how to sort custom properties?

[lgw6237]

sir:
    We develop custom content type,which name is StandardDocument that includes some custom properties;Now,we hope to sort custom properties,such as standardDownloadCount,how to do with it?
my codes are:
                SearchParameters sp = new SearchParameters();

      sp.addStore(storeRef);
      sp.setLanguage(SearchService.LANGUAGE_LUCENE);
      // sp.setLimit(5);
      sp.addSort("standardDownloadCount", false);
      sp.setQuery(spath);

but this codes don't work.