Having thought about this some more, I've decided to rewrite my answer...
Your doubts about the acoustic properties of the harp are understandable. When heard in an orchestral setting, the harp is relatively quiet, requiring careful orchestration if one wants it be heard within a multi-instrument texture. However, a concert pedal-harp is in fact capable of producing quite a large sound; one that is significant in a chamber music setting, if easily obscured in an an orchestral setting. For a good example, listen to how easily Ravel balances the harp with other instruments in his Introduction and Allegro of 1905.
I discovered just how resonant a concert harp is the hard way! While at college, I wrote some pieces for harp and classical guitar, for me and a harpist friend to play together. Following the first rehearsal, I had to significantly rethink the pieces, as the harp was far, far louder than the classical guitar.
And yet, this seems surprising: to look at, a harp seems to basically be a set of strings vibrating within a wooden frame. Intuitively, this should be quite quiet, much like an electric guitar being played without an amplifier. After all, the strings aren't going to vibrate the air molecules around them enough to significantly project their sound. But, as with all (?) acoustic stringed instruments, the sound is not projected directly from the strings themselves, but instead from a resonant hollow body, via a soundboard. Because of the shape and construction of a harp (it looks "see-through" with the strings the most identifiable element when seen from the side) it is easy to overlook the soundboard and body, which are instead a prominent feature of the shape of other stringed instruments. But they are there. Here is some simple information about the construction of a harp.
So, the strings are the elements which oscillate when plucked; these strings vibrate the soundboard, which together with the body of the instrument amplify the sound, which is then radiated to the surrounding air. I'm not sure I would describe the body as the "sound effuser", but I guess that is the part of the harp where most of the sound radiates from.
Years ago, I found a really simple exercise that can demonstrate just how important the body of a stringed instrument is for amplifying the sound: if you lie an acoustic guitar flat, place a sheet of paper over the soundhole and strum the strings, the sound is much quieter. Interestingly though, not all harps have soundholes; but they are used on harps with metal strings, to aid emanation of the sound, as the soundboard is much stiffer to accommodate the increased tension of the strings.