crazy question

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crazy question

Postby Mark » Fri Jan 28, 2011 4:17 am

I am trying to make a speaker bounce up and down whilst suspended in a space. I have the speaker, I have the amp, I have the speaker suspended on rubber bands... but when I drive a signal through it remains (almost) static, even at maximum excursion... which I am guessing is what it's designed to do really... does anyone have any ideas on how to modify this driver so that it will bounce whilst suspended on rubber bands in a space please... (it's for an art project and sound quality isnt an issue really)
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Re: crazy question

Postby Klaus Stock » Tue Feb 01, 2011 4:41 pm

Hi Mark,

Mark wrote:I am trying to make a speaker bounce up and down whilst suspended in a space. I have the speaker, I have the amp, I have the speaker suspended on rubber bands... but when I drive a signal through it remains (almost) static, even at maximum excursion... which I am guessing is what it's designed to do really... does anyone have any ideas on how to modify this driver so that it will bounce whilst suspended on rubber bands in a space please... (it's for an art project and sound quality isnt an issue really)


several factors prevent your system from behaving as desired.

First, there's the resonant frequency of the box-spring system. You might try to stretch the rubber bands by pulling the box down by hand, then release it. It will probably bounce with a period of between 0.2 and 1 second, equivalent to a resonant frequency of between 5 Hz and 1 Hz. A speaker which can reproduce such low frequencies at significant SPL would be too heavy for your application. Plus, the additional weight would mean an even lower resonant freqeuncy, if you want to stick to rubber bands ("bugee ropes", in that case).

Second, the ratio between the moving mass of the driver anf the air is probably much lower that the mass of of the total system. You might try to add mass to your driver. That'll also lower it's resonant frequency and it's efficiency. It may also pre-load the suspension, thereby reducing the possible excursion and probably damaging the driver.


One workaround might be an "acoustic rectifier", which allows fast air movements (produced by the driver) to escape only from the speaker. The resulting vacuum inside the box will have to be taken care of by a vent which allows for slower air movement back into the box. I once witnessed such an effect when a speaker tore itself apart. I estimate that this happend at an SPL of more than 140dB/1m. I suspect that such an SPL, you'll witness how your installation will forcefully be "bounced" out of any art gallery where you try it ;-)


So, who not try "cheating" instead?

Install a small DC motor in your box. Connect it to the speaker wires via a rectifier (or an simple diode, but this will be less efficient). Now attach an arm to the axis of the motor, pointing sidewards. Attach some mass to the end of the arm. When an acoustic signal gets applied to the speaker, the motor will begin to turn. The off-axis mass on the arm will cause the box to react. You might experiment with different arm lengths and masses.

This construction wass used to modulate light effects according to sound. Typically, the motor was attached to a mirror which was then shaken when the motot spun. With the motor's speed proportional to a music signal, it would appear the the light "respnds" to the music. Yup, primitive.

You might also consider to use a more direct approach, by attaching the arm directly to the rubber ropes. You can also use a stereo amp and connect driver and motor to different channels, thereby allowing you to alter motor vibration and sound level independently.


Best regards, Llaus
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Re: crazy question

Postby Mark » Tue Feb 01, 2011 11:34 pm

Hi Laus,

many thanks for your informative reply. I suspect you are correct with regards to the the moving mass of the driver and the air being much lower than the mass of the total system; I have tried your suggestion of adding mass to the driver, but without success (and possible damage to the driver in question)... being a visual artist, I am wondering if there is somewhere I can go to view a diagram of the dc motor setup you describe under cheating?

Mark wrote: Install a small DC motor in your box. Connect it to the speaker wires via a rectifier (or an simple diode, but this will be less efficient). Now attach an arm to the axis of the motor, pointing sidewards. Attach some mass to the end of the arm. When an acoustic signal gets applied to the speaker, the motor will begin to turn. The off-axis mass on the arm will cause the box to react. You might experiment with different arm lengths and masses.


Kind regards
Mark
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Re: crazy question

Postby Speedskater » Wed Feb 02, 2011 1:34 pm

What direction is the speaker driver facing? Top or bottom would be best.

Maybe one of those chair shaker driver's would get it going.

Boating stores have several sizes of very stretchy shock cord.
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Re: crazy question

Postby Klaus Stock » Wed Feb 02, 2011 1:56 pm

Hi Mark,
Mark wrote:Hi Laus,

okay, my mistake ;-)

Mark wrote:being a visual artist, I am wondering if there is somewhere I can go to view a diagram of the dc motor setup you describe under cheating?

I have no picture of the mechnical setup. The last time I've such a construction was probably in the early 1990s, in a "Moon Flower" type disco light effect. However, we decided to remove the "vibrator" from the device when the springs were worn out (the mirror assembly was getting more and more tilted down, until the reflected light beams missed the front lens alltogether and the device went dark). So, while the light effect is probably still sitting on some shelf in the warehouse, so no chance of getting a photo of the relevant part. Nope, I didn't find a photo on the internet either, I guess that none of the manufacturerers of cheap lighting effects want a prospective buyer to see the amazing low-tech internals of their products ;)

However, it's all very simple.

First, the rectifier: I suggest that you'll get an integrated bridge rectifier; no need to solder one yourself from four diodes to save 5 cents or so. I suspect that a B80C800 would be sufficient. B80 means that it can withstand 80V peak voltage, C800 means that it'll cope with a current of 800mA (0.8A). This should be sufficient for the voltage which a home amp outputs, and the current which a small motor consumes. Here's a picture of a B250C1500: http://www.reichelt.de/?ACTION=3;ARTICLE=4620;PROVID=2402. That one will cope even with higher-powered PA amps and also with 1500mA (1.5A). It might cost a few cent more that the B80C800...well, okay, at Reichelt it's actually cheaper than the less potent one. More voltage and current rating is never a mistake, but you probably want to stay away from very heavy units with 30A rating... Rectifiers come in various forms and sizes, so your local supplier might have compatible units which look different. However, all will have four pins, two labelled with a "wave" (~), one labelled with a plus (+) and one with a minus (-). Some may carry the letters "AC" instead of the wave symbol. Anyhow, the (+) of the rectifier goes to the (+) of the motor and the rectirifer (-) to the motor (-). The remaining two terminals (wave/AC) are connected to the power source (the two speaker wires). Really simple. Yup, you may note that your speaker wires/terminals are also labelled with plus and minus (or maybe not), but they really carry AC. Plus and minus on the speaker wires/terminals are only there to help you with connecting all speakers "in phase"; it has nothing to do with plus/minus polarity as known with DC.

Next, the motor. My first thought was something like a motor from a cassette recorder; the ones I remember from my childhood ran with something like 6V DC and they had a disc at the end of their axis, to which an arm could be glued. There may be othe types, though. Turntable motors can a line voltage devices; these won't work for the application in question. A toy motor (like from Lego) might also be an option.

Another motor option might be a case fan, like this one: http://www.reichelt.de/?ACTION=3;ARTICLE=45608;PROVID=2402. It's a 12V DC fan meant to provide airflow in a computer case. Such case fans may come with a two, three or four pole connector. Two wires are for power (typically red/black for +/-), additional wires for a tacho signal and PWM control (can be ignored). If the fan has a moley connector, you might be lucky and the +/- pins from the rectifier can be directly plugged in. Using such a case fan as a motor has an additional advantage as it provides convinient mounting holes and is often relatively light. A serious disadvantage is that these motors typically have very little torque, meaning that they'll take quite long to spin up, especially when mass-loaded. They might not start spinning at all if you choose a direct connection to the rubber rope. However, if you have such a fan lying around, it might be worth a try. You might try to attach your mass to one of the fan blades. If it looks like this going into the right direction, you might experiment with an arm gued to the center piece of the rotor, maybe even cutting off the fan blades so the motor only has to move the mass and not the air.

Powering a fan via recfigier from the music signal is not totally unhead of: some well-known PA speaker manufacturer (I think it was Community) did this in order to provide cooling to the tweeters. Nope, sorry - only cooling, no bouncing ;). A typically Community speaker is so heavy, if it bounces on the floor, it might drop through directly into the basement ;)

Best regards, Klaus
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Re: crazy question

Postby Mark » Thu Feb 03, 2011 8:55 am

Hi Klaus,

I though I'd try experimenting with a different weight speaker and tension bands before I attempted the wiring up of an ancillary motor to this thing. (but I do understand your instructions, thanks again)

You can view some rough video of a (33lb) Exodus Tempest X2 suspended on a metre long 16mm Blackdog power band (Speargun rubber), playing a fadein/fadeout low frequency sweep at the following utube link

http://www.youtube.com/watch?v=U1eBfjzi3ao

... maybe I got lucky with the power to weight/suspension tension this time, because it bounces along quite nicely.

Now I need to work up some sort of frame to install this in!!!

Cheers
Mark
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Re: crazy question

Postby Klaus Stock » Fri Feb 04, 2011 6:23 pm

HI Mark,

indeed, that looks impressive. I suspect that some non-linearities provide enough demodulation to start the "bounce". So the singal's envelope (the fade-in/-out sweep) comes into effect.

Nice job! Thanks for the video!

Mark wrote:Now I need to work up some sort of frame to install this in!!!


Around here (Cologne) 50mm (~2 inch) aluminium pipe is readily available. It comes in lengths of 6 meters and can be easily cut to smaller lengths. Some dealers can also bend these pipes. Now, you probably won't want to get into some Bauhaus design for your frame, especially since it's not very rigid and will act like a spring on it's own, but perhaps it'll provide some inspiriation. It don't anything about prices of aluminium pipes; all I know that there's a stock in our warehouse if I ever need some ;) We use such pipe to suspend speakers or lights (or decoration), in addition to "real truss".

Best regards, Klaus
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Re: crazy question

Postby Mark » Sat Feb 12, 2011 3:07 am

Thanks for your suggestions Klaus... The Bauhaus school IS useful as a springboard for construction ideas.

Regards
Mark
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