These assemblies use a medium size bearing (16mm) to decouple a hifi shelf from surface vibration. They can be used under glass or wood shelves. one set of 20 units available (so 20 bearings + 20 bases) which should be sufficient for at least five shelves, depending on whether you use 3 or 4 assemblies per shelf.
These can also fit directly under hifi components, but if doing so I would recommend using 16mm 'O' ring rubber washers to keep the bearings in place and to protect the underside of your precious kit.
Each assembly is just under 18mm high