This is one of the better and more popular brands. It emits bursts of pulses at different frequencies. I did not find any spider webs inside this unit:
Here is a higher end version of the same thing, with a few extra user selectable settings to allow audible as well as ultrasonic output so it can be more effective against animals:
And a cheapo one that runs directly off the AC line:
I do not gaurantee the accuracy or safety of any information herein.
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