Source: http://en.wikipedia.org/wiki/Tambourin.htm
The tambourine consists of a frame to which small objects are attached, typically small metal bell- or cymbal-like in objects. The frame is rattled or shaken so that the objects smash into one another. These objects are thus small membrane plates.
One variation of this design adds a vellum to the frame, which is then hit like a drum. Such an instrument may be regarded as two instruments in one, with different sets of acoustic properties. The small objects are tensed during manufacturing and remain consistent regarding acoustig properties, while the tension of vellum, made of softer and less dense material may change due to envrinmental influences, such as temperature and moisture.
Source: http://en.wikipedia.org/wiki/Tambourin.htm
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