Recently, we (the okes in the office an’ me) bought two of a device that is supposed to repel insects. Well, not only. The box (as seen on TV) has a statement that it is an “ultrasonic pest repeller” and will repel “rats, roaches, flies, spiders, mice, fleas, ants and mosquitoes”. The device is marketed in supermarkets. Since we are keenly inquisitive acoustics engineers and, since ultrasonics is a part of acoustics, we turned the devices on to see what magical ultrasonic waveform could repel so many troublesome creatures. We duly set up our ultrasonic receivers, calibrated them using a 40 kHz source and looked to see what signal came from the devices. The answer: nothing. No waveform at all. The lights flashed and indicated that all was well . . . but nothing.

Good-looking Gregg, our financial man, would have nothing of our ultrasonic receivers. He said our test was unfair to the device and we should try a test that was more fundamentally African. Zer gut. Thus, he conceived the ‘biltong test’. A piece of biltong was carved into a precise shape until it weighed exactly 100 g. It was soaked in purified water for exactly 720 seconds. It was then placed on a plate that had been sterilised using a 3% solution of sodium hypochlorite.

The sample was then placed outside in the sun on a garden table. The air temperature was 26 ºC and 50% humidity. There was no rain and it was dead calm. We set the ultrasonic repeller on the table and waited for the flies to arrive and settle on the sample.

As we watched from a distance, there was a ‘woosh!’ and a brown streak flashed past. Tommy dog, the office watchdog, had leapt onto the table and scored the biltong.

We repeated the test, this time restraining Tommy and feeding him biltong. As we watched, there was a ‘meow’ and a thump and the office cat landed on the table, eyed and stalked the biltong. We shooed her off. After 20 minutes, the flies had been attracted. So, we switched on the ultrasonic repeller. A short pause and . . . nothing happened. We waited and the flies just did their thing. At a word, good-looking Gregg released Tommy dog, who did his blur act and the biltong vanished. “Yup,” said Gregg, “proof that it does not repel either flies or cocker spaniels.”

We could not get any rats or roaches but we did make a sugar trail, which soon became full of ants. Switching the ultrasonic repeller on did not shift them one bit. A nearby lizard took interest and flicked his tongue and nailed a few ants. “Yup,” said Gregg, “proof that it does not repel either ants or lizards.”

We did proceed further. However, I was disappointed. I opened the devices and looked at the circuitry. For it to be an ultrasonic device, it would have to have at minimum the following components: a 220 V to 12 ac transformer; a rectifier circuit to convert to 12 V dc; an oscillator circuit for the generation of flashing lights; an oscillator circuit for the generation of ultrasonic waveform and an antenna for the transmission of ultrasonic waveform.

The device’s claimed power consumption was 10 W. With losses, one expects at least 1 W radiated power, which, at 12 V, is about an amp. There was no antenna thingy which could begin to carry this current. This, and the field tests and our measurements with our ultrasonic receivers which showed no signal, led us to the conclusion that the device does not work and could never work. A complete hoax. This is a pity. Not only because a device which would repel “rats, roaches, flies, spiders, mice, fleas, ants and mosquitoes” would be real useful but because it would be nice to see some ‘wow’ technology rather than a rip-off. Shame.