Hello got a technical question. 
How much power at 21hz can you put in to the ZV28, and what is the maximum excursion and at what dB
Peak or sweep or white noise. 
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David Lee
It seems you’re interested in a very specific frequency, which is unusual. What you need to know this for, and what are you trying to find out?  
21Hz is a single frequency, so, practically speaking, the answers you seek can’t be established testing with white noise nor ordinary peak measurement criteria. One single cycle of 21Hz takes 47.6ms, so the duration of the test needs to be a factor to establish a qualified answer and a repeatable test. 47.6ms is longer than most peak tests, so a peak test at 21Hz is rather uninformative. To find out what you’re asking, tests must be made using a sine wave. The voltage and current exiting the amplifier have to be measured to establish the power being dissipated through the woofers. 
If the impedance is high at that frequency, the power, in watts, would be lower than the maximum power available from the amplifier because an amplifier’s power output rating is specified at a fixed impedance. (And if there are 2 boxes being tested at the same time, the impedance at that frequency would be slightly different and the tests for power and excursion would yield a different result.) 
At the same time as taking the power measurements, a device must measure the excursion of the woofer at that power. Now consider that tests for SPL have to be done in a very large outdoor environment to eliminate the contamination of reflections on the SPL measurement, which, for 21Hz would require the test take place at the center of a 100 foot diameter circle of unobstructed space. 
Providing that space for the SPL measurement would require the laboratory electrical test equipment and the excursion measurement equipment to be set up and calibrated in that setting. All this is highly impractical unless there is a very compelling reason to answer your question and entirely unnecessary if what you’re trying to establish can be answered in a different way. 
The point is, there will be a limit for each of the parameters, and one of those limits will establish the maximum limit of the others. If the output power from the amplifier were the limiting factor, the maximum excursion and the SPL would be limited by the amplifier. If the maximum excursion were the limiting factor, the power input would be restricted so that the driver would not be called upon to exceed that maximum to cause damage to itself. 
In either case, the resulting SPL output could not exceed the physical limits of the driver nor the output limits of the amplifier. There is also a time limit based on the thermal dissipation capacities of the various parts, specifically the voice coil, but under certain conditions, the output devices of the amplifier. 
I’ll put the question back to you: How will knowing these very specific details help you? What level of detail and specificity would satisfy your curiosity?  
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