Historically, percussionists have maintained that a perceptible difference exists in the responses of aluminum and brass marimba resonators. Brass resonators are thought to produce a fuller and richer sound, whereas aluminum resonators are considered to yield an inferior, thinner sound. Marimba performers will seek out and pay a premium price for instruments equipped with brass resonators, while lower cost aluminum resonators are found primarily on student model instruments. Experiments are being performed to determine if any such difference can be detected in laboratory measurements. This paper will outline experimental techniques used in recording airborne responses generated by repeatable, mechanized mallet strokes. Data analysis will be summarized, and results will be presented showing the nature of measured differences in both overtone structure and response evolution.