In immersion testing of aluminum with water as the couplant, how many inches of aluminum correspond to 1 inch of water to prevent the second front surface indication before the first back reflection?

• A. 2 inches of aluminum
• B. 8 inches of aluminum
• C. 4 inches of aluminum
• D. 6 inches of aluminum

Answer: (C)

In immersion testing, echoes follow by how long sound takes to travel through each medium. To prevent a second front-surface echo from showing up before the first back reflection, you want the travel time through the aluminum thickness to be about the same as the travel time for 1 inch of water. Using typical speeds of sound (water ≈ 4,900 ft/s and aluminum ≈ 16,000 ft/s), the time for 1 inch of water is roughly 17 microseconds. The time to traverse L inches of aluminum is L/12 feet divided by 16,000 ft/s, which is about 5.2 microseconds per inch. Setting 5.2 μs per inch × L ≈ 17 μs gives L ≈ 3.3 inches. Since you usually select a whole number, about 4 inches is the practical equivalent. Therefore, 4 inches of aluminum corresponds to 1 inch of water for this purpose.