From the Desk of Art Noxon

Art Noxon is a fully accredited Acoustical Engineer with Master of Science degrees in Mechanical Engineering/Acoustics and Physics. A Professional Engineer since 1982, he is licensed in Oregon to practice engineering in the public domain with the specialty area of acoustics. A prolific inventor, he developed and patented the iconic TubeTrap, the original corner-loaded bass trap/treble diffuser, 150 other acoustic devices, and counting. Lecturer, writer, and teacher of acoustics, he has presented 7 AES papers, numerous magazine articles, white papers and blogs. He is president of Acoustic Sciences Corporation, the company he founded in 1984.


Room Breakup, Curse of the Audiophile (2007)

Nothing, not even audio, is perfect all of the time.  But most of the time, the audio system seems to behave just fine.  Still, if you crank the volume up just one notch too high, in most systems, something changes and the system no longer sounds musical.  What you are hearing is not cone breakup but room breakup.    Yes, rooms too can be overdriven and more easily than we might suspect.  

There are limits, even in Audio

The world of digital audio seems nearly perfect.  Waveform replication is fantastic.  The standard CD provides a 44k sampling rate which traces waveforms beyond the range of hearing and 16 bid audio depth which provides 96 dB of dynamic range.  Just because we can apply some wonderful signal to the sound system doesn't mean we will hear it while seated in the listening room.     

Every loud speaker driver has  some upper limit to how loud it can be played without the cone breaking up, vibrating is a way that is not part of the music signal.  Cone wobble, diaphragm flexing or over-excursions begin to be heard.  Every amp has an upper limit, above which some distortion, non linear behavior, usually clipping, begins to be heard.  Every power supply has an upper limit in its ability to deliver power, above which fade outs can be heard.  Even power cords provide distortion during heavy power surges due to electromagnetic coupling effects and wire heating.

It should come as no surprise that the surfaces that comprise the audio playback room also has an upper limit, above which, sonic distortion begins to be heard. 

In the electronic chain, distortion effects generally begin to become apparent at sound levels above 95 to 105 dB,A.  However, in the playback room, distortions begin to be heard as much lower sound levels, in the range of 75 to 85 dB,A.    Above this level, rooms no longer stand still, they begin to break-up.  Doors resonate, windows twang, lamp shades ring, walls shudder, floors shake and ceilings thunder.    

The audiophile measures room breakup...hit PAUSE

One of the tests usually made by audiophiles is to play some energetic music for a while at some fairly loud level and suddenly hit the mute button….and listen very carefully.  What is heard is the decay of the residual sound of the room.  It is the self made noise floor that accompanies the music.  This self noise floor limits rapid changes in musical dynamics because when the music stops, the listener continues to hear the room humming along with the music.    

In high performance audio rooms, one of the performance standards by which a room is known and judged is to determine how loud the room can be played before it begins to break up.  Generally this will be in the range of 75 to 85 dB,A.  The audiophile has learned through careful listening, that if the sound level is kept at a "reasonable" level in a room, the whole sound system behaves quite nicely, except that it might get a little too quiet at times.  Even the audiophile can’t resist temptation forever and eventually experiments with turning the volume up.   To some degree, the louder the volume, the more dynamic range becomes available to the music. 

But as with all good things, where a little is very good, a little too much begins to not be so good.  In every room there is some sound level, and it varies from room to room, where something new begins to happen in the room and music no longer sounds clean and clear.  The audiophile is a purest at heart and is not really interested in sullying the listening experience with distortion of any sort and so they just turn the volume down.

What is room breakup?

In nature, most things have some sort of internal friction.  That's why the world tends to stand still. When something gets disturbed, it tends to deform or move.  It's internal friction acts to dissipate the energy of the disturbance and the object quickly regains its rest condition.  This is how the familiar world is put together. At lower sound levels the walls, ceiling and other surfaces of the room have enough internal friction to absorb wall movements induced by pressure from the speakers.  But, inject high power audio into a normal room and something else happens, the surfaces of the room become fluidized.  The surfaces of the room begins to quiver like the surfaces of a rectangular chunk of jello. 

At higher volumes, more power is being pumped into the walls than the natural friction of the wall can dissipate and the walls become energized, taking on a sound of their own "wall thunder". Vibrating walls are not much different than vibrating speakers except they are a lot larger and therefore louder.

Vibrating walls are a form of stored energy, not too different from a vibrating bell.  Structural vibration is like room reverberation:  When the music stops, the walls keep shaking and the room keep reverberating.  Like room reverb, walls continues to sound out until their stored energy is absorbed.

Half the sound emitted from a vibrating wall is broadcast away from the listen room into the rest of the house.  But, the other half, the inside surface of the vibrating wall broadcasts sound back into the listening room until the internal friction of the wall plus the energy transmitted into other parts of the house dissipates the energy from the vibrating wall and it finally comes to rest.     

A room with a steady noise floor, possibly due to some equipment fan or air conditioning, affects how quiet a sound can be heard in the room.  Noise reduces the dynamic range available in the listening room.  It’s like a projection screen in a room with some lights left on.  Where ever the light hits the screen it grays out the black, and the dynamic range of contrast is reduced.  A room with a noise floor, due to acoustic or structural reverberation, also lacks dynamic range. 

Room reverberation occurs at all sound levels.  Leave a door to the kitchen open, off of the listening room.  At low volume levels the kitchen reverberation is there, audible but quiet.  At higher sound levels, the kitchen reverb remains, just proportionately louder.  There is no sound level above which the kitchen acoustic reverb turns on.  It is linear and always proportional to the volume of sound in the listening room.  Room Break-up is not linear.  At low volumes there is no structural reverb but at a higher sound levels structural reverberation is stimulated.   

How to fix Room Break-up

Room break-up is all about sound stimulating resonant surfaces into resonance.  It’s about yelling at a bell and listening to the bell ring.  There are two ways to get rid of the resonant structures that surround the HiFi system.  One is to make the structure so massive that the power from the sound system can’t flex the walls.  This might be a concrete or adobe type walled house. The other way is to add a structural damping feature to the light weight flexible walls. 

In desperation, some audiophiles and as well, recording studio engineers have been known to drill holes in the top part of the wall and blow or funnel sand into the walls. Sand has the magic property of being a very damped material to vibration and it weights a lot as well.  The idea is that if the walls need damping or weight, how about giving them both.  The problem is that it weighs so much and acts almost like a fluid.  Almost like a Chinese Fingertrap and every time sound pulls the wall out, sand trickles down and holds it out.  Over and over and eventually the wall just blows out at the bottom, dumping the sand onto the floor.   

Sometimes people want to just make a massive wall, something that can’t move.  They will add layer upon layer of gypsum board and create a low cost, heavy wall.  What happens with massive walls is the there is no where for the bass energy to go and so, the bass energy stays in the listening room.  The energy that used to shake the walls has become room reverberation, shaking air inside the room.  Room reverberation is a problem at all sound levels.  At least with shaking walls, you could play the sound at lower levels and not be plagued with room reverberation because the wall conducted the low level energy right our of the room.  Massive walls don’t work very well in high performance audio rooms unless lots of bass traps are added into the room so as to get rid of the excessive bass reverberation and return the room to a sensible level of dynamic performance. 

The last option is the only one that works for high power audio rooms.  Add a damping feature to the walls.  This way, when the walls stand still, they still conduct sound out of the room. And when the walls start to move, the damping feature begins to absorb the energy out of the movement.  It the proper amount of damping is added, the walls remain flexible but never can be driven into free vibration.  The more they are bent under the pressure of sound, the more energy they absorb.  The walls become self stabilizing and energy absorbing.  Since the damping feature absorbs the energy of wall movement, the wall gets rid of its energy without broadcasting it throughout the rest of the house or back into the listening room. 

Adding a damping feature to the walls and ceiling, doors and windows of your high power audio room lets you not only break your old sound level barrier, but completely eliminate it from your room.  Now you can enjoy high performance playback with unlimited sound levels, like a recording studio.  Caution:  Limit listening at 95 dB,A to not exceed 3  hours a day.