Top picture: The bedug at Al-Hikam Mosque in Cililin in West Bandung.
Bedung is a musical instrument that is traditionally used as part of the gamelan ensemble. It is basically a double-barreled drum made of wood with buffalo skin/leather covering both ends. Its shape is similar to the kendang but is generally much larger in size. Because of this, the sound it produces is deeper and duller. Another feature that makes it different from the kendang is that it is typically placed on a rack (see picture above) and played with a mallet.
On Java island bedung is also traditionally used in mosques to mark the times of the daily prayers and is sounded before the adhan (call to prayer). The tradition probably began in the old days before the loud speaker was invented. The sound of the bedug could reach farther than the voice of the muezzin (the caller to prayer). This function has long been replaced by the loud speaker. However, many mosques still retain and use the bedug as a matter of tradition.
In addition to the bedug, many mosques on Java island traditionally also use the kentongan or slit drum to mark prayer times and help call Muslims to prayers.
The use of bedug and kentongan in mosques is just another example of many ingenuous adaptations and localization that Indonesians have been making in absorbing foreign cultural and religious elements to suit their own environments and needs.