Swallows are a rare exception in the bird world because they do respond to ultrasonics. Very few bird species (owls being another) can hear into the ultrasonic range, therefore presumably cannot be repelled by ultrasonic frequencies.
The U.S.R. provides a dynamic environment of changing conditions due to it's use of variables, such as frequency mode selections, speaker rotations at varying speeds, and warble rate (cycles per minute).
The U.S.R has the obvious advantage of being able to be used in close proximity to people without causing any disturbance, although it should always be placed at a level above head height, so as to not have any human impact.
It is designed for indoor or undercover usage. Ultrasonic frequency ranges vary according to your environmental circumstances.
The unit can run off mains power (it is 12 volt device and comes with a standard 240 volt adaptor). If required, a longer power adaptor is available (15 metre cord). Power consumption is only 3 watts (in steady mode).
Effectiveness on swallows is consistently good.