Hello again DIY audio community, I've been attempting to improve my ability to test speakers and my lack of a proper microphone got me thinking about some other ways I would be able to test speakers with just my computer and some cords, my idea is a testing cord, a simple headphone jack on each end, one end plugged into headphone output, the other into microphone/source input, partway on the cord, the signal path would be cut, and each end of the cut would be soldered to a set of alligator clips which could then be attached to the terminals of an speaker I desired. My belief is that any form of distortion within the speaker will translate to mechanical movement of the voice coil, by either altering the impedance of the voice coil or by inducing a current through the coil, would it be possible to then use some software to analyze this input as if it were from a microphone without worrying about the quality of a microphone or testing room since the speaker itself is acting as the microphone? Even if the signal might not be able to be analyzed in the same way as the signal from a mic, is there some way to still understand the relevant information from this setup? Am I really just crazy? I'd love to hear your opinions on any of this.
As a general rule it can be difficult to test something by using itself. Would you test the accuracy of a ruler by checking it against itself?
A loudspeaker is a rather lossy non-linear transducer and a room is acoustically quite attenuating so you can't expect to measure distortion in sound output by only looking at its electrical terminals.
A loudspeaker is a rather lossy non-linear transducer and a room is acoustically quite attenuating so you can't expect to measure distortion in sound output by only looking at its electrical terminals.
Sorry, found out my wife was pregnant right after I posted that so I haven't had much time for the forums recently.
You're correct in that you can't measure something against itself, but with either a lead from the speaker like I have or with a microphone, its the computer that's doing the measuring, or at least that's my impression of what's happening. I'll upload some pics as soon as I can, but if you look at the spectrograph of my set up, there is a line of extension of the frequency at 1300ish Hz (please excuse my poor terminology) that matches a peak in freq response when measured by microphone, my impression of this is that when a freq sweep is played through the speaker, if the driver is resonating at any frequency, no matter how good the speaker or amp is, the voice coil will obtain some of that kinetic energy, which will transfer to electrical energy at the coil leads which can be picked up by the Mic input of the computer, and this way there is no microphone that could pick up room modes, resonances, or background noise. There was a lot of "noise" in my readings that matches certain resonances of the frequency being played but I am aware that it could be some interaction between the speaker and the amp/mic circuitry in the computer as well. As soon as I can find it I'll upload a pic of the spectrograph and freq response of the speakers I've tried this with.
J Michael Hajek
LazyH-Online.com
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