HiFi Subjectivist ¦ Audio Forum ¦ NVA User Group

"I like transmission lines so maybe the bass response with a passive radiator would not be too much of an issue for me (if designed properly) however it maybe a solution looking for a problem. Which is the more evil - a passive radiator or a phase changing filter ?"

I have to say that my limited exposure to transmission lines was a mixed blessing. The bass was extended but always a bit behind the music. Worse than a flared port to my ears.

TheMadMick wrote: ↑Sun Oct 03, 2021 1:10 pm "I like transmission lines so maybe the bass response with a passive radiator would not be too much of an issue for me (if designed properly) however it maybe a solution looking for a problem. Which is the more evil - a passive radiator or a phase changing filter ?"

I have to say that my limited exposure to transmission lines was a mixed blessing. The bass was extended but always a bit behind the music. Worse than a flared port to my ears.

Hi MM,

Yes, although the output is in phase the back wave has a substantial distance to travel from the rear of the cone to the vent. As with all things speaker related there are the swings and roundabouts of compromise.

I should have realised but passive radiators work best with low qts drivers ie ones which work in vented boxes. That would make the B&W drivers suitable for a PR. Not sure I can be bothered to take it any further.

Other thoughts have been to trying to try and use a 6.5" as a down firing driver, solely for the purpose of digging a little deeper in the bass than a 5.25" is capable of. Seems a bit like fannying around the edges- 12 inchers would be nice but I won't be going there again as the cabinets are just too big even with isobaric and aperiodic loading for the rear chamber.

The volume required for a 6.5" with a high enough qts for sealed enclosure is too big for the 30cm steel spheres especially when some of the volume is required for the up firing driver even if if is a smaller midrange.

I have thought again about using isobaric for the 6.5" driver but then things get expensive and power is wasted. It does half the box volume required though.

erm , Celestion 15s . Sealed boxes.

Sealed boxes .

Everything else is a compromise motivated by money.

New design. Using isobaric in clamshell configuration for both up and down firing. I have hidden the two out facing magnets in the hemisphere and the tweeter sphere So there are no legs of any kind required, even between the three diferent parts (hemisphere, mid bass sphere, tweeter sphere)



I am very pleased with the design but the isobaric idea has it's own little problems like a resonance in the mid range at a frequency determined by the distance between the two cones. And the driver basket causes diffraction but if a driver with minimal cast frame is used then that can be minimised. My previous design had four legs connecting each part anyway so it's not much worse in that regard.

There is a reduction in the higher frequencies with magnet out because the centre of the cone is obscured by the magnet. I can also get the tweeter a bit closer to the cone of the mid bass. Of course the enclosure volume required is halved

I'm beginning to like this design very much even though it has flaws. Will be expensive if decent drivers are used though. It's still only about 60cm tall.

Have U heard the isobaric clam shell ?

Hey Stu- do u do WhatsApp

Latteman wrote: ↑Mon Oct 04, 2021 7:18 pm Have U heard the isobaric clam shell ?

Yes Peter, I used to have my doped 12 inch drivers in clamshell isobaric. It worked really well and was never the issue with those drivers, the issue being the Qts and Vas were so high I couldn't get the box small enough to be domestically acceptable without a bass hump.

J have never tried the clamshell with a smaller driver but the principal will be the same. I highlighted it's theoretical shortcomings above. Doc used isobaric with doped eight inch drivers run fully open but he had equal volume behind each driver and not clamshell.

Latteman wrote: ↑Mon Oct 04, 2021 7:54 pm Hey Stu- do u do WhatsApp

What's WhatsApp ? I do messenger sometimes which is an extension of Facebook

It is a real shame I don't use my four 12" drivers The options were open baffle or infinite baffle. I don't want to get into the necessary EQ required to make them work in OB. True infinite baffle (mounted in a wall or floor with the backwave having at least 10x the Vas of the driver) was always an attractive proposition apart from the fact they can't ge moved and would need a low pass filter (used as a sub)

There is another option to enable a reasonable sized box. Design the box for a high Q of 1.1 (severely undersized box) and use a high value capacitor(s) in series just before the driver. This cap is in the 100's of uf range and can be calculated using the TS parameters of the driver.

At first glance this cap seems to be just a high pass filter. But there is much more to it than that. Basically it flattens completely the horrible bass hump produced with a Q as high as 1.1 and extends the drivers output even lower in frequency. Sounds like magic eh? There is no free lunch. Group delay is increased a bit and you have a big expensive yucky electrolytic cap in series with the driver. I happen to have lots (over 20) of 47uf 63V film caps which when paralleled will add up to the value required. Ten of them will be 470uf.

I will give it a go sometime with my heavily steel lined Marantz Imperial 7 cabs - remember how this thread started out four years ago?