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Power Vs Efficiency


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Hi,

 

I have had a read of some of the posts on the forum (bored in day job) and noticed that lots of people comment about taking x amount of power to a gig and saying that their speakers are so and so watts and they can say what gig they can do by taking "300w speakers" or something like that.

 

I'm not getting into whether you can or cannot say before the gig what speakers you'll need - you all know your gear and your gigs.

 

The one thing I would say is that nobody seems to have mentioned efficiency of speakers. This rating is described on specs as "x"db/w/m and is the spl of the speaker from one metre away when one watt of power is applied to it.

 

Efficiency is easily as, if not more, important than the power handling of a speaker. Part of the reason is the way "volume" increases with power. As I'm sure you are aware, to get a 3db increase in volume, you need to double the power. So if one is to look at, say, the Carlsbro Gamma 12" tops compared to the ASS SP3 it is easy to see why efficiency is important.

 

The max power of the Carlsbros is 300w whilst the ASS is 250w. It would seem that a lot of people on this forum would suggest that the Carlsbros will be louder. However, the efficiency of the Carlsobro is 96db/w/m whilst the ASS is 100db/w/m. This means that in order to get the Carlsbro to do 100db you need to put more than double the power in it compared to the ASS. The max output of the Carlsbro is 120db at 300w. The max output of the ASS is 126db at 250w. This means that, even if the speaker could go that loud, to get the Carlsbro as loud as the ASS you would need to give it 1.2kw. The ASS is louder with a smaller amp.

 

I have thought about it like this - the efficiency of the speaker is like the engine in a car - the better efficiency the bigger the engine. The power handling is the control on the accelerator - more power handling equals give it more gas. A big engined car will go quicker than a small engined car even if you are just giving it a little bit of gas.

 

I hope my info is helpful. I don't know much and there are people on here who know a lot more than me (Norty303 and Andy Dunn for example), but, discounting all the other variables that affect how "loud" a speaker sounds (don't get me started on distortion!), efficiency should not be overlooked by anybody. It should be your first question when enquiring.

 

Hope it helps

 

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Good point, and yes, efficiency means a lot.

 

However, any decent PA driver should be able to provide reasonable efficiency within its design frequency range, so to my mind the greatest contribution to a speaker's efficiency is the design of the cabinet.

 

There are many different enclosure designs out there, but I group them into three broad groups:

1. Sealed cabinets.

2. Ported (tuned/reflex) cabinets, including bandpass.

3. Horns.

 

Sealed cabinets are rarely seen in bass enclosures for PA work simply because they are too inefficient, and the bulk of typical DJ-type bass speakers are of the ported design. These give a good compromise between cost, size and bass response.

 

The most efficient, but also the largest are the horns - these can have efficiencies up to 10 times that of a typical cabinet, meaning that placing a ported cab and a bass horn side by side, the horn would only require one tenth of the power to produce the same volume output - brilliant!

 

Just to get a taste of the 'big stuff', below is a diagram of a bass horn capable of floor-shaking response to below 30Hz - bear in mind it sits in a corner and is eight feet tall... !

 

http://myweb.tiscali.co.uk/andyspatch/hosting/sensurround.gif

 

Incidentally, tweeters and sometimes also midrange units are often horn designs in PA speakers.

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its not just the spl or watts thats important its the clarity of the input signal.A clean signal will carry better than a distorted one with less power.

 

Not sure what you mean by carry, the function of projection over distance is purely dictated by the spl at the speaker.

 

An interesting feature of our hearing is that we perceive distortion as being louder, so nice big clean sounding horns doing 140db at 1m can sound less loud than some overdriven reflex boxes doing 130db. This may be why people still love some of the older 'brute force' amp like the Crown VZ5000 macrotech, because they actually have a bit more harminic distortion inherent in them than some of the newer designs.

 

There is a fine line between when this distortion sounds good and 'phat' (mate!) and when it starts to spoil the experience. Choice of listening material will also dictate how much you can get away with.

DIY plans and pro audio related technical discussions

www.speakerplans.com/forum

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The most efficient, but also the largest are the horns - these can have efficiencies up to 10 times that of a typical cabinet

 

Indeed good horn cabs are generally more efficient than reflex bins, but I haven't seen any that are ten times the efficiency - the ones I have seen are, maybe, up to 10db/w/m better than a reflex bin.

 

, meaning that placing a ported cab and a bass horn side by side, the horn would only require one tenth of the power to produce the same volume output - brilliant!

 

It doesn't quite work like that. If we take your example figures and assume a reflex bin had an efficiency of, say, 1db/w/m and a horn had an efficiency 10db/w/m and you were going for a volume output of 40db at 1m.

 

Assuming everything else being equal, the horn would require 1,024w whilst the reflex bin would require 8,192w.

 

If you increase these figures to 50db/w/m and 500db/w/m respectively, the power needed for the the reflex bin to put out 500db is (roughly) 1.42 x 10 to the power of 45 watts whilst the horn would need only 1 watt

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...but I haven't seen any that are ten times the efficiency - the ones I have seen are, maybe, up to 10db/w/m better than a reflex bin.

 

I still think I'm right, and you're wrong!! :)

 

A 10dB power incease is precisely that - multiplying the power output by 10 - the definition of a Bel. 20dB is a hundred-fold increase and 30dB is a thousand-fold increase.

 

I also think you may be confusing dB with bytes, looking at the strange figures you give, such as 1,024 and 8,192!

 

Let's look at some figures slightly more representative of the real world than the ones you used:

2 speaker cabinets, one having an efficiency of 97dB with 1 watt at 1 metre, and a horn outputting 107dB under the same conditions.

 

It doesn't matter what power you put into the cabinets, the horn will remain 10dB above the reflex. (Power compression ignored here.)

 

Therefore, reducing the input to the horn to 1/10 of a watt will cause it to output the same SPL as the reflex. It is 10 times more efficient as it requires 1/10 of the power to do the same job. The maths in this simplified example really is that simple. :)

 

Happy New Year! :)

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I still think I'm right, and you're wrong!! :)

 

I've done some more research (I mentioned above that I didn't know everything!).

 

We're both right just looking at it from different angles. You're right that the more efficient (using the 107db/w/m and 97db/w/m examples) would do 97 when 1/10th of a watt is applied.

 

However, in order to get the less efficient speaker to output 107 would require 8 and a bit watts as you have to double the power for each rise in 3db spl. My figures were correct, but I was looking at bringing the less efficient speaker up to the output of the more efficient, rather than the more efficient down to the output of the less.

 

I wasn't aware that it was log to the base 10, but anticipated that it would be natural log.

 

Sharing of knowledge - what forums are all about!

 

 

 

 

 

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