A practical guide to acoustical design of listening rooms and placement of loudspeakers
Listening rooms work best, when they are designed from the viewpoint of the loudspeaker first, and secondly for the listener. The walls near the loudspeaker should work as an extension of the loudspeaker, and the signal should reach the listener with minimal interfering 1st reflections from the room. The reverb should be diffuse and delayed by using many surfaces to reflect the sound before the sound reach the listener. Symmetry must be maintained in the front part of the room.
The existing IEC 268-13 standard for listening rooms attempts to represent a good, but normal living room, taking an average of existing rooms into account, and to serve the development of loudspeakers for home use. This paper has a different goal. This paper attempts to define a standard for the best possible listening room or control room for professional use, but can also serve as a guide for special home theater and media rooms.
Normal listening rooms are rectangular, and many AES papers have calculated the room modes, the reverb time, and tried to work out the best placement of loudspeakers in these normal rooms. Early designs of recording studios tried to aims at an even spread of resonance frequencies, and to maintain an even reverb time of about 0,3 seconds at all frequencies. Later, the rectangular design was replaced by a polygon, and absorbing material several meters deep. Some of the walls was covered with irregular stone surfaces as diffusion.
The users of the studios were not happy, as one control room sounded very different from another control room. So in the early 80’s, a common development by studio engineers was the use of small near field monitor speakers placed on top of the mixing console. They were preferred over the large wall mounted monitor speakers in spite of a much lower output level handling, and less low frequency output. HiFi enthusiasts also started to move their speakers away from the walls and up on stands, in spite of the loss of bass output.
When a large number of critical listeners took the same approach to solving their room and speaker problem. I find it important to analyse what happened.
I have here drawn two rooms, both 5 meter by 7 meter rectangular rooms with a listener at L, and a loudspeaker at LS. Height 2,4 meters. Volume 84 cubic meters.
Room A. With loudspeaker close to the listener (Note the gap of silence between 5 mS and 12 mS)
Room B. With loudspeaker close to the back wall.
Direct sound and diffuse sound.
For a reverb time T of 0,3 Seconds, the absorbing area A in square meters is given by
A = 0,16*V/T = 0,16*84/0,3 = 44,8 ( V is room volume in cubic meters.)
The distance, where direct sound from the loudspeaker and diffuse sound from the room is equally loud, Dc, is given by:
Dc = 0,14 *Qs*A = 0,14 *5*44,8 = 1,6 meter. (Qs is directivity of loudspeaker.)
The wall mounted loudspeaker in room A is 2,9 meter from the listener, and the speaker on top of the mixing console in room B is 1,3 meters from the listener. Comparing these to Dc of 1,6 meters, we find that people prefer to hear mainly direct sound. This gives our first rule:
Rule 1: Increase direct sound over diffuse sound, so that the listener hears more from the loudspeaker and less of the sound from the room.
Since the listener also have a Q due to directional hearing, we can improve the total Q by keeping the diffuse sound coming from the back of the room, so that the listener better can use his/her directional hearing to tell direct sound apart from the reverb in the room.
Rule 2. Place a lot of absorbers in the front half of the room.
But we should also look at the 1st reflections from the walls. Next to the drawings of room A and room B, I have graphically compared the direct sound to the 1st wall reflections, assuming 100% reflection as the worst case. So level is reduced only due to distance. In room A the wall 1 reflections arrive later and they are reduced in level compared to room B. They cause a comb filter effect of additions and cancellations, that changes the sound quality. In room A the initial time delay gap is 8 mS and the level is -8,6 dB, in Room B it is 1,2 mS at -1,1 dB or 4,8 mS at -3,7 dB for the side wall. It is good practice to have more than 10 dB attenuation of the reflections, to avoid severe comb filter effects. We can see that room A is likely fulfil this criterion with a a bit of absorption from the walls, but room B will need a lot of absorption from the walls. This brings us to:
Rule 3. Avoid or delay and reduce 1st reflections.
When the speaker is placed away from the room surfaces the coupling of the bass speaker to the room was now poor, the radiation impedance mainly 4pi. The classical wall mounted loudspeaker had 2pi radiation. and it can therefore play louder. The ideal room should combine the advantage of 2pi radiation with the other rules. When we go lower in frequency and play loud, all bass drivers will reach their limited excursion. So it makes sense to increase the radiation load at very low frequencies and thereby give the bass driver as much support as possible. Therefore, seen from the viewpoint of a wall mounted loudspeaker, the ideal room has a radiation of 2pi in the midrange, and then go smoothly to pi, and pi/2 as the frequency goes lower. This reduces the demand on excursion of the bass driver, and can be set to compensate for the low frequency roll off of the bass driver. In fact a rising response below 50 Hz down to 16 Hz with 6 db per octave was measured in the PUK recording Studio, which I designed according to these rules in 1985.
Any rapid change in radiation impedance will give a change in the frequency response, so to conclude, we have:
Rule 4. Keep the radiation impedance high and smooth and try to optimise the bass driver response.
Rule 5. There must be symmetry to preserve correct stereo balance.
Logical, but often not followed. In the back of many rooms the sound is diffuse, so here the symmetry requirement is not so important, and doors can be located here. In some control rooms, there is a need for a window in the side wall. It should be made small, and placed so that its reflection of the tweeter sound goes to and absorbing surface.
Rule 6. Listing position and loudspeakers should form a 60 degrees triangle.
This is an old practical rule. Wider distance between speaker makes it difficult to have a stable phantom image center. Smaller distance between speaker gives an incorrect stereo image, with to little space between the players. The 30 degree walls for the speakers also reduce standing waves. Here are two possible solutions for the 5 meter by 7 meter room. In both cases the volume in the triangles is large, 1500 liters for the small ones. This can be used to build a infinite baffle for the bass loudspeakers and so we can have large closed cabinets. This space can also be used for 19″ racks for equipment or amplifiers.
We can see from these drawings, that we want to place the loudspeakers close to the window. so that the tweeters intersect at the console armrest. There is not much depth close to the windows, so often the loudspeakers are placed too far apart. So in the design of our AMPSPEAKER system, the midrange and tweeter section was made small, a 25 cm wide and only 25 cm deep module, which can be placed close to the window. The bass unit can be placed where there is more room, as they do not give much directional information.
Rule 7. Avoid standing modes by keeping walls and ceilings and floors non parallel.
A 5-10 degree angle is enough to avoid the standing waves, and this can be difficult for the casual eye to see, and it will not take much space from the room. In these two room designs, the corners for the loudspeakers are at 30 degree, the window can be placed at a 10 degree vertical angel, the side walls are at 5 degree each, and the ceiling can be 10 degree.
Rule 8. Loudspeakers should be above ear height.
The human ear responds very different to sounds coming from above. This rule is too often ignored. Research has shown a 6 dB difference due to height of sound source. Also for different listening positions, the change in vertical angel is minimal, when the speakers are at ear height. Since loudspeakers have limited vertical dispersion, placing them too high give different sound balance depending on whether the listeners are sitting or standing, or moving backwards.
A wide control window has often been the excuse to move the loudspeakers up above the window However a careful layout of the control room window with the 60 degree angel extended will cover most recording rooms. But an attractive design with wide angle vision can be done with the AMPSPEAKER center module placed as slim divider’s of the 3 window planes. Bass units are placed under the windows. Since the speakers are in plane with the windows, there are not 1 reflections from the two of the windows, and the reflection from the 3rd window is absorbed by the side wall absorber.
Rule 9. Use membrane absorbers to control reverb time at low frequencies.
Bass membrane absorbers can be made in the walls using gypsum plates on rafters, with Rockwool glued on as damping. The floor is large in area and it can be used as a bass absorber buy laying a floating 25 mm wood floor on top of a 50 mm hard Rockwool plate. Use 10 cm thick Rockwool bats to control reverb time at mid and high frequencies. Cover the Rockwool with pleasant looking loose woven material.
Rule 10. Bend the other rules to make it a pleasant room to be in for those long listening hours.
This drawing shows a number of common faults in designing control rooms:
- The console is placed far from the speakers, leaving a large useless area in front of the console. and placing the listener to far from the speakers. Also the viewing angel though the window is narrow due to the distance between console and window.
- The front walls for loudspeakers are at 20 degree angle, and intersect far behind the listener. This gives a too wide stereo image.
- The left and right back walls causes 1st reflections. If the back wall was straight, this would not happen, and the standing waves are prevented anyway by the 20 degree loudspeaker walls .
- The window is recessed to give space for speakers, causing 2 diffraction corners, and a resonating cavity.
- Door 1 causes a 1st reflection, when you sit at the left or the right side of the console.
A practical method to check or design a room is to place a light at the tweeter position, and to have somebody moving a mirror around on the wall or ceiling, while you sit at the listening position. If you can see the light in the mirror, then there will be a 1st reflection from that part of the wall or ceiling, and it should be covered with absorbing material.
A practical set of rules for design of control rooms and placement of loudspeakers has been tested in many large and small control rooms with very good results. The stereo image is precise, and is maintained over a large area. The rules can be implemented by building new walls inside a normal rectangular room.
I would like to thank Mr. Poul Ladegaard of Brüel & Kjær for his co-operation with measurements and exchange of ideas.