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RE: Acoustic Suspension: Not the same as sealed box

24.151.10.224

Hi David,

A lot of emphasis has been placed on a floppy suspension for Acoustic Suspension designs, and this is probably due to the fact that Villchur's first design and patent called for the suspension compliance to be 10 times higher than the compliance provided by the box. This requirement was difficult to meet in production as VC centering becomes a problem with very soft suspensions. Even AR ended up with ratios more like 3 to 5 for, what we now call alpha, the compliance ratio in later revisions. Having this ratio high is only important if the suspension is non-linear and it is therefore necessary to swamp it out with the more linear air spring. Modern suspensions are not as bad as they were in the past, so it is common to see alphas of 3 to 5. The mass of the cone, and the total compliance (box and suspension) determince the closed box resonant frequency Fc. When the suspension compliance is high Fc is most strongly dependent on box volume and cone mass. Cone mass becomes the important factor, and the AR-1 woofer had a much higher mass than most in its day. The other key factor is long linear throw in the motor and it turns out that the AR-3 and 3a motor while better than most is far from the best as shown in Klippel testing. The low cost modern Shiva woofer provides about the same Fc, and Qtc as the AR woofer in the same volume. It is fine in an acoustic suspension system as are many others. The original Shiva has been discontinued but the link below is an improved version with an XBL motor and cast frame. The XBL motor provides huge linear throw.

What really hurts most modern woofers is that they have too much magnet, which lowers the system Qtc, and the low bass output around Fc. Many allow for an inexpensive inductor to be used with high DC resistance which brings Qtc back up to an acceptable level.

An interesting property of 2nd order systems (closed box speakers) is that the relative amplitude at Fc is 20 *log (Qtc) relative to the passband. Thus the output at Fc is: 0 dB for Qtc = 1, - 3 dB for Qtc = .707, and -6 dB for Qtc = .5.

I usually look for 12" woofers with a moving mass of about 100g to provide a similar Fc to the AR-1 in the same volume box. Go a bit higher on mass if Vas is on the low side. The Aurasound NS12-513-4A 12" at Madisiound is another 12" that should provide similar performance in the same size box as the AR-1 or 3/3a. There are many others.

Another thing to note is that the high voice coil inductance of the older designs often provided built in baffle step compensation for many of these woofers. However, it is non-linear with high excursion and saturation of the iron pole piece which results in higher distortion. Better to reduce the voice coil inductance and use an external series air core inductor for this compensation. Low Qtc, and no baffle step make modern woofers sound thin/weak in the low bass.

A few benchmarks for vented systems is that a fourth order Butterworth (B4) alignment requires an alpha of 1, meaning that the box size should equal the Vas of the driver. High compliance drivers require a large box to obtain a B4 alignment which is - 3dB at the box tuning (Fb) frequency. Also, Fb = driver Fs for a B4. A B6 peaked filter assisted design with 6 dB of boost requires the same alignment as a B4 but with half the box size. We lose 6 dB at Fb by reducing the box size by one half.

Pete B.



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