Change header logo according to a category

Hello,
is there a possibility to change the header logo according to different categories?
All posts, archives and sub-categories of a specific category should show the same header logo.

I tried this, but it doesn’t work:

add_filter('avf_logo','av_change_logo_url');
function av_change_logo_url($url)
{
    if( is_category('slug') )
    {
        $url = "http://kriesi.at/wp-content/themes/kriesi/images/logo.png";
    }
    return $url; 
}