02.04.15

Best Practice: Home Cinema Design—It’s all in the Engineering

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By Peter Aylett, Archimedia Middle East. As with every technical endeavour in our industry, designing a home cinema should be far more...

Peter AylettBy Peter Aylett, Archimedia Middle East. As with every technical endeavour in our industry, designing a home cinema should be far more about engineering science than simply selecting a set of cool-looking products that you happen to make good margin on. Every aspect of a cinema’s performance is governed by a range of standards and recommended practices. Though it’s extremely difficult to have the budget, space, and interior design freedom to achieve perfection we should always reach a compromise in a calculated way. Here are some of the performance standards to strive for, and how you can achieve them in the real world. [caption id="attachment_8830" align="alignleft" width="283"]Every aspect of a cinema’s performance is governed by a range of standards and recommended practices. Every aspect of a cinema’s performance is governed by a range of standards and recommended practices.[/caption] Ensure you have a bright enough image The SMPTE recommended practice for projected screen brightness is 14fl (Foot Lamberts, which is a measurement of the amount of light reflected off the screen surface). This figure comes from commercial cinemas where owners are keen to ‘turn projector lamps down’ to extend their lives. In the home, most cinema designers aim for 30fl with some people even going as far as 50fl. This level of brightness adds real ‘punch’ to an image. The difficulty in achieving this comes from the fact that bright projectors are very expensive, and published projector brightness figures (in ANSI Lumens) are rarely correct. Once you factor in calibrating the projector, which usually removes lots of its blue light component, and the fact that the brightness from bulbs declines over the lifespan of the projector (these two issues go away with solid state lit projectors, i.e. LED and laser) a 2000-lumen projector should be treated as a 1000-lumen one. The calculation used for calculating screen brightness is: Brightness =  Screen gain x projector brightness (in lumens) / Screen Area (in square feet) Once you do this calculation, you will realise that to achieve 30fl with a projector that has a published brightness of 2000 lumens, the maximum screen size you can have (with a screen gain of 1.0) is 105-inches diagonal. This figure is probably surprising. The same projector on a 160-inch screen will deliver 16fl—still within recommended practices but no way as good a picture. Increasing the gain of the screen is a relatively inexpensive way to solve the problem but always look at brightness uniformity and colour accuracy with higher gain screens. [caption id="attachment_8695" align="aligncenter" width="1024"]Ensure that the projector is bright enough for the screen size you are using. Ensure that the projector is bright enough for the screen size you are using.[/caption] The compromise you will need to engineer depends on the screen size for the room. The recommended practice for HD (1920 x 1080) is that the viewer is seated 3 times the screen height away from the screen. With UHD, because the pixels are much smaller, the viewer can be seated closer. Current practice is around 2.3 times the screen height for UHD. Once the screen has been sized, then the brightness calculation can take place and budget negotiation begin! It is often best to recommend a smaller screen and maintain image quality than go for a larger screen than the projector brightness can handle. Though the size of the screen may be impressive, the image projected onto it may not be. [caption id="attachment_8696" align="alignleft" width="144"]Line array speakers are very efficient and help mitigate propagation loss. Line array speakers are very efficient and help mitigate propagation loss.[/caption] Ensure that the system can handle loud transients The standard for a reference level cinema is that each speaker should be able to deliver a minimum of 105db at the primary seating position. That’s not all speakers together, that’s each one individually. Digital sound formats have discreet channels and it is possible that an effect will be replayed out of a single speaker only. Though more volume usually comes from the front speakers, many mixes in films have VERY loud sounds coming from the surround channels. To give you an idea of what this means from an engineering perspective, a speaker that is 89db efficient (It delivers 89db of Sound Pressure Level from 1W of power at 1M) will need 638W if it is to reproduce 105db to a listener 4M away! In the real world, the 105db reference figure is the first specification that is compromised due to budget constraints as it’s very expensive to deliver. If you use the analogy of a car, we do not buy cars with a maximum speed equal to the speed limit. The driving experience would be terrible with the car straining and screaming at maximum all the time. The same is true of audio systems. For any audio system to sound good it needs to be working well within its performance envelope. As amplifier/speaker systems reach their maximum output they begin to distort and compress— effects that are very audible. The solution is to use more efficient speakers. A 95db efficient speaker at 4M distance would only need around 150W to deliver 105db. Don’t forget the subs. They need to deliver a combined 115db (The LFE channel is mono as is most bass management) at the listening position. A fantastic resource on the advantages of using multiple subs in a system and small room acoustics in general is Sound Reproduction written by Dr. Floyd Toole. Two subs, optimally positions is way better than only one, with four subs correctly positioned being optimal. Read the book, it will reveal all! This is just the beginning The above two pieces of engineering are just the tip of the iceberg when it comes to cinema design. I’ve not touch on speaker positioning, propagation loss, immersive audio, room acoustics, lighting or any of the many other engineering decisions that need to be individually made for your customer’s budget, space and design requirements, CEDIA offer some excellent practical workshop based cinema design courses. I also recommend the ISF (Imaging Science Foundation) and HAA (Home Acoustics Alliance) for excellent and in-depth courses on calibration. Conclusion Don’t be tempted to simply use the same old product set over and over for your customers. By taking an engineering approach, you can justify every specification decision you make, which helps maximise the sales value. Taking the time to learn the engineering and how to apply it ensures that performance is maximised in a very repeatable way for your customers. Peter Aylett is a world-renowned speaker and lecturer in residential technology, and the Technical Director at Archimedia, a multinational high-end residential integrator in The Middle East. He is also currently Chair of CEDIA’s International Technology Council Applied Content Action Team, and a regular contributor to HiddenWires. www.archimedia-me.com