Electrostatic Panel Membrane Heat-Treating Instructions
When type C (capacitor grade) Mylar film is heated to a temperature in the range of 300 degrees F (149 degrees C) its molecular structure attains a predictable and stable configuration. This is referred to as the “critical” temperature. Heating the film slightly below this temperature will not cause the desired transformation and heating it slightly above this temperature can melt the film. As a result, the temperature window to attain this condition is relatively narrow. Sound Lab speakers use this temperature characteristic of Mylar to advantage.
The physical design of the panels is such that when Mylar is heated to the critical temperature it assumes the correct tension for the speaker design. If for some reason the membrane has low tension, heating it to the critical tension will tighten it. On the other hand, if the membrane has a higher tension than desired, heating it to the critical temperature reduces the tension to the proper degree. Thus, the tension in the membrane can be set at the desired point anywhere in the world. It does not need to be done at the factory. The procedure to properly heat treat the Mylar membrane is the purpose of this note. The procedure is simple but special precautions need to be exercised so as not to damage the membrane.
In order to accomplish the heat treatment procedure you will need a hot-air gun that can attain the proper temperature. Most hair dryers cannot reach this temperature. Therefore, we recommend a suitable industrial-rated heat gun. At Sound Lab we use a hot-air heat gun available at Sears, Model 9 27801 Craftsman Industrial Heavy-Duty Heat gun. The heat output of this gun is thermocouple controlled and is adjustable. We highly recommend this tool or its equivalent.
Once you have a heat gun that can attain the required temperature, it is very important to know how to use it so as not to burn holes in the membrane. Just one side of the membrane needs to be treated. In order to do the treatment, the grill cloth must be removed from the side of the panel that you choose to work on. To remove the grille fabric so that it can be replaced after the treatment, using a utility knife, cut the fabric just outside of the line of staples that secure it. It is recommended that only the sides and the bottom of the panel need be cut. Fold the cloth back out of the way during the treatment. After the treatment, replace the cloth to its original position by carefully lining up the cut edges and, using a heavy-duty stapler, staple the fabric back in place.
Turn the heat gun on and let it run for about a minute in order for it to thermally stabilize. Hold it about 20 inches (50 cm) from the speaker. Start moving the nozzle of the gun back and forth in a sweeping motion where the extent of the sweep is about 6 inches (15 cm) at a rate of about one second to go from the left to the right and another second to go from the right to the left. During the procedure, never stop this sweeping motion. If you were to stop the sweeping motion, it would be at the risk of burning a hole in the membrane.
While you are sweeping the nozzle of the gun back and forth, with the nozzle pointed directly at the membrane, slowly bring the nozzle of the gun toward the membrane. If the membrane has visible wrinkles, start in this area. Orient the speaker so that light reflects off of
the membrane in a manner that the wrinkles are clearly visible. As the nozzle of the gun is brought closer to the membrane (keep it sweeping back and forth) watch for the wrinkles to disappear. This usually happens rather suddenly. At this point, do not bring the nozzle any closer to the membrane. You have reached the critical distance where the temperature at the membrane is correct.
Make note of the distance of the nozzle to the membrane when the wrinkles disappeared. Carefully maintaining this distance, and keeping the gun moving in the sweeping motion, go over the entire membrane in over-lapping strokes, even over areas where you may not see wrinkles.
In the event the membrane has too much tension and wrinkles are not visible, set the temperature of the gun to 300 degrees F (149 degrees C) and, with the sweeping motion, slowly bring the end of the nozzle to about 6 inches from the membrane and heat treat the entire membrane. Then, test for membrane resonance. The resonance peak should be on the order of 32 Hz for large panels and 38Hz for smaller panels. This peak may vary a few cycles either way depending on environmental factors such as humidity, temperature, etc. A real-time analyzer is a most useful tool to determine the membrane resonance. If one is not available, listening to a test CD with low-frequency tones or listen to music CDs that have extended bass energy. If no improvement is apparent, repeat the operation with the nozzle at a distance of 5 inches (13 cm) from the membrane. Again, if no improvement is obvious, repeat the operation with the nozzle at 4 inches (10 cm) from the membrane. Do not attempt to go closer than 4 inches (10 cm). If noticeable improvement has still not occurred, contact the factory for further instructions.
Having successfully completed the heat treatment on the panel membrane, it should have the factory calibrated tension.
Sound Lab 11-15-02