Sonic Primary buffs team resistance and also has some enemy debuffs, Sonic Secondary mainly debuffs enemy resistance.
High resistance means you won't take much damage and sonics are good for helping Fire/* and Inv/* tanks hit the resistance cap.
There aren't many sonics about, I have a level 50 on Union, give me a shout if you want to team some time. 
Sonic Barrier - Smash Lethal Toxic resistance Shield
Sonic Haven - Fire Cold Energy Neg energy
Shields have an awesome effect on tanks both combined makes Firetanks near unstoppable without the crashing and with a very fast healing flames, it gives the def sets or regen sets an extra leg to stand on versus alphas or streakbreakers and res sets have a longer fight duration and some shields even if end drained then add sonic dispersion for more resistance.
Sonic Siphon and Disruption field make the enemy guaranteed to feel more damage lowering fight duration.
So thats more fight duration capability to team and less fight duration capability to enemy for a meet in the middle.
Clarity whatever shield you have on stacking or combining with what sonics give you can stay on.
Liquefy for when the paper plane is likely to hit the fan you can use this to interrupt enemy damage output with knockdowns and holds also lowering the enemies defence but most enjoyably the enemies tohit so you make the enemy miss more buying time for players to get themselves together. I add Tohit debuffs to this power Paltry recharge on it but too much recharge would definately over power earth controllers.
Sonic Cage take the one real danger in the mob out of action and get to gank him or her after ya dealt with everyone else.
Sonic Repulsion cast it on a player who then has to be told they have it (mainly cos the symbol is next to only your name) and then watch that player be clueless of what to do with it. If they werent former kin defenders outstanding with repel or perhaps stormies they will usually create problems or do nothing great with it but with this power your actually meant to use knockback to lower the enemies damage output although many sets will hate you for it as you can lower their survivability, efficiency or due to the selected player being not good with it..cause foes to find their way to more foes and make someone with poor perception get extra aggro by following - most skip it, i dont blame them and it can sure haul the endurance out of you which isnt good neither. If someone is in over their head then its my absorb pain type power where i cast it on the player and all mobs around them get knocked about and so have less chance to do more damage and time is bought. If it was knock up people would learn to love it probably.
It is a cracking set and there isnt enough of them around imo.
He will honor his words; he will definitely carry out his actions. What he promises he will fulfill. He does not care about his bodily self, putting his life and death aside to come forward for another's troubled besiegement. He does not boast about his ability, or shamelessly extol his own virtues. - Sima Qian.
The quick answer is it's damage resistance bubbles. 
But you don't see many bubbler defenders ether 
I'd recomend playing a sonic defender, while not so good as other defenders to solo (as most of the powers are team buffs)
They do have a noticeable effect on teams:
At low lv the unenhanced buff of the bubbles is very noticeable on all toons (20% res base number)
At later levels an ice tank, SR scrapper or regen scrapper becomes godly with a sonic defender behind them. Although res based toons (eg invun) gain far less as they will be capping there res.
Squishies will still notice the bonuses though and your large bubble will give them hold&stun protection so many lack, with a single target CM type power for other effects (also great for thoes pesky nightwidow smokebombs )
Don't neglect your attacks if you play a sonic def as once you've got the team bubbled you will not have much to do for the next 5min execpt blast stuff
For a secondary I'd mix it with Psi, elec, dark for there controlling effects or sonic for the added res debuffs and control.
If you want some advice/more info on each of the powers then have a look at Xanthus's sonic guide
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The quick answer is it's damage resistance bubbles.
But you don't see many bubbler defenders ether
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I am seeing so many bubblers these days both as defender and controller. But only Sonic/Sonic defender i know in Union is Diva. Only have one time chance to team up with her and from the way she using powers and held herself in battle i am thinking about making a sonic/sonic defender. But nowadays i have many toons to enjoy tho.
Hi ive been playing for 2 n a half years and i don't know anything about sonic defenders. I've never teamed with one, never seen one, never used one, what do they do? what is there role? From the power list are they a mixture of radiation and force fields? The first power pool seems quite fun to use and i like the concept. I like sonic blast because it has a high single target damage output ( i think ). Could someone please help me out, thank you
and i would mainly use him for PvE.