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The acoustics of windows

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Windows are certainly a nice feature if you want to look at views; but they are detrimental to the performance of a home cinema.

Windows create unwanted sound reflections; they let in ambient light that washes out the picture; and they ruin any attempt to create the look and feel of the cinema environment at home.

Despite this, clients continue to want windows in their home cinemas. So what do we do?

Well, move the home cinema to a room without windows, of course!

You’d be surprised how many projects I’ve done where there were plenty of other spaces in the residence perfectly suited to a home cinema. It might be an extra garage, an attic, a store room, a hall, or an unused bedroom. (After all, who wants the bedroom with no windows?) All it took was a respectful explanation on my part, and the client changed his/her mind about where to put the cinema.

But that’s not going to happen most of the time. Keep the following things in mind when you’re stuck in a room with windows, and you’ll end up with a result that might surprise you – for the better!

1.       Use front speakers with higher directivity.

In simple terms, this means that the speakers focus more sound at the listeners and spray less sound around the room where it can reflect off the windows. The speakers you’ll typically find with high directivity are D’Appolito configurations (vertical stack of woofer-tweeter-woofer), tapered arrays (vertical stack of woofer-midrange-tweeter-midrange-woofer), speakers with horn-loaded tweeters, line sources, or large planar dipoles with the back wave treated. I don’t have space to describe all of those in detail; Google is your friend.

Another useful tool is THX Ultra certification. Most manufacturers don’t publish directivity specifications, but THX takes a close look at that during the certification process. The Ultra specification is designed for larger rooms; hence the speakers have focussed directivity. Make sure you do everything possible to maximise sound quality and clarity from the speakers. Avoid inadequate placement inside cabinets, in corners, or at floor junctions. This is important in any room, but it becomes vital in rooms with windows.

 

2.     Cover the windows with movable drapes or shades.

The drapes could be heavy velour or a sandwich of acoustically open fabric front and back with two layers of flannel inside. If there are a lot of windows, watch out for too much absorption coming from the window treatments. You may need to mix 50% of the flannel area with 50% of thin black-out vinyl layer to provide absorption/diffusion characteristics instead of just absorption.

You can also use micro-perforated window shades. These can either be thin pull-down Mylar (or similar) with very small holes or actual thin micro-perforated plastic sheets affixed to the window with 2 to 3cm of airspace.

3.      Watch out for deficiencies in the windows themselves.

Hardware rattles are a huge problem, and windows often lack sufficient sound isolation properties. In closely-spaced subdivisions, sound leakage through windows can annoy the neighbours and cause all kinds of trouble. Preferably use double-pane windows with dissimilar layer thickness to avoid co-resonance of the panes.

4.      Use thick carpeting and plan to treat the ceiling.

This is particularly important in a room with lots of windows that absolutely cannot be treated. You will need absorption on the floor and ceiling, as there is not enough treatable wall area to keep the sound reflection decay time within acceptable tolerances. For the carpet use a thick natural fibre with open backing, and place a thick underlayment, again made of natural fibres instead of foam or rubber.

5.      Use an electronic equaliser, preferably digital, to fine-tune the sound system for optimal intelligibility and imaging during the final calibration process.

Automated EQ programs will not work for this, as their little microphones can’t differentiate between windows and a kangaroo skin!

6.      Finally, plan on controlling ambient light and light leakage with black-out material.

Ambient light totally kills the picture quality of front projection video systems, plus it ruins the cinematic experience. Light is also bad for flat screens and other direct view displays. Also, if you are starting from a fresh sheet of paper, and your client or architect wants windows in the home cinema, please ask them when was the last time they saw windows in a commercial cinema… There are good reasons for that “omission”.

About Anthony Grimani

Anthony Grimani

Anthony Grimani is president of PMI, Ltd., a home cinema engineering firm, and MSR Acoustics, a manufacturer of fine acoustical tuning systems. MSR is represented in Australia by Wavetrain (www.wavetrain.com.au).

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