Tonal noise and HVAC noise is noticeable in the broadcast studio of WACV- TV station in Ashland, Massachusetts. In turn, this unwanted noise is recorded on the studio’s audio recorder via microphones situated in the studio. At your request, David Coate Consulting (DCC) performed noise and vibration tests in the facility on July 25, 2014.
David Coate Consulting (DCC) performed various tests and analyses to determine what acoustic treatments would be required to meet these design criteria.
The tonal noise is caused primarily by the older of the two station’s transformers. Analysis of the measurement data indicates that structural vibration at harmonics of 60 Hz (mainly at 120 Hz) appears to be the main cause of the noise. In this case, building elements such as the ceiling and wall adjacent to the transformer are vibrating and generating noise analogous to a large loudspeaker. HVAC noise in the studio is also significant.
The new transformer also generates vibration/noise in the studio, but at lower levels than produced by the old transformer. While this does not appear to be an airborne noise issue, that cannot be absolutely confirmed without first installing proper vibration isolators on the transformer(s). The Recommendations section of the report provides details on vibration isolation of these units.
Given the complexity of the situation including the HVAC noise issue, I recommend that various modifications be made in steps incrementally to see how each step affects studio noise levels.
Published “Noise Criteria” for television broadcast studios recommends a maximum of NC 25. With the HVAC and transformers on, noise measurements in the studio indicate NC 28. However, the NC method does not adequately account for tonal noise, so the small increase from NC 25 to NC 28 understates the problem. With HVAC and the old transformer off, the measurements indicated NC 18 which gives a better indication of the extent of influence of HVAC and transformer noise (i.e., NC 18 vs. NC 28).
Noise and Vibration Measurements and Analysis
Airborne Noise from Transformers
In order to compare airborne noise levels of the two transformers, noise measurements of each were taken 3 feet away from each unit. Figure 1shows that the old transformer generates higher noise levels than the new transformer, but only by about 3 decibels at the problematic 125 Hz tone.
As a matter of fact, the higher noise levels at 630 Hz and above were likely caused by another noise source such as the rack of equipment also in that room.
During the field tests, because of the overall higher airborne noise level of the old transformer, I initially suggested replacing it with a new one. That approach may still be warranted at some point, but since the 125 Hz tone is only 3 dB higher and other data (described below) indicates this is a vibration problem, it makes sense to first install proper vibration isolation.
Effect of Mezzanine Doors
The next test employed was to determine the effects of the two doors in the studio leading to the mezzanine level. Figure 2 shows that the two spectra, “doors open” and “doors closed” are nearly identical. Therefore, it does not appear that these openings are a major path for airborne noise from the transformers. As a matter of fact, this lends credence to the probable structural vibration cause since vibration would be unaffected by the doors being open or closed.
Figure 2 also shows the effect of the old transformer turned off. As can be seen in this figure, the 125 Hz tone is essentially non-existent, a full 15 dB lower than with the old transformer on. This data clearly shows that the old transformer is causing the 125 Hz tone in the studio.
The next test was to determine the effects of the HVAC noise. Figure 3 shows the frequency spectrum with rooftop units 1 and 3 on. With the HVAC on, overall levels increased from 27 dBA to 32 dBA. It can be seen from the figure that noise levels at certain frequencies increased. At this time, it is not clear why the 60 Hz peak increased so much. In other words, there is no reason to expect that peak noise levels from the HVAC equipment would line up so precisely with the peaks associated with the transformer noise.
Transformer Structural Vibration-Induced Noise
Vibration measurements were conducted by affixing an accelerometer to the studio wall adjacent to the transformer. These data were used to calculate the theoretical sound pressure level (vs. frequency) that would be generated by the vibrating wall. This data was compared with actual noise measurements 3’ from this wall. The two sets of data are in reasonable agreement, particularly at 125 Hz, which strongly supports the structural vibration probable cause.
A separate airborne noise analysis was conducted to calculate the expected Noise Level Reduction (NLR) of the wall if this were an airborne noise issue. NLR data for a similar 6” wall was used and subtracted from the airborne noise level data measured 3’ from the transformer. This data shows that if this were an airborne noise issue, transformer noise should be at least 20 to 30 dB lower than it actually is.
This separate airborne noise analysis also supports the structural vibration probable cause.
Existing Vibration Isolation of Transformers
The two transformers are currently connected to the ceiling trusses via a metal frame which is not connected to the floor. The bases of the transformers are resting on 4 vibration isolators which appear to be a very high (hard) durometer neoprene or rubber. This type of material is typically used for vibration isolation for much heavier equipment (the transformers weigh about 300 lbs.). Using static deflection data from a similar appearing rubber product, and PSI data versus load data, the deflection of these isolators could be as low as .005 inches. With such a low deflection, much of the vibrational energy from these units is probably being directly transmitted to the metal frame and to the surrounding structure. The precise nature of the existing condition is difficult to determine since the metal frame itself can be thought of as a giant spring, with its own dynamic properties. However, it is likely that the metal frame is very stiff and thus readily transmits vibration as well.
The following steps are recommended in succession to determine the most effective noise/vibration strategy:
1) Re-install the old transformer on the floor with new vibration isolators. The transformer should be placed in a location similar symmetrically to the location (in plan view) of the new transformer. It may be that there will be some benefit to getting the transformer further away from the studio wall. There should be no physical connection between the transformer and the studio wall or ceiling. Any connection via wiring should be carefully examined to make sure that there is no corresponding structural path.
Install 4 Kinetics Noise Control AC-222G vibration isolators on the 4 corners of the base of the transformer. Based on the transformer weight of 300 lbs., the resonant frequency of this spring-mass system would be in the 10 Hz range, well below the forcing frequencies of 60 Hz and 120 Hz. Our calculations show that this should result in a 13- 21 dB improvement at 120 Hz.
2) Depending on the outcome of step 1, repeat the same steps for the new transformer.
3) If the results of step 1 are not adequate, replace the old transformer with a new one.
4) Various penetrations in the studio wall should be completely filled with a massive substance such as plaster or cement grout. Even slight gaps should be completely filled with non-hardening acoustical caulk.
5) The mezzanine doors should be replaced with either one or two solid core doors that fit very tightly. In the existing condition, the airborne acoustical performance of the studio walls is defeated by these two acoustical holes.
6) Further investigate HVAC noise issues. The cause of the 60 Hz noise issue is unknown at this time. Unacceptable HVAC noise can be caused by several independent mechanisms. In some cases, a silencer is needed on the output/intake of the blower unit. Large cross-sectional areas of ducts are usually needed to achieve low air velocities suitable for broadcast studios. Highly absorptive duct liner completely lining ducts can also provide additional noise reduction.