Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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RE: I do agree that designers probably can make the best use of measures, but two questions.

Posted by tomservo (M) on January 5, 2008 at 08:43:53

In Reply to: RE: I do agree that designers probably can make the best use of measures, but two questions. posted by AJinFLA on January 4, 2008 at 19:09:38:

Hi AJ

“Where did that definition come from? Is damping a lossy process? Cheap? What would that have to do with "best"? Best value perhaps?”

That definition is my opinion / conclusion, based on developing radiators from scratch for several different types of (mostly rotary) loudspeaker drivers. Cheap, from hanging around driver mfrs engineering dept's.
Yes, damping IS a loss mechanism; the idea is that it dissipates stored internal energy.
“Cheap” has always been an important factor in manufacturing loudspeaker drivers, keep in mind that after they are marked up by the driver mfr, that cost is multiplied again by the company selling the enclosure.
The cost factor is why many / most of drivers used here, in total or in part come from China now where it is much cheaper to build.

“There are a great many transducer designer that disagree with you here Tom. Not to mention those like Toole, Thiel, Jordan, Linkwitz, etc who have a hand in loudspeaker design. Your soft, lossy cones have their own issues.”

Well that’s ok with me, they may have had different goals, constraints, measurements systems and experience. I can tell you for sure that regardless of market appeal, an exotic driver which also has a large feature in its breakup range, is not as good as one without.
The logic is that this feature is “safe” when above the crossover point, a logic which ignores the less obvious downside.
Yes the “soft” approach has trade offs too, that’s why one has to weigh the importance of all of the properties as a big picture, the best trade off of weight, stiffness, damping and cost.

The issue for the radiator is something like this (again in my view).
At low frequencies, it acts like a piston, one which is ideally perfectly ridged but in reality has some “spring” to it. The force from the VC propagates through the material at a speed high enough that piston motion is retained.
As the frequency increases, two things happen.
At some point, the radiator dimension is large enough to cause directivity, which while on axis may not be visible it is having a strong effect on what the driver does in the room.
A compression driver is something of a different case as it may have significant acoustic damping and when combined with the possibility of its break up modes being at / above ones hf limit hearing, makes metal a more popular choice.

Second, the acoustic dimension of the cone radiator is large enough that piston motion is not retained, that it begins to resonate with a series of non piston axial and radial modes.
These modes can cause a net acoustic gain of the radiator compared to it as a piston and show up as peaks or a broad rise compared to piston motion.
This acoustic gain is not a good thing, even if above crossover, the distortion components the VC makes are amplified as well.
So, with a given motor system and moving mass constraint, the radiator which has less or no breakup rise will be the better measuring driver, having less contribution to the sound.
Actually a good deal of the problem with speakers is they produce more than they are told and usually not all in the proper time relative to the input signal.

The speakers I build for work must keep the cone drivers within the piston band and with an acoustic low pass filter in front of each cone, what sound is still produced above xover is greatly attenuated / blocked from driving the horn.
In this case, the horn alone defines the radiation pattern to essentially a constant angle down to the mouth limit.

I think, if one included the speakers radiation pattern as part of the “measurements” then I would say that it would be possible to pick what would be the most faithful reproducer in a room or certainly identify ones that wouldn’t be..

”This agrees with the (scientific, not audiophile) research done by many. It would be rather amusing to see something like Norm's "hi-fi" speaker measured this way (or any way for that matter). Actually, any measurement of anything audio owned by Norm et al ought to be extremely entertaining. Will never happen of course, as that would reveal....
BTW, do you recall the listening (woohoo) results of Earls Polymide/Paper Summa vs the Aluminum/Glass Fiber Gradient Revolution. I do :-).”

I have not heard Earl’s speakers and I am not what Norm has but the B&C de-25 had a polyester diaphragm and was a very nice driver. Understand, I am not saying there is no room for other materials, just that usually for cones, it turns out treated paper fiber is very hard to beat.

So far as room stuff, fortunately I don’t have to deal with hifi lore, nearly all of our stuff goes in commercial size rooms. Here, there is some “lore” but thankfully there has been a form of blind test which has been used all along, sadly not suitable for music as there is no universal truth in music nor are the room problems as large at home..
For voice systems it works particularly well, you want to compare two systems in a bad room, march 50 people in, add some background noise and have them keep track of how many words they were able to make out from the speaker.
What is known in commercial sound is that with all other things being equal, that your ability to make out words is related to the ratio of direct to reflected or reverberant sound.
The rub is that as the room size increases, the volume of air increases more rapidly than the surface area (where the sound is absorbed) so the larger the room, the greater the reverberant level.
Here, a rather un-hifi like in view, reverberant sound may well sound like a natural voices but if you can’t make out words, who cares how it sounds.
The idea is your trying to maximize the information that gets through.
In a large room, the best way to produce this ratio is through directivity, you confine the sound radiation to an angle which only includes the audience and not the room walls, ceiling etc. This minimizes the reverberant field relative to the direct level.
Part two of the hard part is that each time you double the distance you need to produce 6 dB more energy to retain the same level. Essentially everything “bad” in speaker world increases at an exponential rate or some power of the increase and not at the level of the increase. This makes LOUD and crystal clear, very very hard.

In the home, like in larger spaces, I believe room issues are largely the same except with shorter time constants and lower reverberant level, substitute stereo image for intelligibility.
People with large panel speakers or larger horn systems enjoy the larger direct field that these (directional) speaker produce.
Also, I believe there is a form of coloration, which effects what you hear but may not show up in a response measurement. As the driver to driver and upper to lower range interference was eliminated in the horns at work, it became harder and harder to guess how far away the speaker was when listening to It with ones eyes closed.
In other words, at crossover, when the upper and lower systems are both radiating AND are physically too far apart to add coherently (more than about 1 / 4 wl apart), they may measure properly on axis but they seem to radiate a clues that your ears use to judge how far away it is.
I don’t know if that is in the form of off axis beams or what exactly that does it.
I do know once you get rid of that and get the radiation of a single source, it is much harder to guess how far away or exactly where the speaker is, there is no “depth issue” or minimum distance needed with them..
While 1% of our customers ever experience it, in stereo that same effect makes for a very real stereo image, the best I ever heard.
I guess that’s partly why I like recording stuff too, I want to capture and replay “being there”.
Best,

Tom Danley

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