Testing cabinet vibrations

[Kanga]

I fired up my new bass cabinets yesterday, but with no stuffing yet, and the crossbrace not yet glued in. They're designed to handle frequencies up to 200 Hz using a 10" Scan Speak, and just for interest I tried them running full range with the tweeter of my AE1s handling 3kHz upwards. Surprisingly they don't sound too bad, although the midrange is not that great.

Before I glue the cross brace in and add the stuffing I'd like to do some measurements on the cabinet vibration to check on the effectiveness of the cross brace (see previous post "cross brace not working") and to check for any other problem areas. I don't want to spend much money here, so I was thinking of just getting a stick on guitar pickup (about AUD$25) to act as a cheap accelerometer, and use my multimeter to measure the voltage produced.

Does anyone have any other suggestions for easy and cheap ways of measuring cabinet vibration?

Mick

[Michael Rothacher]

Mick:

Digikey sells an accelerometer for $20 that is very good for speaker testing. The part # is MSP1001-ND. You'll probably need to boost the output with a mic preamp or similar amplifier.

Good luck!

Mike

[Kanga]

Thanks for the suggestion Mike. Unfortunately Digi-key is not in Australia, and while I could import this will probably double the cost. Also, this unit doesn't look like it could be easily attached and detached, and needs and external power supply etc...

Bottom line - a bit too difficult.

Any other simple suggestions welcome

Mick

[Jean]

Kanga,

What scanspeak 10' driver did you use, was it 8565-01 ? If so, how big is your box and tuning ?

tia

[SY]

I didn't check out Mike's reco, but if it's a PVDF film accelerometer, it will work great. Use a thin double-sided adhesive to attach it to the cabinet walls. Be a sport, borrow a scope to use instead of that multimeter. Better yet, use an impulse/MLS.

[jcarr]

You can also use a phono cartridge.

jonathan carr

[Kanga]

Thanks for the further suggestions.

I really was hoping for something fairly simple and cheap, just to get some kind of quantitative measurements rather than "that one feels like its vibrating more than the other one". Going for a proper accelerometer and scope is a bit more involved that I'm prepared to go in for at the moment. Maybe if I have to order something from Digikey in the future I'll get the accelerometer then.

Jean - I used the 25W- 8565-01 in a 50 litre nominal enclosure (less a bit of litre or two for bracing etc.) in a sealed arrangement. Without stuffing this will give a Q of around 0.8, but once I add stuffing this should lower the Q a bit.

I'll give more details once everything is finished, including the active XO etc, which should be later next week.

Mick

[SY]

For simple and cheap, use a stethoscope.

[Kanga]

Sy

You've given me an idea based on the stethoscope suggestion. I already have a tandy (radio shack) sound meter which has the mic capsule at the end of an extension tube. If I can get a funnel with some kind of foam seal around the rim, and hook this up to the capsule via a short piece of flexible tube, I will have a primitive stethoscope which will give quantitative readings in dB!

The funnel can be lightly pressed against the vibrating surface and easily moved from one surface to another.

Mick

[Kanga]

I tried the funnel idea, but unfortunately it didn't work. The funnel seemed to be picking up mostly sound and not much vibration. If I had a reading of 100 dB with the funnel on the cabinet, and then took the funnel off, then the reading might still be 98 dB. These sound like really high readings, but remember that the funnel is acting like a horn.

I really want that accelerometer to play with but just can't justify it at the moment just for "research". I want to get the braces glued in and stuffing added so I can get listening.

Preliminary nearfield measurements with the sound meter showed a -3dB point of around 40 HZ, just as the formulas predict. Theory really does work sometimes....

Mick