"The Audibility Of Cabinet Panel Resonances and Pat. Pend. Method Of Reduction Of Audible Coloration"
by
Albert Von Schweikert, Chief Design Engineer, VSA Corp.
Copyright July 2009 by A.V.S.
Lately, several high profile speaker companies have been conducting advertising campaigns using their cabinet construction methods as the topic of discussion.
One manufacturer of expensive speakers claims that stacked plywood slices using an aluminum baffle is a "magic" cabinet design, yet another claims that pure aluminum cabinets are "The Best In The World" when compared to any wood-based cabinet. A third manufacturer, using resin impregnated MDF, claims that their "proprietary ABC mystery material" is the best choice. Since all of these claims seem to have scientific facts behind them, and since it is well known that inexpensive thin-wall speaker cabinets can contribute a high degree of coloration due to panel resonance, VSA Corp. has undertaken a two year scientific study of panel resonances, their audibility, and methods of reduction. As we wanted to conduct the highest level of analysis, we used a famous university's lab, which is well stocked with laser interferometer, Finite Element Analysis software, and Fast Fourier Transform-equipped computer programs.
This paper seeks to inform the reader of a new Pat. Pend. design now utilized on all Von Schweikert Audio speaker systems, using a triple layer of constrained damping materials with opposing Q factors. See Photo #2 below.
In essence, our cabinet design utilizes a multiple laminate of three different materials, since there is no perfect single material with "magic" properties. In addition, our design goes one step further decoupling of the drive units from the baffle, and in the UniField Series, the baffle is further decoupled from the cabinet proper. This is accomplished by our proprietary use of a visco-elastic gasket that provides a mechanical barrier to vibration transmission. (See pages 4 & 5 to review this isolation technology).
THE AUDIBILITY OF CABINET WALL RESONANCES
Readers will instinctively realize that in order to reproduce a musical signal faithfully, an accurately designed loudspeaker should not add or subtract anything from the musical signal. Speaker systems claimed to be tuned to the orchestra by the use of resonant cabinet designs are nonsense. An accurate speaker should have no sound of its own, including cabinet panel resonances. After all, speaker systems are not "generators" of music, they are "reproducers" of music, which is self explanatory.
Below is a graph showing a 6" bass-midrange driver mounted on a traditional speaker cabinet with a damped baffle design versus an undamped baffle. An accurate loudspeaker must trace the signal as quickly as possible, and any time smear emanating from a ringing baffle or resonant cabinet wall will be highly audible if the amplitude of the vibration is sufficient. In addition, if the cabinet wall resonance falls into a frequency range that is excited constantly by music or the human voice, i.e., 100-400Hz, the resonances will be even more audible. Finally, due to the nature of high Q versus low Q effects, the type of damping utilized will have a large impact on the success of the method. A narrow, high Q resonance found at 1kHz may not be as audible as a low Q resonance found at 150Hz, so the type of damping and its effect on the resonance is highly frequency dependent. For that reason, a wide bandwidth resonance reduction method necessitates the use of several different types of materials, and their individual Q factors need to interact with each other in order to be effective at all offending frequencies. Successful implementation will result in greater clarity.
#1. Comparison of Damped Vs. Undamped Baffles
(http://www.audiocircle.com/image.php?id=20757)
NOTES TO ILLUSTRATION #1, ABOVE
In illustration #1 above, the Red line is the accelerometer-measured response of a mid-woofer mounted on a baffle using a compliant decoupling method. Note how quickly the impulse excitation has been damped. The 6" mid-woofer frame has been mounted onto the baffle using a synthetic clay absorptive gasket, effectively isolating its vibration from the front baffle. In turn, the baffle is decoupled from the cabinet itself, using the same compliant gasket method, thereby reducing the mechanical vibration transfer from the mid-woofer to the rest of the cabinet.
The Blue line is the response of the same driver, hard-mounted on the same baffle, without compliant decoupling. Note how the driver and baffle are still ringing up to 50 milliseconds after the impulse. Since the mid-woofer is mechanically attached to the baffle, and the baffle is mechanically attached to the cabinet, there is a high degree of unwanted transfer of vibration into the cabinet, effectively adding smear (a lack of clarity due to the constant ringing of the cabinet walls). This results in poor sound.
Since the surface area of a speaker cabinet can be up to 10 to 40 times the area of the cones, very little motion of the cabinet walls is necessary to become audible. The implication is that the cabinet vibration might have as much energy as the output of the driver itself, and this unwanted energy must be cancelled if true clarity is desired.