OBs fail to please

I tried my best with one and two open baffles with single 15" vinyl-coned AEIB15s fitted but failed. I  found that the Subwoofer-out socket was controlled by the crossover setting on the Lo/Hi stereo channels. The bass from the baffles went from 0Hz upwards and sounded deep and and punchy. I clamped the baffles to the TV stand to stop them falling backwards under the weight of the driver. It proved hard to stop them rattling. I had hoped that a 25Hz filter would be available with the Sub-out but none was available separately on this channel. Though in my fuzzy logic the setting of 25Hz high pass filters on the other channel should have the same effect.

I spent hours with REW trying to find a balance which would smooth the response between the IB and the OBs. Nothing I tried produced a reasonable response. Having phase adjustment on the SVS Bash amp gave me a huge range of options but all to no avail.When I reduced the crossover point on the CX2310 the OBs lost their top end. When I raised it the speakers became too thin. ThomasW was right that I do need another crossover. Though I still don't think the CX3400 is the real answer.

I discovered a difference between the new and old drivers in the big IB when the speakers were playing. Only the upper (newer) drivers produced the trough in the overall response at 160Hz. It must be something to do with these drivers being raised so far above the others. I tried every combination of BFD filters to try and fill the gap but this area is too far beyond the crossover point. All that setting higher frequency filters did was to cause a hump lower down.

 Here is the mess I captured towards the end of hundreds of sweeps for a 4 x 15" IB, 2 x 15" OBs and one trace of the stereo speakers combined.

Having stripped all the wiring out of the manifold I then had to spend another couple of hours retuning the IB and speakers. I still have a 160Hz trough and another further up but nothing I try will alter this. I moved the speakers around within their physical constraints but again nothing helped.


Traces of the 8 x 15" IB + speakers "retuned" after rewiring. The two lower curves are the two different sets of four, BFD-matched drivers and the upper line the combined curve. The troughs seem immovable.

A frustrating day but another snowstorm kept us at home anyway. The 15" drivers in OBs were not much appreciated by the domestic authorities. These drivers are really too big to live on the stage even in the smallest baffles which will fit. I used 3/4" plywood of about 24" x 18" set vertically with the driver also offset vertically . These baffles were just an old side of a previous manifold which I cut in half. I was really quite shocked how much bass those tiny baffles produced when playing on their own! You would think they'd be emasculated almost to a free air performance but no. Sadly there was no combination of OB and IB which matched each other or the speakers. 

There often seemed to be no connection between the REW response curve and any frequency settings on the Sub-out on the XC2310. With the 80Hz setting on the Hi/Lo stereo controls the Sub-out feeding the OBs was also limited to 80Hz. The Sub-out control setting of 215hz seemed to be completely ignored by the CX. It is supposed to be a summed response. I had hoped for a summed independent  output which I could trim with the low pass control knob but it was hopeless. The baffles were so ugly I didn't bother to photograph them for posterity.

Interestingly(?) the retuned 8 x 15" IB seemed to have gained some extra power. On Felix Hell's "Organ Sensation" 1st track (Guilmant Sonata No1 in D minor Intro and Allegro) at around 6m 40 the air is shredded by a fierce low frequency chord. So out of curiosity I ran it through Spectrum Lab.

For some reason I get a lot of 4-8Hz content showing on this track. These show as distinct tones rather than as continuous ambient noise.  Fundamentals at these very low frequencies are impossible with conventional pipes. 8 Hz requires a 64 foot pipe. 4 Hz requires 128 feet. Very few 64' pipes exist and no 128 footers (at all) as far as I am aware. Though some experimenters have reproduced these very low tones using electronics and very specialised subwoofers. (Thigpen rotary for example)

Even the largest, domestic IBs would struggle to reproduce 4Hz cleanly. Some can certainly manage an 8Hz sinewave though distortion may be rather higher than desirable. Such low frequencies have more value in the way they modulate higher tones rather than existing in isolation. The problem is one of impedance mismatch between conventional driver cones and the air. It requires a  colossal driver area to reproduce very long wavelengths with any degree of efficiency. Thank goodness for harmonics!

This is a 0-60Hz waterfall of the last part of Track 1. The red pillars show high levels of 4-16Hz content. (Warning 360kB enlargement!) The entire "Organ Sensation" CD is riddled with very deep sounding growls where the actual beats of the tone are clearly audible.

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