@Robert. That is a good question. Let’s check it out in WinISD to see what types of differences you could expect. The first graph you will see is the transfer function magnitude. This graph is a little misleading because it isn’t taking into account the sensitivity of the driver, the potential room gain or the power you are feeding it. Nevertheless, I feel this is a good teaching point, so lets start with it. The red is the SB Audience 21 (6 cubic foot box) and the Blue is the Ultimax um18-22 (4 cubic foot box).
Immediately we would assume the Ultimax is the winner. But before we do that, we need to input the power numbers. I went ahead and went with the RMS of 1000w for the UM18-22 and the RMS of 800w for the SB Audience Rosso 21. After double checking that both could handle the RMS in the box size defined, I went to the SPL tab to see how close they are.
Interestingly enough they are almost identical at the lowest bass with the SB Audience easily beating it in the higher bass region. If you are only interested in the high bass region, the SB Audience is the clear winner here. But if you are chasing the subsonic notes, things are not as clear. To clear this up a little more, we will add the same room curve I added for this test to both subwoofers.
Now we can see the SB Audience still outperforms the Ultimax in the upper bass, but the Dayton is more linear in this sized room. Interestingly, if you were to DSP these both flat anywhere from 10hz-25hz they would basically be identical. Meaning if you are chasing the lowest frequencies you could choose either and should get similar SPL results in a closed room.
So why will they be virtually identical when the Ultimax has so much more xmax? That is due to their sensitivity ratings – which in layman’s terms, is used to determine how much power is needed to create sound. The higher number the easier it is to create sound (ie a higher SPL).
THE Ultimax UM18-22 sensitivity is only 88.6 while the SB Audience sensitivity is 98dB. THis means with only 1 watt of power the SB Audience can play at 98dB SPL while the Ultimax 18 will only play ate 88.6dB. In order for the Ultimax to play at the same SPL, you will need to give it more power. With the more power is the need for more excursion (xmax).
So which one would I do? If it was an open room, I would go witht he Ultimax 18. If this is a closed off room, with around 25 feet in it’s longest dimension, I would go with the SB Audience 21. But either are great choices. I hope this helps.