The Howler Alarm is triggered by Redstone and produces a loud audible alarm that can be heard for 64 blocks by default. The range of the alarm can be changed by right clicking on it. The Howler Alarm at its max setting can be heard 256 blocks away. It can be upgraded to an Industrial Alarm to change the color and add lighting effects.
The Howler Alarm can have in default two different settings: sci-fi and default. Fortunately you can create and set your Own Alarm sounds.
The Howler Alarm works well for telling you when your nuclear reactor is overheating (with a heat sensor) instead of building the more expensive Industrial Alarm which works better for intruders. Industrial Alarm
This item can be disabled by server mods - it can still be crafted, but you will not be able to place it.
To add your own sound, put it in the ".techniclauncher/tekkit/resources/newsound/ic2nuclearControl" folder in your appdata folder. Sound should be in ogg format and it's filename should start with "alarm-".