Utilizing the “Free Field” and constructive interference for low-frequency precision.
One of the greatest challenges in underwater acoustics is the “free field” limitation. In environments like lakes or large pools, the water’s surface acts as a near-perfect reflector, often causing signal cancellation. The AKUASOUND 200 introduces a revolutionary method to turn this challenge into an advantage.
The Problem: Surface Interference
When a source and receiver are submerged, the wave reflected from the surface can interfere with the direct wave. At low frequencies (like 63 Hz), this usually requires a minimum immersion depth of 27 meters to avoid signal degradation—a logistical nightmare for small boat operations.
The Solution: Constructive Interference Geometry
The AKUASOUND 200 features opposing emission faces. Instead of fighting the surface reflection, our new method exploits it:
Shallow Immersion: The source is placed at a depth equal to a quarter of the wavelength (e.g., just under 6m for 63 Hz).
Phase Alignment: The upward wave reflects off the surface and rejoins the downward wave in phase.
Amplitude Boost: This creates constructive interference, effectively doubling the signal amplitude at the receiver.
Field Validation: The Lake Bracciano Test
This method was rigorously tested at Lake Bracciano, Italy. Chosen for its 167m depth and silence (due to navigation restrictions), the site allowed us to operate at 63 Hz without bottom reflections.
fig. 9 (both, left and right) – Lake Bracciano bathymetry and area identified for experimentation
Results from the Field:
Logistical Ease: Using a simple work pontoon, the source depth was easily managed between 1m and 6m.
Stability: The shallow positioning improved the stability of the distance between the source and receiver.
fig. 10 – Transmission apparatus Precision: For higher frequencies where the “interference method” is less practical, the system allows for “time gating” to isolate the direct wave, ensuring clean data regardless of the environment.
fig. 11 – Sound source immersion point
By combining Italian engineering with innovative acoustic geometry, the Akuasound system simplifies complex underwater tasks, making high-quality low-frequency transmission accessible and safe.
