Non Destructive Testing involving induced waves into structures by impact is used to evaluate the condition of building materials and to detect flaws.
Sounding is the simplest form of impact testing. On delicate materials such as terra cotta, it is referred to as tapping. In addition to using a hammer, sounding can also be performed by dragging a chain or using a special rotary percussion device. Materials without problems will "ping" at their natural frequencies; materials with problems will "thud" due to flexural vibrations at lower frequencies.
Although the ear is an excellent frequency analyzer, some document sounding by using a hammer with a pressure sensor and a transducer to record the impact wave. The graph of the amplitude vs. frequency is called a mobility plot. Low frequency "thuds" will produce a high slope, high frequency "pings" will produce a low slope. Other sonic differences will also appear on the plot, as compared to a plot from a region without problems.
Impact-Echo (IE) testing uses different hammer weights to induce temporary standing waves ("echoes") between the surface and the bottom of a slab or void. In the graph of amplitude vs. frequency, these standing waves appear as peaks. Differing conditions can cause peaks to shift, or cause multiple peaks.
Ultrasonic Pulse Velocity (UPV) testing is used to measure the speed of a wave through a material. Differences in the arrival time of the wave can be due to problems with the material or with embedded materials otherwise not accessible except through destructive means.
Rebound hammer testing is an indication of the surface hardness of a material. A known weight is impacted against the surface at a known velocity, and the distance the weight "rebounds" after impact is measured. The arbitrarily scaled "rebound number" can be correlated to the compressive strength of concrete or to the proper (or improper) mixing of mortar.
Unlike Electromagnetic Field Testing, impact testing uses pressure (or "seismic") waves induced in the material.
According to classic wave theory, pressure waves in a solid can be P-waves (or "primary" or "compression" or "dilatational" waves), S-waves (or "secondary" or "shear" or "distortional" waves), or R-waves (or "Rayleigh" or "Rayleigh-Lamb" or "Lamb" or "surface" waves).
Advances in computer modeling and analysis have led to new techniques to test multi-layer materials using surface waves, including Spectral Analysis of Surface Waves (SASW), Multichannel Analysis of Surface Waves (MASW), and Multiple Impact Surface Waves (MISW).
Impact Testing is related to other NDT techniques such as Ultrasonic Testing and Acoustic Emission Testing, although the former is more commonly associated with coupling of vibrational waves in metal and the latter involves passively listening for waves produced by cracks actively forming in a material.