Overcoming Wall Shudder
By now, most of us know how bad for your listening experience excessive reflections and bass buildup in the room can be. Have you ever been curious about the walls themselves being a source of coloration?
Learn about that very topic in this week’s excerpt from the essential white paper by ASC founder & president, Art Noxon, PE Acoustical.
by Art Noxon
Wall shudder leads us into a whole new world of listening room control: structural shudder control. When a pressure spike hits a wall or ceiling it delivers a short solid blow to the surface. This vibrational surface twangs back and forth with its own resonant tone. These mechanical reverb times are long, easily over 1.2 seconds. The walls and ceiling of most rooms vibrate too freely to be used for any kind of powerful audio in music listening rooms. Explosive transients in an unconditioned room are not tight and clean, they stimulate structural vibration which creates new sounds that are heard but which are not in the program material.
A sonic boom delivers a huge transient pressure pulse to the roof of a house and when we are inside the house we hear what we think is a sonic boom. However if we were outside in the open, we’d hear something different, the real sonic boom. These two sounds come from the same source but sound very different. What we are really hearing when inside a house starts with the sonic boom but then we have to listen to the after shudder of the house as it calms down from being hit by a fast velvet hammer from the sky. Acoustic testing shows that the sound of a sonic boom is twice as loud inside a house than outside. The noise level inside is really 10 dB stronger inside than outside. This extra 10 dB comes from the sound generated by the extended structural shaking of the structure of the house. Loudspeakers shake houses too.
Let’s take a wall, 8 x 15’ in size. The edges of the wall are attached rigidly to the corners of the room. But the middle area of the wall is free to move in and out under pressure. Assume the area of this moving part of the wall is 5’ x 10’ = 50 square feet or 7200 square inches. Let’s assume the wall barely quivers, shaking no more than with an amplitude of 1/32 inch back and forth. It displaces 225 cubic inches of air with each movement.
Let’s also look at the displacement of a big subwoofer. If it is 15” in diameter its cross-sectional area is 182 sq inches. If its throw is 1.25” its displacement is also just about 225 cubic inches. When this sub is displacing that much air we know it is making loud sound. But when the wall quivers, we didn’t even think about it. The best way to imagine what contractor-grade flexible walls behave like in high-power audio rooms is to imagine a big subwoofer installed in the middle of each wall and a bigger one in the ceiling. There is one real sub in the room that is getting the audio signal. Imagine that this signal is split and run into 5 different reverb circuits. The output of each is amplified to the same power level as the real woofer and fed to the 5 in-wall subs. And now you settle down and light off your system and imagine you are listening to great music…
Want more on this topic? You can read Art’s entire article here.
If you want a quick, effective fix, get yourself some WallDamp Squares, apply them to your walls and ceiling, and add another layer of drywall!