I got bored of my Airzound never having any air in when I wanted to use it - despite charging it regularly, and even buying a new one which very quickly went the same way. So I went for a Stebel Nautilus electric air horn (£22 from Motor World) - 139dB, little compressor built in. It operates off a 12V supply, but can draw 18A, so needs a decent battery. Installed a suitable 12V lead acid battery just behind my pannier rack, suspended the horn unit from the top tube, and installed a push button on the handlebars. End result = bloody excellent, and louder than an Airzound - though would point out I use it very sparingly, but when I do it has the desired effect!!