HELP! Bass-reflex design for vintage University woofer C-15HC

[davidbosch]

I am currently investigating everything I can on the famous vintage C-15HC woofer from University Sound (somewhere around 1970's), in order to construct the matching enclosure and divulge the results in the vintage electronics community (who are eager to know).

So far, I know this:



Freq. response: 18-800 Hz (catalogue)

Fs: 25.5 Hz (currently trying to measure Q, but found problems)

Ideal Cabinet Volume: around 4.22 cu ft (dimensions are: 26.87H x 19.5W x 17.5D inches, in 3/4 MDF)

Power Handling: 60 watt max.

Nom. Impedance: 2-8 ohms (parallel or series, dual voice-coil)



A bass-reflex configuration is in order. I only don't know what cross-section or port length to use. Anyone could help me here?

 

[GM]

Greets!

Well, you don't say how you arrived at this optimum Vb or what you want it tuned to, but to tune it to around Fs with a low vent mach at 60 W requires a ~2" diameter x 1.375" long vent.

GM

 

[Taterworks]

Perhaps for 'optimum vb', he's using the measured Vas parameter - which is used for Vb in the B4 bass reflex alignment. Then just tune to FS. It doesn't really matter what the Q of the woofer is, then - that will just describe the transient characteristics of what he ends up with.

 

[GM]

Perhaps, though I find it hard to believe it would have such a low Vas.

GM

 

[davidbosch]

Sorry for taking too long guys

"Well, you don't say how you arrived at this optimum Vb or what you want it tuned to, but to tune it to around Fs with a low vent mach at 60 W requires a ~2" diameter x 1.375" long vent."

Isn't that vent a little too short for a 25.5 Hz tuning? just guess

The 'ideal Vb' wasn't really calculated, I got it from the original cabinet from University. It is most probably not the ideal Vb but it's a reference point. I've already calculated Qms (3.50), Qes (0.62) and Qts (0.53); now I'm about to calculate Vas.

What is the best method you'd employ?

 

[GM]

Greets!

Well, we do what we can when we can.

No, it's fine with what little info I had to work with and can be made to work with a bit of damping still, but with the latest info it jumps to 3.5" dia. x 6.25" long to get a low vent mach. Don't start making a vent yet though because I figure Vas is going to be considerably >4.22 ft^3, which will increase the vent size yet again and the 'ideal' vented cab will be a TL ML-TL or possibly a ML-horn since typically these worked 'best' in 12-15 ft^3 cabs, but I'm getting ahead of myself.

I prefer the sealed box method if for no other reason than I wouldn't want to stick a bunch of oily clay on a vintage diaphragm to do it the added mass way.

GM

 

[davidbosch]

Hey!

Is the first method for measuring Vas described here http://sound.westhost.com/tsp.htm
accurate enough?

Will having the the Vas, Q's and Fs be enough to model the right port?


There is a fact, I want it to be a bass-reflex enclosure, and even the original designer set this version, the C-15HC, unlike its predecessor, the C-15W, for a vented enclosure rather than a horn-loaded one.

 

[GM]

Greets!

Yes, it's the one I recommended.

Yes.

Well, the 'fact' is this driver's Qts is too high for a simple bass reflex to be its optimum alignment, but if you're convinced it is, then why not just clone the factory cab?

Or are you wanting to calc its T/S 'ideal'?

Or 'best' alignment tuned to Fs?

Or.....?

GM

 

[davidbosch]

I could clone the factory cabinet but only the Vb, 'cause I haven't got the port's dimensions.

But, yehp, I want to calc the T/S model in order to get a more accurate result.

I have doubts on the Q results, I'll check the steps and the measurement.

What would be the 'best' alignment tuned to Fs?

 

[GM]

Greets!

OK, then do you know what it's tuned to?

Anyway, 'best' depends on its specs, the system's output impedance and the room's gain curve at the listening position, so at 'best' about all you can do is find an alignment that can be EQ'd ~flat in-room.

Historically I've found cabs at least = Vas tuned to at least = Fs work 'best' overall as they typically had plenty of adjustment, but they were all high Vas drivers. The current trend of making them stiff (low Vas) to keep the cab small don't fare so well in these type alignments, typically requiring either long well damped vents or preferably massive PRs tuned extremely low, so reverse tapered quarter wave tubes (TQWT) or mass loaded (vented) variants (ML-TQWT) is the logical alternative.

GM