A total of 227, or roughly one out of every five teachers who reported to city schools for the first day of school last year, were no longer teaching in the system on Sept. 5, when the schools kicked off the 2006-07 school year. More than half of those -- 138 -- had resigned or left of their own will, and 46 retired.
Ultrasound and Arthropod Pest Control (2001), an extensive Kansas State University study, confirmed that ultrasonic sound devices do have both a repellent effect as well as reduces mating and reproduction of insects. However, the results were mixed, and ultrasonic sound had little or no effect on some pests.ome animals, such as bats, dogs, and rodents, can hear well into the ultrasonic range. Some insects, such as grasshoppers and locusts, can detect frequencies from 50,000 Hz to 100,000 Hz, and lacewings and moths can detect ultrasound as high as 240,000 Hz produced by insect-hunting bats.
Small mammals like Dogs, Cats etc. responds to 22-25 kHz US and Rats to 60-72 kHz. Insects like Mosquitoes, Flies responds to 38-44 kHz. What is Ultrasound? The hearing property of human ear is limited. Human ear can perceive sound with in a frequency range of 20 Hz to 20 kHz. The sound frequency below 20 Hz is Infrasonic sound and above 20 kHz is Ultrasonic sound. Human ear is not sensitive to US because the eardrum cannot vibrate as fast as US. Ultrasound has high frequency, so the wave is very powerful.