How to convert response of AVAssetExportSession

I am using AVAssetExportSession for Audio Recording with assert and here is my code to convert AVAssert to AVAssertExportSession.

AVAssetExportSession *exportSession = [AVAssetExportSession exportSessionWithAsset:self.asset presetName:AVAssetExportPresetAppleM4A];
exportSession.outputURL = [NSURL URLWithString:dataPath];
exportSession.outputFileType = AVFileTypeAppleM4A;
exportSession.shouldOptimizeForNetworkUse = YES;
[exportSession exportAsynchronouslyWithCompletionHandler:^{
    NSLog(@".... Audio... %@",exportSession);
}];

It gives me output like this

<AVAssetExportSession: 0x177f4b30, asset = <AVURLAsset: 0x18981f60, URL = file:///private/var/mobile/Containers/Data/Application/8BB39AD5-EEFB-4AF1-A913-B26C5C072E61/tmp/1422861622SCVideo.0.m4a>, presetName = AVAssetExportPresetAppleM4A, outputFileType = com.apple.m4a-audio

Here i just want URL to NSString.

Help me for this