I recently swapped out twin 4″ SB12NRX25-8 with rubber surrounds for twin 4″ SB12NRXF25-8 with foam surrounds in a M-T-M configuration. The purpose was to compare the performance as mid-range drivers in the band 150Hz-2.5kHz. SB Acoustics claim "Foam surround for maximum transparency and midrange resolution"
https://sbacoustics.com/wp-content/uploads/2021/03/4in-SB12NRXF25-8.pdf
From the rubber surround version data sheet "High damping Butyl rubber surround for non-resonant sound character"
https://sbacoustics.com/wp-content/uploads/2021/03/4in-SB12NRX25-8.pdf
The foam surround appear much stiffer than the rubber surround which is very flexible. This seems to be confirmed by the foam Fs = 63Hz vs the rubber surround Fs = 57Hz. The two driver appear identical otherwise. Surprising to me, the foam does sound better than the rubber to my ears in this purely mid-range application. More natural and less "shouty" at all volume levels.
I don't buy the claim that foam was used for cost saving reasons.
Like many others I have had foam surrounds disintegrate in the past (KEF B139) and have avoided them till recently. My inclination is to use rubber surround drivers if large excursion is likely but my recent experience make me willing to use foam mid-range specific drivers for sound quality reasons.
Not always the case as my comparison above described.
I agree with robustness and longevity. Rubber is elastic thus stores and returns energy.
Also keep in mind that some modern foam surrounds are actually made from foamed rubber, not the traditional polyurethane foam, which had a reputation for deteriorating quickly under more unforgiving environmental conditions. Foamed rubber can also be "dialed in" by density to exhibit certain desired dampening characteristics. This can be used to control cone resonances, breakup and reflected components back into the cone edge (midrange bump), plus the material is lighter in mass than many thinner solid rubber surrounds. Scanspeak has some 7" drivers with foamed rubber surrounds.
I prefer midrange drivers with foam surrounds, mainly becuase they're typically less dampened in the upper midrange and usually tend to sound more open. This makes cone design much more critical, since the surround won't dampen much of the less desired resonance and breakup modes higher up.
I prefer midrange drivers with foam surrounds, mainly becuase they're typically less dampened in the upper midrange and usually tend to sound more open. This makes cone design much more critical, since the surround won't dampen much of the less desired resonance and breakup modes higher up.
The frequency response plots for the rubber and foam surrounds of the SB Acoustics 4" drivers appear identical below 200Hz but the foam surround response appears more even in the frequency range 200Hz-4kHz and has better dispersion.
Interestingly only the 4" Norex cone drivers are offered with foam surrounds. All of the larger cone drivers of the extensive catalogue have rubber surrounds.
The foam surround does appear to be a darker hue than the typical polyurethane foam surrounds of yore but I can't determine if they are foamed rubber. I do agree they sound more open, especially on vocal recordings.
Interestingly only the 4" Norex cone drivers are offered with foam surrounds. All of the larger cone drivers of the extensive catalogue have rubber surrounds.
The foam surround does appear to be a darker hue than the typical polyurethane foam surrounds of yore but I can't determine if they are foamed rubber. I do agree they sound more open, especially on vocal recordings.
My error, it was the ABR for a KEF Cadenza, BD139 which is the same chassis as the B139.
https://www.vintagechief.com/kef-bd139-sp1082-passive-radiator-22172
https://www.retroginger.com/kef-cadenza-70s-classic/
The Cadenza was similar to the original R104 which is nothing like the R104.2
https://www.hifiengine.com/manual_library/kef/model-104.shtml
Comparing the specs of the foam surround version against the rubber surround version, the biggest disparity is the compliance Cms = 1.18 mm/N for the foam vs Cms = 1.49 mm/N for the rubber. Assuming the spiders are the same, the rubber surround produces the higher compliance. This supports the conclusion the the foam surround optimises mid-range performance for this design. Other manufacturers may do things differently.