Using LAUD with ===ACCELEROMETERS

We are asked from time to time about how LAUD might be used with an accelerometer to measure loudspeaker cabinet vibrations. An accelerometer is used to detect vibrations induced on its body, rather than those propagated in air. The process of using an accelerometer with LAUD is quite simple.

Figure 1: Waterfall plot of cabinet vibration on baffle of small speaker - this one has a significant resonance at 220Hz and 310Hz
If the accelerometer is electrically compatible with the LAUD input circuitry's impedance and sensitivity, you can use the accelerometer essentially the same way as you would a microphone. Just feed the accelerometer's signal to where your microphone normally connects and drive the speaker with a stimulus signal (with the cal probes sensing the speaker's drive signal). Measure the result from the accelerometer mounted on the cabinet face you are investigating, rather than from a microphone placed in front of the speaker as you would normally do. You are in effect measuring a frequency response, but rather than one "from drive signal to sound output", it is "from drive signal to cabinet vibration" -- with the less measured in general, the better. When using an accelerometer into LAUD's mic inut, you should temporarily disable your mic correction file (by the menu sequence: [* File Micdat ], then type "NONE" and press the [Enter] key.
A simple piezo element, as might be removed from a cheap piezo tweeter, can be used effectively as an accelerometer. Even with its high impedance badly mismatched by the FIJI's (or Mic/Probe Preamp's) input, the piezo's output level is still adequate for making comparative measurements of the vibrational characteristics of loudspeaker cabinets. The main difficulty with this technique is that you can't be sure which of the response peaks that you see are from the piezo and which are due to the cabinet (and cabinet vibrations do tend to be very peaky). Another problem is in finding a way to mount the piezo to the cabinet -- you can't press the piezo with much force onto adhesive or wax without breaking the ceramic disk. One approach that works well is to glue the pizeo to a small piece of wood, and then attach the wood to the cabinet with clay or wax.  But there's no reason to put yourself through this kind of trouble.