Driver impedance plotted against frequency

In discussion about IBs on the AVF subwoofer forum it was suggested that manifolds reduce the frequency of the natural (free air) resonance of the drivers. This was something I had not heard of before. The idea was that the mass of air in the manifold would load the drivers sufficiently to reduce Fs (which is measured in Hz). The immediate question was how one would go about measuring this change.

Small signal testing using REW with a series resistance in the speaker leads would provide an impedance curve for the individual drivers. The cable which normally connects the SPL meter to the sound card Line-in socket is connected across the speaker terminals. REW then reads impedance against frequency. The resonant frequency of the drivers should be clearly shown by a peak in the frequency plot.

I used a small audio amplifier to provide the test signal after removing all the heavy cables from the IB's EP2500 power amp to the drivers. The big amp was unnecessarily powerful for testing when a 30 watt audio amp would do the job nicely. Naturally I pulled the mains plug from the socket on the back of the Behringer amp to avoid short circuits between the naked ends of the IB's speaker cables. It is all too easy, after making changes to the wiring, to switch everything back on without thinking. I am as prone as any other to being on auto pilot when it comes to my audio system.

The resulting curves dip downwards at driver resonance due to the polarity of my hasty wiring exercise. The impedance curves of the individual drivers were then superimposed using "Measured" in REW. It seems the new driver's individual Fs hovers around 13Hz. A trifle under the expected 15Hz but I am certainly not complaining.

Now onto the problem of measuring the reduction of Fs due to the manifold loading all of the drivers simultaneously. My fuzzy logic suggests that, individually, the drivers would not be loaded enough by air mass to notice my very large manifold even existed. So I connected all four drivers in parallel using lighting flex for a 1.8 Ohm (DC resistance) load. I wanted to see if there was any reduction of the combined Fs on a small signal. It seemed unlikely given the tiny driver excursions involved. After checking driver polarity with a 9 volt battery I used a 100 Ohms variable resistor in the positive leg of the cable from the speaker terminals of the audio amp. This was to ensure a clear impedance peak at the combined driver frequency resonance.

Another REW sweep and I had the answer. No obvious change in Fs due to all drivers being driven on a small signal.

The question is how any manifold air mass loading might take effect. Would it be linear and based on driver excursion? Does manifold size and opening affect the driver's Fs?

The curves above are interesting. Blue is the impedance curve of all drivers driven in parallel with the door out to the IB enclosure open. Green is with the door closed. The door is right beside the manifold so it short circuits the intended infinite baffle dimensions when open. The total distance from front to rear of the divers is no more than a few inches via the hinged side. These old glazed doors hang on the original antique hinges which makes for a 3/4" wide gap at the hinge when the door is open. When the opposing pressure waves and rarefactions meet they should cancel each other out below a frequency which depends on the distance they have had to travel. This roughly equates to the diameter of an open baffle of similar dimensions to the short circuit between front and rear of the driver cones.

Just out of curiosity I closed off the mouth of the manifold with a piece of plywood to see the effect on impedance. The remaining open slot of about one inch width seemed to do nothing serious to the shape of the (closed door) curve but slightly reduced its level. I had made no changes to the REW settings or the test amplifier during these last three sweeps. I have raised the curve by a couple of dB using REW's trace adjustment to match the level of the earlier curves to aid comparison. The new trace is the red curve.

I am no nearer to proving a reduction in Fs due to manifold loading but have learned a few more things about my IB system with these REW impedance tests. Not least where that 12Hz frequency response peak probably comes from. I have been trying to discover a dimension in my home which would boost 12hz by boundary gain. I even wondered whether the living room downstairs was acting as Helmholtz resonator using the open stairwell as a port. Imagining the nearby IB manifold was driving the system like a huge reflex enclosure. Rather like blowing over the open top of an empty bottle. It now seems there is a much more likely connection between the driver's natural resonance and the frequency peak at 12Hz. Suggesting that the drivers are not quite critically damped by the EP2500 at system resonance.

BUT! Let's not jump to any false conclusions. Having just re-read my earlier posts I realise that my 32Hz drivers also exhibited a 12Hz peak when boosted at 20hz with the BFD. So it must be a room effect after all. I shall have to close off the stairwell with large piece of ply to see if I can kill that peak if only to satisfy my curiosity. I do know that my AV room is rather prone to exaggerate diesel rumbles from the distant road and delivery vehicles in the area.

My gratitude goes to
aktiondan (a member of the HT Shack) for the clever suggestion of impedance measurement using REW and a series resistance. The original HT Shack thread was back in June 17 '07. Dan's original posts are well worthy of careful study. A direct link appears below:

If clicking on the graphs doesn't provide a large enough image to show the detail then try the CTRL+ trick to magnify the whole page. CTRL0 will return the page to normal. CTRL- will shrink the page. Back click to return to the text.