To amplify sound
When Belfry refused to cooperate, Prime Evil attempted to persuade the bat by locking him up in the Closet of Fear. However, Belfry escaped with the help of the Ghostbusters and used the Spectral Amplifier against Prime Evil, turning Hauntquarters into a ruin with one amplified Belfry blast. The blast conveniently also blew a fuse in the Spectral Amplifier, causing it to self destruct and explode.
The Spectral Amplifier first appeared in the episode Outlaw In-Laws.
To amplify sound by a billion times.