I keep thinking about this acoustic lens idea. In the document that shows 4 different shapes and sizes of JBL horns with lenses, the lenses have different widths for the same 800 Hz cutoff. But different horizontal dispersions. The wavelength of the cutoff seems accidentally as wide as the lens itself. So maybe the width needn't be too extreme if the desired dispersion is not too wide.
I would still cover as much of the entire driver as you can. Introducing a path length/time delay for the treble part of the response is like making it less perfectly aligned two-way system. How much and how audible, I am not sure, but why if you can avoid it? Extend the fanes a bit up and down and you are there with the large mock-up.
I would still cover as much of the entire driver as you can. Introducing a path length/time delay for the treble part of the response is like making it less perfectly aligned two-way system. How much and how audible, I am not sure, but why if you can avoid it? Extend the fanes a bit up and down and you are there with the large mock-up.
Worth a try covering the whole driver for testing, but i can't imagine you'd notice that time misalignment? Or at least I don't think myself could notice it... 1.6 inches extra path. 1100 ft/s speed of sound = 13200 in/s. So the time added is 1.6/13200 = .000121 seconds = .121 ms delay... right?
Seems to me the dispersion is only set by the angle of the cutout and angle the plates are at, as thats all that affects path length and thus all that affects the bending of the wavefront... But the width of the plates may well have to do with the lowest frequency cutoff of the lens.
Jesse
Seems to me the dispersion is only set by the angle of the cutout and angle the plates are at, as thats all that affects path length and thus all that affects the bending of the wavefront... But the width of the plates may well have to do with the lowest frequency cutoff of the lens.
Jesse
Correct, though every 'FR' driver I've tested has audible mids/HF TL and breakup modes launching somewhat randomly on the necessarily flexible diaphragm, so a lens that covers all of the radiating surface, and then some to control reflections off the surround, baffle cutout is a good plan IME and one reason why I suggested the full size, deep grill with built-in waveguide and if necessary you can overlay specific area to the grill cloth's inside to further damp/diffuse pattern till you get a ~flat power response over most/all of the listening area or at least the 'sweet spot'.
If not doing polar measurements, then at least get a cheap mechanic's stethoscope [as little as $4 in my locale] to create a crude frequency launch map to find the worst offenders that's comb filtering with the whizzers and often on different areas of the diaphragm!
When dealing with wide range/'FR' drivers, IME this is a 'must have' tool along with a 1 Hz frequency generator unless one can either do [now nearly dirt cheap] polar plots and/or tune a ~ full range mids/HF musical instrument by ear.
Amazon.com: Mechanics Sonarscope Auto Engine Hearing Device Pinpoint Tool Stethoscope: Industrial & Scientific
GM
If not doing polar measurements, then at least get a cheap mechanic's stethoscope [as little as $4 in my locale] to create a crude frequency launch map to find the worst offenders that's comb filtering with the whizzers and often on different areas of the diaphragm!
When dealing with wide range/'FR' drivers, IME this is a 'must have' tool along with a 1 Hz frequency generator unless one can either do [now nearly dirt cheap] polar plots and/or tune a ~ full range mids/HF musical instrument by ear.
Amazon.com: Mechanics Sonarscope Auto Engine Hearing Device Pinpoint Tool Stethoscope: Industrial & Scientific
GM
nice probe-o-scope - how does P10 find the exact placement where to paint the patterns with his cone treatments? On some drivers I can identify a hot area on the inner cone diameter when tapped - but not on others - tone and amplitude of the tap seemed the same. (perhaps my hearing is just too insensitive)
GM - what's TL? I've read your and others posts about such breakup modes, but I'm not sure if my ears are tuned to hear such things. Or is that something more noticeable on a test of some kind but not typically audible with music?
I certainly do have a mechanics stethoscope, but I'm not sure how I'd use it to create a frequency launch map? Put it directly on the cone? (Yikes!) You've gone over my head again, was this explained in some detail in a prior thread somewhere? I'll do more reading later to try and catch up with the rest of the class.
Thanks,
Jesse
I certainly do have a mechanics stethoscope, but I'm not sure how I'd use it to create a frequency launch map? Put it directly on the cone? (Yikes!) You've gone over my head again, was this explained in some detail in a prior thread somewhere? I'll do more reading later to try and catch up with the rest of the class.
Thanks,
Jesse
An incrediblely useful tool for finding cabinet resonances too.
An externally hosted image should be here but it was not working when we last tested it.
dave
007 -one last (?) thing to try vs your slant plate lens What is the ideal directivity pattern for stereo speakers?
P10 - - it would be great if someone would make a video explaining the "tap test" - I'd imagine it would apply (at least in the inner diameter) to such things as Bastanis using little felt disc furniture pads.
P10 - - it would be great if someone would make a video explaining the "tap test" - I'd imagine it would apply (at least in the inner diameter) to such things as Bastanis using little felt disc furniture pads.
I definitely heard a "hot, bright" ring area on several drivers - just barely outside the whizzer circumference IIRC. Going outwards, I lost track on a driver - my hearing was damaged 40 + years ago so takes more input than what one would have with perfect hearing. It would probably take a separate mic on a stand and quite close to the driver to illustrate for a video. Are there any verbal descriptions as to what to notice when doing the tap test? What are the best instruments for tapping? IIRC, the plastic pen tip for applying the little blocks of paint is frequently used.
That's what the scope is for.
A driver's HF response is primarily dictated by the VC's diameter and why the pioneers used 0.75-1" VCs on large diaphragms to get a really wide BW [<40 ->13 kHz with a 12"] with the tradeoff of course being low power handling, though with a high Qt, large Vas, efficiency compensated.
A driver is normally pistonic until the BW is < the VC's diameter and why simming programs use a flat disc, then begin radiating [TL/'traveling' modes] outward towards the surround where they get damped and finally beaming directly off the VC or driving a small piston [dust cap/DC] into its own set of modes.
So we got this driver generating all these different polar responses ~simultaneously plus unless each section is perfectly damped, there will be breakup modes scattered over its surface, which will be flexing/distorting and comb filtering with its normal output.
Whizzers are an attempt at confining much/all of the critical mids isolated for max speech intelligibility, allowing the diaphragm to be more rigid, damped [pistonic] for its lower BW down to at least 125 Hz and preferably an octave lower where the lowest male vocals roll off.
Ofttimes, the manufacturer must compromise to meet the needs of a specific apt and that's where doping, varying material thickness and/or laminating differing materials, etc., come into play to get the right amount of pistonic action at one area of the diaphragm and be in TL or even in 'controlled' breakup mode elsewhere. Before advanced CAE, et al, it truly was an art, now just hard science.
At this point, 'art' takes over to fine tune it to a very personal preference since we all hear the same, yet not so much, hence Bud's patterns to control TL modes, others seemingly random dot patterns to 'hit the high spots', so to speak, etc..
Early on, my test tones were limited to a CBS test record, so 1/3 octave was the best I could do early on, but once I built a Heathkit tone generator I realized I had been way overdamping most times, so ideally need a pink noise or pure tone 1 Hz frequency generator, then use the scope to find where a frequency is loudest and if one spot is obviously louder than another, then it needs a little Dammar or similar to damp it [use shellac, lacquer or similar to 'brighten' an area] or maybe find you need to dope a large band all the way around the diaphragm, whizzer or DC.
FWIW, I used an airbrush for detail work and a touch up gun for large areas, heavier coatings. Bud's patterns are nice looking, but for those of us lacking in patience, steady hands/whatever, 'dusting' to painting on a band depending on amount of damping required works just as well and save the dots/dashes for DCs or you might try adding a stick-on five point star in its center, a tweak that has worked well for me on smaller drivers.
GM
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