A vibration generator for a portable electronic device includes a speaker driver and a counterweight retained by a housing to form a closed assembly. When a communications transmission is received by the electronic device, an input force signal is transmitted to the speaker driver to apply a force at a frequency substantially below its resonance frequency. Air sealed within the vibration generator between the speaker driver and the counterweight transmits the force signal inputted to the speaker driver to the counterweight with minimal dissipation, thereby causing the counterweight to vibrate at substantially the same frequency as that of the input force signal. The vibrations of the counterweight alert a user that a communications transmission is being received.