They should sell two versions of it... One version as it currently sold, and another version with a very proper back chamber... Charge an extra $80 or something for it, whatever is appropriate and profitable. I agree with you, this seems like a missed opportunity for ScanSpeak.
Here's two shops that sell a rear chamber for the D7608:
https://frequence.dk/hifi/72-mellemtone-enheder/729-d7608-bagkammer-kabinet/
https://compacbel.be/boutique/#!/news/products/scan-speak---rearcover-13cm
https://frequence.dk/hifi/72-mellemtone-enheder/729-d7608-bagkammer-kabinet/
https://compacbel.be/boutique/#!/news/products/scan-speak---rearcover-13cm
I believe they omitted a chamber because the driver was designed mainly for OEMs to be used in their commercial designs. The mids weren't really supposed to be marketed to us DIY guys, as we don't have access to the pro engineering resources and testing facilities they have. SS just figures people know what they're doing using these mids and probably has them in their main catalog to be accessed as repair parts.
Hopefully my efforts will make this driver more attractive to other hobbyist builders who normally would shy away from a driver who's specs indicate its difficult to work with.
Hopefully my efforts will make this driver more attractive to other hobbyist builders who normally would shy away from a driver who's specs indicate its difficult to work with.
I measured the impedance of D7608's new out-of-box, both free air and with an improvised rear chamber of about 0.3 L (and I mean improvised: driver's frame hand pressed unto the opening of a 5 cm wide duct tape roll core, itself filled with a sheep's wool - just to get the relative difference ballpark). No enclosures built yet. Await the consensus here...
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@Lojzek I have a pair of these chambers. They were designed for used with the amazing Vifa P13WH-00-08. They don't really work with the D7608 from a physical fit POV.
Its possible this one aspect could have misled most people into thinking the D7608 was difficult to work with if the intended enclosure didn't optimize its FR on the lower end.
Its possible this one aspect could have misled most people into thinking the D7608 was difficult to work with if the intended enclosure didn't optimize its FR on the lower end.
They look way too big. Chamber usually needs to be small enough to provide air pressure protection at lower frequencies.
@Draki That looks close to what you should be seeing with the chamber. If you look at that 2nd shallow impedanxe peak in both curves, the chambered curve has a substantially diminished 2nd peak. This is desired and what you should be aiming for when optimizing the dampening right behind the felt.
You'll find that just squashing the felt in the chamber isn't what you should be doing. There should be dampening right behind the felt and then some airspace between that and the rear chamber boundary. This is necessary to slow down the airspeed to avoid the secondary standing wave, as indicated by the 2nd shallow impedance peak. The You'll want to try completely suppressing that 2nd peak.
You'll find that just squashing the felt in the chamber isn't what you should be doing. There should be dampening right behind the felt and then some airspace between that and the rear chamber boundary. This is necessary to slow down the airspeed to avoid the secondary standing wave, as indicated by the 2nd shallow impedance peak. The You'll want to try completely suppressing that 2nd peak.
Fun approach! 😉 Works, tho. The seal must have been good as there's no small glitches in the impedance curve. Looks like that volume is big enough; too big? Would smaller volume protect against over-excursion at lower frequencies or push Fs too high?
@mikessi That chamber is WAY too big for this mid. I dont know why even vendors believe this is the correct intended chamber for the D7608. It just isn't and I know this having worked with Vifa engineers back in the 90s. The main guy who designed most of the dome drivers told me the chamber was intended for 5" cone mids. There was no mention of the D7608 here, but maybe people assumed this is the correct chamber for it just on the basis it sort of fits.
It's possible, and also a high Qts value would not help either. This is all rather less important, for one has to shape the high pass
curve, after all.
@Draki The target Fb is right at 340 - 360 hz. That is where the airspace right behind the dome and before the felt becomes reactive, coupling with the chamber air space. It effectively cancels out that 2nd shallow peak. That's the trick to getting the most effective dampening effect which does the most good without killing most of the natural low end boost before rolloff.
I know you're working on a chamber with toroidal shape for the d7608. Have you tried a conical shape with progressive denser damping down to the end (point)?
@Lojzek You'll need to aim for a Qtc of 0.7
Thats going to give you the correct compliance and rolloff slope along with optimal reactive dampening to get rid of the secondary smaller impedance peak. The Qtc just happens to be a symptom and not a main cause for getting the best possible low end output (in quality and quantity along with good mechanical power handling).
Thats going to give you the correct compliance and rolloff slope along with optimal reactive dampening to get rid of the secondary smaller impedance peak. The Qtc just happens to be a symptom and not a main cause for getting the best possible low end output (in quality and quantity along with good mechanical power handling).
@mikessi Yes, been there and done that with multiple variations. This mid doesn't respond to the typical TL style chamber designs. That 2nd peak is where the bad stuff happens. Its right in the place where H3 will peak if left untreated. This is something quite a few mid domes suffer from, ie. the fancy Esotar Dynaudio, Volt VM753 and even the mighty ATC SM75-150(S).
Hey guys, I have to get some other important stuff done. I'll try to get back to this thread a little later on.