Why the Neutron™ May Be Used for MIMO Modal Testing

A Childhood Story of Modal Testing

There was an innate intuition in me drawn to resonance as it pertains not just to music, but everything else. On a minor vacation to northern Michigan in what was colloquially known as ‘up-north,’ I had with me a learner’s bow. A cheap fiberglass ordeal with a nylon string affixed to the upper and lower recurve, it was an easy draw and provided endless entertainment amongst the inherent boredom of the family cabins. I mean, a 12-year-old child can only watch his father play solitaire so many rounds before proclaiming, “Welp, I’m gonna go shoot the bow now!”

After perfecting the projectile arch and obtaining an adequate cluster on my target, I noticed the sweet sounds emanating from the bowstring after a successful arrow release. After each shot, I would attempt to deaden the string noise by touching it with my finger. Eventually, I would land on a nodal point and, voilà, a harmonic! A glorious multiple of the natural resonant frequency was available at the touch of a finger and I was off marking all of them with a sharpie; completely abandoning the whole point of the bow: Shooting arrows.

Mapping the harmonics of the string revealed roughly four distinct sounds that could be almost infinitely modified. By applying compression or tension on the bow itself. I didn’t know it at the time, but I was exciting primary modes on the string and changing the boundary conditions by compressing the bow. Fundamentals of Modal Testing.

If I was better at being a child-prodigy (or a child-prodigy at all) I may have mathematically simulated the string-modes and made my harmonic marking predictions by calculating measurements instead of discovering them by experimental analysis. Since I had all the time in the world, seemingly, the latter option was just fine.

My Adult Experience with Modal Testing

Many years after mapping the modal properties of a bow string, I was on my first remote engineering campaign while working with a satellite manufacturing facility. We tasked a vendor with Modal Testing of a new structure, where the goal was to produce an experimental modal analysis on our new structure and correlate the real-world measurements with the Finite Element Model (FEM) that lived strictly in the computer space. Much like Kevin Flynn, former employee of ENCOM, entered Tron and brought with him the abilities of the physical world, the real-world measurements of a structure were brought to the digital model to make it more useful. Albeit without the cool Light cycle racing of the hit movie.

The test we supported required many high-dollar implementations: A massive, isolated mass to ensure predictable boundary conditions. Many discreet shakers to excite the structure with incoherent inputs that can produce quality Frequency Response Functions (FRF’s.) A large data acquisition system to take many hundreds of channels of data.

All these things, combined with many years of experience from the personnel running the test, produced a quality result: A tuned digital twin (Computer model) capable of future redesigns that would suit our customers’ needs for years to come. Costly, but worth it. A true Modal Test.

ARS Neutron™ Efficiency

In my position at Acoustic Research Systems (ARS), I’ve been part of a very different kind of task. Designing, validating, and implementing the most efficient family of acoustic devices (possibly) that have ever been designed. Fortunately, there were more brilliant minds than mine who designed and brought to fruition the devices, so I was left more to the validation and implementation portion of the process.

To make the Neutron™ efficient, the driver configuration was made in such a way that the waveforms emanating from the front of the device were in-phase and well-timed at all the crossover points. This was one of the contributing factors to how a single device could achieve launch-vehicle levels (~145 dB OASPL) using roughly the energy available from a US wall-outlet. Impressive stuff, indeed.

It turns out that the same stuff that makes the Neutron™ efficient also make it perfectly suited to providing well-correlated forcing functions using only air as the transmission source. Much like one would attach a shaker (or multiple shakers) directly to a structure to provide a discreet forcing function to measure modal response via FRF’s, a single Neutron™ (or multiple Neutrons) may be used as well.

In recent experimentation, we’ve found that using the Neutron™ to excite large structures is not only possible, but perhaps desirable when compared to legacy options. Since no time alignment issues exist between separate speaker outputs (Subwoofers, Mid-drivers, and High-Frequency drivers,) the Neutron™ can provide a single, full-range output to excite components and frequencies to meet or exceed the shakers and impact-hammers of yore.

MIMO Modal Testing

In ARS Direct Field Acoustic Noise (DFAN) testing, multiple outputs are controlled individually to more accurately match the ideal diffuse acoustic field, which is defined as an infinite number of incoherent sources exciting a Device Under Test (DUT). This requirement of our acoustic test rig means that we cannot rely on summation of devices, something that traditional systems need to achieve high-level tests. Here, again, the same stuff that makes a good Diffuse Acoustic Field in terms of uniformity and timing are really good at providing quality Multiple Input, Multiple Output (MIMO) modal tests excited by only acoustic noise.

Though much is yet to be proven about this method, it seems that an acoustic modal-testing method may provide multiple advantages over traditional methods:

• Non-Contact – Unlike shaker or impact-hammer testing, acoustic MIMO testing needs only air to transmit the forcing functions for the entire component/structure. Modes on sensitive components such as mirrors or lenses may benefit especially from this type of excitation.
• Lower Cost – Since DFAN testing is already required of many DUT’s, adding a few minutes for modal testing is a trivial ask when compared to shipping the entire unit to a separate facility where traditional modal testing is performed.
• All Modes Excited – Impact hammers and shakers only excite highly mass-effective modes, which require massive portions of the structure to react to the direct input or the excitation device. Acoustic testing may excite modes that are not typically revealed due to unique coupling characteristics of the Acoustic Devices to the DUT.
• Correlated Forcing Functions – When a device is as well-tuned as the Neutron™ is, it may be possible to use each device, or groups of devices, to produce an accurate forcing function. This feature will produce FRF’s that are much more accurate than those processed with just a microphone or average-field input, which is common for Operational Modal Analysis (OMA) processed from acoustic input.

Proving validation for this type of testing will sufficiently fill my calendar for at least the remainder of 2025, and I can’t wait for the results to validate, or disprove, our research.

Conclusion

Far from the child that mapped the harmonics of his bow with a sharpie, and the annoying songs this action produced, I still enjoy the adventure and wonderment that dynamic testing provides. Hearing, feeling, and (now) seeing the dynamic response of a component or structure still evoke the same awe and emotion that I’ve found in my own personal experiences. I realize this may elevate me to a level of nerd that some may recoil from, but I’m proud to be a part of such things and glad to have found a family of like-minded individuals at ARS to relate to.

We have much to research, publish, and present on this in the months and years to come, so please stay tuned and share in our excitement. Our first publication will arrive with an experiment co-researched with ATA Engineering at this year’s International Modal Analysis Conference (IMAC). I hope it resonates with you as it has with me. Pun intended.