The stimulus polarity can be rarefaction polarity, condensation polarity, or alternating polarity. The acoustic pressure wave method analyses the rarefaction waves produced when a leak occurs. He attributed condensation to cold/wet air and rarefaction to the interaction of hot/dry air. All other substances are derived from it by condensation and rarefaction. The classical elements change into one another by a process of condensation and rarefaction. Sound is a pressure wave, which consists of alternating periods of compression and rarefaction. One compression and one rarefaction make up one longitudinal wave. The pressure antinodes correspond to the locations with the highest amount of compression and rarefaction. In 1704, he invented the U-tube manometer, a device capable of measuring rarefaction in gases. This index is discussed further in Rarefaction (ecology). Modern construction of guitars is an example of using rarefaction in manufacturing. This rarefaction effect has given rise to the misnomer "vacuum bomb". A natural example of rarefaction occurs in the layers of Earth's atmosphere. Rarefaction polarity is recommended for clicks. Rarefaction curves are necessary for estimating species richness.