Do soundproof windows need angled glass?

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of Record-Producer.com.

Tuesday February 7, 2006
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If you have ever visited a professional recording studio, or even just seen pictures, you may have noticed that the window between the control room and recording area has angled glass panes. Is there a good reason for this, or is it just for show?

Control room window

If you have ever visited a professional recording studio, or even just seen pictures, you may have noticed the window between the control room and recording area.

There has to be very good soundproofing between these two areas so that the engineer can have the monitors as loud as he or she wants, but no sound energy leaks through into the recording area to be picked up by the microphones. Also, you don't want sound leaking directly from the recording area to the control room, otherwise you wouldn't be able to judge properly from the monitors whether the microphone selection and positioning was right.

So the studio window is commonly made of three panes of thick glass. Commonly the panes are all of different thickness, but this is an issue for another time.

What you will also notice however is that the panes are all set at different angles, and none are vertical. Surely there has to be a good reason for this?

One possible reason is that somehow it improves the sound isolation capabilities of the window. The theory is that there could be standing waves between the glass caused by the direct reflection back and forth. However, BBC research has shown that there is virtually no advantage of angling the panes with respect to sound isolation.

The difference it does make however is in the control room and recording areas individually. Flat surfaces are always acoustically disastrous as they cause strong reflections that are not dispersed. And of course windows are always flat, and tend to be large. So it is in fact a good idea to consider where this reflection is going, and preferably the outer panes will be angled so that sound is reflected towards absorbent or diffusing acoustic treatment.

Also there is a purely visual aspect. Vertical panes create light reflections that travel directly back to whoever is looking through the window. And three thick panes mean there are six surfaces where reflections will occur. So the seeing through a window with three thick, vertical panes is not exactly clear. But if the panes are angled, then the light reflections are directed away from the observer. Consequently the view can be almost as clear as if the panes were not there.

So, angling the panes does not affect sound proofing, but for both acoustic and light reflections it is very worthwhile.

Image: Jeff Wilson CC BY 2.0

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