***** Acoustic noise in the DET Lab at different supply and return fan speeds *****

The scope of the test was to evaluate the acoustic noise at different supply and return fan operating speeds, determine the configuration corresponding to the minimum noise level, and monitor the system parameters during overnight operation.

The morning session was dedicated to preliminary investigations. The HVAC system was progressively shut down, including switching off the supply and return fans, isolating the water distribution (valves located in the CEB hall AHU room), and closing all supply, return, and outdoor air valves.
Tapping tests were also performed on the sections of the supply (bottom) and return (top) air ducts entering the interspace where the inverters are located.

The system was restored to its initial configuration before the fan speed tests.

During the afternoon session, supply and return fan speeds were tested by keeping one fan fixed at a given speed while varying the other; the fixed speeds were 24 Hz for the supply fan and 13.8 Hz for the return fan.

*** The supply and return fans were left at 27.5 Hz and 13.8 Hz, respectively, as a potentially low-noise configuration, to monitor acoustic noise and environmental parameters overnight. ***

Detailed actions are reported in the attached file.  A dedicated analysis will follow.

Comments on the test of the DET AHU supply and return fan speeds.

Figure 1 and Figure 2. Spectrograms of the microphone in DET lab (*EDB_MIC*) during the main actions carried out in the morning and afternoon, respectively.

Figure 3. The plots show the environmental parameters in the DET lab during the fan speed test and over the following period, with the selected fans configuration (supply @27.5 Hz, return @13.8 Hz) kept overnight. Temperature, relative humidity, and pressure signals do not show significant fluctuations, indicating that the chosen configuration provides stable environmental conditions.

Figure 4. Increasing the supply fan speed results in a clear increase in acoustic noise, as observed in the RMS levels across different frequency bands (less clear in the 1–16 Hz band). When varying the return fan speed, the effect on acoustic noise is generally weaker. However, an increase in the RMS level in the 16–512 Hz band can be observed at higher return fan speeds.

Figure 5, 6, 7.  With the return fan fixed at 13.8 Hz, reducing the supply fan speed down to ~22.7 Hz leads to acoustic noise levels that, up to a few hundred Hz, approach those measured with the system switched off (dashed black curve). When varying the return fan speed while keeping the supply fan fixed at a lower value than the reference (red curve, supply@31.8 Hz, return @13.8 Hz), a slight reduction in acoustic noise can also be achieved.

Figure 8. Reduction factor of the acoustic noise with respect to the reference configuration (supply @31.8Hz, return @13.8 Hz) during the test.

Figure 9. The figure shows a comparison of the acoustic noise inside the DET Lab during the last site-wide HVAC system switch-off (elog 68269). Switching off the DET AHUs (blue curve) reduces the acoustic noise inside the lab. A further reduction is achieved when the supply and return AHUs of the CEB hall are also switched off (green curve).

The table reports the air velocity and airflow values measured during the test.

** Configuration left overnight and during the weekend

Duct area = (100 × 35) cm² = 0.35 m²
Airflow = duct area × air velocity × 3600

In the table below are reported the latest actions performed on the supply and return fan operation.

In Figure 1, a slight temperature drift of approximately 0.1–0.2 °C is observed over time, together with oscillatory behavior in the humidity control loop.

*** Erratum ***

Due to an offset between the Kieback & Peter interface and the inverter, the supply and return fan speeds reported were incorrect.
The values in the table below have been updated, and an additional frequency step has been added.

Since yesterday 12:00 LT (April 29) the DET AHU is running in MANUAL mode (fixed fan frequencies) at slightly reduced rate of the SUPPLY fan (28.5 Hz instead of usual 31.5). Ambient parameters (TE, HU, PRES) look stable enough (see attached) so we decide to keep it in this configuration untill Monday.

A few more observations concerning the test of April 23:

• at some point, while the AHU was off, we stopped the water flux by closing the valves located inside the CEB AHU room. Unespectly we measured an increased acoustic noise in the range 50 to 100 Hz (Figure 1)
• between 8:49 and 11:13 UTC, while the AHU was off, we closed all air valves of ducts (all set to 0%). In principle we would expect some change in the position of low frequency spectral peaks, if they were associated to acoustic modes of the air ducts. We did not notice any change (Figure 2).
• acoustic spectrograms show some peaks changing frequency, mostly above a few hundred Hz. They seem correlated to the frequency of the SUPPLY fan (Figures 3 and 4). Yet, we then notice that similar ones are present also when the HVAC was running in MANUAL modes (that is when both fans frequency fixed): Figure 5. Could they be associated to the air flow regulators instead?