SPL-12 Volume Test

[mrogowski 6]

They are in stock now. If you live in the US, order through SSA - they also include shipping with the price.

I should have those response graphs done up this weekend.

Best,

Mark

[Tommythecat 0]

Box tuning turned out lower than expected as the impedance measurement shows below. The port tuning ended up being 40Hz and not my expected 46Hz. For interpretation, the dip between the two humps is the box tuning. The lower hump is the peak of the port and the upper hump is the peak of the driver in the box.

I have emphasized what I find to be a common mistake among many people. The impedance "humps" you see are not characteristics of one part of the circuit or another. The impedance shape comes from two things:

1) The impedance peak from the drivers' mechanical resonance

2) The impedance dip caused by the port

The dip is really "dug out" of the impedance peak by the port resonance. That is why the dip is centered on the tuning frequency of the box.

It's really a bit more complicated than this explanation, but it would take a entire primer on loudspeaker modeling to uncover what's going on here.

Edited November 29, 2007 by Tommythecat

[mrogowski 6]

Interesting, thanks for posting. I would love to hear the basis behind what you stated. I, just like others, am always open to learning something new...

Oh, and why do you think ThomasW is a moron?

Best,

Mark

[Tommythecat 0]

Interesting, thanks for posting. I would love to hear the basis behind what you stated. I, just like others, am always open to learning something new...

Oh, and why do you think ThomasW is a moron?

Best,

Mark

Understanding what's physically happening with a loudspeaker is something of a feat. I certainly haven't found any comprehensive and yet perfectly understandishable explanation of it. It's something I hope to cover in a lecture next semester - hopefully I can get it down to 1 hour including prerequisite knowledge (simple differential equations, circuit analysis, etc.). Even beyond an hour audio-visual extravaganza it takes some effort and hours of problem solving/coding to start to truely understand (the kind of "understand" when light-bulbs go off in your head).

I think it's funny that someone like ThomasW could make a statement such as "ported enclosures give you 3db more output over sealed" and get away with it. What does that statement even mean? I tried to pose the question to him at Htguide and was promptly banned and the post deleted. You can search for it because he's said it more than once - but it's a waste of time.

I'm not a big fan of ignorant, blanket statements that run rampant in DIY audio. That and my mommy didn't love me enough. That's why I think he's kind of a moron or at least willfully ignorant.

[Aaron Clinton 2,372]

SSA loves you.

[mrray13 30]

I'm not a big fan of ignorant, blanket statements that run rampant in DIY audio. That and my mommy didn't love me enough. That's why I think he's kind of a moron or at least willfully ignorant.

^^sig worthy..

i'm still laughing after reading that. it's an instant classic.

wheeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

[mrogowski 6]

Understanding what's physically happening with a loudspeaker is something of a feat. I certainly haven't found any comprehensive and yet perfectly understandishable explanation of it. It's something I hope to cover in a lecture next semester - hopefully I can get it down to 1 hour including prerequisite knowledge (simple differential equations, circuit analysis, etc.). Even beyond an hour audio-visual extravaganza it takes some effort and hours of problem solving/coding to start to truely understand (the kind of "understand" when light-bulbs go off in your head).

I think it's funny that someone like ThomasW could make a statement such as "ported enclosures give you 3db more output over sealed" and get away with it. What does that statement even mean? I tried to pose the question to him at Htguide and was promptly banned and the post deleted. You can search for it because he's said it more than once - but it's a waste of time.

I'm not a big fan of ignorant, blanket statements that run rampant in DIY audio. That and my mommy didn't love me enough. That's why I think he's kind of a moron or at least willfully ignorant.

Hmmm...

Keep in mind, this is DIY, and not all folks are engineers. Perhaps that is why you end up seeing such blanket statements like those posted by Tom and others (there are plenty as you are aware - I too am guilty of this).

The more I think back to my R&D days, the more I recall getting rather vague explanations from the VP of Engineering - certainly not because he didn't know the technology, but probably because it would take too long to explain everything behind it. I was therefore left to seek clarification elsewhere and draw my own conclusions.

I think Tom does his best to explain the concepts in layman language. I personally wouldn't chastise him for the odd hiccup though. I do however find your posts getting deleted over at HTGuide a bit strange, and rather abnormal. Maybe he was just having a bad day...

So, I/we shall wait until next semester to see if you are successful in delivering your lecture. Perhaps you can then explain what exactly is happening.

Best,

Mark

[Tommythecat 0]

Well, I dont have a full report right now - but I figured I could throw something out there. I don't want to come off as some know-it-all who doesn't seem to know-anything. I guess I owe this thread a response.

Our first graph shows the Driver's free-air impedance response and a bass-reflex enclosure response. Typical results. Free-air response has a single impedance peak which comes from the mechanical resonance from the spring-mass relationship between the spider+surround and moving mass. The vented response has two-peaks but they cannot be attributed to a certain single thing.

(scaled for emphasis - not absolute values)

This graph (is mislabeled because I'm lazy) and it shows some interesting things. We have the:

1) port resonance impedance - rising to a peak at the desired tuning frequency

2) the driver's mechanical impedance - a parabolic curve with a minimum at the driver's Fs

3) combined response of the two above plots

Now we see the combined response plotted against the (electrical side) impedance. This should illustrate where the impedance valley and peaks come from and why the valley is centered on the tuning frequency.

Technical Chatter:

Why are these graphs inverted?

What you have to keep in mind is that what goes on mechanically seems to be inverted from what we electrically measure. When the mechanical impedance is "reflected" into the electrical domain it is related by the factor of Bl^2 / Z. So our drivers mechanical impedance (as seen in figure 2) appears as a parabola mechanically, but electrically it appears as a hump.

What about the peaks being directly related to driver/port?

I believe the only way this could be arrived at is how the port and enclosure losses can affect the impedance peaks.

Because our model is an ideal - we can and should account for some of the friction/leak/etc. losses that occur. For a closed box we account for box losses (Ql) and for a ported box we account for both box (Ql) and port (Qp) losses. These losses must be measured and compared against our model to be sure of their absolute values - but there are some general values that can be used to predict the reponse.

When you model a bass-reflex system and vary the losses (Q's) you start to see that the Ql will affect the lower peak and the Qp will affect the upper peak. This has lead some to infer that upper peak is "created/controlled/or something" by the port and the lower peak by the driver(box would be more accurate). But this isn't the case.

If you simply model a loudspeaker and tune the box below the driver's resonance you'll see that changing the Qp will affect BOTH impedance peaks. It's something of a coincidence that one loss factor changes one of the peaks in a system where the box tuning frequency is higher than the driver's Fs.

[mrogowski 6]

Thank you Tommy. I can grasp a majority of what you said, but just like the many AES papers/reports, I will have to read it a couple more times for it to sink in. And since it is 6:30am, I have yet to devour my first cup of coffee What you state here is in fact, similar (with a bit more added) to the aforementioned.

Keep it coming if you are so inclined to do so...

Best,

Mark

Oh, and I doubt anyone thought of you as being a 'know it all'.