Critical Listening Rooms

A series of articles on how to design and plan the acoustic treatment of rooms for recording music and for critcal listening.

Critical Listening Rooms

Postby Scott R. Foster » Fri May 09, 2008 3:19 pm

Critical Listening Rooms

A room designed to listen to music needs to have balanced and smooth reverberations, and controlled early reflections with regard to the listening position. Also, with typically sized room there is a need to moderate the reverberant field so that music can be sampled in a natural sounding environment which has some interplay of reflected after tones, but which is not overly reverberant to a degree which “muddies up” the sound. This is especially problematic at lower frequencies where small rooms have low modal density. If your room is less than about 7,000 cubic feet consider using all broadband treatments - at least 4" thick.

A listening room should not be acoustically dead, but resonances should be control all the way across the band [lows, mids, and highs] so that the user can be assured he is listening to the music, not the room. A completely dead listening room will not allow you to sense and interpret the decay products of the music that will exist in “real” environments.  For example, if the room is used as a mixing suite/control room, a too dead acoustic treatment scheme would probably cause the user to introduce too much added reverb to mixes, and on playback, sampling music from other sources will not be as spacious lush as intended. The goal of acoustic treatment in such rooms is to cause the absorption of sound at a fairly even rate across the spectrum so that neither end of the spectrum is allowed to be absorbed unduly in relation the rest of the spectrum.  For example, if all of the walls were covered with a material which absorbed high frequencies, but left low frequency content more or less intact, the room will sound lifeless in the higher frequencies, but still unnaturally “boomy and muddy” in the lows.

Here is a list of 4 core concepts for acoustically preparing a critical listening environment in a typical room– these concepts are applicable to studio control rooms, audiophile listening rooms, and home theaters - and generally apply to rooms where the prime listening position will be closer than about 11' [+/-3.5 meters] to one or more of the walls. Rooms larger than this range generally can be rendered high quality listening environments with less treatment than smaller rooms.

1. Lay your system and listening position out symmetrically so that the stereo image is balanced – try and acoustically render the left half of the room as an exact mirror image of the right half of the room, especially the part of the room and system components in front of the listener. Orient your system layout toward the room half which is most compatible with rendering symmetry in front of the listener – both in the shape of the room, and in your ability to place speakers and acoustic treatments. Use broadband absorbers for all acoustic panels so that they work on all frequencies [lows, mids, and highs] in a linear fashion [evenly / smoothly].

2. Try to avoid placing either the listener or the speakers against the walls, and also avoid the points in the room at 50% or 25% of room depth off the front wall - for both listener position and speakers. Generally, try to place the listener in a range between 32 and 40% of room depth off the front wall. A good place to start, if available, is approximately 33% of room depth measured off the front wall. For example, if your room is 14’ deep [168 inches/4.25 meters], and it is practical to do so, try setting your listening position up so that it is about 56” [+/-1.42 meters] off the front wall. An equivalent alternative would be to try and set up the listener about 56”[+/-1.42 meters] off the rear wall, if you need a listening position behind the primary, sample this location. If 33% does not work try sliding back a bit.

3. In placing treatments, first smooth out the non-linearity of very low frequency resonances by installing thick corner mounted treatments. As a standard guide for treatment, consider adding a pair of vertically stacked 4' tall - 4” thick - corner panels in each of 4 room corners [total = 32' linear feet of corner mounted bass traps such as the SCA ]. StudioTips Corner Absorber If vertical corners are not available, then use any available horizontal corners [ceiling/wall or floor/wall corners]. Try to maintain symmetry when placing these corner panels – especially toward the front of the room. If you cannot install 32' of corner panels of 4” thickness [either vertically or horizontally, or in some combination of these the two aspects], or if you will have very high amplitude low frequency sound sources, then use thicker panels in the corners you do have available. In such circumstances where you need to get the highest possible performance from each corner mounted unit use the a thicker 6” panel, or better yet, use SuperChunks or dual panel arrays - see the Acoustic Treatments section of this FAQ for examples of such units. Treatments

4. Finally, treat the Early Reflection Points with additional broadband absorbers [minimum 4” thick]. As a standard guide for treatment, and assuming a panel size on the order of 2’x4’ [0.6 m x 1.2m], consider hanging at least a pair of panels on the front wall, at least one panel each on the side and rear walls, and at least 3 panels on the ceiling. Locate these early reflection point panels so as to mask the first reflection points where sound from the speakers will hit the wall or ceiling and bounce directly to the listener. A typical application might use 8 reflection point panels - a pair on the front wall, one on each side wall, three over on the ceiling, and one on the rear behind the listener - all masking the 1st reflection points. Also, it is good practice to try and keep equipment below ear level - use short racks (28" or 70cm) to minimize reflections toward the listening position, and arrange fixtures such as the mix desk or coffee table to minimize early reflections. This will stabilize and clarify the stereo image.


Once all of these tasks are complete, stop and sample the results before adding more absorptive elements.  The goal is to leave the room as “live” as possible while diminishing its flaws sufficiently to allow accurate listening.  Only add additional treatments if the room remains too reverberant, and in this case add broadband panels incrementally [perhaps two or three additional panels at a time], and in as symmetric a fashion as possible, spread more or less evenly around the balance of the room [walls and ceiling] or to better reduce early reflections to secondary listening positions.

Controlling reflections to the listening position (or mix position) means arranging the monitor speakers and the mix position in conjunction with the surfaces of the room so that the initial impulse of sounds coming from the speakers arrives at the mix position by a path un-garbled by early reflections off the room’s surfaces. The goal is to control early reflections of sound so that the loudness of the direct sound from the monitors overwhelms the energy content of any reflected source arriving at the mix position during a critical time period during which the ear and mind form enduring impressions of a sound’s character from the information contained in this first impulse.  

The direct sound from the monitors always arrives first because it has the shortest path.  Reflected sound have longer paths to the listener.  When multiple copies of the first impulse arrive at the ear close together in time, the multiple sources “smear” the sound, making it indistinct, and rendering the task of critical listening more difficult. The goal of arrange acoustic treatments so that all reflected paths which are near the same path length as the direct path are either re-directed, or absorbed is called early reflection control. The time gap during between the arrival of sound at the ear from the direct source, and the arrival of the first of the strong reflected sources is called the initial time delay or “ITD”.  

Because sound travels at 1,130 feet per second [343 mps], reflected sound paths with a total path length of 22’ [6.7 meters] will show up at the listening position about .020 seconds later. Sound arriving .020 seconds after the initial impulse, or later, is not perceived as part of the initial impulse, but rather as an echo [reverb] – this is called the Haas effect. Too much sound arriving earlier than .020 seconds can cause problems by distorting the listeners sense of the tone and timbre of the sound, and slurring the stereo image by disturbing the listeners ability to take locational cues.  

Avoiding the Haas effect, by controlling early reflections is accomplished by creating a system where the ITG is at least .020 seconds.  The expedient of building a listening room that has a rear wall at least 11’ [+/- 3.5 meters] behind the mix position and then covering the surfaces at front end of a control room which would otherwise reflect sound from the monitors to the ear of the listener with absorbers is a well explored method of assuring an ITD of at least .020 seconds. A common mistake is to use absorbers for early reflection control that are thin panels of absorbent material, that only work on mid-band frequencies and higher, but because listeners take locational cues all the down to about 100 Hz, this not best practice for typical circumstances.  It is usually best to use a broadband absorber for early reflection control [a unit which works on lows, mids, and highs].

The “mirror trick” is a way to identify the location of the reflection points on the room’s surfaces [boundaries].  This technique involves one person at the mix position looking at a mirror held flat to surfaces at the front of the room by another person – if the mix person can see the monitors in the mirror, then this is good place to put absorption material to defeat early reflections to the mix position.  

There are other methods to achieve the goal of early reflection control which involve faceting the walls of the room so that early reflections are bounced away from the mix position and instead toward the rear of the room, but such steps are only practical in cases where a purpose built room in to be constructed, and even then, the trade off of reduced room volume for early reflection control is rarely efficient versus the use of broadband absorbers [either built into the room – or modular]. There is no problem in small room acoustic you can’t cure with a bigger room – making the room smaller to address one of the room’s challenges often causes more problems than it solves.

In rooms which are toward the larger end of the scale [about 5,000 cubic feet or more] it may be beneficial to use some diffusion instead of absorptive elements. An efficient way to accomplish this is the polycylindrical. You might for example use a "poly" instead of rear wall absorber in a medium sized or larger room and manage to have a sharp stereo image with no muddiness and keep the room a bit more "live". See the Types of Treatment article in this FAQ for more on "polys".
SRF
Scott R. Foster
 
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