From a New Zealand SAR team newsletter:
"Last year the members of the Hamilton YSAR Group conducted a series of whistle tests during their weekend camps.
They found that the Storm Whistle was one of the best performers with an average range in the NZ bush of 400 metres.
This is 60% further than the standard SAR whistle that many people carry. Its range was 250 metres. Although the [Storm] whistle is slightly larger than the SAR whistle it is lighter and its bright fluoro orange colour makes it easier to find if dropped on the ground."

Opinions & test details:
http://www.ysar.org.nz/technology/Other.htm