strange measurements

mashaffer · 29th June 2006, 06:04 AM

Bouyed by the succesful measurement of the TS parameters on my 15" MI speaker I took a shot at one of the RS 5" whizzer drivers (looks like a fostex with the tan cone and brown surround) and got very strange readings. When measuring the voltage across the resistor (in series with the driver) instead of getting a null indicating Fs the voltage just kept dropping gradually all the way from over 300Hz down to 30Hz. Now I know this thing doesn't have an Fs that low and I was able to measure the Fs of 45 on the MI speaker so...

Any ideas what might be going on? In both cases the speaker was suspended horizontally between two wooden chairs. Thinking that it might be interference from the chairs (which are of course proportionally closer to the cone) but holding it suspended vertically didn't seem to make any difference. Could it just be that the light weight speaker needs to be clamped in position?

mike

Ouroboros · 29th June 2006, 06:51 AM

If the voice coil has a fairly high inductance then you might expect the impedance of the drive unit to increase as the frequency increases. Surely even on a 5" unit the Fs will be well below 300Hz? I would normally expect to see a low impedance at very low frequencies, rising to a fairly well defined peak at the fundamental resonance (the sharpness of the peak being a function of Qes), then dropping untill the inductance of the voice coil causes a gradual rise with frequency.

29th June 2006, 06:04 AM	#1
mashaffer diyAudio Member Join Date: Jun 2004 Location: Indiana	strange measurements Bouyed by the succesful measurement of the TS parameters on my 15" MI speaker I took a shot at one of the RS 5" whizzer drivers (looks like a fostex with the tan cone and brown surround) and got very strange readings. When measuring the voltage across the resistor (in series with the driver) instead of getting a null indicating Fs the voltage just kept dropping gradually all the way from over 300Hz down to 30Hz. Now I know this thing doesn't have an Fs that low and I was able to measure the Fs of 45 on the MI speaker so... Any ideas what might be going on? In both cases the speaker was suspended horizontally between two wooden chairs. Thinking that it might be interference from the chairs (which are of course proportionally closer to the cone) but holding it suspended vertically didn't seem to make any difference. Could it just be that the light weight speaker needs to be clamped in position? mike

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29th June 2006, 06:51 AM	#2
Ouroboros diyAudio Member Join Date: Dec 2003 Location: Nottingham UK	If the voice coil has a fairly high inductance then you might expect the impedance of the drive unit to increase as the frequency increases. Surely even on a 5" unit the Fs will be well below 300Hz? I would normally expect to see a low impedance at very low frequencies, rising to a fairly well defined peak at the fundamental resonance (the sharpness of the peak being a function of Qes), then dropping untill the inductance of the voice coil causes a gradual rise with frequency.

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