I recently helped in designing closed spaces – a multiplex and a recording studio. He wanted to know the how-to for basic acoustics.
While reading my response to his emails today, I decided to put it up on the blog as many will benefit from this information. I admit, it’s not an ultimate acoustics guide, but a starting point for more research, especially for beginners who want to understand basic acoustics. These tips have been mentioned for a multiplex movie theater, but applies universally.
These are some factors you need to consider for acoustics in any closed space. The figures recommended here are for a movie hall and can be varied according to the space you are designing.
1. RT60: The recommended RT60 is 0.8-1.2 seconds. Make RT60 calculations using best available specs for all the material you use.
2. STC – Sound Transmission Class is also known as Structural Noise Isolation. It should ideally be around STC 50/STC 60.
3. Since the movie halls are situated next to each other, you need to pay special attention as you wouldn’t want sounds from a loud action movie spilling into the next theater playing a quiet romantic movie. I suggest a double concrete wall between the two halls, and not sharing HVAC Ducting between the two. Make sure the exit and entrance doors don’t open directly into the same lobby or corridors.
4. Standing Waves: This is usually a problem in small closed spaces. There are different modes of standing waves – axial mode, tangential mode and oblique mode. Axial mode is the most prominent and can be avoided with non-parallel walls*. In case of parallel walls, calculate the frequency of the axial mode and use absorbing material on the parallel walls accordingly.
5. Noise Criteria (NC): This is the measurement of internal noise or ambient noise present within the space – eg. HVAC, equipment noise etc. This should not exceed NC 30 to NC 35.
6. HVAC noise could be your biggest concern, so design this with great care. Treat the ducting with absorbing material like glasswool.
7. Lobby noise can be avoided if you design good doors with air sealing. Consider Vestibule doors.
8. For external noise apart from STC, check for doors, windows and HVAC paths, especially if you have railway or airplane noises near your site.
9. Make side walls and back walls absorptive, especially where the speakers are likely to fire most, and back wall for slap back.
10. There are many treatment materials available in the market, the most common being gypsum boards of various specs and glasswool. You can combine both along with air gaps to get a great acoustic treatment. You can consider various forms of the Helm-Holtz resonator (a quick search on the Internet will explain this to you). There are companies like Auralex that manufacture foam diffusers with detailed specs of their products that you could easily use to calculate factors like RT60. These foam diffusers could greatly add to the aesthetics of the space you are designing depending on the budget.
11. There are various standards such as THX®, that certify a space for acoustic related criteria. This is essential for movie theaters to test aspects like the NC level, STC rating and reverberation time. You can hire a certified consultant to meet these standards and get your site approved.
I can assure you that this is not as baffling as it looks. It may take a couple of days the first time you try to calculate such information, but once you understand the basics, it will get much easier. I suggest you begin by trying to do work this out on your own as one dummy project could teach you a lot.
Here are a few links for starts, there are many such tools available online that you could use if you do not have the required software. However, do the calculations manually at least once to understand the basic factors as this will help you decide room treatment materials and design.
Room Mode Calculator (Standing wave, not very relevant for large spaces)
*Avoid parallel walls whenever possible as it will reduce internal reflections and room modes. The ceiling and floor is not parallel due to the gradient on the floor for seating. Consider this point for designing acoustics in studio and auditoriums as well. Read up on Standing Waves for more info.