New Product Development - 2402/075 - Can You Help?

[toddalin]

So the size of the blades has no relation to the wavelengths of the frequencies the driver is emitting?

There is no mention that I recall in the JBL literature on the size of the blades. However, the spacing of the blades is addressed and our spacing is good to 20KHz (1/2 wavelength).

However, if the size of the blades does have a relationship with frequency, than we should be covered there too. The 075 is used from 2400 Hz up while the HL-92 is used down to 800 Hz. Our blades are ~20 percent smaller, but would not be expected to be used until over double the frequency. At double the frequency, the wavelength would be 1/2 as long and the blades would be half the size. But remember that the 075 and LE-85 have about the same top end extension. Also, the 375 used a world of different size blades, all the way from the rather small H-91 size to the ~3 foot across plates used by Keith Emerson..., so again, we should be good.

[toddalin]

Initial impressions with the RTA are that the lens does flatten out the peak and extend the lower range of the tweeter. On axis response seems to be down about 1.5 - 2 dBA overall, with most removed in the area of the peak, and volume actually increased at the lower end (<7KHz) maybe partly because of the blockage, but most probably because the energy is now spread over a wider area. The increase in dB at the lower end could be due to the "loading" of the horn. This may really help with the D130/075 systems.

I need to get some serious on and off-axis measurements and will probably even put the HL-92 lens in front of the tweeter to note what it does differently.

Is there anyone in the Orange County CA area that has the D130/075/N2400 load to "try this on" and offer impressions?

Thanks

[toddalin]

OK, did some on-axis comparisons including RTA pictures. Pink noise and graphic is from the Behringer Ultracurve 2496 using the Behringer ECM8000 microphone at a distance of 1 meter.

The 075 tweeter is totally original (never opened and hardly used back in the day) and works perfectly as designed and is representative of the line. The pink noise is passed through two 3 mfd caps in parallel (actual measured 6.18 mfd), and is representative of the 6 mfd caps used in the JBL N2400, N2500, and N2600 crossovers used with the D130 woofer and 075 ring radiator. The tweeter displays the typical curve inherant to this model. Three shots in rapid succession. Note that even though the crossover point is ~2,500 Hz, there is really nothing below 4KHz relative to the the upper range.








Now we simply slip the slant plate assembly over the tweeter and repeat the process. I did turn up the volume approximately 2 dB to compensate for the on-axis volume diference, but the 075 has so much volume to spare. Note that the curve is much broader and flatter with usable volume all the way down to the crossover point.

And this isn't even getting into the broadening of the horizontal plane (beaming).

[toddalin]

Further testing indicates that we can reduce that dropout above 4KHz by angling the plates outwards a bit more. Testing also shows that the JBL slant plate placed over the tweeter has similar effect, except that (interestingly) it doesn't provide as much boost between 2.5-4kHz, nor quite as much cut up around 10K. The JBL plates frequency response more closely resembles the tweeters without the plates. Also, the JBL plates are about 1 dBA louder than my plates, perhaps due to bleed through, and, the thinner plates block less of the opening. The dropout above 4KHz is still there, but not quite as much. Angling my plates out a bit brings the the volume to the JBL plates in that region, but retains the benefit of the increased bass and flattened highs.

I also think that perhaps with the JBL plates, the plates are half as thick so more sound actually goes through them, rather than around them, relative to my fatter plates and some of this "direct" path is getting through making the JBL plates a bit louder and more of the natural tweeter-like (hence turning my plates out a bit better simulates this "bleed through").

I've done another set of revised angles to hold the plates in CAD and need to cut them out and see the difference.

[ivica]

Further testing indicates that we can reduce that dropout above 4KHz by angling the plates outwards a bit more. Testing also shows that the JBL slant plate placed over the tweeter has similar effect, except that (interestingly) it doesn't provide as much boost between 2.5-4kHz, nor quite as much cut up around 10K. The JBL plates frequency response more closely resembles the tweeters without the plates. Also, the JBL plates are about 1 dBA louder than my plates, perhaps due to bleed through, and, the thinner plates block less of the opening. The dropout above 4KHz is still there, but not quite as much. Angling my plates out a bit brings the the volume to the JBL plates in that region, but retains the benefit of the increased bass and flattened highs.

I also think that perhaps with the JBL plates, the plates are half as thick so more sound actually goes through them, rather than around them, relative to my fatter plates and some of this "direct" path is getting through making the JBL plates a bit louder and more of the natural tweeter-like (hence turning my plates out a bit better simulates this "bleed through").

I've done another set of revised angles to hold the plates in CAD and need to cut them out and see the difference.

Hi toddalin,
Neglecting the thickness influences to the sound levels and internal reflections, You are aware that changing the plates angle, You are changing the delay of the sound, so You are changing the 'sound lenses' focal characteristics, and their off-axis dispersion...

regards
ivica

[toddalin]

Hi toddalin,
Neglecting the thickness influences to the sound levels and internal reflections, You are aware that changing the plates angle, You are changing the delay of the sound, so You are changing the 'sound lenses' focal characteristics, and their off-axis dispersion...

regards
ivica

Agreed. But I'm only looking at changing a couple degrees and if this provides sonic benefit, so be it. To my ear, frequency response is the primary focus, with such things as dispersion and imaging coming after, and if it can be bettered, so be it.

Also, don't think that JBL had a monopoly on using slant plates placed infront of their speakers. There are actually several manufacturers who put them in front of their tweeters with different shapes and angles.

[ivica]

Agreed. But I'm only looking at changing a couple degrees and if this provides sonic benefit, so be it. To my ear, frequency response is the primary focus, with such things as dispersion and imaging coming after, and if it can be bettered, so be it.

Also, don't think that JBL had a monopoly on using slant plates placed in front of their speakers. There are actually several manufacturers who put them in front of their tweeters with different shapes and angles.

Hi toddalin,

I have to agree with You, that You have to be the only respectable measure of Your taste.
I am only 'talking' about technical issues.
For sure JBL is not the only company using acoustic lenses of different kind.

Regards
ivica

[toddalin]

Today I got a nice pair of 3/8" clear stands cut. 3/8" acrylic is not easy to cut, even with a 50 watt CO2 laser, and it tooks some experimentation to get a nice cut. Then it takes several tries, with minor adjustments to the size of the piece, to account for the melting of the piece in the final product to get a nice tight fit, but not too tight.

I'll get some pics up in the next day or so and will offer these through the "For Sale" forum with pre-order sales through the forum getting a price break..., and as a bonus, if legal, a pair of JBL monogram coasters in 3/8" clear acrylic.

[toddalin]

Pics are up with a special price for JBL Forum members. These came out nice. Check them out in the "Lansing Product Marketplace" section.

Thanks,

Todd

[Ed Zeppeli]

Pics are up with a special price for JBL Forum members. These came out nice. Check them out in the "For Sale" section.

Thanks,

Todd

It's been fun watching their development. I hope you find a few takers.


Cheers,

Warren

[ivica]

Pics are up with a special price for JBL Forum members. These came out nice. Check them out in the "Lansing Product Marketplace" section.

Thanks,

Todd

Hi Todd,
Very nice, indeed, but what about 2405H (Ferrous magnet type, as is has larger diameter and weight).

Regards
Ivica

[toddalin]

Hi Todd,
Very nice, indeed, but what about 2405H (Ferrous magnet type, as is has larger diameter and weight).

Regards
Ivica

That's why the feet have the the contours to accomodate the ferrite magnets. Weight is not a problem.

Today I made up a couple sets of the 1/16" slant plates for testing. Because these are thinner and more prone to flex, I also made some spacer/stiffeners that keep the plates from sagging out of position. I also made a couple different slope angles to represent the angle I used with the 1/8" plates, and one with a bit less downward slope, which showed response improvemements in the 4KHz range with the 1/8" plates.
It's getting there.

[toddalin]

This shows a pair of the slant plate assemblies hot off the laser. Note the protective coating on all pieces to keep them from scratching.



"Baby we were born to run." It would seem that the US Postal Service had us in mind when they designed their "If It Fits, It Ships" box.



A bit more testing for the better angle with the 1/16" plates and we should be good to go.

[ivica]

This shows a pair of the slant plate assemblies hot off the laser. Note the protective coating on all pieces to keep them from scratching.



"Baby we were born to run." It would seem that the US Postal Service had us in mind when they designed their "If It Fits, It Ships" box.



A bit more testing for the better angle with the 1/16" plates and we should be good to go.

Hi Tod,

Very nice and profi work. congratulations !

Regards
Ivica

[toddalin]

I assembled a couple sets with the 1/16" plate material. I did one set in the original angle and another set with the angle raised about 4 degrees.
When I put them up next to the JBL plates, I found that my initial angle was about 2 degrees lower than the JBL angle, and the revised angle was about 2 degrees higher. This actually makes a difference in the overall sound, but changing from 1/8" plates to 1/16" plates made more difference.


With the 1/8" plates, the "swish" of pink noise became more of a "swoosh." The JBL plates didn't do this and the "swish" was still distinctly a "swish." The 1/16" plates, especially with the higher angle, left the "swish" a "swish" but added the lower range as the 1/8" plates did, more so than the JBL plates. The lower angle added a bit more in the lower range, but took out a bit more over 4kHz.


I've revised the angle to essentially match that of the JBL plates and will go with that. The overall frequency response improvement way exceeded expectations, and I've not even begun to look at the increase in horizontal dispersion.