3l3phantstomp wrote:I was on Talkbass.com and offering some thoughts about smaller rigs based on my experience, and someone rolled in and said something to the nature of Isobaric cabs only using/needing half the wattage of normal cabs, and that I was a numbnu7...I was completely confused. I think I obsorbed all of A.hun's passionate explainations here and learned a lot, but I can't underderstand the isobaric uses half, for I just assumed Isobaric meant sealed in tandem or something. Does that mean a 1200W amp can use the SP212 (rated at 600) and little fear should be involved? I'm not worried about my rig, but I want to understand and the folks at TalkB can be a bit...abrasively condescending at times...
Well that isn't my understanding of it. In fact I think the opposite is true - a twin driver isobaric cab is 3dB LESS efficient than a single speaker cab with the same type of driver, so requires DOUBLE (not half!) the power to reach the same sound pressure levels. Of course the two drivers mean that the power handling is double that of a single speaker cab so that given a powerful enough amp it can go just as loud.
The advantage of an isobaric cab is that you can get the same frequency response from a cab with half the volume of a standard single speaker cab.
The two drivers operating in tandem exhibit exactly the same behavior as one loudspeaker in twice the cabinet. The cabinet is defined as the space behind the rear driver. The volume of air between the speakers has no acoustic function so that the saved space is less than 50%. All other aspects are unchanged like resonant frequency and maximum SPL. The new driver will have the same resonance frequency, Qts, distortion, excursion etc. as one driver with the same applied signal. Because the impedance is also halved, this performance is achieved with twice the power. The new efficiency is thus 3 dB lower than with one loudspeaker. The reason for the unchanged resonance frequency is simple: the new combined loudspeaker has twice the moving mass compared to the single driver but also half the compliance because of the doubled suspension.
The result is that the coupled driver pair (iso-group) can now produce the same frequency response in half the box volume that a single driver of the same type would require. For example, if a speaker is optimized for performance in a 40 liter enclosure, one iso-group of the same speakers can achieve the same low frequency extension and overall response characteristics in a 20 liter enclosure. The aforementioned volumes exclude the isobaric chamber. If the iso group is placed in the original 40 liter, the loading will be incorrect (if the 40 liter was a correct loading of the loudspeaker).
So no, the SP212 is a 600W rated cab, and will need 600W to get to the same SPL as a standard (larger) 300W cab with the same driver type would. (And no it really doesn't mean you can safely drive that cab with a 1200W amp.)
The 'numbnuts' poster at Talkbass was either saying something else or is plain wrong!
(Could you link to that thread?)
I found you on THIS ONE
where someone else said pretty much what I'm saying - these isobaric cabs put out less SPL* (for the same power input anyway), though the same maximum SPL given double the power.
*Saying 'half the SPL' is wrong though; everything else being equal -3dB is the equivalvent of half the power not
half the volume
which happens at 1/10 the power or -10dB.
" onclick="window.open(this.href);return false;
I'd say that if someone was both wrong and
insulting than it is probably worth reporting their post to the Talkbass moderators. Not everybody out there knows what they are really talking about, but they should at least be keeping things polite...