dB calculations

Re: dB calculations

Mitch Miller said:
Am I to understand that 10 watts into a 10" speaker will produce less dbSPL than the same 10 watts into a 100" speaker?

Aside from physical differences that result in forces such as friction and larger mass, 10W should produce the same amount of SPL in any speaker (efficiency not withstanding).

I think there's a whole lotta confusion regarding units of measure. dB SPL is not o let the electrical power being consumed by the driver, but also the mechanical resistance of the cone, cabinet design, and environmental factors such as other drivers nearby.

I think a pure theoretical world, double the power is an additional 3 dB, regardless of the units. In the real world, though, that may result in more than +3 dB SPL.

Yeah?
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It gets A LOT more complicated than that.

A larger speaker will narrow its response at a lower freq than a small speaker. So the "overall energy" changes with freq over area.

If all you are concerned with is the on axis response, and if the sensitivity is the same for both drivers, then the same power in will result in the same SPL (assuming no reflections etc that would affect the total SPL)

I am not aware of any cases in which doubling the power alone will result in more than a 3dB gain.

Power is watts. What other units are you talking about?
 
Re: dB calculations

Ivan Beaver said:
It gets A LOT more complicated than that.A larger speaker will narrow its response at a lower freq than a small speaker. So the "overall energy" changes with freq over area.If all you are concerned with is the on axis response, and if the sensitivity is the same for both drivers, then the same power in will result in the same SPL (assuming no reflections etc that would affect the total SPL)I am not aware of any cases in which doubling the power alone will result in more than a 3dB gain.Power is watts. What other units are you talking about?
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Nifty topic... now there are four different situations discussed here:#1: single speaker; double the power to it from an amp= 3Db output gain. #2: single powered speaker + identical single powered speaker = 3 Db output gain.#3: coupling effect of multiple speakers, that changes the polar pattern outputs in a more directional mannor, such as seen in a line array, where the vertical axis is narrow, but the horizontal is wide (inverse square law drop changes from 9Db/doubling distance to 6Db drop/doubling distance.#4: coupling effect of multiple speakers, not in correct phase, or out of time alignment - effecting combined total output pattern in a destructive mannor. I don't think anyone has covered the coupling effect of multiple speakers, and coupling effects of walls, floors and corners yet on this topic, but that may be too off topic.
 
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Re: dB calculations

Declan Slater said:
I don't think anyone has covered the coupling effect of multiple speakers, and coupling effects of walls, floors and corners yet on this topic, but that may be too off topic.
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The coupling effect of multiple speakers (and reflections off of walls/floor etc) is dependent on both the distance and the freq.

The lower the freq the further away devices can be and still couple. As you go up in freq the distances get very small very quick.

When talking about combfiltering (contrary to popular opinion) it is NOT the distance the devices are away from each other, but RATHER the difference in signal arrival from each of the devices to a particular location that matters. When you change locations, you have changed the difference in arrival times, therefore changing the combfiltering freq. Which (again contrary to popular opinion) are both ADDITIONS AND Subtractions. Of course the subtractions FAR outweigh the dB difference of the additions.

The maximum addition will be 3dB will the maximum subtraction could be infinitely deep.

As usual with MOST audio questions, the FIRST question should be "At what freq?". Because all sorts of different things happen at different freq.

The "rules" for 30Hz are completely different (in terms of additional speakers-boundaries etc) than at 2Khz.
 
Re: dB calculations

Ivan Beaver said:
Power is watts. What other units are you talking about?
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Obviously he's talking about the Newton Metre per Second. Keep up, Ivan. 8)~:cool:~:cool:

This whole thread reminds me of a math assignment my brother got a few years ago...

280440_445604252152256_1400978130_o.jpg

To which a friend of mine, and the best IT professional i know, responded:

While we're on the topic of stupid proofs, let's prove math is pointless

Let a and b be arbitrary, non-equal numbers.
Let t be their sum.

Then,
a+b=t
(a+b)(a-b)=t(a-b)
or, a²-b² = ta - tb
a² - ta = b² - tb
Completing the square:
a² - ta -t²/4 = b² -tb - t²/4

Now we can "un-expand":

(a-t/2)² = (b-t/2)²

Take the root:

a-t/2=b-t/2

or, a=b and this assignment was insulting and stupid for anything above a middle-school student.
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Re: dB calculations

Max Warasila said:
View attachment 11943
Only compared to some...
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It all depends on what your reference is.

However the sad thing (I cannot speak on the sub in question as I have no experience with it), about many subs and the "max SPL" spec is that often times it is NOT in the actual intended operating range of the sub (say below 100hz)

Since most subs have a louder output at some higher freq (some close to 2KHz), some manufacturers will use this number as their spec for "peak output".

But the actual SPL in the intended range can often be quite lower.

So while the manufacturer is not "lying" in terms of SPL output (the sub can produce that SPL at SOME freq)-they are not "telling the truth" as you would expect the product to be producing.

THAT is why it is IMPORTANT to be able to see where the actual numbers are coming from/based on etc.

You have to be sure to compare apples to apples.

Kind of like when Sony first came out with the "pocket radio". It would not fit in normal sized pockets (like people would expect it to), so the sales men all wore shirts that had LARGER pockets on them, so they could demonstrate how it would fit in a "pocket".

Often what the consumer "thinks they are getting" is different than what they ARE getting-despite the numbers.

Oh the power of marketing----------
 
Re: dB calculations

Ivan Beaver said:
So while the manufacturer is not "lying" in terms of SPL output (the sub can produce that SPL at SOME freq)-they are not "telling the truth" as you would expect the product to be producing.
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And if you call them on it, they say, well our factory supplied DSP setting shelve it up, to be flat ;-)

I was asked a few weeks ago to measure a multi-thousand dollar raw frame subwoofer for a customer that questioned the mfg spec sheet. The real sensitivity of the woofer in the 20-100Hz region was 15dB off of what the mfg claimed. So companies are still up to this trick, even with the number of individuals with their own measurement systems now.
 
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Re: dB calculations

Mark DeArman said:
And if you call them on it, they say, well our factory supplied DSP setting shelve it up, to be flat ;-)

I was asked a few weeks ago to measure a multi-thousand dollar raw frame subwoofer for a customer that questioned the mfg spec sheet. The real sensitivity of the woofer in the 20-100Hz region was 15dB off of what the mfg claimed. So companies are still up to this trick, even with the number of individuals with their own measurement systems now.
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The sad thing is that most people just simply believe the "simple numbers" and never even bother to test or check for themselves.

And even more sad is that I have found MANY people (who claim to be "audio professionals") can't even read a simple response graph.

If you ask them the simple question "Look at the following graph and let's say the sensitivity is 100dB-at what freq is the response 3dB down?" they can't tell you. :(

I guess that is why so many manufacturers don't even bother with showing response graphs. They just hope customers will accept the numbers given.
 
Re: dB calculations

Ivan has it right. The problem is that most people want a simple answer to a question that really isn't that simple at all.

SPL is calculated as a weighted average over a wide frequency range. There are several different weighting curves used for various different purposes.

See the graphs here: http://en.wikipedia.org/wiki/A-weighting

The "A" weighting is the most frequently used. As you can see, this is how a double 15" with a horn can end up with an SPL the same as a 2-way 8" speaker ;) The lows are practically discarded in this weighting. The A weighting was designed for use in determining hearing damage (thus the reason LF is discarded in the weighting).

If you were to cut the HF out using a cross-over then used an A weighted SPL measurement, it would be practically useless IMO.

As for the original topic, I generally figure that center clustering a pair of subs will increase the output enough for it to be noticeable to anyone who was listening for it compared to having the subs far enough apart not to have any coupling.

I like the +3db number to go along with my "easily noticeable" definition. Center clustering subs also has the advantage of providing a more even distribution of sub frequencies over a wider area than having the subs stage left and stage right.
 
Re: dB calculations

Scott Bolt said:
SPL is calculated as a weighted average over a wide frequency range. There are several different weighting curves used for various different purposes.
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Many manufacturers provide a sensitivity plot vs. Frequency for boxes or OEM speakers using a reputable piece of test equipment, not a Averaging SPL meter. And probably 75% of manufacturers at least measure the sensitivity with the proper tool and only report the one 'peak' number.

This is exactly what we have been talking about in this thread. The whole idea behind a Averaging SPL meter, is measurement of uncorrelated noise signal. Like you said, the most common use of these devices is compliance testing for local and federal laws. These devices were not designed for measuring anything about a loudspeaker(s) and no professional really does use them that way, except to make sure their show is complying with the local laws.

It is easy when looking at definitions online, to just read past a statement like, "uncorrelated noise," and assume that the math might still work out. It just doesn't work out; no more than playing pink-noise through a speaker, taking the FFT, and assuming that what you see is the frequency response of the box.
 
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Re: dB calculations

Scott Bolt said:
Center clustering subs also has the advantage of providing a more even distribution of sub frequencies over a wider area than having the subs stage left and stage right.
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But the problem with center subs is that that very often you end up with a very different levels throughout the room.

It is much louder close to the subs than at seats further away.

Sometimes split subs (even with the interference) provides an overall more even response.

It really depends on the situation and what is expected as to what approach is best-for THAT situation. What is right for one is wrong for another.
 
Re: dB calculations

Mark DeArman said:
Many manufacturers provide a sensitivity plot vs. Frequency for boxes or OEM speakers using a reputable piece of test equipment, not a Averaging SPL meter. And probably 75% of manufacturers at least measure the sensitivity with the proper tool and only report the one 'peak' number.

.
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I would argue that a large majority of manufacturers do NOT provide sensitivity vs freq plots. Some provide freq-but there is no reference to sensitivity-the vertical scale is just in dB from a 0 line. And what IS that 0 line? The rated sensitivity? Something else?

And I guess my biggest "gripe" is that when a manufacturer shows a sensitivity and freq response plot-yet the "simple numbers" are very wrong when compared to that same graph.

I have seen them 15-20dB OFF (especially in low end of the freq response simple number). That is the whole subject of a paper I am working on about how to read and understand spec sheets and what "games" are often played. And the "major players" are not immune to doing this :(

So a person who does not have a graph (or understand how to read it) takes the "simple numbers" and "compares" it to products that have legit specs and makes a very uninformed decision. But that is EXACTLY what those manufacturers are counting on-and it works (for getting sales). But does not help the consumer.
 
Re: dB calculations

Ivan Beaver said:
I would argue that a large majority of manufacturers do NOT provide sensitivity vs freq plots. Some provide freq-but there is no reference to sensitivity-the vertical scale is just in dB from a 0 line. And what IS that 0 line? The rated sensitivity? Something else?
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Yeah 'many' was probably an overly optimistic word. How about 'a few', I was meaning some less than the 75% number.

Of course the reference level is another whole argument. For instance, I get where the AES was going when the wrote the new AES2 standard. Although 99.9% of amplifiers are close to c.v. I don't like the, "let's fix it at 2.83Vs".

Personally I like to publish the Impedance vs. Frequency plot, choose a actual impedance mid-band for the speaker (not on a resonance if present, come on people!!!), calculate the 1W drive level, and then produce the Sensitivity vs Frequency plot at 1m. For subs I like to do it ground plane, so the customer can actually verify the specs.
 
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Re: dB calculations

Primoz Vozelj said:
Can you give an example of situation where L-R is better than center cluster? I love my job but if there's one thing I hate, its L-R subs.
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As I said-if you want even SPL across the seats-a center sub (unless flown high enough) is NOT going to do that.

It is going to be much louder near the subs than further away.

OF course some people don't care about even coverage (typical clubs), and they expect it to be louder closer to the stage.

It depends on what the design goal is.

Go to a really wide coverage situation and the center sub idea quickly falls apart.

If you want to "get hungup" on the the interference of subs and ignore the overall SPL, then center subs is the way to go.

But to me-it is more about trying to provide a "pretty close to the same" experience for everybody.

In a install (non nightclub type), my "go to" design is a center FLOWN sub. BUT NOT on the ground. I try my best to avoid that-unless there is no other option.

As usual-it depends.
 
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