1.2 inch round, 8 ohm .4 Watt speaker for HO & larger scales. It's frequency response is 550 to 13,000 HZ. It fits perfectly in Atlas and Proto 2000 "A" frame speaker mounts. When adding a QSI Revolution "A" or Soundtraxx Diesel Tsunami to an Atlas Silver series with the plugged "A" frame speaker mount, these speakers fit those mounts perfectly. NOTE: the photo of this speaker shows it in an Atlas "A" frame mount. This mount is not included, only the speaker.