The audiences are not taken into account on the search results page (web part). Managing the security would be the easiest way to implement this. If you really needed to perform audience targeting, consider what it is you are trying to do. Do you simply want to filter the content by audience post-page load. If this is the case then a custom web part that wraps rows with the applicable audience would work and could be filtered/unfiltered without a page load. In most cases what you really want is pre-filtered or targeted content based on the defined audience. In this case you would want to find a way to include the audience as part of the actual search query so that it is pre-filtered prior to being returned for rendering.