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Author Topic: Proposal for a new figure of merit for subs - Output Density  (Read 6919 times)

Chris Van Duker

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Proposal for a new figure of merit for subs - Output Density
« on: February 02, 2014, 04:33:46 pm »

If you consider that Hoffman's Iron Law puts a limit on how low and loud you can get from a given amount of power and enclosure volume in a vented cabinet, and that actual vented cabinets can only approach that limit, then the only variable left to affect actual output from a cabinet is power handling (minus thermal compression) -- you could use this information to produce a single figure of merit to represent the output-density of a sub cabinet. You could compare subs with that one figure, plus a -3dB point (to see if the sub meets your program material needs).

For example, if we decided that a "perfectly efficient" double-18 which had 2kw power handling per cabinet, an external size of 18 cu ft., an F3 point of 35Hz, and -2dB power compression at 1/8 power scored a "1.0" on this hypothetical scale, then increasing the power handling of the drivers to 3.6kw and reducing the power compression to -1dB would score the cabinet closer to a "2.0", representing 3dB more output. A cabinet with a lower F3 doesn't have to be as efficient to keep the same figure of merit, so comparisons should be done between cabinets with the same or similar F3 to be meaningful.

My thinking on this is that it allows for a better comparison of apples to oranges: medium-sized vented, large tapped horns, and very large conventional horns could all be compared on a single metric of output to truck space.

We could also stipulate that it's calculated on the basis of the cabinet's power handling, or the point at which it reaches 20% THD at any point in its passband, whichever is lower. Further, we could stipulate that a powered cabinet automatically gets 15% or so added to its score, since separate amp racks take space, too.

Any interest out there in something like this?
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Ivan Beaver

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Re: Proposal for a new figure of merit for subs - Output Density
« Reply #1 on: February 02, 2014, 05:55:04 pm »

Part of the problem is trying to get actual agreement on what is power compression and when it takes place.

Heck-we can't even get manufacturers to have their spec sheet agree with the -3dB on the printed specs and their own measurement.  I have seen some measurements that are close to 15dB lower at the stated -3dB spec-in relation to the rated sensitivity.

I can see all sorts of ways to "fudge" numbers.

What does -3dB really mean?  Is it a single point in the response?  or a broadband change?

How long is the noise applied and increases to get this number?

What is the test signal?  AES-IEC-flat pink etc.  Is there a highpass filter used?  If so-at what freq? slope etc?

You could take the same loudspeaker and apply different sets of the above parameters and get different numbers.

So until there is a standard test-there would not be any real way to compare.

Just like freq response-there is no standard where the sensitivity number comes from-what the -3 and -10 dB numbers come from and so forth.

The nice thing about "standards" is that there are so many to choose from.

I would hate to try to introduce a new standard-when we still don't have one for something that is much easier to measure than power compression.

When you say the F3 of the cabinet-is that the number on the spec sheet or something else?

Most manufacturers don't even publish a simple response graph.  How are you going to get them to give data on how loud the cabinets can actually get------

I agree that it is a good idea-but exactly how it gets carried out is a different story.

Kinda like the "max spl" figures.  Just because it can get that loud-does not mean you want to listen to it that loud.

I REALLY wish there were more standards that everybody would use-to make comparing cabinets much easier.  But I don't see that happening in my lifetime anyway.

Until customers start making manufacturers actually prove the performance-the numbers have to be taken with a grain of salt.

And manufacturers will simply make up numbers that come from somewhere (who knows where) that they think the customers will believe and allows them to sell more cabinets.

Sorry for the rant.
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Ivan Beaver
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Art Welter

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Re: Proposal for a new figure of merit for subs - Output Density
« Reply #2 on: February 03, 2014, 12:02:14 pm »

My thinking on this is that it allows for a better comparison of apples to oranges: medium-sized vented, large tapped horns, and very large conventional horns could all be compared on a single metric of output to truck space.

Any interest out there in something like this?
Ivan has mentioned most of the problems with trying for agreement, but even using a specific measuring protocol won't apply to all types of program material, power compression comes from heat, and voice coil heating depends on excursion and frequency, different program material can reveal different shortfalls in driver/cabinet combinations.

A single metric on truck space is also problematic, volume of a cabinet does not insure it will fit well in a "standard" truck, and with various truck widths and height available, you still have to determine if the cabinet dimensions fit your transportation.

For a fixed installation, reducing power needs by using larger, more efficient designs would save money in both initial amplifier cost and ongoing electrical expense, power density would not be my most important metric metric there.

I would like to see distortion and output levels at various long term voltage levels with the measurements from 1/10th rated power up to the rated power level and bandwidth,  2.83 or even 28.3 volts don't give a real world view of subs rated for 90+ volts.
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Ivan Beaver

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Re: Proposal for a new figure of merit for subs - Output Density
« Reply #3 on: February 03, 2014, 06:56:45 pm »

Ivan has mentioned most of the problems with trying for agreement, but even using a specific measuring protocol won't apply to all types of program material, power compression comes from heat, and voice coil heating depends on excursion and frequency, different program material can reveal different shortfalls in driver/cabinet combinations.


Agreed.  Something like a kick drum puts a very different demand on loudspeakers than say EDM.

The former is more likely to be excursion limited while the later is more heating limited.

And of course the freq response needs are completely different.

Trying to come up with a "single metric" to be able to compare different speakers is not so easy-once you start to look at all the variables and usages.

For example the "standard" test signals (AES-IEC etc) actually have a LF and a HF rolloff.  The HF is probably still somewhat valid-but NOT the LF.

The LF may have been fine for the music at the time they were developed-but todays music is VERY different in terms of freq response-duration of energy and so forth.

So just because a particular loudspeaker meets a certain power rating-does not mean it can actually be used with that power in todays music.

Trying to describe a complex question results in simple wrong answers.
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Ivan Beaver
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Richard Turner

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Re: Proposal for a new figure of merit for subs - Output Density
« Reply #4 on: February 03, 2014, 11:10:55 pm »

I understand what you are discussing but I doubt its feasable so far as another spec listed on cutsheets from sales reps in the near future.

In a day and age where MI grade box makers are slapping 1000watt power handling ratings on anything and everything.  If this did become a useable spec from the pro manufactures how long before the bottom feeders make it less menaingful?
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Ivan Beaver

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Re: Proposal for a new figure of merit for subs - Output Density
« Reply #5 on: February 04, 2014, 07:37:02 am »

I understand what you are discussing but I doubt its feasable so far as another spec listed on cutsheets from sales reps in the near future.

In a day and age where MI grade box makers are slapping 1000watt power handling ratings on anything and everything.  If this did become a useable spec from the pro manufactures how long before the bottom feeders make it less menaingful?
I would argue that it is not just the "bottom feeders".

Many of the products from the most respected "big guys" who are at the very top of the industry don't provide meaningful specs and specs that are very wrong.

Yet people just swallow them "hook line and sinker" because they are "so and so".

Most people don't even both to do simple tests to see if the specs are correct or accurate.

The simplest one that is wrong in soooooooo many cases is the -3dB point in freq response.

VERY easy to measure.  Heck in fact look at the freq response GRAPH provided (if it is provided) and compare the numbers to the printed numbers (-3dB) and see if they match up.

I have seen cases that is 6dB or more off.  If you don't think 6dB is much-just go to any large rig and ask them what it would take to get 6dB more.

The truly sad thing to me-is that with a large number of products that are coming out (from the big guys) there is little to NO data.  Not even simple spec numbers.

I guess you are just supposed to "believe" that the product will be fine for the job-----------

Just swallow this little pill with the kool aid and all will be fine-------------------
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!

Tommy Peel

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Re: Proposal for a new figure of merit for subs - Output Density
« Reply #6 on: February 04, 2014, 11:39:10 am »

I would argue that it is not just the "bottom feeders".
I guess you are just supposed to "believe" that the product will be fine for the job-----------

Just swallow this little pill with the kool aid and all will be fine-------------------
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This is your last chance. After this, there is no turning back. You take the blue pill - the story ends, you wake up in your bed and believe whatever you want to believe. You take the red pill - you stay in Wonderland and I show you how deep the rabbit-hole goes.

--Morpheus, The Matrix(1999)
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Tim McCulloch

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Re: Proposal for a new figure of merit for subs - Output Density
« Reply #7 on: February 05, 2014, 10:33:22 am »


I took both pills.  Now all I have is insomnia.  And a fear of rabbits... ;)
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Chris Van Duker

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Re: Proposal for a new figure of merit for subs - Output Density
« Reply #8 on: February 05, 2014, 08:08:39 pm »

My thinking behind the original suggestion was that there's a lot of "received wisdom" when it comes to end-users' ideas about the merits of different types of subs. One of those, I think is, "yes, horn subs are more efficient, but they're also so large that you can actually get more output from the same volume of ported cabinets". I don't know how many people actually do the math to see if that assumption is true.

As for the problem of stretching the truth on spec sheets, it's not really a technology problem at all... it's a human nature problem. I can't see any real solution outside of either making new laws, or having an industry-funded independent organization to do testing. The latter option seems more viable to me.
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Ivan Beaver

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Re: Proposal for a new figure of merit for subs - Output Density
« Reply #9 on: February 06, 2014, 08:31:19 am »



As for the problem of stretching the truth on spec sheets, it's not really a technology problem at all... it's a human nature problem. I can't see any real solution outside of either making new laws, or having an industry-funded independent organization to do testing. The latter option seems more viable to me.
But I wonder how many manufacturers would actually send in speakers to be measured-even if it was free.

The results would likely be not what they want on the spec sheet.

And unless everybody does it the same way-the results can be all over the place.

Heck I just ran some basic numbers of a well respected manufacturer.  If you use their peak power rating and their sensitivity rating (no telling where the sensitivity rating came from-no response graph), then you come up with a peak SPL number.

Yet they just added another 6dB (I guess for the hell of it) to their already "peak" number to get a number to put on the spec sheet.

And people are using this 'inflated" number saying how loud the sub gets----------------

Yeah-just do whatever looks good to sell product--------  No matter if it is the truth or not.  :(
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!

ProSoundWeb Community

Re: Proposal for a new figure of merit for subs - Output Density
« Reply #9 on: February 06, 2014, 08:31:19 am »


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