Dealing with tile complaints: Hollow-sounding tiles can mean poor installation.

Test for hollowness.
When you receive a tile failure complaint like cracking and lifting, one of the first and most easily conducted tests you should perform is to check whether the tiles sound hollow. This can be a indication that the tiles were poorly laid, in which case the so-called failure is not the responsibility of the tile supplier or manufacturer but of the installer. In general hollow-sounding tiles are an indication that the adhesive does not have a good bond between the tiles and the substrate or that there are hollow voids of missing adhesive under the tiles. As the floor was used and subjected to foot traffic, the tiles that were nor correctly laid may have cracked or even lifted off the floor (debonded).

What bad tillers do.
The number one way for a bad tiler to cut corners and keep materials costs down is to use the bare minimum of tile adhesive. Typically this is done by putting a blob of adhesive under the corner of each tile and one in the center. This is called spot-bonding and is completely contrary to all local and international standards. The tile thus has an empty void underneath and is un-supported.

One problem leads to another.
Spot-bonding does not meet industry standards for the 80% coverage required for residential floors or the 95% coverage required for commercial floors, exterior applications and interior wet applications. Spot-bonding also reduces the bond strength of the tile and its attachment to the substrate, making it more susceptible to stress and causing debonding under certain conditions. Voids that occur with spot-bonding become pockets for water and moisture to collect, which could lead to efflorescence and other moisture-related problems. The lack of support beneath the voids also make the tile more vulnerable to cracking damage from live loads such as falling heavy objects, heavy equipment, rolling vehicles, loaded dollies, carts and even normal foot traffic.

How to check.
A simple way to check for the sound in question is to tap on the tiles with a hard object such as a thick coin or a steel ball bearing. Chains or special sounding devices can be used for larger areas. If the tile is well bonded (i.e. attached to the concrete substrate), it will have a high-pitched sound. If you hear a lower-pitched or a hollow sound, this indicates that  tiles have either debonded or were never bonded to the substrate.

But...
A low tone is not an absolute indicator of debonding however. A low-pitched sound can also occur if the tile is installed over a less-dense substrate like wood, a sound-control mat or other membrane. Notice if the hollow-sounding tiles are grouped together or are the spread out randomly. If an entire tile installation sounds hollow it may be a result of the type of substrate that was used and not an indication of a problem. Generally there are other symptoms like loose or cracked tile and grout. Consider the solid-sounding tiles: do they have the same problem but haven’t debonded yet and might at a later date?