Official Luthiers Forum!

You know what would really help is if you all that are doing this could give us newbies a list of what equipment you are using and part #s and where we can buy them. Are are you all using the Bradley generator? What speaker are you all using? What amp are you using? etc. I've been screwing around for 2 days now trying to find speakers and looking for used amps and then not being sure if they will work. Jody bought a speaker and now you say it might not be putting out enough? And how about some pictures of you setup? So far only Darryl has described his setup that he got for free and it works but maybe he could give us the name of the computor speakers and the 4" speaker he has. How about a little help here?

I use a laptop to generate the sine wave. (an Itouch app might be better)

I've used, a fender amp with the speaker removed and held in my hand. I didn't like that at all but it worked.

I find a fairly small self powered computer speaker works fine. You don't need a big speaker, in fact a smallish speaker is better because you can direct the power to the part of the top that wants to move. That's also the reason why handheld is better than fixed underneath.

I use the little foam pads that my wife uses to remove makeup for the supports. I find that the position of the supports is important. Just like tap tunning, if you don't hold the top at the node you don't get a clear tap. If I don't put the pads on a node I don't get a clear pattern and need more power to drive the top. I really don't like using a lot of power because pure sine wavs are really bad on the ears and standard ear protection doesn't handle low frequencies very well.

I use a metalic glitter. It makes a mess, pretty much forever, but it's what I remember Al using.

You also need to find the right frequencies. Being able to do all of this takes some skill. Al is good at this and he makes it look easy. The students that I watched do this didn't find it as easy to do as Al makes it look.

One thing that I like to do is record a tap of the plate and find the resonance frequencies. That saves some time finding the right frequency. But I can also imagine that, especially if you are new to this that would just confuse and complicate things.

Most important, protect your ears. I don't feel that I can say that enough.



Interesting, I never saw it done braces without plates.

I just laid a small end table upside down and pulled some rubber bands across the legs. Laid my little JBL box speaker that has a 4" speaker in it facing up and at the bottom of my unbraced plate suspended on the rubber bands. I had my computer hooked into the TV input of my reciever and gave it a go. I started to get a pattern some what like the 3 horizontal pattern and I turned up the freq. and the volume some and started to get another pattern and then started to smell something burning. I turned down the volume and noticed some smoke from the speaker. Unplugged the speaker leads from the speaker an took it out into the shop. I'm not to worried about the speaker because I think it might have had a crack in it as I wasn't using it because it made a nasty scratching sound with a low bass sound when watching DVD's. I have my old Boses 901 on the system now. But when I switch back to A speakers the right side is now dead. Bummer, maybe there's a fuse that got blown but I don't know if there is one inside or not. What's the deal with that?

Now that's not right. What I meant to say is that I never saw it done on guitar shaped plates without braces. Is there a reason for doing this?



I don't know what's up with your speakers but there is a known "characteristic" of old speakers where the foam that holds the cone in place decays over time, it's like it dry rots in place. It's normal. When it happens you get a scratchy sound. Eventually the cone has nothing to hold it in place. Strong sine waves can't help this. Of course, I don't know if this applies to your situation or not. Hang in there, it sounds like you're making progress.

Well here's the picture of the speaker out of the box and there isn't anything at the edge. These are only 9 years old. Is that considered old? I might as well open up it's mate and see what's going on with it.
I was checking out the reciever and I have it sitting on top of the 901 and when I spun the speaker to get to the back of the reciever I must have loosened the wire as it came off the speaker terminal.
That's a relief, I really was getting pissed about thinking I blew the right channel.
So John( that's my middle name) you're saying that I should be able to go to Walmart and buy the 35 watt computer speaker system and get it to do the job?
Any one know why is smoked?

Seems like I read, somewhere, that doing sine wave stuff on a speaker is really hard on it. Stereo speakers are designed to handle music, not hard wave stuff. There is a bit of a difference. I could be wrong though.

Also, most stereo speakers are pretty wimpy. They are usually protected by some kind of crossover system, and other speakers mixed into the system. A guitar speaker would be a little more sturdy.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Yep, that looks like foam rot. From your description of the DVD sound it was well on it's way, the sin wave just put it out of your misery. 9 year, that's on the young side, i guess. My speaker was 20 years old when it happened but everyone said that was way on the old side.

I used the computer speaker because I had it sitting around doing nothing and it worked well enough for me. If I had to buy something from Walmart I would get something like;

http://www.walmart.com/catalog/product.do?product_id=11968793

You don't need the duel cone, you could use something cheaper. As I remember it, there was nothing too fancy about Al's speaker. Think of the speaker as a fuse for your ears, you'd rather it blow out first.

I use the amp you have for a trial. Later if you want you can replace it when you have a better idea of what works best for you.

Although I'm sorry you're having these problems, I think it's very helpful at teaching everyone to have a healthy respect for what they're dealing with here.

Well Alan is using a 30 watt Radio shack mid range speaker in the DVD. 8 ohm.
I'm wondering if the speaker that Jody bought would be the one to use. And there is a cheaper on then that. I'm going to keep my eyes open for a used reciever. I could use a little better sound system in the shop so I might as well use it to tune the tops. I can get a brand new 100w per channel reciever for $125. So maybe I'll just use system until I get back to work and can afford something. And there's always my Crown IC 150 and D 150 amp. and preamp. that's sitting there doing nothing at the moment.
http://www.parts-express.com/pe/showdet ... er=290-372

Used to go to Radio Shack a lot for electronic pieces and parts but I just looked at all of their speakers on the web site and didn't see anything suitable. Maybe they still have some inexpensive replacement speakers in the store?

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Steve Smith
"Music is what feelings sound like"

Some of the midrange speakers don't go much below 500Hz. I'd be looking for something like these that have a low frequency response. I found these at http://www.simplyspeakers.com/14replacements.htm. I've never ordered from them so don't know anything else about them but I'm sure there are others out there.


Wow, somewhere in there I became a Koa guy

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Steve Smith
"Music is what feelings sound like"

That top speaker is the same one that Jody bought. And it only has a 30hz. lower rating then the $10 one I posted.

I missed Jody's link. I'd go with the $10 one for sure, at least to start.

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Steve Smith
"Music is what feelings sound like"

Yeah, I don't want to smoke the $20 speaker!
Jody's was $26 so you link is a bit cheaper. That would make up for the shipping.

well I would say the amp I am using is suspect.I have a 6x125 watt surround amp, but I realy dont want to use it for this, radio shack has a 2oo watt reciever for $99.99 , they say 100 watts per channel,I asked the guy how many watts would go to each speaker , he said about 50, that should be enough . I think I will wait a few days and buy that, especialy since it has the turntable pre amp , it would be nice to pull out some of the old vinyl and listen to it again. Plus I feel better about spending $100.00 on something new rather than spending money on used electronics,especialy cheap electronics , at a yard sale.Thanks Jody

That sounds like the kind of thing I remember. A plain old speaker. You would think it couldn't get a low enough bass but it really does work. I remember trying a 12" guitar speaker, now that could get bass, but it was too big to get the power where you wanted it, also, if you need that much power, you're doing something you shoudn't be doing.

If you get the ohm rating within a factor of 2 you'll be fine. If you have an old boom box lying around ripping the speaker out of that would work. The boom box could be you amp too.

I already tried that with my old sony that's in my shop for sound now and it didn't work for me. We need to give some concrete direction here and no more of the probably will work stuff. That's can be costly and very time consuming and very frustrating. This is my 3rd. day into chasing the might work and probably. I'm on my way to get a speaker locally that I think will work and checking out some more thrift stores for a reciever.
http://speakerex.com/5300.html
I'll let you all know if it works.

Define too big please.

The easiest way to drive a top is with a hand held speaker. I use a 5" car speaker I picked up at WalMart for around $20. I connect the speaker with a 4 foot cord to the output of an old Crate guitar amp (25W). The signal generator feeds into the guitar amp input.

• Place your top on four foam pyramids about 2 inches tall and 1-1/2 inches square at the base. I use the spongy foam packing material. Place the four pyramids in the four corners of the top.
• sprinkle glitter over the top.
• With one hand, hold the speaker about 1/2 inch above the center of the lower bout.
• With the other hand, control the signal generator. Start at about 60Hz and raise the volume to the level just below where it starts to distort. This is quite loud and you should be wearing hearing protection.
• Raise the frequency slowly until you see the glitter start to vibrate.
• Fine tune the frequency until you get the maximum vibration.
• Move the speaker around the top keeping it about 1/2 inch above.
• You should start to see lines of glitter form.
• Where a line of glitter reaches the edge of the top plate, move one of the foam pyramids under that spot.
• Once your pyramids are placed, move the speaker all around the top to get the best lines.
• Draw the pattern you see in your log book and note the frequency.
• sprinkle new glitter or redistribute the old glitter evenly over the top.
• Slowly raise the frequency until you see the glitter vibrate again and repeat the process. You will have to move the pyramids around for each frequency and some will be much more active than others.

I hang my speaker from a spring on the shelf overhead, so the speaker floats about 6" above the top when I'm not using it. I let it float between note-taking, and it's easier to pull it down than to pick it up and hold it up when I go in search of the next mode.

Pat

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formerly known around here as burbank
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http://www.patfosterguitars.com

The boom box technique worked for me when I was in a pinch once. I don't give advice that I haven't tried, Chris.

Mike Maher;s advice is good. That works. Like I said before, there is a bit of skill and trial and error involved with this.



Anything over 6" is too big. But this is opinion, not fact, like I said, I used a 12" guitar speaker for a time- it's not the way to go.

Hang in there, you'll get it.

Maybe when Al gets back he can "approve" of a low cost set of componets to make this easier.

The thing is that not every boom box is the same as far as wattage goes. That's pretty general. I was just at the speaker store and they had a signal generator that they connected to my spaeker that was driven by a 20 watt pa amp. with 2 channels.
I'm wondering if this would work but I can't tell if the output is 4 or 8 ohms.
http://cgi.ebay.com/Pyle---Mini-2x75W-S ... 35004r3666

The 'ohms' rating on a speaker, or amp, refers to the impedance of the unit. Ohms is the measure of resistance in a DC circuit: how much pressure (voltage) it will take to get a certain amount of current to flow through it. With alternating current, which the signal from the amp is, things get a little more complicated, so they talk about 'impedance', but the measurement is still given in ohms.

You've GOT to match the impedance of the speaker and the amp to get good performance. If the speaker impedance is much higher than that of the amp, there won't be much current flowing in the circuit, and you won't hear anything. If the speaker impedance is too low, there will be too much current through the power amplifier and you could blow it. Ask me how I know.....

Automotive stuff is often rated at 4 ohms, while home audio used to all be eight.

With the right impedance you should be able to blow your eardrums nicely with 12 watts or so, and get nice patterns on a free plate. In fact, it usually doesn't take that much power for free plates, but you might want more if you're going to look at the patterns on the assembled instrument.

One problem with big speakers is that they tend to overlap the boundaries of the antinodal areas of the modes. Remember that the phase of the motion reverses when you cross a node line, so that driving on a line will cause the driving force to cancel out and give you no motion, no matter how much sound you've got. That's why I go with the smallest long-throw woofer or mid-range speaker I can find that will handle the power. Ideally you'd like to drive at a point, but in practice a 4" speaker works pretty well for most of the stuff we do. RadShack doesn't carry speakers any more, and what I'm going to do when the ones I have blow out is a quandary.

The biggest problem for beginners always seems to be finding modes. Until you get some practice they can be pretty ellusive. One thing I'd suggest would be to get a piece of expanded styrene bead board and go looking for the modes on that. Your guitar top will probably be different from mine, but bead board is bead board. The only caveat here is that the closed cell insulation foam that you get at the lumber yard won't work as well: they produce that stuff through rollers, and that gives it a grain that makes it stiffer in one direction than the other. The same is true of the cardboard faced foam board you get at Staples, and Masonite. Plexi is pretty good, but can be hard to drive, and glass has such low damping that you can skate right past a mode without seeing it. Besides, foam board is so much easier to cut up! Bead board is pretty easy to tell by looking at it, and it's almost always cut to size with a hot wire that leaves a telltale pattern, sort of like bandsaw marks.

I got the speaker I posted above and connected it up to my home stereo and did a little practice. At 70hz. I could't turn up the volume to much before the speaker started to chatter like. But as I increased the hz. I could get the speaker volume louder. I didn't max it out and as far as volume but I noticed the magnet was starting to get warm so I shut it down. Is the heat from over pushing the speaker? It wasn't really very load but I could see where it would mess up you ears without plugs or muffs. I stopped by and upolstery place on the way bak and got a scrap piece of 1" X 3" X 20" foam to make the legs from.

Amps can easily handle a factor of two of impedance mismatch, Al.

By the way. I finally got my DVD today.