Ideal TB 881s TL geometry

[kneadle]

Here are the specs I used, downloaded from Partsexpress when I ordered the things.

I measured them using SW, and if they're different, I didn't note it, so they must not have been too different.

Dave

Re 6.87 [ê ]
Fs 111.15 [Hz]
F1 81.75 [Hz]
F2 153.99 [Hz]
Zm 37.25 [ê]
D 62.00 [mm]
Qms 3.60
Qes 0.81
Qts 0.66
Bl 3.31 [N/A]
L1K 0.60 [mH]
L10K 0.24 [mH]
Ms 1.86 [g]
Vas 1.40 [l]
dBSpl 86.36 [dB]
Cms 1.10 [mm/N]
Ma 16.00 [g]
FsMa 35.88 [Hz]

[kneadle]

A little context: this is a branch-off from another thread. I'm having difficulty simulating ideal geometry using MJKs worksheets.

With that being said, I've come up with something close using the ML TQWT, but I was hoping for a more simplified TL.

Here's the geometry for that:

L=24in
Zeta=.5
SO=2
SL=2
Density=.25
rport=.5
Lport=1.0

It has some rippling in the mids, but what really bothers me is that the simulation shows the 881s reproducing way down below 80Hz. That seems unreasonable for a driver with an Fs of 111Hz.

Dave

[GM]

Greets!

Why? As Qts goes up, so goes cab Vb since the motor is getting progressively 'weaker' with increasing Qts, hence must rely increasingly on the cab's trapped air mass to do the 'work' of driving the vent, ergo a lower tuning is required to flatten out this higher mass system.

Anyway, your sim looks very good for these specs. The way I figure them yields more gain, but less smooth looking FR, though I've built numerous examples with various medium Q drivers and they sounded fine to all n' sundry so as always YMMV:

L = 32.1875"
Zd = 0.3201
SO/SL = 12.894"^2
density = 0.25lbs/ft^3
rp = 0.75"
Lp = 0.75"

Normally though, a tapered pipe is a better choice for these since you get the gain without so much 'ripple' from the vent's output:

L = 32.1875"
Zd = 0.68
SO = 1.919"^2
SL = 23.833"^2
density = 0.5lbs/ft^3 at the top, 0.25lbs/ft^3 for the rest
rp = 0.5"
Lp = 0.75"

Of course a golden ratio cab nets the smoothest response for a given stuffing density, so consequently has the least gain:

L = 12.06"
Zd = 0.46233
SO/SL = 7.4375" x 4.625" (34.4"^2)
rp = 0.5"
Lp = 0.75"

Note that all these assumes vanishingly low series resistance, IOW an SS amp with sufficiently large speaker wire so if tube driven then either aperiodic or sealed will be required for best overall performance.

GM

[kneadle]

Hi GM,

Thanks for the work. A few followup questions:

1) for SO/SL, you use a notation I'm unfamiliar with. Are you using MJK's mathcad worksheets? If so, how are you entering the information in that way? I'm using the Mathcad 8 sheets.

2) How does an SS amp compare to a typical GC? I have a half dozen GCs under construction, simply to replace my consumer-grade amps. **see edit below**

3) Actually, smooth FR is a goal, here. I've been building TLs without paying much attention to that, and I just want to take advantage of a really, really good driver. The modified 881s has excellent potential. Why not get it all?

Thanks,
Dave

** Edit ** By "SS" are you referring to "all" solid state stuff? Or to a particular genre as opposed to chipamp?

[kneadle]

I just talked to Mark McKenzie. He informs me that the Qts of a warmed up 881s will drop to .59 or so.

Dave

[planet10]

Dave,

A chipamp is SS, so you will be fine

dave

[kneadle]

True; but this community has a forum for solid state and another one for chipamp. Just checking.

Thanks.
Dave

[bwbass]

Originally posted by kneadle
1) for SO/SL, you use a notation I'm unfamiliar with. Are you using MJK's mathcad worksheets? If so, how are you entering the information in that way? I'm using the Mathcad 8 sheets.

He's expressing the cross sectional area in real units, where in the MJK sheets there's just a ratio (or coefficient) for Sd. Divide GM's number by the driver's Sd to plug it into MathCad.

GM - Do you use Augspurger's math?

[MJK]

He informs me that the Qts of a warmed up 881s will drop to .59 or so.

OK, you lost me. If the voice coil temperature rises won't the resistance also rise. If the resistance rises so does Qes and therefore Qts. How does the Qts of a warmed up driver drop? Maybe I am missing something obvious.

[kneadle]

Perhaps I omitted something? A broken-in driver? Will that result in changed mechanical and electrical values?

Thanks,
Dave

PS--and will .07 result in a substantial difference in simulation?