By Howard Brody, Ph.D.
You have seen and probably tried those little gummy, rubber like things that are placed in the strings of your racket in order to damp out vibrations. They cost a few dollars and weigh a fraction of an ounce. They come in all sorts of shapes and some people even make their own out of rubber bands or surgical tubing.
Do they do any good or are they just a placebo?
Experiments have shown that they damp out string vibrations rather well but they do essentially nothing to vibrations of the frame. Does this make them useful, or are they another gadget that you don’t need?
When the ball hits the racket, the strings deform. The ball then is propelled out by the strings snapping back. But the strings overshoot and oscillate for a while with a frequency that depends mostly on their tension and the size of the racket head. The frequency is in the 500 vibrations per second range and that is what you hear after a shot - the ping of the strings. (The musical note A is 440 vibrations/sec). A damper mounted in the strings does a good job in reducing the vibrations very quickly, and instead of a ping you hear a thud. Some players like the feel of the racket (or possibly the sound) with the damper in place. Others have tried it and do not find it necessary. There is no medical reason to use these devices. There is very little energy involved in string vibration (the strings weigh about ½ ounce) and these oscillation should not be the cause of any damage to your arm, with or without the damper in place.
Tests have shown that a single damper is very effective eliminating string vibrations, so it is not necessary to use multiple dampers, as some people do. The device must be placed outside of the last string of the head, to conform with the ITF and USTA rules.
Frame vibrations are a different story. The racket weighs 20 times as much as the strings and when it deforms and then snaps back, there is a great deal more energy involved. The frequency of frame oscillations runs from 100 to 200 vibrations per second depending on the frame weight and stiffness. Experiments have shown that the currently available dampers, when placed in the strings, will do absolutely NOTHING to reduce or damp out frame vibrations. Your hand is probably the best frame vibration damper around. The tighter you hold the handle, the sooner the vibrations die out (with your hand and arm absorbing the energy). Unlike string vibrations, which occur every time you hit the ball, frame vibrations are not a problem when the ball strikes near the center of the head (the location of minimum vibration is called the NODE). The farther the impact is from the node and the more flexible the racket, the bigger are the frame vibrations. When you hit the ball hard and near the tip (where the racket is most flexible), you may be tempted to look down at the court to see if any of your fingers have been shaken off of your hand and are lying on the ground.
Even though there is no clinical evidence claiming to show that racket vibrations do or do not cause the dreaded tennis elbow, there is the possibility that these vibrations may aggravate your arm. Therefore, you should make every effort to hit the ball close to the nodal point of the minimum vibration, which is near the center of the head of the racket.
Tests have shown that a single damper is very effective in eliminating string vibrations, so it is not necessary to use multiple dampers, as some people do. The device must be placed outside of the last string of the head, to conform with ITF and USTA rules.
Dr. Howard Brody is a professor of Physics at the University of Pennsylvania and member of the USTA Sport Science Committee.