Two waves nearly alike in frequency would do the same, and generate tones at the sum and difference frequencies. In the audible range, these additive frequencies create a cacophony from the original tones. But move the fundamental tones into the ultrasonic range, say 40 and 41 kHz. Now, only the difference frequency of 1,000 Hz falls within the audible range, he said, while the sum frequency, the harmonics, and the original tones remain unheard. The audible message, be it music or voice, modulates the ultrasonic carrier.
Air then demodulates the message back out to the hearer.
The Long and Winding Load by Michael Valenti Heavy and long construction equipment and large industrial loads tax the capabilities of most semi-trailers. Nicolas Industrie of Champs sur Yonne, France, designed its Nicolong semi-trailer to handle loads weighing up to 80 metric tons. The telescopic trailer is 36 feet long in the closed position and 98 feet in the open position. The Nicolong semi-trailer is equipped with a hydraulic weight adjustment system to enable operators to handle long, heavy loads more easily. The Nicolong trailers are also articulated, with three to eight running axles, depending on the degree of freedom desired.