Thinking differently about room treatment

Introduction

The conventional approach to room treatment is often adopted without considering or being aware of the options I'm going to present here. The main key to getting it right is to consider the speakers and the room together as a system. Many of the ideas presented here are based on the work of Dr Earl Geddes, however, the opinions expressed are my own.

The conventional approach

In a typical dedicated or semi dedicated room, the conventional approach to room treatment usually involves the following:

• bass traps (typically removable)
• absorption panels for first reflections

The result is usually a fairly dead sounding room. The cost is quite high and the result often removes ambience from the room and takes some of the life out of the music in the name of improving imaging.

While this approach can work quite well, an alternative is presented here which many will find more satisfying.

Considering speakers and the room together

Speakers and the room together form a system. The relationship between the two is critical. Conventional speakers are often designed for their on axis response, while neglecting the power response. As a result, the off axis output which is radiated to the room has a different response. The ambient sound is therefore coloured relative to the direct sound. For this reason, room treatment is often used to reduce the damage.

An alternative approach is to first start with speakers that are designed to interact with the room in a way which does not require damage control. Some options include:

• controlled directivity speakers with waveguides
• open baffle speakers with behaviour approaching constant directivity
• omnidirectional speakers which radiate evenly in all directions

Some notable examples include:

• AI Audio loudspeakers or their Gedlee kit versions
• Linkwitz Orion or the Nao
• Linkwitz Pluto or B&O Beolab

What are we trying to achieve?

If we start with any of these speakers, we no longer need to be concerned as much about reducing reflected sound in the same way. The sound radiated by the room now matches the character of the direct sound. So what are we trying to achieve now?

I suggest the following guidelines:

• high level of bass damping
• modest amounts of diffusion
• little if any absorbing panels
  
• careful speaker placement

The bass challenge

Firstly, the room itself should provide a high level of damping in the bass range. Idealy this should include the entire envelope acting as a large bass trap. Installing multiple layers of plasterboard (drywall) joined with a flexible adhesive such as Liquid Nails will achieve this goal. If the existing structure is very solid and undamped (masonry/concrete), then false walls and ceiling is a good place to start.

When starting with a room with significant damping, the problem of room modes is much easier to deal with. In addition to starting with a well damped room, it's also advisable to place multiple bass sources in locations determined by measurement. Three bass sources will typically provide a good balance of price and performance.

Read more about the multi sub approach >

Broad band treatment

Most domestic rooms can be considered acoustically small. They require different treatment to large spaces such as a commercial theatre of concert venue. Unlike these venues, it's preferable in a home environment to retain as much reverberant energy as possible. As a result, we should start with diffusion. It's important to avoid placing diffusers too close to the listening positions. Nearfield placement results in poor performance. Ideal locations include the wall behind the speakers, ceiling and side walls.

Experimentation should be used with absorbing panels. When forced to sit with a wall close behind, it may be worthwhile placing absorbing panels on the wall. Idealy such panels should be as thick as possible and relatively dense to be effective over a broad range of frequencies. Panels that are too thin will only work at high frequencies.

Speaker placement

In nearly all cases, speakers should be given room to breathe. Firstly, the tweeter should be as close as possible to seated ear level. They should have at least one metre clearance behind and to side walls, but preferably more. In the case of omni speakers, Linkwitz recommends that they be placed wider and closer to the listener than open baffle speakers.

Read more about omni placement at Linkwitz lab >

Geddes loudspeakers have unique recommendations for toe in. Geddes recommends 45 degree toe in relative to the side walls. This means the axis of each speaker will cross in front of the listening position. As a result, the listening position will be off axis. This is discussed in an online discussion at DIY audio. View thread >

This recommendation may or may not apply to other speakers.

Conclusion

Ideally the decision to purchase speakers should be made while also considering room issues. If your room isn't a dedicated room and treament isn't an option, then it becomes even more important to choose speakers with a well behaved polar response. In this case especially, omni, open baffle and controlled directivity designs should be seriously considered.

More information is avaible in the white papers and scientific papers on the Harman International website >

Why two subs isn't enough - Part 1

Introduction

Yes, you heard it right - two subwoofers isn't really up to the task. I know what you're thinking and no this isn't about insane output. The primary reason for using three subs is actually all about sound quality! The increase in headroom and output is also a key benefit, but not the main reason. Of course, not everyone has a dedicated room. The context of this post is a serious dedicated home theatre or music room in which we are aiming for accuracy and fairly serious output.

The problem with one subwoofer

There are two problems with attempting to do the job with just one subwoofer:

1. Output - the excursion and thermal power requirements are expensive and detrimental to accuracy
2. Room acoustics - room mode issues aren't properly addressed

First, let's consider output. High output at low frequencies requires moving a lot of air. The typical solution is providing drivers with increasing excursion capability. From a driver design point of view, this leads to expensive drivers and low efficiency hence a great deal of power is required. This in turn means larger voice coils with higher inductance. Again, keeping inductance down increases the expense further. Many high excursion drivers give up so much efficiency that their thermal rating is exceeded long before they run out of excursion.

If the driver is placed in a sealed box, the efficiency will be very low at 20 Hz. In many cases it will be as low as 70 db 1w 1m! Compared to a driver in a vented box, the excursion requirement is doubled. If we place this ultra high excursion driver in a vented box it now becomes very difficult to deal with vent chuffing with air moving at high velocity. At very high excursion it become impractical to design a vented box. The typical solution then becomes using two passive radiators. This removes the vent turbulence problems, but further increases the cost.

Of course, all of these challenges can be navigated and balanced - this is a part of good design. However, the purpose of this post is to present a sensible alternative.

Simulated example

Let's consider an example to illustrate. Simulations have been performed in Win ISD pro alpha.

1. 3 modest subs with Peerless XLS (12") in an 80L vented box tuned at 22 Hz with a modest 300 w input each.

2. One sealed sub with Exodus Audio Maelstrom-x (18") in a 200L sealed box with 1.5kw input power.



Max SPL chart - Red: 3 x 80L Peerless XLS 12" vented Green: Exodus Maelstrom-x 18" sealed

On paper it doesn't look like much difference, but in fact in an actual room the difference in output could in fact be easily increased. In a room with substantial gain, different filtering could be used to get more output in the top end, with room gain brining up the bottom end to match. The efficiency of each Peerless driver is about the same as the larger Maelstrom, and the total radiating area is greater.

The Maelstrom uses 33mm one way excursion to achieve this output, but the Peerless drops down to 5mm at tuning with a maximum of 12mm at 30 Hz. As a result, the Peerless can be used in a vented box and air velocity remains at an acceptable level with a practical vent.

Please note: the point is not to attempt to show that either driver is superior, but to illustrate a different approach to achieving a target. Both are excellent drivers which are also very different. Many would in fact use the Maelstrom in a very large vented box tuned below 20 Hz. Used in this way with a very powerful amplifier it could achieve things which the three Peerless subs couldn't.

So it can be seen that the three modest subs can actually achieve more output than the single large sub. They do it with less power and excursion, and will often fit more easily into the room.

The real advantage, however, is the way in which three subs can integrate into the room. This will be covered in Why two subs isn't enough - Part 1