Please note the phrase “Non-Inductive.” You can make inductive power resistors at low cost, but they have limited use in audio. They change how they work with different frequencies. This one doesn’t.
Koz
They have a piezo transducer (speaker) too, but it’s good all the way up to 3000Hz.
Koz
If all you need to do, is test a bat detector, 100 mW will be plenty.
At that level, no risk of burning something out, even when the amp would oscillate badly.
I wonder if there’s a trick to tune piezo beepers up. These are something lke 90dB @ 4 KHz @ 5V. Could you detune to let’s say 40 kHz?
Some electric hand dryers use ultrasonic detectors at 40 kHz. One transducer creates a series of 40 kHz tones and the other transducer listens for echoes. It’s a common form of proximity detector. They used to be available as hobby kits - perhaps still are.
Ah yes, here’s one that can be used with a Raspberry Pi http://uk.rs-online.com/web/p/products/7813020/?grossPrice=Y&cm_mmc=UK-PLA--google--PLA_UK_EN_Semiconductors-_-Semiconductor_Development_Kits&mkwid=suqDMDFeg_dc|pcrid|88057061283|pkw||pmt||prd|7813020&gclid=CKya0vmr0ssCFUE_GwodW70NEw
and here’s a DIY breadboard version: http://www.jameco.com/Jameco/workshop/JamecoFavorites/proximity.html