--- title: "Microwaves versus acoustic or seismic waves" subject: "RF and Microwave Engineering" chapter: 1 key: "semester_7/rf_and_microwave_engineering:microwaves-vs-acoustic-waves" source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/microwaves-vs-acoustic-waves site: BCT Engineering --- # Microwaves versus acoustic or seismic waves RF and Microwave Engineering · Chapter 1: Introduction - Asked in: 2072 Asoj, 5 marks; 2072 Magh, 5 marks (asked 2 times) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/microwaves-vs-acoustic-waves - Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-1 ## Question What are the advantages and disadvantages of microwaves over acoustic waves (merits and demerits of microwaves compared to seismic waves)? ## Short answer Microwaves are electromagnetic, travel at 3 x 10^8 m/s through vacuum, have huge bandwidth, long range and weather immunity. Acoustic/seismic waves are mechanical, slower, need a medium, but penetrate water and earth better and are cheaper. Microwaves are absorbed by water/earth and need line of sight. ## Answer 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. | Feature | Microwaves | Acoustic/seismic | |---|---|---| | Nature | Electromagnetic | Mechanical | | Speed | 3 × 10⁸ m/s | 343 m/s to about 6 km/s | | Vacuum | Travels | Cannot travel | | Under water | Strongly absorbed | Good (sonar) | | Bandwidth | GHz | kHz | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why can microwaves be used for satellite links while acoustic waves cannot? A. Microwaves are mechanical waves B. Microwaves are electromagnetic and need no medium; acoustic waves need a material medium C. Acoustic waves are faster D. Microwaves are absorbed by vacuum Answer: B. Microwaves travel through vacuum at c; acoustic/seismic waves need a medium. (Key point: Medium) 2. Roughly how much smaller is signal delay for microwaves than for sound in air? A. About 10 times B. About 1000 times C. About 10^6 times D. About the same Answer: C. c = 3 x 10^8 m/s versus 343 m/s gives a ratio of about 10^6. (Key point: Speed) 3. Why are sonar and seismic waves preferred under water and underground? A. Microwaves are absorbed by conducting media such as sea water and wet soil B. Microwaves are too slow C. Sonar uses higher bandwidth D. Microwaves cannot be generated Answer: A. Conducting media absorb microwaves strongly, while acoustic/seismic waves penetrate them. (Key point: Penetration) 4. How does the bandwidth of microwave and acoustic channels compare? A. Microwave: kHz; acoustic: GHz B. Both are GHz C. Microwave: GHz range carriers carry Mbps-Gbps; acoustic channels only kHz D. Both are kHz Answer: C. GHz carriers allow very high data rates; acoustic channels are limited to kHz. 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