Please Help With Power Handling In Cabs

[fusionbassist1]

Hello everyone,

I've started to doubt what I've always believed about power handling and stuff, nothing wrong with being corrected when it's needed.

My beef is about power handling and my paranoia about blowing my brank-spank new speakers (once I get them of course...it's always the way).

I always thought that each cabinet in a multi-cab setup had to have a 'power handling RMS' higher than the amplifier its self. An example would be a head of 500watts being used to power one 15" cab speaker that's got a handling of 400watts and a 2x10" that has a handling of 300watts. In my head that wouldn't be wise as the speakers in both cabinets would be getting pushed really hard. I'm half expecting someone to point out that the load from the amplifier is spread out between each individual speaker but i really need someone to TELL ME THIS so I know for sure.
(i know the impedence things btw so in this scenario we'll take it the amps are all wired correctly for their impedence etc)

Could someone clarify this for me and please tell me exactly what the deal is if I really have got it wrong. It'd be sh*t to get new gear then have to go have it checked out really quickly due to me not setitng them up correctly.

Cheers guys 'n' gulls,

FB.

P.S. I'm getting a topic running to inquire about smaller cabinets for 5 string basses so I'd much appreciate it if you dropped a word in.

[Bill Fitzmaurice]

The power rating of drivers is a thermal rating, and does not reflect real world power handling capability. For electric bass that is usually defined by displacement, how much power may be applied before the cone reaches the end of its travel limit, and that is usually only 25 to 40% of the thermal rating.
In the vast majority of cases if you exceed the displacement limit there will be a lot of distortion, and the cure for doing so is simple. Turn it down. If you don't apply enough power to create distortion, by either overpowering the drivers or pushing the amp beyond its limits, the likelihood of blowing drivers is remote.

[Kiwi]

Bill, what would be your take on a situation where you have a 400w valve amp that sounds fine with one 4ohm Acme 2x10 cabinet but the lower frequencies distort when two 2x10 cabinets are plugged in? Are the cabinets testing the amp, or is the amp testing the cabinets?

The amp has been serviced and checked out by a trusted amp tech who says that the amp is fine and putting out roughly 360W. The impedance switch was switched over from 4 to 2ohm when the set up was changed.

The cabinets have also worked fine with solid state amps.

[Bill Fitzmaurice]

[quote name='Crazykiwi' post='83964' date='Nov 5 2007, 08:52 AM']Bill, what would be your take on a situation where you have a 400w valve amp that sounds fine with one Acme 2x10 cabinet but the lower frequencies distort when two 2x10 cabinets are plugged in? Are the cabinets testing the amp, or is the amp testing the cabinets?

The amp has been serviced and checked out by a trusted amp tech who says that the amp is fine and putting out roughly 360W.

The cabinets have worked fine with solid state amps.[/quote]
Short of using testing gear that you probably don't have the best way to see if amp headroom is the culprit is to drive the second cab with a second power amp of at least equal power. If two amps works better you know that insufficient power is the problem.

[fusionbassist1]

Thanks for that Bill.

Just one thing, to get the best performance possible out of a bass cabinet and amplifier, should the power handling be as close to, if not over what the amp's able to shed out. If I've understood correctly this should keep the speakers a decent distance away from it's maximum travel limit.

Have I got this correctly?

[Bill Fitzmaurice]

[quote name='fusionbassist1' post='83976' date='Nov 5 2007, 09:19 AM']Thanks for that Bill.

Just one thing, to get the best performance possible out of a bass cabinet and amplifier, should the power handling be as close to, if not over what the amp's able to shed out. If I've understood correctly this should keep the speakers a decent distance away from it's maximum travel limit.

Have I got this correctly?[/quote]
Ignore the cabs power rating. If it goes as loud as you need without distorting it's powerful enough, if not chances are you need a larger or better cab or a second cab. As far as cab to amp power ratios are concerned, thanks to the volume control you can never have an amp that's too powerful. Having plenty of reserve power at hand is never a bad thing.

[fusionbassist1]

ah ok then, i was just thinking in terms oh looking at cabs via tinterweb before trying them out and in turn purchasing them.

[Bill Fitzmaurice]

[quote name='fusionbassist1' post='83985' date='Nov 5 2007, 09:41 AM']ah ok then, i was just thinking in terms oh looking at cabs via tinterweb before trying them out and in turn purchasing them.[/quote]
Whilst manufacturers provide wattage ratings as the primary guide to performance said ratings are the least reliable indicators of same. And, since the indicators you need to make valid comparisons from specs only are not available, your only recourse is to get thee into Ye Olde Music Shoppe and try until you find what you want to buy. Then you find the best deal, which, BTW, may not be on the internet. Web salesmen can't cut deals.

[gypsymoth]

if you can't try every speaker you are considering, I would suggest using a cab with an RMS rating at least double your amps RMS output- and quadruple would be better still. it doesn't hurt to have a high rated speaker, as long as it sounds good and has the volume you need.
an arced voice coil can make a real mess of a tube amp, especially a big one that was running hard.

[Bill Fitzmaurice]

[quote name='gypsymoth' post='84016' date='Nov 5 2007, 10:29 AM']it doesn't hurt to have a high rated speaker,[/quote]Sometimes it does. Output is limited primarily by driver xmax. Where that's concerned a 150 watt Eminence Basslite S2010 will put out more than a 350 watt Kappa Pro 10. There are other pertinent factors as well, for instance the driver Qts. In the case of the S2010 versus the Kappa Pro 10 the very low Qts of the Kappa Pro chokes off its response over an octave higher than the S2010. In most cases drivers with very high power ratings are accompanied with low Qts values, so if you buy a cab with a very high power rating in the quest for more bass output the opposite may be the result.

[fusionbassist1]

[quote name='Bill Fitzmaurice' post='84129' date='Nov 5 2007, 06:16 PM']Sometimes it does. Output is limited primarily by driver xmax. Where that's concerned a 150 watt Eminence Basslite S2010 will put out more than a 350 watt Kappa Pro 10. There are other pertinent factors as well, for instance the driver Qts. In the case of the S2010 versus the Kappa Pro 10 the very low Qts of the Kappa Pro chokes off its response over an octave higher than the S2010. In most cases drivers with very high power ratings are accompanied with low Qts values, so if you buy a cab with a very high power rating in the quest for more bass output the opposite may be the result. [/quote]

Sorry but "Qts." ?? I suppose (this may sound very thick here but I'm trying my best not to) 'response' as you put it is how well the speaker amplifies and re-produces frequencies created by the guitar.......

You really seem to know your stuff when it comes to speakers Bob. Thanx for sharing. I'm guessing you're in the trade then?? Or used to be. Could you please comment on speaker area as I'm sure that more sound is usually thrown out of a cabinet (or set of cabinets) with a large speaker area than one with small area. as in a 8x10" cab on it's own has 80" speaker area and a 2x10 paired with a 1x15 has 35" speaker area, unless I'm missing some very basic math here.

I'm genuinely learning stuff here by the way, so please continue, this is useful stuff.

[Kiwi]

[quote name='Bill Fitzmaurice' post='83974' date='Nov 5 2007, 02:09 PM']Short of using testing gear that you probably don't have the best way to see if amp headroom is the culprit is to drive the second cab with a second power amp of at least equal power. If two amps works better you know that insufficient power is the problem.[/quote]

Cheers, it just so happens that I do have a second valve power amp rated at 400w which I could try out. Looks like loading/unloading at next week's band rehearsal is going to be a major mission.

[Bill Fitzmaurice]

[quote name='fusionbassist1' post='84147' date='Nov 5 2007, 02:02 PM']I'm sure that more sound is usually thrown out of a cabinet (or set of cabinets) with a large speaker area than one with small area. as in a 8x10" cab on it's own has 80" speaker area and a 2x10 paired with a 1x15 has 35" speaker area, unless I'm missing some very basic math here.[/quote]
You're only thinking in two dimensions, whereas the maximum output of a driver is determined by the product of cone area (Sd) and maximum excursion (Xmax), the result being the three dimensional parameter of displacement (Vd). You can find explanations of driver parameters and how they affect performance here:
[url="http://www.eminence.com/resources_data.asp"]http://www.eminence.com/resources_data.asp[/url]

[fusionbassist1]

I haven't read it all but that seems like a very good explanation apge of what exactly's going on in the mighty world of speakers.

Cheers!!

[bass_ferret]

The daddy of all speakers is either the Cerwin Vega 18 inch Earthquake with 2.5 inches of cone excursion or the EV 30 inch that I think was used by Accugroove in a silly cab.

[gypsymoth]

"as long as it sounds good AND HAS THE VOLUME YOU NEED"

yes, I'm quoting myself - and although I certainly appreciate your technical and theoretical expertise ....

it is pretty darn easy to read the Spl charts on the speaker manufacturers websites, and determine if they have the volume you need (comparitively speaking) and the frequency response you desire - along with a realistic for the application watt rating. there is a three way balancing act for these factors (which can be overcome to a certain extent by money/technology) where increasing one reduces the other.

sure, it's a black art, as is cabinet design, but realistically there are some pretty simple principles to both that can be applied by the regular buying public, without the need for software or special test apparatus. and the manufacturers supply the data so people CAN make a reasonbly well informed decision without designing and building a box for a specific speaker.

the smaller the speaker (10's) the more obvious these tradeoffs are - go for a wide range and you lose sensitivity/volume.
maximize one aspect and you lose on two.

my gamma pro 10's were chosen for their 300 watt rating and their wide frequency response - they are not very loud speakers. if I was willing to accept a narrower frequency response I could have had a louder speaker.
the trade off is not such a concern with the 15" pro series. a delta or kappa pro 15 has it all (some would prefer more bottom and less top end), as does a delta pro 12 - but a kappa pro 12 loses high end response.
read the supplied charts carefully, compare the provided information, and you shouldn't be disappointed.

[fusionbassist1]

[quote name='bass_ferret' post='84191' date='Nov 5 2007, 08:20 PM']The daddy of all speakers is either the Cerwin Vega 18 inch Earthquake with 2.5 inches of cone excursion or the EV 30 inch that I think was used by Accugroove in a silly cab.[/quote]

i noticed accugroove at the bass gallery, 20"+ speakers..........us bassplayers need roadies......and bill dickens doesn't really look like he does much jogging imo.....