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Watts vs. Max SPL: What Do These Numbers Really Mean?

What is going on here? Speaker efficiency, differences in the way they rate power and output?

By PSW Staff December 6, 2013

Editor’s note: Following is an interesting discussion from the Live Audio Board (LAB) here on ProSoundWeb. It’s been lightly edited for space and consistency only.

Question by Sean Thomas: I’m doing some research on buying a new line array system. I’ve looked at 10-12 different brands and models, and have graphed a bunch of specs on paper so I can keep track of them all. I found that one brand is 1,500 watts RMS and 136 dB peak output, a 3-way system. Another brand is 750 watts RMS and 135 dB peak output, (also) a 3-way system.

What is going on here? Speaker efficiency, differences in the way they rate power and output? How should I interpret these specs?

Reply by Gordon Brinton: I would say it’s mostly that as well as a few other factors. Box design and materials can change specs somewhat. Facts and conditions about the testing environment are rarely given, but can also affect results. Furthermore, if you pulled 10 identical drivers off the store shelf and tested each one, you would probably get 10 slightly different results.

There really needs to be a standard created for reporting speaker performance to consumers. It’s about as reliable as advertised gas mileage in cars. Clear as mud.

Reply by Jeff Harrell: It can be either one. Are the power ratings AES/RMS or peak? If they don’t say, you need to call and ask. It could also be sensitivity. I was looking at two different 18-inch woofers. One has a sensitivity of 98 dB, 1w/1m , the other has a sensitivity of 93 dB, 1w/1m. Also, what frequency are they measuring them at?

Reply by TJ (Tom) Cornish: Yes, yes, and with a grain of salt. Speakers aren’t (intentionally) heaters, so wattage will never tell much of the story. The best you can do is to look at frequency plots and try to figure out how they are measuring and reporting things. At the end of the day, though, you need to get your list down to your top couple choices and get demos to hear them yourself to really get a sense of things. Just because a speaker can do 136 or 140 dB or whatever, doesn’t mean it sounds good at that level.

“The nice thing about standards is you have so many to choose from.” There are a number of standards that are applicable for both amplifier and speaker performance. The problem is to try to define what you are actually measuring – simple peak SPL? At what frequency? Just one, or some kind of average over a band? And for how long before it blows up? Does it have to sound good, or just be loud? How do multiple boxes in a system affect the output? Some standards give a formula for some of these questions, but not every user has the same goals.

Spec sheets are nice, but at the end of the day, we’re buying speakers, not a kitchen mixer. The best standard to use is our ears.

Reply by Paul Bell: I suggest that after you sift through all the specifications on paper that you try to do a side-by-side comparison of the two top cabinets. Your ears should be the final determining factor.

Reply by Ivan Beaver: And when looking at “simple numbers” (max SPL or sensitivity, for example) you have to know where the number comes from. If you look at the frequency response of a loudspeaker (that does not have a lot of smoothing applied), you will realize that some frequencies are louder than others. Sometimes quite a bit louder (10 dB is not out of the question in some cases).

So if you were the one “charged” with coming up with spec numbers – what do you choose? The points that are the loudest? That looks real good on a spec sheet – and is not lying (as the speaker will produce that, and just that freq) at the SPL stated. Or do you choose a number that is somewhere “in the middle” that gives an idea what the loudspeaker can actually do, SPL-wise?

It depends on what you are trying to convey to the purchaser – a marketing number or a useable number? So you need more data (than a simple number) to get a real idea of what the usable max output is. Without that data, you have to rely on a pure guess on your part, or better yet, actual experience with the product as compared to another product.

Spec sheets are a great place to start (assuming you have enough real data to actually look at), but you have to look a bit closer to get a real idea of what is going on. Or you can take the numbers as presented and “hope” the product will produce the results you are hoping for…


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Andrew says

Haha yes, I had an PA
loudspeaker that stated it could handle 155DbM okay, I got curious
(speaker is 1kw)

Staring of 890W
not even close 99dbm was red
Next step full power of amp 1.6Kw channel

Max power received 109Dbm

Fireworks were amazing tough...
Its not the power or DB's.... It's the sound and the giveaways of them.

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