Wall Shudder in the Listening Room

Does your sound system generally give great results, but certain tones tend to cause an overwhelming boom, sometimes accompanied by a rattle? Audio-minded people are mostly aware of room modes, and often mistakenly blame their room dimensions for this unwelcome and irregular event. So let’s ask another question: have you ever heard of wall shudder?

Did you know that your walls can act like big drums, or even like huge subwoofers? The frequency/note at which they will tend to “ring” depends on many factors, but the cause is clear. As is the solution, which we will reveal in the coming weeks.

This week we enjoy a deep-dive on the subject by ASC founder & president, Art Noxon, PE as he identifies and explains a source of sonic coloration that you may not be aware of. Enjoy this excerpt from an amazingly thorough white paper that we link below.

Wall Shudder

This 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.

Illustrations of wall vibrations

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.

front and side view volume displacement illustrationn

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.

This pretty much describes the reality of listening to high power audio in normal houses. Not only does the sub shake the surface of the room, but since the surface is connected to the rest of the house, it shakes the rest of the house, and usually the walls of the neighbor’s house. My focus was always to deliver great sound to the listener. Yes, making and selling products was important because it kept the company doors open, but the real goal was not about selling product, it was about making great rooms, rooms that really worked. This wall shaking problem had become the next sound barrier to good sound. It simply had to be dealt with.

Before we address this, let’s look at the alternative, a room without shaking walls. This might be a concrete room, similar to what is common for residential construction in Europe and Asia. If we have a room whose walls don’t shake, we have in effect, a racquetball court. The sound in this type of room is just about as bad as it is in a reverb chamber. Since the walls don’t flex, all the sound dumped into the room stays in the room.

The only way to get rid of it is to absorb it, using lots of giant bass traps. Personally, I’ve never achieved satisfactory success using [in-room] acoustics to convert a dedicated, sealed concrete room into a high performance listening room, and I’ve tried…. However, concrete rooms that are typically residential are not so impossible to set up and get sounding good because some of the bas buildup is leaked out of the room through openings, such as windows and lightweight doors, open doorways, halls, stairs and even closets. However, there is one big exception, which we’ll soon cover.

-AN