Impedance chart, need explanation

[bentoronto]

Why does it have 2 peaks ? 52 Hz corresponds to the lowest point between the 2 peaks. Is this correct ?

Much valid tech talk early in the thread, but not much plain talk. The baby can get lost with the bath water.

Yes, correct. The basic BR concept is to construct a box tuned to the resonant freq of the driver, roughly 52 Hz in your case. When that is the case, the box restrains the movement of the cone and the minimum point is around 52 Hz, as you have carefully recorded. And consequently, traditionally the impedance curve you posted was a sign of proper implementation.

But all this is pretty dim engineering. The first problem is that any mechanism that goes bonkers at 52 Hz would never be used below maybe 65 Hz. But we drive our drivers outside their useful operating range and always have, with the possible exception of the concept of the AR-1, 1955, with a 12 Hz driver in a smallish box*.

While BR concept addresses the problem of the driver self-destructing at the bonkers-point (and slightly reducing the chance of booming loudly at a freq high enough and loud enough to be quite noxious), it still leaves an acoustically open box sitting behind the driver.That leads to all kinds of distortions with stuff coming out of the port when it messes up the sound above, mid-point, and below resonance.

That's the traditional model. There are various ways to try to tame the BR idea and there are many applications where it makes sense. But beautiful music reproduction is ordinarily not one of those applications.

Hope that helps address the opening the question.

Ben
*I've posted curves for my ancient AR-1 a few times.

 

[just a guy]

The basic BR concept is to construct a box tuned to the resonant freq of the driver, roughly 52 Hz in your case.

The ported box tuning frequency is a design choice. It doesn't have to be tuned to fs, in fact tuning to fs is rarely a good idea. Tuning at (or near) fs is the first step towards a max flat design that will sound quite boomy, "loose", "slow", with "droning bass", to throw out a few "audiophile" terms you might be familiar with.

Exact tuning for this type of max flat alignment is rarely right at fs anyway, usually it's a few hz higher or lower depending on qts. The only time tuning right at fs is proper for this type of max flat alignment is when the driver has the optimal qts.

Tuning a good margin below fs and designing for a moderately rising response will almost always provide a more balanced in room sound. As I've told you dozens of times the max flat alignment that you describe here is the reason you don't like ported boxes. A properly designed ported box won't be so overly boomy in room.

But all this is pretty dim engineering. The first problem is that any mechanism that goes bonkers at 52 Hz would never be used below maybe 65 Hz. But we drive our drivers outside their useful operating range and always have, with the possible exception of the concept of the AR-1, 1955, with a 12 Hz driver in a smallish box*.

Using resonance to enhance low bass is not dim engineering, it's actually pretty brilliant.

The mechanism does not "go bonkers" at resonance, it's actually very well controlled, in fact more controlled (less cone movement) than at any other frequencies.

The useful operating range is quite wide, it extends well below fs (when designed for a tuning below fs). There is absolutely no disadvantage to tuning below fs.

While BR concept addresses the problem of the driver self-destructing at the bonkers-point (and slightly reducing the chance of booming loudly at a freq high enough and loud enough to be quite noxious), it still leaves an acoustically open box sitting behind the driver.That leads to all kinds of distortions with stuff coming out of the port when it messes up the sound above, mid-point, and below resonance.

The "acoustically open" port is really not a problem. As I mentioned in my response to the other guy, there shouldn't be much of anything besides the desired resonances coming out of the port. This is easily controllable both mechanically and electronically. With a crossover point of 80 hz there's not much danger of anything bad coming out of the port.

The port does not mess up sound above or at resonance (unless poorly designed like you advocate). And below resonance, when used properly with a high pass filter, there's nothing there to mess up.

That's the traditional model. There are various ways to try to tame the BR idea and there are many applications where it makes sense. But beautiful music reproduction is ordinarily not one of those applications.

Clearly you've never heard a properly designed ported box. Like I keep telling you, if you design a ported box to have the same frequency response as a sealed box, it will sound very much like a sealed box.

Ben
*I've posted curves for my ancient AR-1 a few times.

Yes you have, and while your low power in room eq'ed frequency response doesn't look too bad it's only a very small part of the story.

With a driver that has maybe 4 mm of xmax and a tiny sealed box the max spl is necessarily going to be VERY low. About the same level as a modern bookshelf speaker.

While it was a revolution at the time to have a small box subwoofer, it's place is in the past. We've moved on quite far in driver technology so it's now possible to have all kinds of different small subwoofer alignments, all of which could easily eat your AR-1 for breakfast in terms of performance.

 

[Ron E]

You should probably start a new thread, as pretty much everything since post #8 no longer has to do with the original question of vented box impedance interpretation.

All of this started with someone's rather smug, vague, biased and silly notion that:

In the case of using a resonant box to counter-act the natural motion of the driver, it (viewed in isolation) tells you not much about sound output in the range around the bumps. Odd to try to make accurate sound when the cone motion is poorly related to the sound output, eh.

The current debate seems to have more to do with positive and negative opinions about vented boxes and such technical terms as "floppy" and "bonkers", and "all sorts of stuff" and the like. There isn't even any point worth arguing in any of that.

 

[sumaudioguy]

Let us make this really easy!

The idea is the electrical impedance curve as measured in a vented system is a good representation of the driver behavior. Fact is sometimes it is and most of the time not so much so. Rather than argue and discuss this modeling method and subject lets us do something far more useful and easy.

Just look at them! In a vent a small lightweight flap in the vent will show the air motion of the vent or even better, a passive radiator. Place a small dot on the passive near the center and a similar small dot on the active driver. The dot should contrast the driver material to make it easy to see. Now use a variable frequency oscillator with the level set so cone motion is several millimeters at 100Hz and then slow sweep down in frequency. First will come a frequency where the active driver reaches and maximum peak to peak amplitude. Going below this frequency the active driver amplitude will decrease as the coupling to the port comes into play. The active driver will reach a minimum excursion. This is the frequency of greatest loading of the active driver into the resonator system. At some frequency the vent or passive radiator will reach its maximum excursion. This is the frequency the vent or passive is tuned to. Please understand maximum loading frequency and maximum amplitude of the passive radiator need not be the same frequency. Below the lowest of these three identified frequencies the active becomes "unloaded" and the passive is not near it coupled resonance so out of band low frequencies cause excess active driver motion.

All of this is really easy. A piece of scotch tape or cellophane tape hanging in the port works pretty well to observe air displacement distance inside the port. Marchand has a free online computer based oscillator program works with any sound card. Am assuming everyone has a power amplifier and a speaker they want to know more about so here it is. This completely avoids the pesky electrical analog analogy by taking direct measurements anyone can do. These measurement can then be put into your software to look at the response and so on. Or at least I can do this with my software so...

Just so you know, if the minimum amplitude of the active driver occurs at the same frequency as the maximum amplitude found in the port the sympathetic coupling is at its most efficient for those two drivers. The free air Q of an ideal driver for a port is 0.383 which allows 8th order (4 aligned second order systems) Butterworth alignment of the coupled system for best results. The free air resonance of the active should be the port frequency with the box frequency being 1.414 times the free air resonance. This provides the most perfect alignment.

Is this sort of an answer to the question of the impedance curve? The impedance curve is in reality kind of useless on most drivers in vented systems. It might tell you something but, that is largely dependent on how well that particular driver follows the electric analysis. Some drivers are amazingly perfect and others off by more than a factor of two. It is interesting to set up to repeatedly sweep and measure the impedance curve while moving the cabinet around in the room or moving furniture nearer or further from the vented system. With these the room plays a large role.

 

[Ron E]

The free air Q of an ideal driver for a port is 0.383 which allows 8th order (4 aligned second order systems) Butterworth alignment of the coupled system for best results.

Yes! 4 simultaneous days per rotation!

 

[bentoronto]

...the most perfect alignment.

Again, an illuminating post.

For most of a century, since the reverse engineering of the Altec A7 Voice of the Theater (see sumaudioguy avatar) it has been clear you don't need to adhere to the "traditional" theory of BR tuning that I outlined. You can mix-and-match as you please according to taste.

Which leads to the question of what's best since there are various competing criteria. Some folks emphasize peak loudness. Or efficiency. Or flat swept FR when tested in your local park. Or some arbitrary Q value with mathematical advantages. Others, like me, want to get the best house-curve, least boom, or lowest distortion.

And you need to (a) know there are choices besides what your sim commands you to obey and (b) wisely choose your criterion.

Ben

 

[just a guy]

Again, an illuminating post.

Maybe illuminating for you (are you learning something?) but it's so basic it's pointless. Really? We need to have it explained that the port is moving more air that the driver at tuning? Come on.

For most of a century, since the reverse engineering of the Altec A7 Voice of the Theater (see sumaudioguy avatar) it has been clear you don't need to adhere to the "traditional" theory of BR tuning that I outlined. You can mix-and-match as you please according to taste.

Which leads to the question of what's best since there are various competing criteria. Some folks emphasize peak loudness. Or efficiency. Or flat swept FR when tested in your local park. Or some arbitrary Q value with mathematical advantages. Others, like me, want to get the best house-curve, least boom, or lowest distortion.

As far as I can tell you've never designed and built ANYTHING except for cutting a hole in a board and calling it an OB "subwoofer". (Even that was not designed per se, and hardly qualifies as a build.)

The only design advice I've ever seen you give is to put a driver in the largest sealed box you can, sometimes you advocate making it leaky sealed and sometimes adding stuffing.

I've NEVER seen you describe anything but max flat ported designs, and even then only to argue they are "dim engineering".

As clearly pointed out in post 13 you don't even understand simple concepts like sealed box qtc (with your "whomp up" at resonance in near IB comments) so it's not believable or remotely accurate when you say you design for "the best house-curve, least boom, or lowest distortion."

And you need to (a) know there are choices besides what your sim commands you to obey and (b) wisely choose your criterion.

Ben

We've been over this numerous times Ben. Simulators don't command anything. None of the simulators I've EVER used even have a single default alignment. The closest I've ever seen is WinISD with it's suggested starting points which can (and usually should) be tweaked.

Apparently your entire experience with simulators is that you've seen a rudimentary crappy online "simulator" that asks for a couple of t/s parameters and spits out a single box recommendation.

You shouldn't even be talking about simulators, your experience with them is clearly near zero, you clearly don't understand even the most basic things about them, and you very clearly don't know how to interpret the results they provide.

As such you are not qualified to choose the proper criteria for any given design goals, as you have so frequently proven.

 

[planet10]

Not mentioned yet, but relevant, is that T/S parameters are dynamic, they are curves not scalar. One of the problems with typical BR boxes is that they are tightly tuned, and as the T/S change with the dynamics of the music (or the setting of the voolume control) the box moves in and out of tune. The more "horizontal" the T/S curves of the driver, the less problem you have.

dave

 

[just a guy]

Not mentioned yet, but relevant, is that T/S parameters are dynamic, they are curves not scalar. One of the problems with typical BR boxes is that they are tightly tuned, and as the T/S change with the dynamics of the music (or the setting of the voolume control) the box moves in and out of tune. The more "horizontal" the T/S curves of the driver, the less problem you have.

dave

Not sure what you mean by "tightly tuned" and "horizontal t/s curves" and "moving in and out of tune" but when you say t/s change with dynamics and volume setting it sounds like you are talking about power compression.

In that case, sure, power compression will change the t/s parameters in a way that requires a larger box for a given response curve. So a box design based on cool driver t/s will be too small for a hot driver.

But this is not something that afflicts only ported boxes. All box types suffer from power compression and the resonant types without a sealed chamber (ported, tl, 6th and higher order bandpass, tapped horns, back loaded horns, etc) will all experience basically the same shift with thermal increases.

Even the sealed chamber alignments (sealed, IB, flh, 4th order bandpass) will experience issues with thermal increase and changing t/s parameters. For example, in a sealed box with a hot driver qtc will be higher than if the driver were cool.

So this is a universal issue. It's not just about ported boxes. Sealed boxes lose their composure (frequency response changes) too, all enclosure types shift pretty dramatically with increased thermal range.

 

[planet10]

Not sure what you mean by "tightly tuned" and "horizontal t/s curves" and "moving in and out of tune" but when you say t/s change with dynamics and volume setting it sounds like you are talking about power compression.

No i am not talking about power compression althou the change in VC R at high levels will exacerbate the situation.

Tightly tuned is simply the tuning you get with a typical tubular vent. It is tuned to a specific collapse of the T/S to scalars, as soon as the T/S change the tuning is no longer optimum. A measure that gives the vent a wider tuning helps reduce that problem.

A horizontal curve is just that… as the variables that influence the T/S change the curves remain close to horizontal (ie the scalars obtained by collapsing the curve tend to stay similar).

dave

 

[sumaudioguy]

Curios post. A huge amount of the problem is attempting to measure T/S parameters by analyzing the impedance curve. Occasionally this works however, far more often than not the results are not very close (half to double the Q as opposed to actual Q) to the actual driver character. I have previously outlined the correct way to measure T/S parameters over in the lounge.

For those who want the lowest boom and least distortion then the Q=0.383 and resonance tuning outlined above provides the tightest coupling and Butterworth flat type response. This also turns out to achieve the highest efficiency for the correct box over any other alignment. Clearly due to the most efficient coupling. Of course this will require either the big port or a passive. I prefer to not ever tune below 40Hz due the reasons stated in an earlier post. If the actual T/S parameters are measured as I outlined, then the finished box design will come in extremely close to the theoretical performance at normal to very loud listening levels.

In advance I will say using the impedance curve and an added mass measuring a driver at a few milliwatts is a complete waste of time and the most popular way to determine T/S parameters incorrectly. There are several other ways to measure T/S incorrectly which are also popular. My post in "The Lounge" is the only accurate way I know of. Historically through about 60 different woofer designs using this method the finished design performed within 3% of the calculated theoretical values.

 

[Ron E]

All you need do is look at some of the power compression measurements of subwoofers and find that Dave's statement is mostly hogwash. He implies that the box becomes mistuned, which would imply a change in frequency response.

These measurements show the changes in SPL with changes in drive level, as well as revealing the point where those changes become a problem, often due to using a port that is too small. You don't see much change in frequency response, so the effect Dave likes to talk about is not significant.

 

[Ron E]

For those who want the lowest boom and least distortion then the Q=0.383 and resonance tuning outlined above provides the tightest coupling and Butterworth flat type response. This also turns out to achieve the highest efficiency for the correct box over any other alignment. Clearly due to the most efficient coupling. Of course this will require either the big port or a passive.

I would say almost all of that is wrong.

 

[Ron E]

FWIW, here is sumaudioguy's parameter measurement thread:
http://www.diyaudio.com/forums/lounge/164047-completely-impressed-saddened-diyaudio-2.html

Nothing wrong with measuring Bl directly, nor in accurate measurement of DC resistance. I haven't ever seen a 100% error (as he claims) using small signal measurements. Fairly common to see 5-10% errors, the lumped model isn't perfect.

 

[planet10]

He implies that the box becomes mistuned, which would imply a change in frequency response.

Differing FR as shown in all the bass/midbass driver tests in Voice Coil?

dave

 

[bentoronto]

How come we don't see distortion measurements with impedance? Might be interesting, eh.

Attached is an impedance curve and THD for my home-brew electrostatic interface box (mostly a 1:100 step-up transformer) and ESL panel. Horizontal bars 20dB apart. The distortion is nearly all 3rd harmonic. Weird.

Ben

 

[Ron E]

The question is not whether or not there is a difference, but how much 😉

In voice coil, Vance does LEAP sims, not measurements, IIRC.

 

[Brian Steele]

The question is not whether or not there is a difference, but how much 😉

Exactly. It's not if there's a difference, but if the difference makes a difference.

BTW - I have come across one instance where a driver failed to measure properly using small-signal t/s params measurement (Vas, Qts, Fs didn't match published specs). A popular 18" PA driver too. However, BL, Mms, etc. worked out to be fairly close to the published params. Even more curiously, the impedance curve in box, measured with the same DATS, measured as expected.

 

[just a guy]

How come we don't see distortion measurements with impedance? Might be interesting, eh.

Maybe if done properly, not like whatever you are showing here.

Attached is an impedance curve and THD for my home-brew electrostatic interface box (mostly a 1:100 step-up transformer) and ESL panel. Horizontal bars 20dB apart. The distortion is nearly all 3rd harmonic. Weird.

Ben

First, what is the point of this?

Second, impedance is measured in ohms. The vertical scale on the graph says db. Did you just transpose the impedance curve over the distortion measurement? If so, why? The impedance curve really doesn't mean anything without a properly labeled vertical axis, IN OHMS not db. Also, are you sure this is a measured impedance curve? If so, are you sure it's done properly? It doesn't look like other esl panel measurements I've seen and for that matter the distortion measurement looks a little fishy as well.

What spl level was the measurement done at? I can't imagine your antique esl panels do much more than 90 db at max volume and you probably measured somewhere around 70 db since you live in an apartment (I assume surrounded by neighbors around your own age) and that's probably about all the spl you can get away with. If you use these panels from 140 hz and up as you previously stated and you measured at 70 db then it looks like over 100 percent distortion up into the several hundred hz frequency range and not so great up even at higher frequencies. Also, odd order distortion is the most irritating so if your distortion is mainly 3rd order AND it's quite high relative to the measured spl, it looks like these speakers suck pretty bad.

I don't know, maybe I'm interpreting your data incorrectly, but it seems pointless except to show that your speakers have terrible distortion problems.

 

[sumaudioguy]

I would say almost all of that is wrong.

This was a widely understood condition back in the 1970's and is a fact. One you obviously do not know Ron E. If one plots efficiency versus tuning the highest efficiency achievable is using this exact alignment. I can clearly tell you have spent little time with real speakers. I have done over 60 different woofer system designs and what I have written is all fact. Using my method for measurement the need for "Grandma's Fudge Works" as described in a paper by Richard Small are not needed because the error of measurement are minimal compared to the very erroneous method propose by Small to measure T/S parameters. It was a good idea that did not usually work in the real world with errors as large as I mentioned with otherwise perfectly acceptable drivers. One very easy test to estimate how closely an impedance measurement will represent the mechanical system is to observe the difference between the mechanical resonance frequency and the electrical reflected resonance frequency. The more these to frequencies are different the less accurate the Small method is. If the frequencies are identical then the Small method works well. If these are only 2Hz apart (for a woofer) then the Small method is functionally useless. Do you Ron E even know how to do this measurement or any of you other experts care to jump in here and explain?

Do not even say these two frequencies are always the same unless you wish to show you know almost nothing of transducers in the real world and instead rely on very faulty simulations of little use. I am trying to bring a little light to this impedance discussion with simple test and insights many can understand and not bury it in ignorance and religious beliefs as so often is the case on DIY. This is especially true with heavy users of simulations with almost no laboratory testing of their models or results using real cabinets with real speakers in them. Hearsay and myth more than fact and results.

You will notice here offerings of simple methods to check out your tuned sympathetic system which give irrefutable results that do not rely on hocus pocus or the need for mysterious test built into some test and measurement machine. Direct observation of cone motion (you can even measure the displacement with a ruler at various frequencies to determine the response!) can only tell the truth and does not rely on anything other than the researchers ability to see and measure and not some bogus electronic test system based on very dubious methods never shown to actually work outside extremely controlled environments and test conditions. Simplification of the most important measurements to produce accurate results leads to a lot less confusion about what is happening with a woofer system ported or not. This is what I am try to bring to this thread.

"The fact that an opinion has been widely held is no evidence whatever that it is not utterly absurd; indeed in view of the silliness of the majority of mankind, a widespread belief is more likely to be foolish than sensible." —Bertrand Russell, modern philosopher. For all you that prefer simulation results and then claim this is what happens in the real world without any real world validation of that method or results. Simulated speakers should be used to only to listen to simulated music in a virtual world.

"When a true Genius appears in the World, you may know him by this infallible sign: That the Dunces are all in Confederacy against him."—Jonathan Swift Here here I say! No truer action than this applies far to often at DIYaudio.