How to make your home theater sound good.
“Implementation Guide” companion to the CEDIA/CTA-RP22 Immersive Audio Design Recommended Practice for Home Theaters
CEDIA (the home theater and automation installers association) and the CTA (the consumer technology association) have taken a few decades of best practices about dedicated home theater design and construction and summarized the audio aspects in one document. It is available for free to anyone with an interest in reading it: RP22 Download Page
They have organized it around 21 design goals, and four levels of performance for each goal. The traditional "reference" level is level 3. But for many of us, level 1 would be more than enough to be satisfied. So it's great they have put rigor into not just a single level of performance.
At first blush, this list might seem like a recipe book.
A closer look at the details behind each performance parameter reveals a very articulate explanation of the performance goal, and relatively little detail about how to achieve it. That's fine: The document was not designed to tell you HOW to achieve these goals in a comprehensive manner
This post tries to collect additional "how to" guidance for enthusiasts. Results are not guaranteed, but hopefully we can help each other up level and better predict what factors will make the most improvement to our personal screening rooms.
IMO, this guide produces better results than some of the traditional tools like: The academic, technically correct, allocentric speaker layout, as well as Dolby's official (and free) design tool that uses angles to position speakers for a single listener in a mastering suite.
Some are easy to understand how to implement: Parameter One says that a reference level (3) theater will have no seats within 4 feet of a wall. That tells you the goal AND how to achieve it.
Some are difficult to achieve: Parameter Fifteen is that a reference level (3) theater will ideally have a background noise level of NCB 18. That's super non trivial. And RP22 does not explain how to achieve that in enough detail that you could direct a contractor in the building process (or do it yourself) without additional knowledge.
The additional knowledge, in most cases, is "out there."
Intrepid enthusiasts can use this post to help in finding that additional knowledge. This post is not "complete" and if you have additions that will help make it more complete and or accurate, please share them.
The goal here is a guide to how to achieve the performance levels outlined in RP22 when the methods are not self evident or explained in actionable detail in RP22. This post attempts to explain how to meet the performance goals of RP22, one objective at a time:
1. Seating/Room Wall Distance
Self explanatory. RP22 specifies a distance.
2. Decoder Channels
Self explanatory.
3. Screen wall speaker locations
Self explanatory.
4. Maximum SPL differences between screen wall speakers
This is achieved not just through speaker sensitivity/power handling/output but also through amplification and through distance differences, as well. Use a calculator to assess the speakers, such as the Peak SPL Calculator to see whether the speakers are within the correct range of one another. Example from RP22 doc.
5. Maximum horizontal angle between adjacent surround speakers
On an accurate drawing, use a protractor. In the room, the “compass” app on a smart phone can aid in the visualization.
6. Maximum SPL differences between surround speakers
This is achieved not just through speaker sensitivity/power handling/output but also through amplification and through distance differences, as well. Use a calculator to assess the speakers, such as the Peak SPL Calculator to see whether the speakers are within the correct range of one another. Example from RP22 doc.
7. Wide speaker deviation from median angle.
On an accurate drawing, use a ruler and then a protractor. In the room, a measuring tape, and then the “compass” app on a smart phone, can aid in the visualization.
8. Upfiring speakers allowed
Self explanatory.
9. Allowable vertical angle.
On an accurate drawing, use a protractor. In the room, there may be some “level” apps on a smart phone can aid in the visualization, but I use a laser level/distance device.
10. Max spl difference between upper level speakers
This is achieved not just through speaker sensitivity/power handling/output but also through amplification and through distance differences, as well. Use a calculator to assess the speakers, such as the Peak SPL Calculator to see whether the speakers are within the correct range of one another. Example from RP22 doc.
11. Number of speakers allowed outside of zonal recommendations
Self explanatory.
12. Screen speakers spl capability
This is achieved not just through speaker sensitivity/power handling/output but also through amplification and through distance differences, as well. Use a calculator to assess the speakers, such as the Peak SPL Calculator to see whether the speakers can reach sufficient output levels. Example from RP22 doc.
13. Non screen speakers spl capabilities
This is achieved not just through speaker sensitivity/power handling/output but also through amplification and through distance differences, as well. Use a calculator to assess the speakers, such as the Peak SPL Calculator to see whether the speakers can reach sufficient output levels. Example from RP22 doc.
14. Bass spl capabilities
CEDIA hasn't defined how this is calculated/modeled, AFAIK. There is a thread where some folks are trying to figure it out: Translating subwoofer output measurements (eg, CEA2010)... but in the interim, Audioholics has done a lot of early leg work on this: https://www.audioholics.com/loudspeaker-design/subwoofer-room-size and user Mr. Wolf at AVForums in the UK May have put together the most useful tool at https://www.avforums.com/threads/how-much-subwoofer-output-capability-do-you-need-in-your-system.2379927/ with their excellent explanation:
“A home cinema system's subwoofer output requirement is, to a greater extent, a function of listening level, room volume and the low frequency content of the movies being played. Two information sources currently exist that advise on a specific subwoofer model's suitability for particular room sizes. These are THX and Audioholics.
THX has its Select, Ultra and Dominus programmes under which it tests and certifies subs as being capable of achieving cinema reference level in 2,000Ft3, 3,000Ft3 and 6,500Ft3 rooms respectively.
THX’s certification method isn’t published but a requirement is widely believed to be that frequency response must be flat to 20Hz. So to be THX certified, a sub must be able to hit 115dB reference level peaks with minimal distortion at 20Hz. Very few manufacturers choose to participate in this programme these days so unfortunately very few THX certified subwoofers exist. You will however find THX certified subwoofer models from Monoprice, Perlisten and M&K.
In contrast, AV guru website Audioholics (“AH”) assign room size ratings to the many subs it reviews and measures based on its estimate of their ability to hit 115dB at 31.5Hz (and 109dB at 25Hz) in rooms of different volumes from under 1,500Ft3 to over 5,000Ft3.
This is a significantly easier performance target to hit than THX’s as most subs have far more output capability at 31.5Hz than they do at 20Hz. AH’s rating method is simply to add 9dB to CEA-2010 RMS@2M output at 31.5Hz and adjust this result by +/-6dB per halving/doubling of room volume from a 3,000Ft3 baseline due to a change in sound pressure, e.g. 1,500Ft3/6000Ft3 rooms add/subtract 6dB from the result due to doubling/halving of volume.”
And then this excellent chart which takes into account that most people won’t listen at reference volume (level 3 in RP22) so it shows subwoofer capability above and below that at different frequencies, in different room sizes so you can target levels 1 through 4 in RP22:
But when it comes to what frequencies RP22 cares about they simply define LFE as 20 to 120 hz implying the suspected THX spec is closest to their own.
By the way, for all but the most edge cases, using ported subs instead of sealed subs tends to give you more than 5 times the output/impact when all other aspects hold constant (driver and amp) other than (of course) cabinet size. Again, user Mr Wolf at AVForums provides some nice detail at https://www.avforums.com/threads/how-much-subwoofer-output-capability-do-you-need-in-your-system.2379927/post-31129739 and this chart:
15. Background Noise Floor
See: Soundproofing does not exist... but Isolation can.
16. Seat to seat frequency response variance among screen wall speakers.
17. Seat to seat frequency response variance among non screen wall speakers.
18. In Room bass extension -3db frequency cutoff point.
See #14 which covers this down to 20hz.
19. Frequency response below the room's transition frequency.
20. Seat to seat frequency response variant from the RSP below the room's transition frequency.
21. Level of early reflections relative to direct sound.
Dolby, many rooms designers like Erskine and Grimani, and countless custom rooms have used RT60 or reflected decay times to assess room acoustics. That approach can still produce excellent results: How Many Panels and Traps Do I Need?: RT60, RDT, REW...