Microwaves versus acoustic or seismic waves

Microwaves are electromagnetic waves (300 MHz–300 GHz) that travel at the speed of light, c = 3 × 10⁸ m/s, even in vacuum. Acoustic and seismic waves are mechanical waves. They need a material medium and are slow: about 343 m/s in air, about 1500 m/s in water, and a few km/s in rock.

Advantages of microwaves over acoustic/seismic waves

1. No medium needed: microwaves travel through vacuum, so they serve satellite and space links. Acoustic waves cannot.
2. Very high speed: signal delay is about 10⁶ times smaller, so radar ranging and real-time communication are fast.
3. Huge bandwidth: carrier frequencies in the GHz range carry Mbps to Gbps of data. Acoustic channels carry only kHz of bandwidth.
4. Long range in air: line-of-sight links of 50 km or more, and satellite links of 36,000 km. Acoustic waves in air die out within metres to kilometres.
5. Directive, compact antennas: λ is in cm, so narrow beams come from small dishes.
6. Weather-independent below about 10 GHz: microwaves pass through cloud, fog and smoke. Acoustic waves are disturbed by wind and temperature gradients.
7. Fine resolution for imaging and radar.

Disadvantages of microwaves compared with acoustic/seismic waves

1. Poor penetration of water and earth: microwaves are absorbed by conducting media such as sea water and wet soil. Sonar (acoustic) is used under water, and seismic waves probe deep underground for oil and earthquake study.
2. Line of sight: blocked by hills and buildings, while low-frequency seismic waves travel through the earth.
3. Expensive equipment: microwave sources, waveguides and analyzers cost far more than transducers and geophones.
4. Rain and atmospheric loss above about 10 GHz.
5. Health and safety limits on radiated power.
6. Licensing: the spectrum is regulated, while acoustic methods need no licence.