A Sonic Tsunami: Born in the front end of your listening room.
December 9, 2016
Comment recently added to Sunday Morning HiFi #4 blog by Spencer Holbert in Absolute Sound on room acoustics:
Over 30 years ago I invented the TubeTrap which naturally led to my being involved in a seemingly endless string of room setups, that continues through today. Over the years certain “rules” evolved. I feel I should expand on the first stage of room acoustics that Spencer is talking about, the “proper placement of speakers in the room”. From my experience it’s not about pacing off the distance between the speakers, listening position and room walls, typically following the Cardas formula or someone else’s formula. That part of setup comes later.
The first part of setup is to figure out WHERE the speaker/listening triangle is going to be located in the room. When we begin to set up our own room, we find we are setting the listening triangle based on values which are not acoustic values, but instead, room decor values. About half of all customer proposed room setups I’ve worked on have the room being played sideways or backwards, not because it sounds better, but because it looks better. In Audio, it’s about the ears, not the eyes….. we dim the lights when we listen anyway.
Rules of thumb when orienting the listening triangle in a room:
1) Symmetrical hard surfaces behind and to the side of the speaker end of the triangle.
2) Irregular part of the room, kitchen and hallways are located behind the listener.
3) Ceiling best slopes up and away from speakers.
4) If room is narrow, put speakers midway along long wall, play across short dimension.
What is the basis for this “setup” routine? It is based on one thing only: Bass wavefront control during the two different stages of its life, the one before you hear the wavefront and the one after you hear the wavefront. Fundamentally we want to room to help the speakers:
A) Launch a clean wavefront and
B) Once we hear it, get rid of it.
Nothing is worse than having to listen to old chunks of bass wavefront crashing around the room. One more thing, We are talking here about the initial stage of the bass wavefront, which is within the first 50 ms or so of it’s life. We are not talking about modes, which is a relatively slow process to create, taking about 500 ms, where some bass energy eventually gets stored in very symmetrical tonal patterns, filling the whole room.
To do the early wavefront control, we focus on:
1) Do not overly compress or hold back the bass wavefront during early stages of bass wavefront expansion away from the speaker……keep speaker out in the open
2) Vent and disrupt the bass wavefront as it is approaching the back wall.
Notice, I’m not talking about ordinary “acoustics” yet. By that I mean treble range acoustics; early reflections, sound absorption, carpets and sound panels and such….. which is the easy stuff. Fitting the listening triangle into the room is about bass wavefront control, which is the hard part of sound to get a grip on.
Once the listening triangle is positioned well within the room with respect to base wavefront expansion, then we add Bass Traps to minimize unwanted buildup of bass wave reflections. That’s why we add bass traps to the side walls up in the front of the room, because that’s where the first buildup of bass energy takes place, side to side up in the front of the room. It’s a lot better to absorb the bass buildup in the front of the room, where it is very loud, because the amount of sound power you absorb is proportional to how loud the sound is that you are absorbing.
The last thing any of us want to experience is the uncontrolled consequence of a powerful buildup of vertical and side to side bass energy up in the front of the room as it begins to slowly expand towards the back of the room and in so doing, washes over and consumes your listening position in a virtual Tsunami of chaotic bass energy.
Well, that’s enough for now.