Further, we created a proof-of-concept eavesdropping device that would record audible conversations and transmit the recordings to a nearby surveillance team over an inaudible ultrasonic link.
When we placed a second inaudible ultrasonic device in the area, we were able to create interference — technically called " intermodulation distortion " — between the two signals that made similar sounds to those recorded in Cuba.
We were even able to control the volume of the audible sounds by varying the strength of the ultrasonic signals. Without additional evidence, our research does not identify what actually happened in Cuba, but it provides a plausible explanation for what might have happened, even if the eavesdroppers were not trying to harm people.
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One way speed of light in one reference frame measurement 16 hours ago. Thermal effects are secondary to ultrasound energy being converted into heat in the tissue indirect effect of ultrasound and non-thermal effects are secondary to the alternating positive and negative pressures generated by the wave direct effect. In comparison, quiet conversation is 40 dB, a railway diesel engine passing at 45 mph at feet is 80 - 85 dB and a rock band is dB 4.
There have been a few publications describing harm to fetuses exposed to elevated levels of ambient noise, particularly industrial noise , specifically in the aircraft and textile industries, but while there have been reports of impaired hearing in infants who were exposed to ultrasound in the womb, several rigorous studies have disproved that notion Furthermore, a study of fetuses exposed in utero to vibroacoustic stimulation 12 and a recent study of fetuses exposed to noise generated during an MR exam of the pregnant women 13 showed no ill effect on the auditory system.
There have been some reports of being able to hear a "hum" during transcranial ultrasound. This may be the pulse-repetition frequency PRF , but, if so, it would be described as a higher pitch, and probably not a "hum".
To our knowledge, this phenomenon has not been investigated. Although the report mentioned above suggested that diagnostic ultrasound is detectable at measurable levels in the uterus, there is no independently confirmed, peer-reviewed, published evidence that the fetus actually hears the PRF, responds to it or is harmed by it.
Does this statement reflect a real risk? What does it mean if this statement is scientifically true? The fear is, of course, that this will raise the temperature of the fetus. Thermally induced teratogenesis has been demonstrated in many animal studies, as well as several controlled human studies A temperature increase of 1.
It is important to note that diagnostic ultrasound was not the source of the temperature elevation in any of these studies. Some believe that there are temperature thresholds for hyperthermia-induced birth defects hence the ALARA [as low as reasonably achievable] principle , but there is some evidence that any positive temperature differential for any period of time has some effect, in other words there may be no thermal threshold for hyperthermia-induced birth defects In experimental animals the most common defects are microcephaly with associated functional and behavioral problems 17, microphthalmia and cataracts.
Is ultrasound audible? Sounds with a frequency of 20 kHz and higher are referred to as ultrasound or ultrasonic sound.
High frequency sound is sound of which the frequency lies between 8 and 20 kHz. High frequency sound with a frequency over 16 kHz can hardly be heard, but it is not completely inaudible. High frequency sound and even ultrasound in the lower frequency zone up to 24 kHz can be audible if the sound level is high enough. The sound threshold the sound level where sound can be perceived rises sharply once the frequency and therefore, the tone becomes higher.
Younger persons hear high frequency sound better and their hearing range is greater toward the high frequencies.
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