TL design for Altec 604H driver

[cesout]

Aside from some opinions to the contrary, I wish to explore the use of a low Qts driver in a TL design. I solved the equations in the MJK's classic alignment tables document using the TS parameters for the Great Plains Audio 604H driver and here are the details:

So = 3.15 which makes the cross sectional area approximately 550 sq in. I also detuned from the driver resonance of 30Hz to 40Hz (I think I read somewhere that shifting the TL tuning 5-10 Hz smooths the response ripples?) to shorten the line to a little over 7 feet. I think the TL pipe volume is around 33 cu ft. How much different will this sound from a ~9 cu ft BR cabinet?

A 24" sonotube gives me about 450 sq in cross section so I have thought about using some. If I use the sonotube, will the 20% reduction in CSA adversely affect the system performance? The alternative is a box cab approx 18 x 29 x TL length (height).

What happens if I close down the open end some with a vent? Does this make it a 'mass loaded' TL? I gather there are some fuzzy differences between traditional BR cabs and MLTL designs. I have never seen many specifics regarding all the TL design variations.

I've seen the design on 6moons.com for the same speaker and it is much smaller than the solution I derived from the tables. I don't want to make the speaker any smaller unless it makes no difference.

Does this sound like a viable method to utilize this speaker?

[GM]

Greets!

Well, it shifts them lower, so they are for a longer period, ergo requires more damping to quell, which I guess can be construed as to equate to smoother.

TLs are much more highly damped with a more IB-like response than BRs.

Reducing a TL's Vb just means it will have a higher F3 and lower Q roll off below it.

There's two ways to make a MLTL (aka tower or column speaker), either by sticking a vent in the terminus or using a high aspect ratio TQWT (aka TQWP) to 'neck' it down to the vent area.

A bit more than 'fuzzy'. A BR alignment assumes the internal air mass has a uniform particle density, but it morphs to a 1/4 WL resonator with increasing height to CSA (cross sectional area = width x depth) ratio.

The 6moons speaker is a MLTL designed to be flush with a rear wall and is a truncated version of the much larger floor standing one I normally recommend. As such it needs the wall/floor boundary gain to somewhat offset the loss of LF gain BW and driven with a high output impedance to round it out. IOW a classic vintage alignment. What was once old is yet new again.

Anyway, the beauty of a low Fs, Qts driver is its tuning flexibility for a wide range of alignments, so what are your performance goals and how big can you tolerate?

GM

[cesout]

Thanks for the reply, GM. I want maximum performance and I am fortunate to be able to accomodate something sick in size. I recently bought Mathcad and have Martin's worksheets and have discovered the original So figure I came up with a little off. I used one of the worksheet to calculate BI based on other TS parameters and came up with a slightly revised figure. As it turns out, the cross section of the 24" sonotube is exactly (to 1 cm^) the value of So*Sd so that alone is driving me towards the ML TQWT design using the 24"sonotube. Not to mention the quest for real bass.

The 30Hz tube length (112" less 5") models response out to 20Hz but it is a little bumpy (+/- 2.5db) around 80 - 200 Hz. I then shortened the tube to 60" for a 50Hz tuning and it goes to 30Hz and is pretty smooth through the intended range (<1500Hz). The TL impedance curve (2 peaks at resonance) straddle the IB impedance curve pretty evenly with the 50Hz pipe. With the 30Hz line, the higher TL resonance lines up almost evenly with the IB peak. Shouldn't the IB peak bisect the TL peaks? I think I have noticed other designs meeting this condition.

As far as the bumpy response region for the 30Hz pipe, I suppose I could tweak the driver placement and maybe play around with the port dimensions to fix the response. I achieved a better curve, albeit not going as low, by de-tuning the pipe a little upward in frequency. I would like to keep experimentation confined to the simulations and do the build once right the first time.

Comments graciously accepted.

[GM]

Greets!

You're welcome!

Hmm, apparently we're working with different specs since using Altec's I get a ~540.639"^2 SO/SL (~26.237" diameter) for a 30 Hz Fp. Even with GPA's series II specs I get ~489.402"^2/24.962", so are you using measured specs? Regardless, I'm curious how you plan to adapt a Sonotube to an offset driver layout or why you would even want to.

The theoretical ideal is to ~completely damp the driver's Fs peak, so Fp = Fs/Qts in a heavily stuffed end loaded pipe, ergo with a low Fs, Qts driver this limits it to a mid-bass alignment. For tuning around Fs, it doesn't matter whether you damp the impedance peak or not with a very low output impedance, but with a high output impedance you typically want the Fs peak to damp the upper TL or MLTL peak, so that the amp 'feels' the twin peaks for best LF gain and why a low power SET tube amp can to some extent perform like a much more powerful SS amp.

Anyway, most folks seem to prefer speakers with plenty of mid-bass 'slam', so short of building a large BLH, this means either a high Fb (Fp) or an under-damped alignment. Some of us prefer a more IB-like response except with gain, so a tuning at/below Fs in a MLTL or TQWT cab that has at least a Vb = Vas since this is what the driver wants to 'feel', then use selective damping to quell any Fb (Fp) peaking.

Bottom line, unless you know exactly how the driver will perform in various alignments in your room and how it should look in a sim, you may not be able to 'build once right the first time'.

GM

[cesout]

I am using the provided TS parameters from GPA. I didn't have a BI parameter and took a stab at it using an Altec figure, around 16 or so. Thats where I came up with the original 550 sq in So. I used the TS consistency check BI worksheet to derive the BI parameter from the others. Here are the driver parameters I am using:
Re = 6.8 Ohms
Qmd = 8.49
Qed = .270
Qtd = .262
Vad = 16.35 cu ft
Vd = 19.20 cu in
Fs = 30Hz
Sd = .113 cu meter (~175 cu in)

Those are all the parameters I have for the GPA driver. Using those parameters with the consistency worksheet, I came up with a value of 23.171 for BI. This made the So figure more like 452 sq in.

I imagine the actual measured driver parameters would probably be a little different and would be more suitable for inclusion in the models. Assuming the parameters solve the CSA larger than the 24" sonotube, scrap the idea? What if somehow the CSA does work for the sonotube, scrap anyway? What are the problems you see with using the sonotube?

The sonotube seemed like a good idea based on some other designs I have run across. I was going to mount the driver on a baffle/coupler into the side of the tube at a suitable offset. I suppose I need to be careful about any transitional CS from the driver baffle/coupler to the pipe. I guess there is only one way to find out. Also I was thinking of turning the whole thing 'upside down' with the vent at the top of the pipe and the driver situated at about 35" off the floor. I don't see a problem with this except I loose the port coupling with the floor. A sonotube seems like a ready-made enclosure of the proper dimensions and is relatively inexpensive. A similar wooden cabinet would be much more complicated for a test of the design. I would like to prove/disprove the concept before moving too far ahead.

If I am reading you right I shouldn't tune the line higher than Fs of the driver but right at or below Fs. Is this right?

What I want from whatever design I end up with is a cabinet that extracts every bit of performance possible from the driver. I like my bass clean and tight and real. I don't want 2nd harmonics 'masquerading' as fundamental frequencies.

I have some UREI 809 12" coaxial monitors and they sound excellent except they are missing the lowest octave, at least. They are the baby to the 813 monitor which used some additional 15" woofers to help out the low end, all in a 10 cu ft or so BR cab. I would do a BR but I've heard that TL is the way to go for better transient response and so on.

What would you do to best accomodate these drivers? Why or why not sonotube? A wooden cabinet 104 inches tall? Scaled down? I thought I was on to something.

[cesout]

I forgot to mention that I have built some Linkwitz Phoenix speakers recently and they sound very good. I am most impressed with the low end and smooth, no holes response. They seem quite accurate and I am pretty well satisfied, but there is something missing. It doesn't quite have the life-like quality coming from them as the UREI's being driven with Transcendent Sound's Beast OTL amplifiers. The UREI's driven with an SS amp is good, but nothing like the OTL tubes. I haven't tried the Phoenix panel with the OTL as I only have two channels (two monoblocks). I don't think that would get me there.

This UREI-TS combo reproduces the timbre of instruments and voices so realistically it is startling. I am not quite sure what it is but I like it. You know it when you hear it. Over the last 35 years or so I've listened to Altec, JBL, Klipsch, Advent, Martin Logan, Tannoy (not co-ax) in short a lot of speakers. I like the sound of a coaxial driver. Whatever other problems they may have, phase coherency and 'imaging' isn't one of them.

This is why I want to build something around the 604 coaxial driver and I would like to keep the design to just the single 604 per side and no subs, etc. Fewer problems. Given I am using the sole 15" driver, I am trying to squeeze every drop of bass performance possible.

What say you?

[GM]

Greets!

When the info is available, Sd can be accurately calc'd by Vd/Xmax = 19.2"^3/0.2 = 96"^2 (~619.354 cm^2), then:

B*L = 0.0001*SQRT((rho*(c*0.0001)^2*Sd^2*Re)/((2*PI*Fs)*(Vas*0.001)*Qes))

where:

rho = 1.21 kg/m^3

c = 342 m/sec

Sd = cm^2

Vas = liters

B*L = 12.333 N/A

Otherwise, you estimate Sd as best you can, then:

Cms = Vas/(Sd^2*rho*c^2)

where:

Vas is in m^3 (liters*0.001)

Sd is in cm^2

c is in meters/sec*0.0001 to get Cms in mm/N

then:

Mms = 1/((2*pi*Fs)^2*Cms)

BL = SQRT((2*Pi*Fs*Re*Mms)/Qes)

GM

[cesout]

GM, I get a little confused sometimes with the nomenclature, i.e. Mathcad vs traditional. Sd as I mean to communicate it is the surface area of the driver cone, not the displacement. I think GPA specifies the displacement with the term Vd. My bad with respect to specifying cu in.

What about the other questions?

[GM]

Greets part deux!

Well, 'Sd' is the driver's *effective* piston area, which is much reduced in a coax, particularly a horn loaded one. 'Vd' is the driver's swept volume displacement at Xmax, both being standard nomenclature/definitions that pre-date T/S, though not common in the DIY community's lexicon before then AFAIK.

I'm working on it, I just copied this info from one of my previous posts on the subject to give you something to work with in the meantime. I do what I can when I can and if I'm not fast enough, then feel free to search all the forums that me, MJK, Scott, Bob Brines and others have helped folks with pipe, etc. design on.

GM

[cesout]

I didn't intend to sound impatient. I am grateful that you are taking time to work on some of my questions for me.

With respect to cone area in the coax, I understand that the HF horn on the front side will cause some stopping down of the open piston area, but isn't the back of the cone pretty much like any other speaker? I suppose whatever is 'loading' the front transfers to some degree to the back as it is just two opposite sides of the same membrane.

Thanks for your replies.