***** 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?

Some comments on the test involving the opening of the valves in the DET laboratory are reported below. The main actions performed during the test are summarized as follows:

• The AHU operating mode was changed from automatic to manual. In this configuration, the supply and return air fan speeds were fixed at values of 27.4 Hz and 13.8 Hz, respectively.
• All supply and return air distribution valves were switched from automatic mode, corresponding to the specific openings reported in Table of elog 68181, to a fixed opening of 5%.
• Additional tests were performed by varying the opening percentage only for the valves serving the Mini-Tower Area A (DET area), while the remaining valves were kept fixed at 5% opening and the fan frequencies were maintained at fixed values.
• At the end of the test, all valves and the AHU were restored to automatic operation.

Figures 1, 2, 3, 4 show zoomed views of the acoustic spectra measured by the microphone ENV_EDB_MIC during the transition from automatic operation to fixed fan frequencies and valve openings. The frequency ranges displayed are: Figure 1 (1–100 Hz), Figure 2 (100–400 Hz), Figure 3 (400–700 Hz), and Figure 4 (700–1000 Hz).

The colored curves correspond to three different operating configurations of the AHU system:

• black curve: supply and return fans, as well as all valves, operating in automatic mode.
• red curve: supply and return fan frequencies fixed at 27.4 Hz and 13.8 Hz, respectively, while all valves remained in automatic mode.
• blue curve: same fan configuration as the red curve, with all supply and return air distribution valves additionally set to a fixed opening of 5%.

Figures 5, 6, 7, 8 report the corresponding reduction factors with respect to the reference configuration (AHU operating in automatic mode).

Overall, fixing the supply and return fan frequencies does not significantly modify the acoustic spectrum, although a moderate reduction is observed in several frequency bands. A more pronounced effect is obtained when all supply and return air distribution valves are set to a fixed opening of 5%.
The ASD comparison shows a reduction of several broad acoustic structures, particularly below 400 Hz. This observation is confirmed by the reduction factor analysis, which reaches values between 2 and 5 over extended frequency regions. Above 400 Hz, the impact of the modified AHU configuration becomes less evident.
Moreover, the time-frequency maps of Figure 9 and 10, show the presence of several non-stationary spectral features above 300 Hz whose amplitude and frequency evolve throughout this part of the test. Consequently, the large reduction factors observed at specific frequencies are influenced by the temporal evolution of these spectral structures in addition to the changes introduced in the AHU operating configuration.
Overall, the results suggest that the reduction of the valve openings has an impact on the acoustic environment than fixing the fan frequencies alone, providing a measurable reduction of the acoustic noise level, particularly below 400 Hz.

**** Mini-tower A area (DET area) ****

Figures 11, 12, 13, 14 show zoomed views of the acoustic spectra in the same frequency ranges reported previously, measured while varying the opening of the MiniTower A return air valves (50%, 90%, and 5%) and keeping the supply air valves fixed at 5% opening. 
The acoustic spectra measured during the MiniTower A valve scan show that all tested manual configurations provide a reduction of the broadband acoustic noise with respect to the nominal automatic operation (black curve), particularly below 400 Hz. The configuration with the return valve set to 90% opening (blue curve) generally provides the lowest acoustic ASD over a large fraction of the investigated frequency range. In contrast, the configuration with the return valve set to 50% (yellow curve) introduces several additional narrow-band features above 400 Hz.
The time-frequency analysis (Figures 15, 16, 17) confirms that several spectral structures appearing above approximately 400 Hz are correlated with specific valve configurations and evolve following the changes introduced during the test.
Moreover, Figure 17 shows that the group of spectral lines observed in the 600–1k Hz region during the configuration with the return air valve set to 90% disappears approximately five minutes after switching the valve opening from 90% to 5% (10:15 UTC). 

Figures 18, 19, 20, 21 show the acoustic spectra measured during the last part of the test, where the return air valve opening was kept fixed at 5% and the supply air valve opening was varied between 50%, 90%, and 5%.
The comparison shows that the configuration with both supply and return air valves set to 5% (red curve) provides the lowest acoustic ASD over most of the investigated frequency range. This reduction is particularly visible between 100 and 400 Hz, where several broad structures observed in automatic mode and in the configurations with larger supply valve openings are significantly reduced. The same configuration also reduces the level of several spectral structures, especially in the 300–650 Hz region as shown in the spectrograms Figure 22, 23, 24.
In contrast, the configurations with the supply valve set to 50% (yellow curve) or 90% (blue curve) show higher acoustic levels and are often comparable to, or above, the automatic-mode reference.

An additional TF map (Figure 25) during the restoration of the MiniTower B valves to automatic operation (11:45 UTC) shows the reappearance of several narrow-band spectral features that had previously disappeared when both MiniTower A and MiniTower B valves were operated in manual mode. The temporal coincidence suggests that these structures may be associated with the airflow distribution of the MiniTower B branch. This behavior is particularly visible for the spectral features observed between approximately 300 Hz and 650 Hz.

**** Summary ****

Figures 26,27,28, 29 show the acoustic spectra for the nominal automatic operation (black curve), the configuration with all valves fixed at 5% opening (gray curve), and the two most favorable Mini-Tower A configurations identified during the test: supply valve at 5% with return valve at 90% (red curve), and supply valve at 5% with return valve at 5% (blue curve).

• The comparison shows that the nominal automatic operation is not the quietest configuration.
• The configuration with all valves fixed at 5% opening also provides a reduction in some bands, but it introduces additional narrow-band structures, especially above 400 Hz. This suggests that forcing all valves to the same low opening is not necessarily the optimal acoustic configuration.
• Among the tested configurations, the most favorable ones are those with the Mini-Tower A supply valve fixed at 5%, namely supply 5% / return 90% and supply 5% / return 5%. However, the nominally identical 5% / 5% configuration obtained during the return-valve scan (return fixed at 5% and supply varied) resulted in lower acoustic levels than the 5% / 5% configuration obtained during the supply-valve scan (supply fixed at 5% and return varied). This indicates that the acoustic response may depend not only on the final valve openings, but also on the sequence of valve adjustments and/or on the airflow stabilization time.

The results suggest that an optimized valve configuration could reduce the acoustic noise in the DET area with respect to the nominal automatic operation. Further dedicated tests are required to verify the reproducibility of the most promising configurations and to assess their long-term impact on the stability of the clean-room environmental parameters.