok, thanks.

[url]http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&category=23785&item=7320979019&rd=1[/url]

would that be good with a couple of speakers that i have?? i dont really know the specs of the speakers but they are big ?? lol

thanks

If the speakers that you have are 8 ohm speakers, they will need to be rated at at least 200 watts each.
If so, yes.
But it really does depend on the specs of the speakers.

well it damage the speakers if they arnt the right specs.. even if just testing them starting like at a low volume??

if the impedance matching is way off, things might get damaged independant of the operating level.

Feedback and Monitors
 

Hello boys and girls,
First post here - hope I'm not screwing it up.

I'm in my first band; I sing lead and play rythm guitar. We have a bass player who also sings backup, and a drummer, and we play rock/pop-punk. I've never known anything about sound equipment before, but have recently purchased a bunch of stuff and I'm learning.

Here's my questions-
1.) I have a Yamaha EMX5000 combo mixer/pa and a pair of Yamaha S115V (15") main speakers. 500 watts or 1,000 watts bridged power. We will only run 2 vocal mic's through them. Should be plenty powerful for local bar gigs, I hope. But, in practice, I STILL can't hear my vocals. So I'm considering a monitor. I guess it makes the most sense to get the Yamaha 15" monitor, right? Any thoughts on that?? In that case, I guess I will have to bridge the 2 mains, right? How is that done? Through a basic speaker cable?

2.) Are there some basic principles for setting up the stage sound equipment that helps avoid feedback?

I know this is all probably very elementary to most of you. I appreciate your help! Rock on~

trust me, speaking in intelligble english has already but you in the top 5% of newbies.

[QUOTE]1.) I have a Yamaha EMX5000 combo mixer/pa and a pair of Yamaha S115V (15") main speakers. 500 watts or 1,000 watts bridged power. We will only run 2 vocal mic's through them. Should be plenty powerful for local bar gigs, I hope. But, in practice, I STILL can't hear my vocals. So I'm considering a monitor. I guess it makes the most sense to get the Yamaha 15" monitor, right? Any thoughts on that?? In that case, I guess I will have to bridge the 2 mains, right? How is that done? Through a basic speaker cable? [/QUOTE]

15" monitor would be great, but a little unnecessary if you're only putting vocals through it- a !2" one would do you fine.

Bridging the amp (if it is explicitly possible) would be done with a switch on the amp. but since you could pick up a powered 12" monitor for not too much more than a high end 15" passive, i'd advise doing that.

Sacto-Grrrl:
Welcome to the forums.
First off; 2x500 watts should be plenty enough. Very suitable for pub gigs.
But, of course, while on stage with the rest of a band, if may be difficult to hear yourself. And, in that case monitors might be a good idea.

Even tho it is possible to split your PA to handle both main mix and monitoring systems. I am enclined to advise against it. Because doing so will reduce the poweroutput of your main speakers and will prevent you from making use of the stereo dynamics.

Instead, purchase yourself a powered speaker. Something like this:
[url]http://www.peavey.com/products/browse.cfm/action/detail/item/91259/number/00392610/cat/98/begin/1/112PM%3F+Floor+Monitor.cfm[/url]
Run this off one of the Aux send outputs on your PA.
And because it is self powered you wont have to worry about the loadings off your PA, or bridging.
It defiantely makes it easier.

Steve:

1. It wont be 'twice as loud' per-se. Although it will be considerably louder.
Because of the extra speaker area and increase in power. You may have an increase in loudness of about 6dB.
And although 6dB may not sound like much in words, it may very well sound like a lot when you are standing infront of it.

(for it to be 'twice as loud' there'll need to be an increase by about 12dB)

With this +6dB increase in mind. You may find that you'll be able to reach similar volumes as you used to have while the level meter on your PA was reaching 0dB (with just the two speakers). But now with the level meter reaching around -6dB.
So, think about how loud your setup was when the level meter was up at 0dB. With the extra speakers on, it may be that loud while the level meter is at -6dB.

2. Those 4 core Speakon connectors are for use in larger PA setups where you [B]biamp[/B] the speakers. That is you have a seperate amplifier for the high frequencies and the low frequencies.
Those outputs on the back of your speakers are just for the parrallel connection on additional speakers. Your speakers cannot be biamped.

You can just use a meter or so 2 core cable, with the NL2FC connectors. For each speakers. Like you have shown in your diagram.
There will be no need to replace those long speakers cables of yours.

How you have drawn the diagram is spot on correct. :)

Hey there guys,
Just recently posted a question to Chris aka Aes820, but ill also send it here in case others can help aswell or learn from stuff im going through.
I have:
1. Powered mixer with 2 x 400w @ 4 ohm or 2 x 200w @ 8 ohm.. and
2. i currently have 2 x 200w rms @ 8ohm loud speaker cabinets connected...
and i use the PA to connect mic and semi accoustic guitar..and play with a drummer, and a lead guitarist who uses his own amp.

Recently i posted that the vocals still werent that loud..or that i had to really crank the levels up close to max to reach acceptable vocal levels in comparison to drums and electric guitar. And got Chris to listen to it (i live in the same city and gave him a test listen), so i could atleast know if there was a problem.. thankfully its all good, ie. as loud as it should be. Thanks again for that Chris, i appreciate the time u took to help out ;)

So BASICALLY I WANT MY SYSTEM TO BE LOUDER (eg vocals so i dont have to strain and be able to hear myself) WITHOUT GETTING EVEN CLOSE TO MAX...

So what ill most likely do is keep what i have already, and get myself 2 more 200rms @ 8ohm speakers, and connect them so i have 2 loudspeakers on L and 2 on R..... this will essentially make me use all the power of my amp, ie. each side will have 400w rms @ 4ohm.
Heres a pic to explains what i have in mind:
[url]http://s25.yousendit.com/d.aspx?id=35M0N70PYJVEU2G64LGN31FHCV[/url]
As the pic suggests, the cool thing about this is that if infact we decide to gig (currently just havin fun jamming but wanting good sound quality), i could even hook up an active/powered monitor speaker to the preamped connection.

A couple of questions:
1. Will this new setup i want (2 extra speakers as the pic shows) mean approx twice as loud how i can get it now? im hoping to be able to get it a bit louder than how we have it now, but at a comfortable level, without even getting close to the clipping light...

2. The only drawback to a system like this one i want (as in the pic) is the weight of it all (4 speakers instead of 2). Thankfully the school i work and jam at has good storage facilities for me so its not that bad. But..is there anywhere one can get single 400w rms @ 4ohm speaker cabinets (with both subwoofer and horn in the one cabinet)??ive only seen one on the net after extensive searching, and that was just a subwoofer.. itd make things so much easier if i could use 2 x 400w rms @ 4ohm speaker cabinets.... meaning just 2 speakers instead of the 4 im currently gonna do.

I was actually thinking of getting a single 800w rms speaker (sub and horn) if they exist, and just bridge it, so that id have only one mother of a speaker to carry round..
But for possible future gigging this one speaker approach probably wouldnt work too well.. guess id be better to half it and have a speaker on each side.. which again is why id love 400w rms @ 4 ohm speakers :)

My Initial goal (and i guess it still stands today) was to get a POWERFUL System, so that vocals could be heard clear and loud over the drums and lead guitar, and to be able to do this very comfortably (using bout 1/2 - 3/4 of max of system), but with the SMALLEST, LEAST amount of equipment. Maybe i need to pay abit more, but money isnt a great issue.. providing its not too much ;)

well thats about it.. any help would be great, and im hoping someone learns from my stuff aswell... after about 2 months of researching this PA stuff.. it is beginning to make good sense. And thanks again to chris and the rest of the helpful guys on this forum.
take care peeps,
steve ;)

[QUOTE=Aes820]Steve:

1. It wont be 'twice as loud' per-se. Although it will be considerably louder.
Because of the extra speaker area and increase in power. You may have an increase in loudness of about 6dB.
And although 6dB may not sound like much in words, it may very well sound like a lot when you are standing infront of it.

(for it to be 'twice as loud' there'll need to be an increase by about 12dB)

With this +6dB increase in mind. You may find that you'll be able to reach similar volumes as you used to have while the level meter on your PA was reaching 0dB (with just the two speakers). But now with the level meter reaching around -6dB.
So, think about how loud your setup was when the level meter was up at 0dB. With the extra speakers on, it may be that loud while the level meter is at -6dB.

2. Those 4 core Speakon connectors are for use in larger PA setups where you [B]biamp[/B] the speakers. That is you have a seperate amplifier for the high frequencies and the low frequencies.
Those outputs on the back of your speakers are just for the parrallel connection on additional speakers. Your speakers cannot be biamped.

You can just use a meter or so 2 core cable, with the NL2FC connectors. For each speakers. Like you have shown in your diagram.
There will be no need to replace those long speakers cables of yours.

How you have drawn the diagram is spot on correct. :)[/QUOTE]


Hey chris, thanks for the quick reply hahah well that post u replyed to was bit fkd up cause i accidently (in the middle of finishing and editing) pressed the submit before id finished.. so i just deleted it an sent the finished version :)
Still thanks for the help.. great to hear i can use the NL2FC cables :) and just buy some shorter ones of the same kind ;)
And thanks for explaining the volumes of sound and ofcourse the confirmation of the pic ;)

Quick one on the Volume levels..u say to be twice as loud id need an extra 12db...
So whats more important for greater volume, db or watts??
these speakers i have are 96 dB@1watt/1metre.. well thats the SPL says, whatever that means..
If i happened to find some 200w rms cabinets with higher dbs.. thatd be better for volume right??
Ive noticed alot of active or powered speakers having high dbs eg 126db
IS that why these little speakers are so fkn loud hahah
Ill probably get one for a monitor oneday.

I was actually thinking of getting 2 powered speakers to replace my passive cabinet speakers, but thatd defeat the purpose of my powered mixer hehe..
which leads to the following question..
Would an option be to simply buy a mixer (not powered) + 2 powered speakers (300w with 120+db each).. i guess the only drawback to that would be ultimate future flexibility and having to use a few power outputs.. instead of the one im using now.. ie. just to power the powered mixer. Would this option be louder with a smaller package..if u know what i mean??

anyways dinners ready hehe.. talk later thanks again,
steve :)

[QUOTE=Aes820]^ It'll ruin it after time. It is normally fine to run vocals through a guitar amp. But after a while, (about 9 months or so) your amp will sound like crap.
As in, dont use your 'good' amp.

The range of frequencies of the human voice, and the levels of the vocal mic constantly overdriving the input stages of the guitar amp, does eventually take its toll.

For those in bands who are a bit strapped for cash and cant afford a proper PA. Just go to a second hand store and buy an old cheap guitar amp. Use that until you can afford something more permanent.[/QUOTE]

i hate my dad so much....hes like ya were gonna need a mic and an amp for a speech....now i dont have any speakers or cabs or anything i just have a 65w guitar amp, i use my mics for recording with headphones....

and also, my guitar amp is blown because we used it as a mic amp a couple times against my approval....so now he's saying well i payed for it its my amp so im gonna use it......now im taking off and he has no clue how to set anything up i hope he breaks everything

That '96 dB@1watt/1metre' thing describes the speakers efficiency. For passise speakers, like yours, you'd expect anything under 100dB@1w/1m. Although the more the better. Those speaker you've got are very good speakers for what they are.
And I think you'd be hard pressed to find more efficient ones.

Those two powered speakers that I have got have an efficiency of 126dB.
Because, of course, they are active and they are internally biamped.

They are 300 watt speakers each. Although 220 of those watts are for the sub driver, and 80 watts are for the tweeter. So, that's what i mean by biamped. One 220 watt amp just for the sub, and one 80 watt amp, just for the tweeter. But 300 watts all up.

And, heh, you've heard how loud they are, for just little speakers they pack a mean punch. If anyone does ever want to buy some powered speakers for monitors or even FOH speakers. They are a great option.

How loud you'll actually be counts on a lot of things, not just the watts, but also the amount of speaker area you've got, the efficiency of the speakers and of course the natural acoustics of the room you are in.

I would normally tell people that 2x200 watts would be more than enough for jamming with a band. It is unfortunate that you don't feel as if it is enough for you. I hope others who are looking at PAs are not in a similar situation as what you, are and are let down if I tell tham that thier setup would be enough and they find that it isn't.

If you go off the "10 watts per person" rule, which I guess is an industry standard. You'd would think that a total output of 400 watts from your PA would be enough for a room with 40 people in it.

But, even tho powered speakers are good, you don't have much room for upgradability with them. It's totally up to you because it is your money, but just see how you go with those extra two speakers for your setup. It may very well be plenty loud enough.

Then, perhaps if you do later buy some powered speakers you could use them just for practising on their own. And add them onto your bigger PA for gigs. Just an option, i guess.

[QUOTE=PainKiller8191]i hate my dad so much....hes like ya were gonna need a mic and an amp for a speech....now i dont have any speakers or cabs or anything i just have a 65w guitar amp, i use my mics for recording with headphones....

and also, my guitar amp is blown because we used it as a mic amp a couple times against my approval....so now he's saying well i payed for it its my amp so im gonna use it......now im taking off and he has no clue how to set anything up i hope he breaks everything[/QUOTE]

its unlikely that putting a mic through it twice would blow a guitar amp, realistically.

noo theyve done it like fifty million times....i had just realized it damages it a couple weeks ago when my speaker blew

[QUOTE=Aes820]That '96 dB@1watt/1metre' thing describes the speakers efficiency. For passise speakers, like yours, you'd expect anything under 100dB@1w/1m. Although the more the better. Those speaker you've got are very good speakers for what they are.
And I think you'd be hard pressed to find more efficient ones.

Those two powered speakers that I have got have an efficiency of 126dB.
Because, of course, they are active and they are internally biamped.

They are 300 watt speakers each. Although 220 of those watts are for the sub driver, and 80 watts are for the tweeter. So, that's what i mean by biamped. One 220 watt amp just for the sub, and one 80 watt amp, just for the tweeter. But 300 watts all up.

And, heh, you've heard how loud they are, for just little speakers they pack a mean punch. If anyone does ever want to buy some powered speakers for monitors or even FOH speakers. They are a great option.

How loud you'll actually be counts on a lot of things, not just the watts, but also the amount of speaker area you've got, the efficiency of the speakers and of course the natural acoustics of the room you are in.

I would normally tell people that 2x200 watts would be more than enough for jamming with a band. It is unfortunate that you don't feel as if it is enough for you. I hope others who are looking at PAs are not in a similar situation as what you, are and are let down if I tell tham that thier setup would be enough and they find that it isn't.

If you go off the "10 watts per person" rule, which I guess is an industry standard. You'd would think that a total output of 400 watts from your PA would be enough for a room with 40 people in it.

But, even tho powered speakers are good, you don't have much room for upgradability with them. It's totally up to you because it is your money, but just see how you go with those extra two speakers for your setup. It may very well be plenty loud enough.

Then, perhaps if you do later buy some powered speakers you could use them just for practising on their own. And add them onto your bigger PA for gigs. Just an option, i guess.[/QUOTE]

Hey there peeps,

Thanks for the info chris.. that biamp stuff was something i didnt quite understand, since lately ive been looking and researching on Powered Speakers. I will definitely get one bit further down the line for a monitor.. thatd be a great use for it… after listening to the one u had, im sold 

Well as u guys may know, ive gotta powered mixer with 2x400w @ 4ohm or 2x200w @ 8ohms. Been using 2x 200w rms @ 8ohm FOH speakers which for us was just not quite loud enough. Well id have to use most of its power to get the level that we need…

So what i was gonna do was buy 2 more of the same speakers, and add one of these 200w rms @ 8ohm speaker on each side (in parallel), meaning id have 400w rms @ 4ohm on each side… using the full capacity of the powered mixer (2x 400w @ 4 ohm) rather than the 2 x 200w @ 8ohm im currently using. But thatd mean 4 x 18.5kg speakers to carry around... tough on the arms.. plus a couple more speakon cables id have to buy.. other part of town ahhahah
Well, its still an option…

But get this… been doing about a week research looking for 4ohm speakers, so that I could get heaps of power, and only need 2 loud speakers all up. Finally found some in Australia.. :

Samson RS215 2x15 350watt Speaker Enclosure
RS215
Power Rating (@ 8Ω): 350 Watts RMS, 600 Watts Program
Nominal Impedance: 4 Ohms
Frequency Response: 35 Hz – 20 kHz ± 3 dB
Sensitivity: 101 dB SPL @ 1W/1m
MAX SPL: 128dB
LF Driver: 2 x 15-inch heavy-duty driver
HF Driver: 1-inch exit, 44mm Mylar diaphragm compression driver
Protection: Internal overload Lamp on HF driver
Weight: 80 lbs. 36.32 kg
Price $530.00 Australian

My option is to either get the 4 x 200w rms @ 8ohm speakers (400w on each side) as mentioned at top, or get these 2 x 350wrms @ 4ohm speakers. The weight will be the same, but having 2 instead of 4 will be easier… me thinks 

Also, these Samson 350w @ 4ohm speakers have a higher SPL or 101db @ 1w/1m compared to the 8ohm speakers I currently have which have 96db @1w/1m.

So basically its either the 2 x 400w rms (4 x 200w @ 8ohm speakers) at 96db @ 1w/1m….
Or 2 x 350w rms (2 x 350w @ 4ohm speakers) at 101db @ 1w/1m….

Im not asking anyone to make a decision for me, but id like to know what u guys think…
ie. Which would be the best way to go… also which would be loudest.. does the extra 5db make a difference…

And finally.. this is probably a silly question..and one I should have asked to begin with.. but chris… will it be ok for me to simply connect the 2 x 350w 4ohm speakers in my amp….. the same as I did with the 8ohm speakers…. ie. Is a parallel connection (2 x 8ohm speakers) the only way one must achieve the 4ohms?? Yeah silly question hahaha

Well finally like to say.. first sorta gig will be this Thursday.. and im **** scared hahaha im a teacher at this school.. atleast its not infront of an audience as such… its at expo night.. meaning all these activties will be goin on in our gym… but me and my half assed band will be on stage doing about 5 numbers hahah

Heres the set list (all covers) in no particular order:
Patience
Kockin on Heavens Door
Have U ever Seen the Rain
Kashmir
Civil War
Stairway To Heaven..
Funny thing is that I know the words to only 2 of these songs.. ill be reading the words off a sheet for rest hahaha how professional ;)

Btw we did a sound check today..and fkn amazing sound.. the reverb is amazing.. don’t need any on the mix 

Take care,
steve

Getting some 4 ohm speakers is another good option for you.

They have 2x15" speakers in them. So they will be very bulky. And maybe difficult to transport. Check out their physical dimensions, and see how you go.

The difference in performance between the two 8 ohm cabinets compared to the one 4 ohm cabinet may only be very minimimal. May not be noticeable at all.
I think the main factor you'll have to take into consideration here is the ease of transorting your rig.
Have a think about it, but if it were me I would be more inclined to go for the seperate 8 ohm cabinets. At least you can mount two of them up onto stands, something that you may not be able to do with the 2x15 4 ohmers.

But good luck with the gig this thursday.

Yeah youre right chris.. thats my biggest factor... having the power i need.. but together with the ease of transporting.

Hey chris do u know of any sites to calculate approx loudness of speakers based only on SPL and Max DB and Wattage (not taking the surface area or venue into consideration).
I mean its gonna probably be minimal difference, but would a set of 4 x 200w rms @ 8ohm speakers (each with SPL of 96db 1w/1m dont know the max) be louder than 2 x 350w rms @ 4ohm speakers (each with SPL of 101db 1w/1m and max of round 128)???
Will that extra 5db make some kinda good difference??

Id much rather only 2 set of speakers in terms of transport, but as u say..the samsons are at 2 x 15" which means pretty much the same size.
I have seen some other 350w rms @ 4 ohm cabinets with only the one 15" woofer included, but they cost a bit more.. thatd be great for transport. Still have to suss it out a bit.

The advantage of the 4 x 200w 8ohm speakers i guess is.. that i can position those speakers better.. in many ways etc... as well they have a 5 year warranty.

Still weighing it all up.. but the db is a factor at the moment.. as the 4ohm speakers ive seen so far have a db spl around the 101 mark..and max of 128

back to work,
thanks again chris,
cheers,
steve

Ok, so, I do vocals in a metal/hardcore/math rock/shoegaze band (yes, we actually do mix all of that). I.. scream.

Anyway, I don't have a big budget. But, I need some active monitor speakers for my vocal amplification. Something that's about as loud as the guitars. Or even a bit louder. I don't know what amp our guitarist uses, but it's an average sized amp - far from the size of a practice amp.

What would be a fairly good budget priced active monitor. Let's say... a 100 watt one. Or 150?

The way that the efficiency of speakers are noted is somewhat misleading for 4 ohm speakers.
This 1 watt / 1 meter thing is an idustry standard measurement, used for the testing the efficiency of all loudspeakers.
However this is usually taken in reference to 1 watt being equal to 2.83 volts.
But. With 4 ohm speakers, seing as they have only half the impedence of 8 ohm speakers, they require less voltage to do the same thing.
So 2.83 volts through a 4 ohm speaker would infact be equal to 2 watts (that's a +3dB difference).

Now, in english.

To compare the efficiency between the 96dB 8ohm speaker with the 101dB 4ohm speakers, you'll have to take into consideration the difference in ohms between the two, by deducting 3dB off the efficiency of the 4 ohm speaker.

Therefore that one 4 ohm speaker compared to the two 8 ohm speakers will have a difference in efficiency of only 2dB. Not 5dB.
But, that's not all that bad news.
Given that these 4 ohm speakers 2dB are more efficient. They will go about as loud while pumping out 100 watts as what those two 8 ohm speakers would while pumping out about 160 watts.
They will require less power to seem just as loud as the less efficient speakers would.

If you can cope with the fact that they will be harder to transport around, these more efficient speaker cabinets would be a good buy.

As for any actual equations for figuring this all out, I don't know of any, sorry.
But I did find this:
[url]http://www.electrosound.com/Speaker_Facts/v5no8.htm[/url]
It explains it technically but in good detail.

Has anyone heard of wharfedale PA gear?

[QUOTE=Kosta]Ok, so, I do vocals in a metal/hardcore/math rock/shoegaze band (yes, we actually do mix all of that). I.. scream.

Anyway, I don't have a big budget. But, I need some active monitor speakers for my vocal amplification. Something that's about as loud as the guitars. Or even a bit louder. I don't know what amp our guitarist uses, but it's an average sized amp - far from the size of a practice amp.

What would be a fairly good budget priced active monitor. Let's say... a 100 watt one. Or 150?[/QUOTE]
A couple of these would probably do the job.
[url=http://www.peavey.com/products/browse.cfm/action/detail/item/91259/number/00392610/cat/98/begin/1/112PM%3F+Floor+Monitor.cfm]Link[/url]

But, even tho active speakers make it easy, they arn't usually budget priced.
Those Peavey ones arn't exactally huge, but if you point them righ tat your face they'll be right.

But if you can afford a few extra dollars, you should definately look into these ones:
[url]http://www.questaudio.net/prod_qsa200.shtml[/url]

I havn't heard much about wharfedale gear, sorry.