A/V RoomService Ltd. – Law Office vs. Community Theater Noise Case Study
Scope: In this condensed case study of law office vs. community theater, A/V RoomService was requested to perform acoustic analysis for the purpose of defining noise transmission from the theater venue into the law office space as a result of complaints of noise disturbances by the law firm. We visited the site prior to testing in order to learn more about the structure and to interview individuals from both parties regarding the noise problem. It was discovered that there are two common walls being shared by the parties. After our initial investigation, it was determined that we would perform ASTM E-336 (Measurement of Airborne Sound Attenuation between Rooms and Buildings) tests to evaluate the sound isolation partitions of the two common walls. We also plotted Noise Criteria Curves for the office space. All equipment was calibrated on-site prior to testing. It was also determined that we would conduct our tests after hours in order to avoid outside traffic noise and office personnel and equipment from corrupting our data.
Test 1: The Noise Isolation Class (NIC) ratings (NIC rating was calculated in accordance with ASTM 413) for this series of tests were conducted between the theater lobby (sound source side) and the adjacent office space (sound receiving side). The NIC results ranged from an NIC-57 to NIC-59 and included 7 microphone locations in the office space. NIC ratings of this level are considered acceptable for typical noise levels generated between the theater lobby and adjacent office space. This range of NIC results would be estimated to translate to STC (Sound Transmission Loss) performance levels in the low to high 60’s. It should be noted that while NIC ratings of this level are considered adequate, they do not assure a sound proof environment. As noise levels increase there will always be a possibility that some level of noise transmission to the adjacent space can occur, especially in the low frequency range.
The noise reduction decibel levels for each microphone location indicated no serious decibel deficiencies over the frequency spectrum of 125 hertz to 4000. A general dip in noise reductions from approximately 250 hertz to 1000 hertz was noted, but is typically indicative of this type of construction. The NIC rating based on the average of all 7 receiving microphone locations achieved a NIC-58 (see graph 8) and agrees with the observations noted above. During our inspection of the basement, there was some evidence of noise transmission occurring via the floor joist, however this path did not appear to be significant.
Test 2: The average NIC value based on 6 microphone locations over the 1st and 2nd floors indicated a NIC value of 61. (See graph 15) Again, this performance level is considered to be at an acceptable level for this type of theater separation wall construction. Again, the projected STC levels for NIC ratings of this magnitude would be expected to achieve STC rating from the low to high 60’s.
As was noted above, no wall is soundproof and will transmit noise through it relative to the noise levels and frequencies being generated on the opposite side. A wall of this performance level, as with the lobby wall, is expected to adequately attenuate most generated noise levels from the theater. However, there will be times when high level mid frequency noise and low frequency noise achieve a decibel level which will penetrate the wall and be heard in the business space. The principle to understand is that annoyance levels, while significantly reduced, cannot be eliminated. For example, a poor noise control wall may have an annoyance level of 75%, while a high performance wall construction only 10%. Both walls will have potential complaints, but the quieter wall will have overall fewer complaints.
Test 3: This test looked at the noise reduction levels associated with the double doors separating the main theater seating area from the lobby on the 2nd floor. No sound gaskets were noted on the doors and as a result achieved a NIC rating of 22. (See graph 16.) The noise reduction curve indicates significant reductions in the mid to high frequency range which would indicate that the lack of door seals are limiting the doors ability to reduce noise transmission. As a result noise levels from the main theater to the lobby would be expected to be elevated due to this condition.
Test 4: During our recording of noise in the adjacent office space, it was observed that intermittent noise was coming from the roof area. While the exact location and source of this noise could not be identified, a noise spectrum of the noise was taken when it was noticeably on and off. The comparison of these two spectrums indicate increase noise levels indicative of motor harmonics. This spectrum indicated increase ambient noise levels at 80 and 160 hertz as noted on graph 17. After a quick investigation, we did note that the new HVAC generators for the theater were mounted directly to the rooftop with no type of isolation system in place.
1a) After studying the data gathered, as well as experiencing the environment personally, our first recommendation is to install an electronic noise masking system into the office space. The reasoning behind this simple solution is due to the quiet noise floor inside the office space (after business hours). With such a low ambient noise floor, small noise introductions become noticeable, where under normal office conditions they would not. It should be noted that it was necessary to conduct our test after hours in order to eliminate noise introduced from other than the source of concern under test, i.e.; the theater. During normal business hours traffic noise and office personal and equipment occupy the noise floor. We understand that sometimes office personnel might be working after hours. It is then that the noise floor would be so low as to notice small noise introductions. It is also after hours that the theater would be producing the majority of their high noise levels. We noticed while performing our tests for noise floor in the office space that you can hear yourself breathe, swallow or stomach making noise.
Noise Criterion Curves (NC) are a series of standard curves of octave-band sound spectra in a system for rating the noisiness of an indoor space; a measured octave-band spectrum is compared with this set of curves to determine the NC level in the space. The determiner is the highest NC curve tangent to the noise spectrum. The lower the number, the lower the noise floor. Expectable levels for executive offices are NC 25-30. The average ambient noise level of the office space next to the theater without the roof noise was NC-20 as indicated in graph 18. This equals an approximate A-weighted sound level of 23-28 dB. With the introduction of an electronic masking system to raise the ambient noise floor levels, small noise introductions would become unnoticeable.
1b) Along with the masking system it is recommended that door seals be installed to all doors in the theater including the bathroom doors which share the common wall with the offices. We noted significant levels of noise escaping from the theater through the double doors separating the theater and the second floor lobby.
1c) All AC receptacles, switches, HVAC grills and any other penetrations above and below the drop ceiling, both in the theater and the office space, must be sealed with acoustic caulk. We noticed in particular that the outlet on the west office wall common to the theater had audible noise as well as airflow passing through it.
“We believe these are the correct recommendations based on our findings and are surly the most cost-effective and least corrupt solution to the problem.”
A/V RoomService, Ltd.
2a) In addition to the above, should it become necessary, a secondary wall would be built, on the office side, where the common walls to theater and office space is located going up the stairs. This wall should extend from the roof joists above the drop ceiling down to the second floor and under the stairs of the first level. See attached wall construction material and installation recommendations.
2b) As above, applied to the common wall of the theater lobby/office hallway that runs north and south.
3) If the above is still unacceptable, it is due to low frequency noise vibrating the building structure. The only path left to treat is the floor of the office space. Installing a floating floor system would “break” the last physical connection to the theater, thus isolating it from the theater. See attached floor construction material and installation recommendations.