Placement of absorption materials

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Placement of absorption materials

Postby archive » Mon Apr 12, 2004 4:25 am

From: "Dan Nelson" <dprimary@h...>
Date: Sat Jan 29, 2000 1:54 pm
Subject: Placement of absorption materials

oops I guess I never put the article up on the site

Placement of absorption materials

Reducing First Order Reflections

In the mix position you will want to setup what is known as a
Reflection Free Zone (RFZ). In the RFZ you will want to eliminate any
early reflections of sound. By reducing the early reflections you
increase what is known as the initial time delay (ITD) gap. This gap is
the time between the arrival of the direct sound and the first
reflections in the room. A 20 ms ITD gap is a good minimum target. The
purpose of increasing the the ITD is to allow you to hear the time
domain localization cues from the recording room. The controls room's
ITD gap needs to be longer then the recording room's ITD. This allow
allows you to hear the reflections in the recording room better.

Any surface within 11 feet of the mix position could cause a first order
reflection, but treating every surface within 11 feet of the mix
position is not possible or necessarily desirable. The International
Telecommunications Union (ITU) and the European Broadcasting Union
(EBU) recommends levels of reflections earlier than 15 ms relative to
the direct sound
should be treated to reduce them to at least 10 dB below the direct
sound for all frequencies in the range 1kHz to 8kHz. In the area of the
room between the monitors and the mix position any surface that has a
21.5 foot complete path between the monitor and the mix position can
also cause a reflection.

One way to find the areas that need to be treated, will require two
people and a mirror (a plastic one is preferred since you may have to
tape it to a pole to reach some areas)
To help find the placement location have one person move a mirror along
the side walls of the control room with the other person siting at the
mix position. Any point the monitor can be seen from the mix position in
the mirror mark those points on the wall. Repeat this process for the
ceiling and back wall. Any of these points that are less then 11 ft from
the mix position will cause early reflections and is where you will
want to locate absorption materials. In the front half of the room
measure from the monitors to the marked reflection point to the mix
position if it is less than 21.5 feet be sure to treat those areas as
well. You may want to treat an area larger than what you have marked,
since you wonít always be in the exact mix position.

The reason for the two measurements is, if you measured 22 feet from the
speaker to the back wall and back to the mix position your reflection
time is still under 19 ms because the monitor is a few in front of you
the sound has traveled 3-4 ms before it reached your ears so the
reflection will reach you in 15-16 ms. In the area in front of the mix
position you could have a reflection from something more than 11 feet
away reach you in less than the 19ms since the spot is is less than 11
feet from the monitors.

Dan Nelson
Posts: 4697
Joined: Sun Mar 21, 2004 6:26 am

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