Impedance plot for Saburo?

[Ty_Bower]

Has anyone ever measured the impedance vs. frequency profile for a Saburo? I'm just curious if this is typical. I guess I didn't expect to see so many peaks.

[rjbond3rd]

1. May I ask what tool you used?

2. Interesting that there are ~5 peaks, and ~5 big bends in the horn. I guess that's obvious though!

3. Would it be correct to say that the F3 is ~40hz? That's just a wild guess from your impedance plot.

4. How do you like them?

[Scottmoose]

7 major bends actually.

No, quite correct. Welcome to the laws of physics where when mouth size is compromised over the optimal for a given Fo, the acoustic impedance is no longer fully resistive & we're relying more on resonant action. Bends aren't particularly an issue, as you can see from the attached. Top = Saburo horn unfolded, expanding cascade of straight manifolds. Bottom = unfolded horn with same St, Sl & basic flare profile but with continuous expansion. Short of damping them per a TL, this applies to any pipe or 'cone' / expanding pipe with a compromised terminus area. And most are (unless you've room for a horn with a 50ft^2 mouth CSA. Per cabinet).

[Ty_Bower]

Originally Posted by rjbond3rd

1. May I ask what tool you used?

The software is Speaker Workshop. I built a Wallin Jig2 for the soundcard interface. I'm just using the SigmaTel audio built into my laptop.

2. Interesting that there are ~5 peaks, and ~5 big bends in the horn.

I was interested by the peaks as well. That's what prompted my post. I don't understand their significance, but I'm trying to wrap my brain around what Scott said. I've measured the impedance curve for a typical ported (bass reflex) enclosure. You get two peaks, presumably one from the woofer and one from the port. I've also measured some aperiodic enclosures (Dynaco A25). They only show one peak.

3. Would it be correct to say that the F3 is ~40hz?

I'm not sure that can be assumed. I don't believe the impedance peak necessarily aligns with the low end cutoff. I've played some test tones, and these guys seem to run out of steam fast below 60~55 Hz. I'm trying to get a test microphone set up, but it is a struggle to get it to work with Speaker Workshop. I think I'll give up and try Arta for a while.

4. How do you like them?

The jury is still out. They are very curious animals indeed.

[Scottmoose]

Depends on the room but F3 will typically be 40 - 45Hz.

Aperiodic boxes are basically (leaky) sealed cabinets, so yes, they will only have a single impedance peak. A BR cabinet should possess two. An expanding pipe will possess a variety depending on length, flare profile, throat & terminus areas unless it is impedance matched down to its design cut-off, i.e. full-sized.

[rjbond3rd]

Fascinating, gentlemen! The Harvey is still one of my all-time favorite speakers (built by Phil Townsend) and I've been curious about the Saburo.

Scott, many thanks. Would it be too simplistic to think that each peak/dip roughly corresponds to the QW resonance of a manifold (not that it necessarily matters in the larger picture)?

[hm]

Ty_Bower,
thank you very much for your measurement, now i know what membran movement
is happend and that a tube amp will have a problem in comparison to my real double horns,
thanks.

[Scottmoose]

Only a bad one (as in valve amp). The vast majority of people who have built these cabinets run valve amplifiers to the best of my knowledge, and most have been quite happy with the results. As far as I'm concerned, that's all that matters.

Why the 'real'? They're completely different enclosure types. Your boxes (at least, the ones I assume you are refering to) run twin drivers in a bipolar configuration, each loading a highly compromised damped horn / expanding pipe differentially tuned to fill in the harmonic nulls. Saburo is a large, forward firing bifurcated catenoid horn (somewhat less compromised, though these things are relative) with an MTM style terminus configuration. Trying to compare them is a bit like comparing a snake with a baboon: pointless. Not that it makes much difference given that Saburo is obsolescent.

You will have noticed, I trust, the rather accurate correlation between the graphs posted above?

Scott, many thanks. Would it be too simplistic to think that each peak/dip roughly corresponds to the QW resonance of a manifold (not that it necessarily matters in the larger picture)?

No problem.

No, it's not got much to do with the manifolds per se; that was the point I was making with the above two graphs. Forget folds for a moment: just assume it's all straightened out i.e. heading in the same direction. Top plot is just that, lower plot is a horn with the same throat & terminus area as Saburo, and the same basic flare profile it approximates, but with continuous (i.e. smooth) expansion. Not much of a difference, right? Most back-horns are not impedance matched down to cut off. They can't be; you need huge mouth sizes to do that, so they rely on resonant action on the bottom end until they reach impedance-matching higher up.

[rjbond3rd]

Hi Scott, thank you!

I've never seen (or at least noticed) harmonics on an impedance plot, and it looks like that's what is there (e.g., on your top graph, peaks seem to be multiples of ~30, the dips multiples of ~45).

So I gather that "impedance matched" would mean "designed to have a flat impedance" (sorry to be dense)?

[Scottmoose]

Not exactly. As noted above, it is in reference to compromised terminus size. When terminus area of a horn is undersized, as it almost invariably has to be for bass horns, you get a standing wave (supersonic shockwave) reflected back along the horn path. Horns are basically impedance matching devices, viz. matching the high impedance of the diaphram to the low impedance of the air, and thereby improving efficiency as the driver is 'seeing' a constant resistive load at all frequencies in its functional BW. Nominally horn terminus circumference should be a minimum of 1 wavelength of the lowest frequency to be reproduced in order to avoid this; depending on localised SoS for a 40Hz horn you're looking in the order of termini areas of 50ft^2+. More to it than that as you need to consider boundary loadings, whether a particular horn is reactance annulled (back horns usually aren't, although it's technically possible); you also need to be careful in other areas -Keele has demonstrated that beyond a certain mouth size efficiency will begin to drop away.