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Old 11th April 2003, 11:16 AM   #11
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A fairly easy and cheap accelerometer can be made by using a piezo disk as found in those “music making” postcards. You need to connect a fet buffer as source follower close to it. A BF245 will do the trick. A G-S resistance between 10M an 100M and a source resistance of 10K and a 9V battery, e voila. Glue the disk to a fiber ring from the toolbox of your local plumber and stick it with double sided adhesive tape to the place to be measured. Hook it up to your soundcard and some FFT software will do the rest.

These piezo’s measure acceleration from very low frequencies up to fs (usually 3 KHz).

Cheers
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Old 11th April 2003, 11:35 AM   #12
F.Ultra is offline F.Ultra  Sweden
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Perhaps stupid , but wouldn't it be possible to put fine grains of sand on the cabinet (with the speaker lying down) and observe how much and where the graind would move the most ? Where the grain would move the most would be where you should place the bracing since that would indicate a resonance node.

Just my silly thoughts
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Old 11th April 2003, 12:11 PM   #13
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Quote:
You can also use a phono cartridge.

jonathan carr
Hi Jonathan, this one could get quite expensive if we'd take one of your's !


An alternative to an accelerometer is a stroboscope, triggered by the driving signal, which makes the vibrations visible.

Regards

Charles
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Old 11th April 2003, 12:39 PM   #14
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Thumbs up !

hey i find this very interesting! i have always wondered how to cheaply measure box resonances and effects,
and to find out how much sound is added from my subs from the box not just speaker harmonics!!!



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Old 11th April 2003, 06:13 PM   #15
jcarr is offline jcarr  United States
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Charles: No, you don't need anything fancy. In fact, I would suggest an MM or high-output MC with a fairly flat frequency response. Just don't get it too close to the speaker drivers

The tonearm can be cheap as well, since the cartridge is not required to move very much (apart being able to cue the cartridge onto the point on the cabinet that you want to measure).

Really, a cartridge works quite well for measuring cabinet vibrations - you should try it.

jonathan carr
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Old 12th April 2003, 12:08 AM   #16
Kanga is offline Kanga  Australia
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Thanks for all the further suggestions.

Re the sand idea - I've seen an article done about using this technique for assessing the nodal characteristics of violins. I think that this technique would be good for seeing where the nodes are, however getting quantitative measurements could be difficult.

Quote:
A fairly easy and cheap accelerometer can be made by using a piezo disk as found in those “music making” postcards.
Pjotr - sounds good, if you have more of an understanding of electronics than I do What FFT software do you know of that is suitable (preferably freeware).

Quote:
An alternative to an accelerometer is a stroboscope, triggered by the driving signal, which makes the vibrations visible.
Charles - I don't have easy access to a strobe, but I'm still interested. Can you explain a bit more how this works. Do you need some kind of reflective surface on the vibrating surface?

Quote:
Really, a cartridge works quite well for measuring cabinet vibrations - you should try it.
Jcarr - how do you arrange the tonearm? I assume that the measured surface needs to be horizontal, and you have a tonearm attached to the edge of the cabinet using bluetak. Unfortunately I sold my turntable a while ago, but I may be able to buy a cheap second-hand one.

Mick
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Old 12th April 2003, 12:39 AM   #17
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cooledit 2000 does FFT !

i have a full version,its useful.

similar to oscilscope also!

perhaps the demo does it too.

i used it to measure the bass information in my fav songs before deciding what my sub should go down to in hz.

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Old 12th April 2003, 05:07 AM   #18
TheoM is offline TheoM  United States
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Default In the cab, on the cab, from the cab

I have done a lot of work with the 25 dollar contact mics which are used as guitar pickups for acoustic guitars. I use them to measure the resonance of wood for designing solid body electrics.

I'm not sure that in this case the contact mic, piezo or otherwise, will measure what you need to measure. Certainly it measures sound traveling inside the wood - but I'm not sure that sound gets transmittted to the air from the cab wall since the wall would have to vibrate in such a way as to move air and that's not what the contact mic will measure. I'm still a bit confused about this - in the wood vs being transmitted to the air... but lets assume for a minute that what you really want to measure is vibration that makes it into the air.

I might suggest measuring how the cab moves the air close to it by using a regular microphone and a spectrum analyzer in your pc. The issue here (and in the other case) would be to subtract out the sound in the environment produced from the front of the speaker. I might do this by measuring a sweep at the front of the driver, plotting it as a gain normalized reference, and then comparing a similarly normalized curve measuring the box from a distance of like 1 mm. If you did this outdoors (anechoic), you would see that the two curves were different - and the high spots on the box measurement would be the places you had resonance problems in the box which were being transmitted to the air.

Seems like a pain but all you need is a mic, and shareware. I've never done this, but it seems like it would get you the result you are looking for.
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Old 13th April 2003, 05:51 AM   #19
Kanga is offline Kanga  Australia
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TheoM - thanks for your suggestions. From graphs out of the loudspeaker cookbook, it looks like cabinet vibration levels are 60 - 70dB down from the acoustics levels, so unfortunately while your microphone idea sounds appealing, it might be difficult to separate the cabinet vibration noise from the sound coming directly from the driver. ie the same problem I had with my funnel.

With cabinet vibrations, the type of vibration obviously will affect the sound being generated. You could have vibration perpendicular to the surface, which will generate sound, or parallel to the surface, which won't. You can also have surface waves (ie like ocean waves) which will generate sound too. If the cabinet is vibrating in a higher order mode, then one side of the cabinet may be going in while the other may be going out. This may produce sound that cancels itself out, depending on where you are located. So measurement location would be important here.

I think overall that the accelerometer or guitar pickup would be a good way to measure, even though it may not tell the whole story.

Mick
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Old 13th April 2003, 02:20 PM   #20
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Hi Mick,

The FFT software I use is Audiotester. It is fairly simple to use and contains a decent signal generator. The free trial version is full functioning but is limited to 20 min. runs. You can also make waterfall plots with it (3D FFT’s). This is very handy when investigating cabinet resonances. It gives you an idea of the decay time of the resonances and not only of the freq. peaks. It is the cheapest program I’ve found that does the job quite well. Here a tread of a discussion about freeware and shareware FFT programs.

------------------------------------

Some time ago I did investigate optimal damping between the LS box and the wooden floor. For that I needed also an accelerometer to see how much vibration went to the floor.

I experimented with low-cost piezo guitar pick-ups from Ibanez. They worked but the frequency response was not low enough, these ended somewhere around 50 Hz. Much depends on the input impedance of the amplifier. The higher it is the lower the freq. response will go. But I got problems with hum.

Also a cheap omni-directional electret microphone capsule worked well when glued to a small metal disk, upside down so the hole to the membrane is closed. But they were a bit too noisy and did not go below 20 Hz.

So I made my own accelerometer with a low-cost 20mm round piezo disk. Attached a sketch how I did it. The disk is mounted between fiber rings. The rings were mounted between 2 disks made of bare epoxy PCB material. All was glued together with 5 min. epoxy glue. On top of the top plate is the fet buffer mounted. This was hardwired and glued with 5 min. epoxy on top. The buffer was finally shielded with 50 um copper foil connected to the ground. The ones I made went far below 1 Hz and had a fairly high output.

Many electronic part shops sell these disk bare fore around 1$ or hidden in piezo-buzzers. Also a disk from an old Motorola piezo tweeter is useful and the already mentioned piezo’s from those music postcards.
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