Once the mounting surface material, shape, and size are known, the exciters can be placed. Exciter placement is critical to achieving uniform excitation of the surface while maintaining wide dispersion. For the best results, exciters should be placed near the center of the mounting surface, but should be offset from each edge of the surface so that the distance from the exciter to the edge is not an even multiple of the distance from the exciter to another edge of the panel, in order to avoid the buildup of standing waves.
For mounting exciters to a flat rectangular panel (a common application), the width of the panel should be less than 4/5 of the height, or vice versa, and good results will be given with the exciter mounted a distance of 2/5 of the panel width from one side of the panel, and 3/5 from the other side of the panel, with the same relationship used for exciter placement with respect to the panel height. This relationship will provide a satisfactory offset to reduce the buildup of standing waves on the panel.
If multiple exciters are needed for a panel or surface, it is not recommended to space the exciters evenly across the surface. This will result in very narrow sound dispersion at upper midrange and treble frequencies. Instead, place the exciters together in a group, with exciters at unequal distances from the panel edges and from one another.
Installation of most exciters is simple. An adhesive pad is provided with some exciters, which should be applied to a smooth, clean area of the surface. For other exciters that provide screw mounting via a mounting plate, it is still recommended to apply an adhesive such as “super glue”, hot melt glue, or double-sided tape to the mating surface of the mounting plate in addition to the mechanical fasteners, to avoid rattling or buzzing of the mounting plate against the surface. If attenuation of upper frequencies is desired (typically not desirable), this can be achieved with an inductor electrically in series with the exciter, or by adding a compliant pad (such as a sheet of rubber) between the exciter and the surface.
Edge Termination and Damping
As described earlier, prior to being radiated into the room by the vibrating panel surface, acoustic waves first travel through the material of the panel itself, as though it were its own acoustic environment, with its own speed of sound. When these traveling waves in the panel material encounter a different acoustic impedance (such as a different material or a panel edge), some or all of the traveling wave is reflected and propagates in a new direction across the panel. In some instances it may be desirable to reduce this reflected energy to improve clarity and transient response, and this can be achieved through controlled termination of the panel edge, or by applying a soft damping material to the panel itself.
The benefit of compliantly suspending part or all of the panel is a reduction in the internally reflected energy, because some of the energy is transmitted into the compliant material and does not get reflected. Compliant suspension of the panel edges may be accomplished using a soft material like foam tape, a silicone adhesive bead, or a flexible rubber member.
Other means can be used to control the vibration of the panel. For example, the corners of a rectangular or polygonal panel may be rounded to help reduce long-decay reflections. Linear or curved braces, point masses, and damping elements (such as felt buttons) may also be added to the panel to control its vibration, but these are generally not necessary except in highly-optimized applications. Engineering modeling software can aid in the application of these more advanced panel damping features, or the placement and use of these items can be determined experimentally.
Additional Tips
- Using a larger panel or installing the exciter to a larger surface will provide deeper bass response.
- A non-magnetic material should be chosen for the exciter mounting surface; mounting the exciter to a magnetic material may interfere with the magnetic circuit of the exciter’s motor assembly and impair performance.
- Suspending a panel at a central location will reduce bass response; exciter placement should be chosen to be between central mounting points and panel edges, instead of placing the exciter close to the central mounting point.
- Exciters are slightly less efficient than conventional loudspeakers. As a very rough guideline, expect to replace one speaker with two exciters in retrofit applications.
