Development of an Innovative Acoustic Factory Acceptance Test for a Gas Turbine Enclosure

Challenges

Developing a meaningful acoustic test in the factory environment presented several significant challenges:

Incomplete Acoustic Boundary Conditions

The GT enclosure was fabricated separately from all intake and exhaust systems. As a result:

• Large openings remained where the intake and exhaust modules would normally interface.
• The enclosure floor was completely open, as the concrete plinth was not present in the workshop.
• Temporary sealing of these apertures would not accurately simulate the final in‑situ acoustic performance.

Inapplicability of Traditional Transmission Loss Testing

The ideal test—a Transmission Loss Test—requires:

• A fully sealed enclosure
• A calibrated internal noise source
• Calibrated Class 1 sound level meters measuring internal and external sound levels

However, due to the incomplete acoustic boundaries, performing this test would produce non‑representative and invalid results.

Highly Reflective Workshop Environment

The workshop environment contains many hard, reflective surfaces located close to the enclosure. This reverberant field would introduce:

• Strong sound reflections
• Inaccurate external measurements
• Invalid transmission loss calculations

Given these constraints, a traditional approach was impossible. A different type of test was required—one capable of operating reliably despite the environmental and acoustic boundary limitations.

Test Requirements

Any alternative method had to satisfy three core criteria:

• Immune to room acoustic reflections
• Capable of testing with partially sealed/temporary boundary conditions
• Able to demonstrate structural acoustic integrity and highlight flanking paths

Solution: Acoustic Surface Imaging Using Sound Scanner P132

To overcome the environmental and structural limitations, we developed a new testing methodology using our advanced Sound Scanner P132 acoustic camera. Operating from 125 Hz to 10 kHz, the camera covers the essential audible frequency range for assessing both low‑frequency and high‑frequency leakage paths.

Principle of Operation

The acoustic camera generates visual noise maps similar to thermal imaging:

• Red/yellow indicate high-intensity noise
• Blue/green indicate low-intensity noise

Similar to a traditional transmission loss test we use a loud calibrated noise source inside the enclosure with the sound scanner positioned externally, facing the exterior enclosure walls. The system captures a spatial “slice” of the sound field at a fixed distance from the sensor array. Using a laser distance measurement tool, the camera can be precisely focused on the external surface of the enclosure, allowing us to visualise the sound field directly at the enclosure skin, independent of room reflections.

Benefits of the Acoustic Camera Method

Immunity to Reflections

Because the camera visualises the sound field at the enclosure surface, reflected energy from the workshop environment is not included. Any reflections from surrounding surfaces such as nearby walls observed in the scene, can be visually identified and discounted.

Compatibility with Temporary Seals

Where temporary seals are installed:

• Expected hotspots around these areas can be visually identified and discounted
• Any unexpected hotspots outside these locations highlight potential structural flanking paths

Visual Verification of Acoustic Integrity

By imaging all surfaces of the enclosure, we can:

• Identify any unexpected leakage / flanking paths & implement remedial actions
• Confirm that the enclosure construction is consistent with expected performance
• Provide clear, visual evidence the enclosure is free from flanking paths

Results

The acoustic camera produced highly detailed images that accurately pinpointed noise escape. During the assessment, several small sealing issues were identified around the personnel doors. These were promptly rectified before final sign‑off and shipment to the customer.

Images shown include a pre‑rectification image, where a distinct hotspot is visible around the personnel door seal, indicating an acoustic leakage path. The other image is the post rectified image, where the leakage path is no longer visible.

Conclusion

The new acoustic FAT method successfully demonstrated that:

• Acoustic testing is achievable even when boundary conditions prevent traditional transmission loss measurements
• The Sound Scanner P132 camera provides accurate, reflection‑free surface acoustic data
• Structural flanking paths can be identified, verified, and resolved prior to dispatch
• The GT enclosure is free of acoustic flanking paths and is thus fit for purpose

This innovative approach now represents a viable testing option for similar enclosures manufactured under workshop conditions.