Certain frequencies suffer from higher attenuation than others, due to the mechanical resonance of gas molecules [8, 9].
This is partly explained by topographic effects that can significantly amplify ground excitations at certain frequencies and distances from the relief edge (e.g. [8]).
At the mud casing interface, because the acoustic impedance of the casing is much higher than that of the mud, only acoustic waves with certain frequencies can penetrate from the mud into the casing.
Motivated by both spring and string ideas, we investigate whether the contributions to the sound from certain frequencies can be canceled by the appropriate gradient pulse sequence design.
Similarly, public safety officials want the FCC to evict cellphone carriers from certain frequencies to minimize interference with police and firefighter radios.
Part of the reason we perceive sounds as being behind us is that the external part of the ear filters out certain frequencies and lowers the volume from sounds that are from the rear.
Figure 1 displays the spectrum mask of the IEEE 802.16 signal specified in the WiMAX standard[16], where about 25-dB attenuation is required between the reference carrier power and all unwanted spurious emissions at certain frequency offsets from the operating bandwidth.
While the FCC can stop Americans from transmitting on certain frequencies at home, it can't control what happens overseas, where wars tend to be fought.
