Hello. Can you explain a step by step process of how you were conducting the experiment so we can hopefully determine where in the process things are going wrong? It sounds to me as though you are testing to see if the sound changes on the same string whether or not the corresponding key is depressed. This is not correct.
You want to test this: If you strike a key (key 1) and make its corresponding string vibrate, will that motion cause another string for a different key (key 2) to vibrate? In order to test this, you must study the concept of harmonics (an introduction to this concept is on the topic idea page).http://www.sciencebuddies.org/science-fair-projects/project_ideas/Music_p012.shtml
Based on harmonics, by depressing key 1, there is a mathematical way to determine which strings could be triggered to vibrate simply by another string vibrating. The hammer must be lifted (hence the requirement of you SOFTLY depressing key 2 to release the hammer without vibrating the string) so that the string is free to vibrate if, in fact, this theory is correct.
Please write back with more information as to how you are carrying out your experiment, and we'll go from there.