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HOME RECORDING

August 1, 2001 "BEST OF" ISSUE

THE BAFFLING WORLD OF HOME RECORDING

BY ROBERT DENNIS

Originally published in January 2000 as a restricted article, this article has fairly complete plans on making and using baffles in the home recording studio. - Bob 

Part of the reason that professional studios get such a good recorded sound is through the use of sound-absorbing baffles.   Here are plans for inexpensive baffles you can make for home recording and suggestions on what to do if you can't even afford the "cheapie" baffles. 
HOW TO USE BAFFLES
Before we get into building baffles, let's do a quick review of how baffles are used in recording.
In professional recording, loud and soft instruments are often placed in "isolation rooms." These rooms are adjacent to the recording studio and are connected by windows and doors to the studio. At home, any other room may be used but it should have a minimum amount of hard surfaces. Often musicians have difficulty playing together if they do not have a good line-of sight with other musicians. If we run out of rooms, or the use of rooms results in poor line-of sight with other musicians, we place the instruments in the same room and achieve isolation by distance and by use of sound-absorbing baffles.
Because of the reflective nature of sound we can usually achieve the best sound isolation by placing the absorbent baffle behind the very loud or very soft instrument. Directional mics are placed so that they point into the baffle; this rejects any sound coming into the area because the back of the mic doesn't pick up sound well. The sound absorbing baffles prevent reflections of sounds from bouncing into the front of the mic. The musician is placed between the mic and the baffle, with the musician's sound projecting into the front of the mic. Figure below shows this set-up.

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PLACEMENT OF BAFFLES AND MICS FOR ISOLATION

CONSTRUCTING BAFFLES

The two drawings below show the construction of an inexpensive baffle that I designed for a studio.

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SOUND ABSORBING PANEL

BIRD'S EYE VIEW OF COMPLETED BAFFLE

The sound-absorbing panel is made by putting together a frame of 2x4 studs.  The drawing shows it 96 inches (8 feet) tall, but this can be modified for your ceiling height.  Over the frame, a 8' x 4' solid Masonite panel is tacked to the frame.  This Masonite should not have holes (don't use pegboard).  Strips of building insulation are put between the 2x4 frames.  (Use the type that does not have any cover paper and wear rubber gloves when handling the fiberglass insulation.) The exposed fiberglass side of the panel may be covered with a cloth or with plastic screen material to both hold the fiberglass in place and to prevent musician contact with that itchy fiberglass.   
The base is made out of 3/4 x 8 pine and has a special shape as shown.  The sound absorbing panels are bolted to the base with two (or 4) quarter inch bolts.  Small castors are installed on the bottom of the bases.  Finally the two mounted sound-absorbing panels are hinged so that the baffles can open and can close per the bird's eye view.  We used a 4 foot continuous (piano) hinge to connect the baffles.  The two baffles can "open" to about an angle of 120 degrees because of the shape of the base.  (This provides stability to the baffles when they are open).   The baffles can close-up and take up minimum space when they are not in use.
Helpful Hint:  I learned this the hard way.  When picking out the wood, make sure the lumber is straight and not warped (I knew to do this).  Construct the baffles right away and "seal" the wood with a good paint or shellac - this can prevent the moisture from getting into the wood and warping it. (Guess how I found this out.)  Remember that basements tend to be damp.
WHAT TO DO IF YOU CAN'T AFFORD (OR DON'T HAVE ROOM FOR) BAFFLES
The home recordist is usually working with a limited amount of space.  Often the live musician performing is in the same room as the console and the musician is playing in the corner or across the room.   There's not a lot of room for big baffles, like there is in a larger professional recording studio.
Big baffles can be expensive, even when built with minimal costs, and for what?  To take up most of the limited space that a home recordist has - naturally.
So I decided to design a home baffling system that the home recordist can actually use.  The system I designed will fit into any setup and cost a mere $3.00 (or $5.00 if you don't watch costs).   Probably everything you need can be picked up at your local dollar store. 
One takes a corner of the room and hangs blankets 6 to 8 inches away from the wall.  This "L" shaped area has reduced leakage of other sounds in the room because the sound waves have to travel through the blanket to the wall and back through the blanket to form a sound reflection.   The air space between the blanket and the wall helps make the blanket effective.  
So you get those hooks and clothesline that will hold the blankets.  You use things like clothespins to keep up the blankets.

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Needless to say everyone has blankets and you can't use your sleeping blanket while you're recording.   You would want to use the thickest ones or even have two layers of blankets {two thin blankets work better than one thick one, by the way].
So your little corner becomes your baffled-off area and things sound better there and have less leakage
THE SOUND ABSORBING QUALITY OF AIR
Air actually is a sound absorber.  It doesn't have as much sound absorbing ability as fiberglass or even the wool-blanket, but it has sound-absorbing qualities none the less.  The blanket baffle works because the sound has to go though the blanket and then a layer of air before it reaches the wall.   After bouncing off the wall it goes though air again and then the blanket again.
The thickness of the baffle determines the lowest-frequency that will be affected by the baffle.  Lower frequencies have longer wavelengths, therefore they need thicker baffles to absorb the sound.  Putting the blanket 6 inches away from the wall makes the baffle effectively 12 inches thick.  Generally sound-absorbing material can have maximum absorption for frequencies with wavelengths four times the thickness of the baffle (or less).  There will be noticeable sound absorption for frequencies with wavelengths eight times the baffle thickness.

LOWEST ABSORBING FREQUENCY FOR A THICKNESS OF SOUND-ABSORBING MATERIAL

FREQUENCY/WAVELENGTH

PARTIAL ABSORPTION

MAX ABSORPTION

1000 Hz / 1 ' 1.5"

1 3/16 Inches

3 3/8 Inches

500 Hz / 2' 3"

3 3/8 Inches

6 3/4 Inches

250 Hz / 4' 6"

6 3/4 Inches

13 1/2 Inches

125 Hz / 5'

13 1/2 Inches

27 Inches

63 Hz / 10'

27 Inches

54 Inches

From the chart, it can be seen that the blanket baffle can reduce leakage for frequencies down to about 125 Hz. 
THE SOUND ABSORBING WALL
Some studio will make one wall entirely sound-absorbing to help with the acoustics.  In a future Alexander Magazine issue we plan an article on the when and why this is a good practice.  Studios often buy sound-absorbing foam that is about 3 inches thick and place it on the wall.  Although there is an angled pattern to the foam that helps absorption, it can be seen from the above chart that the foam will have limited bass absorption.  It can also be seen that placing half-inch thick carpeting on the walls really doesn't work to absorb even midrange sound. 
A much more effective way of making a sound-absorbing wall would be to mount the acoustic foam on pegboard, and the mount the pegboard 3 to 6 inches away from the wall.  This use of the foam will reduce sound reflections well into the bass frequency range.

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Copyright 1993, 2000, by Robert Dennis, ALL RIGHTS RESERVED

Published in Recording Engineer's Quarterly and Alexander magazines with permission

USE OF THIS ARTICLE SUBJECT TO USER AGREEMENT