Measuring driver excursion

[Chrisbee]

Hi

I have been playing around with REW's sinewave generator option.(again)

Measuring cone excursion is a bit of a trial. (and error)

I set up a lab retort stand with a dowel clamped vertically in front of my IB drivers. Then I made a sort of flag indicator out of folded masking tape pointing straight at one of the driver dustcaps.

I used a handy measuring device to set up a safe 10mm distance between the tip of the flag and the dustcap then started the 10Hz sinewave signal.

That was when the dowel started vibrating to match the vibrations of the subwoofer. Grr.

The dowel was quickly replaced by a long length of stainless steel tube. I remade my flag indicator and measured carefully to the dustcap again.

This was much better but it seemed difficult to get anywhere near +/-10mm without the sound output levels getting very silly indeed!

The driver cones seemed to be thrashing in and out wildly even at this rather modest excursion. I'm not very brave when it comes to watching my driver cones slowly destroy themselves.

So, how do you measure excursion safely when testing your own subs?

[drew01484]

Hi
The only thing i can think of, if your dust caps are made of somthing like plastic or mettle that wont mind you sticking somthing to them (with sticky tape or a gum type glue that will peal off without causing damage,

You could stick a small light rod like a tooth pick or a straw to the center of the dust cap and set a rest with a ring atached to the end to allow the rod to slide back and
forth, a fue inches away @ the end of the rod stick a bright red mark and behind all
that in the background get some sheets of paper and draw on it a black and white scale with 1 cm increments or you could use inches if you r living in the dark ages
you can then get down at eye level with the rig and read off the scale to see how far the driver extends.
I think that you should be able to see it with the naked eye but if not you could
set up a web cam, record it and then slow it down.

Please be carefull, could do without a lawsuit No but seriously am just thiking outloud hear i have never thought of a need to test this let alone tryed it.

Good look with it
Drew......

O probobly better if the material that you use for the rod is softer than the dust cap
that way if something go's wrong the rod gets bent not the dust cap... andyou ........

[Chrisbee]

Thanks Drew

I think I prefer my fixed point and soft distance sensor.

Sticking objects to the dust cap might cause problems I'd rather avoid.

I have the feeling there is a simpler way that hasn't yet occurred to me.

Listening for voicecoils hitting the magnets isn't an option I much care for.

I have a small laser pointer if only I could decide how best to use it for this task.

[SteveCallas]

Yeah, I'm pretty sure lasers that measure distance would be the most accurate and safest way to do something like this.

[Chrisbee]

SteveCallas wrote:

Yeah, I'm pretty sure lasers that measure distance would be the most accurate and safest way to do something like this.

I was thinking of something very simple, repeatable, accurate and foolproof.
Like a narrow laser beam lighting something only when it reaches a particular point in space.
(i.e.The center of the dustcap at the desired maximum excursion)
I suppose it doesn't need anything more complicated than pointing the beam at 45 degrees ahead of the dustcap onto a suitable thickness of material. (which is then removed)
When the dustcap travels that distance forwards then the beam will strike the center of the dustcap.
A white sticky paper dot (or a sequin) can mark the center spot to aid observation.
The beam will simply pass beyond the dust cap until the required excursion is reached.
Isolating the laser from mechanical vibration might be a bit of a problem.
There isn't much that stays still in an all wooden structure when an IB really gets going.
Anybody else feeling suitably inventive?

[Chrisbee]

The laser worked fine provided it didn't move around with the vibrations in the floor.

[ISLAND1000]

Did you have a specific reason to measure the excursion limits or was it just curiosity? If it was just curiosity, make a fine pen mark on the side of the voice coil and have a second diyer watch the fine pen mark and make measurements as you carefully depress the cone IN to it's limit and then continuing with your fingers wrapped around the edge of the basket embracing the cone, gently push OUT to it's limit. Take the measurements and have a beer on a job well done.
If it's more than curiosity that's moving you forward then I'm thinking you need a lazer device or . . . . . who was it that had designed a speaker testing rig and was offering to test ALL the speaker parameters anyone could want for a reasonable fee, he thought?

[Chrisbee]

ISLAND1000 wrote:

Did you have a specific reason to measure the excursion limits or was it just curiosity?

I'd like to measure the maximum dynamic excursion during normal programme material. When the house sounds as if it is being destroyed around my ears the most important question (for me at least) is whether my 4 x 15" IB drivers are extending themselves beyond their safe excursion limits. (A bit like my sentences I suppose)

[collo]

I've wondered about this too. It would be reassuring to know how far my drivers are being pushed.

A laser shone onto a mirror that is at 45degrees would allow the beam to fall onto a fixed scale at the side.



The mirror could be held on with some blue-tac or some hot melt glue.

[ISLAND1000]

Geez, that almost looks too simple.

[shr-t]

I was thinking something similar to what collo suggested but slightly different. If you happen to have a woofer with a level part in the middle, like this one, you could just attach a thin mirror (aluminium foil might work) to the level dustcap and point a laser to that mirror at an angle. Then read the laser dot from a scale on the other side. Determining the excursion in mm from the reading you get from the scale is just geometry. See attachment for a quick-and-dirty scetch of what I had in mind..

[Chrisbee]

shr-t

I think your mirror would have to be large to remain in the beam at any reasonable excursion. Your target would have to be translucent or transparent to make the beam visible. The beam would then be capable of eye damage.

Collo's idea has more merit in that it will record any excursion. The downside is having to attach an extended object to the dustcap. This makes the driver and reflector stalk too vulnerable. IMO.

[shr-t]

Chrisbee wrote:

I think your mirror would have to be large to remain in the beam at any reasonable excursion.

Depends on what you consider large, at a 45 degree angle to the laser the mirror diameter will have to be at least the same as the excursion to be measured. So for +-20mm, you'd need a mirror that is at least 40mm in one direction.

Chrisbee wrote:

Your target would have to be translucent or transparent to make the beam visible. The beam would then be capable of eye damage.

Well not really, the scale can (in theory) be any distance from the sub so one can fit between them to read the scale.

Chrisbee wrote:

Collo's idea has more merit in that it will record any excursion. The downside is having to attach an extended object to the dustcap. This makes the driver and reflector stalk too vulnerable. IMO.

Fix the laser at a 45 degree angle and the scale perpendicular to the mirror and you'll get a reading that is double the excursion. Just divide the number by two and you get your result.

I drew it on paper to see if it works. Here's it:

[Chrisbee]

Perhaps 50mm mirrors will become the norm on subwoofer drivers? 25mm Xmax?

The theory is good but the mirror is just too large for normal drivers with conventional dustcaps. A narrow strip of mirror would do but what would it be made of to reflect a narrow beam with any accuracy onto a target? It would require flatness and any flexure would make it impractical. Glass would probably modify the driver's behaviour due to its weight and might be a potential risk if it detached itself. I suppose space blanket foil could be glued to a suitable cone shape. There is a trend towards bowl shaped driver cones with inverted dustcaps so the potential is there to apply a reflective foil directly. Mirror polished aluminium cones are not impossible?

[Bent]

how about a strip[ of reflective aluminum tape> the kind the tin bashers use...
you could put a peice on the dustcap, or cone.

[alan monro]

Why make a simple measurement difficult ???? With the speaker at rest, put a piece of wood across the frame at front . With a piece of paper mark off the distance between the wood and dust cover , power your speaker to your maximum , using the marked paper , slip it down so that you can just hear the dust cover touching the paper , mark it , measure it . Remember there is a out and in movement in respect to the speaker at rest whilst measuring a sign wave . We are measuring the out movement that should be the same as the in movement , therefor , multiply your measurement by two and this is your cone excursion at that frequency . K ind regards,Alan.

[Chrisbee]

alan

You must have been in the rest room when we were discussing dynamic excursion.

We (I) want to monitor maximum excursion on real programme material. (DVD films)

[alan monro]

Crisbee , With my superbly great intellect and intelligence , I can work out that my method (being the best of course) applies to all discussed instances . Also , some of my best ideas occur in the rest room . Kind regards , Alan.

[Chrisbee]

alan monro wrote:

Crisbee , With my superbly great intellect and intelligence , I can work out that my method (being the best of course) applies to all discussed instances . Also , some of my best ideas occur in the rest room . Kind regards , Alan.

Dear Solomon,

Since I do not share your marvellous gifts would you be kind enough to explain how your method does any more than check Xdestruct? (not to mention the considerable increase in noise to signal ratio as it does so) With the greatest respect, Sir, where is the flexibility in your method to monitor lesser excursions? I fear a steady state sinewave measurement does not allow for the most common of LFE dynamic peaks. (explosions et al)

Yours in awe etc, etc,
Chrisbee

[Blaser]

alan monro wrote:

some of my best ideas occur in the rest room.

[Chrisbee]

alan monro wrote:

some of my best ideas occur in the rest room . Kind regards , Alan.

That'll be the rays they beam down from The Large Magellanic Cloud in outer space!

They get concentrated by the small dimensions of the typical Ozzy outie.

I solved that problem with a cooking foil hat with an angle of exactly 55 degrees!

The foil must work because I haven't had a single good idea ever since.

QED!

[phaseshift]

Actually, the laser comments are correct. What I use in the lab is something similar to this- http://www.keyence.com/products/vision/laser/lc/lc.php

It was cheap, about $6K for the whole setup. Maybe another $2K for the fixtures for the driver and the laser head and you are set. If you do your setup solidly enough, you can scan the laser across the cone (or dome) surface and get a confirmation of modal issues, breakup etc... Pretty useful, especially on a woofer or mid that is giving you fits and does not line up with your FEA.

I would not go too crazy about adding a mirror or anything other than maybe a spot of correction fluid to the DC as the mass of a mirror or something like that will impact your mechanical model and therefore the excursion to some degree. A little white dot is all it takes even with a low end displacement sensor and with the later stuff, you can measure more accurately than you really need if all you are looking for is simple displacement. You could also find this second hand or a lower cost, lower resolution version for maybe $1K if you look.

[tenzip]

OK, are we trying to dynamically measure excursion at all points of the programme, or just max excursion?

If it's just max excursion, get yourself a 1" wide strip of cardboard, and punch a hole in the middle with a paper punch or whatever method you pre