The Invisible Speaker – PUI Audio Exciters

The Invisible Speaker

For over 100 years, the dynamic loudspeaker has undergone many changes in shape, style, impedance, and materials, but one constant has remained: a speaker’s diaphragm—the main part that moves air—needs to be directly coupled to open air for any appreciable sound to be created in volume.

This requirement of a speaker opening (for efficient use) can introduce undesirable aspects into products and devices where speakers are integrated. Be it the changing aesthetics of an otherwise slick, sleek design or the introduction of the product to hostile environment variables such as rain, dust, vandalism, or sunlight exposure.

With the introduction of PUI Audio’s new line of Audio eXciters, many of the challenges that design engineers now face when integrating audio can be overcome without sacrificing output.

Exciter Planar Dispersion

Exciters rely upon the voice coil being coupled directly to the inside surface of an object, transforming that object’s outer surface into an invisible, protected speaker by imparting vibration from the exciter’s moving mass into the surface. The vibration of the surface modulates air in such a way that sound is created in a planar-wave fashion. What this means is that (similar to planar and ribbon-type speakers) on and off-axis frequency response is nearly linear without even drop-off.

Listeners can be positioned up to 90° off-axis of the excited surface and experience the same sound quality as a listener positioned directly in front of the excited surface.

Typical speaker dispersion is such that frequencies above 5 kHz tend to become attenuated, starting at angles as little as 15° off-axis. Most exciters are also easier to implement than similar-size speakers as they do not need to be chambered or mounted into an “optimized” enclosure for exceptional low-frequency reproduction.

Many product designers mount speakers on bosses or stand-offs that prevent an annular seal from being created around the perimeter of the speaker’s frame or diaphragm. This introduces cancelation as the front of the speaker’s diaphragm is free to interact with the rear of the speaker’s diaphragm, creating a loss of low-frequency reproduction. Exciters not only use the interior of a device as an enclosure, the coupling with a surface creates an annular seal, preventing cancelation.

Most PUI Audio eXciters employ 3M™ VHB™ (very high bond) double-sided tape on their mounting rings for quick, safe and secure surface coupling. If your product is subject to shock, extreme heat, or extreme cold, it is recommended to use a mounting feature/bracket to take the mass of the exciter off of the VHB. Please contact PUI Audio for more information.

Frequency Response Measurements

Material type and thickness alter frequency response and affect the sound pressure level. Below are frequency response measurements taken from the ASX07008-WP-R when mounted in the same location on 2 mm (black) and 4 mm (orange), 190 mm diameter plates made of the same PLA material. The thinner material has more overall output from 1 kHz to 2 kHz, while the thicker material exhibits more output below 1 kHz.

The ideal material to use with an exciter would be lightweight, dense, and able to bend. The density of the material affects high-frequency performance above 4 kHz (treble). Typically, a higher density material will yield more high-frequency output. A material’s ability to bend affects low frequencies from 10 Hz to 250 Hz (bass) and mid-level frequencies from 250 Hz to 4 kHz (midrange). Materials that are more flexible will typically yield higher output in these ranges.

Use the average sound pressure level (AVG. SPL) rating on the specifications to select by sensitivity. Multiple exciters may be needed to be used on heavier materials. Some materials, such as metal panels, may impart their own sound to the output of the exciter, which may produce unintended audio output effects. Contact PUI Audio for assistance at reducing these effects.