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Fixing Your Room Acoustics

Room acoustics is one of the least understood aspects of hifi. Even scientists have a hard time predicting what will make a room sound good (or bad) and by most accounts, we're stuck with what we've got anyway. How can you make your room sound the best with the least amount of effort?

There are many factors that affect how a room sounds - the volume, the ratio of dimensions, construction material, wall material, furnishings and where you site the speakers and listening chair. All of these factors work in an integrated fashion, so changing one will usually affect how the others work. 

When we talk about room acoustics we are usually starting from a particular problem we want to solve. Some rooms are boomy, some are bright, some have bad speech intelligibility, some require very large speakers, some sound lifeless, some overly punchy. We have all experienced some of these problems at some point.

BASS AND ROOM DIMENSIONS

Bass waves in rooms act somewhat like water. You can imagine that a bass pulse moves from one side of the room, hits the far wall and comes back eventually colliding with the next pulse in the music. At certain points the meeting waves will become much stronger and at others they will become much weaker. This is called a room mode and it is entirely related to the dimensions of your room. For example in a 12 x 16 x 8 ft. room, which is very common, has a strong bass boom at 70Hz which is right where most people consider "boomy bass zone". Placing a tower speaker, which tends to have too much output around 70Hz, will result in sloppy, boomy bass in this room. To make matters worse, the bass waves will cancel at uneven multiples of the boom certain frequencies will be very quiet, further giving you the perception of a bad and uneven bass. You can observe this effect easily. Play some electronic music with deep continous bass notes. Walk around the room as these notes are playing. You will notice that at some places the note will be very loud and at others it will be very quiet. It's still the same note in both places but the room is affecting how loud it is. These are the room modes. 

A useful room mode calculator can be found here:

http://www.bobgolds.com/Mode/RoomModes.htm

Using the room calculator will give you an idea of what kind of bass response you will want from your speaker. Most people get sucked in by larger towers because they have "full range" reproduction, but in practice this is wasted and actually detrimental in most typical rooms. Do not shoehorn large speakers into small rooms. The dimensions of the room are not sufficient to hear lower bass notes at all! 

By now you're probably asking how you can fix a badly sized room. The easy answer is that you can't! Short of adding an enormous amount of dense material such as high density foam or lossy membranes which vibrate in sympathy with the bass modes to sap energy, you can't effectively tune bass in your room. You can equalize but this often ruins the purity of the rest of the sound and only very advanced digital signal processing which addresses amplitude and phase is effective. Many people find EQ introduces drawbacks in more subjective qualities like "musicality", "grain" or imaging. 

The simplest fix for room modes is to choose a speaker that is sympathetic with your room (generally smaller is better) and then experiment with placement to get the right amount of reinforcement and the least amount of cancelation.

The correct speaker for our example room would be one that subtly starts to roll off at 70Hz and can be kept a fair distance (say 2ft from the walls), probably with a front port. 

As a general rule, consider tower speakers for rooms with minimum dimensions larger than 12 ft (the ceiling applies too!), otherwise the speaker will have to be very well controlled (making it expensive). Square rooms or rooms with divisible dimensions (such as 8 x 12 x 24) will have terrible boom and cancelation effects. 

Placement in the room also drastically changes bass output of a speaker. The closer to the wall or corner, the more bass reinforcement. If you have a small room, a smaller speaker will simply give you more options of placement. You will be shocked at how much difference a few inches can make in the smoothness of the bass response. As well, changing which wall the speakers are against will drastically change the bass response. Experiment with all possible options. Where you sit can have big effects too. Try to keep the speakers and listening position in an equilateral triangle but move the triangle forward or back, and side to side within the room to find the most even bass.

Concrete rooms or rooms with thick heavy walls have the worst bass modes because they keep all bass energy in the room. A thin and poorly constructed steel stud and drywall construction allows quite a lot of energy to be absorbed or passed through the wall (think of noisy neighbours) which actually smooths out the bass modes inside the room. If you're building a room, keep this in mind.

MIDRANGE

Midrange is not affected by room modes and is generally not affected by room acoustics at all. The most important quality is to have the speaker produce extremely accurate response through the midrange band (about 500Hz to 5KHz). The human ear is extremely sensitive to frequencies in this region. Make sure to not have anything obscuring the path between the speakers and listening position as this can hugely affect the amount of midrange arriving from each speaker. Think of someone talking (which is all midrange) with a piece of paper or curtain in front of them - most of the sound is easily lost. Obstructions are the enemy of good midrange.

TREBLE

Treble is almost as tricky as bass but luckily it is easily tuneable. Treble can be thought of as similar to how light travels and reflects off a mirror. The sound in the treble band (5KHz to 20KHz) is very directional and will bounce off hard surfaces like walls, bookcases, floors, ceilings, tables etc. Even small items such as lamps can have huge interactions with treble frequencies. The reason is that the wavelength of treble frequencies is very short so they don't tend to bend around things as large bass waves do. They hit things. 

If you've positioned your speakers in a symmetrical pattern so they are equidistant from your ears and from all walls there is a good chance that the treble information from each will reach your ears at the same time and in the proper amount. This complex interaction between two balanced speakers allows us to place instruments in space between the two speakers. On a good speaker this will be perfect and you will be able to clearly point to an instrument that was recorded dead centre or at 1/3 right or left of centre. 

However, not all information from speakers goes straight to your ears. A lot of the sound goes past or to the side where it hits the wall and bounces back creating reflected sound. These direct reflections will eventually come back and be heard by your ears at different times and from different directions which can destroy the perfect illusion of space in front of you.

If too much sound is bouncing around it will smear imaging and in many cases cause the overall sound to be bright and fatiguing. The trend amongst speaker manufacturers is to make speakers that are already too bright (to make them sound more "exciting" during sales demos) so in a room with too much sound reflection, these speakers can be extremely irritating. Certain wall surfaces such as glass or drywall which are popular in modern condo style apartments are atrocious when it comes to treble information and in many cases will echo because there are so many reflections. This destroys imaging as well as intelligibility of speech and transients. Drums especially can blur together so badly that you can't hear individual notes anymore. Voices can have terrible "esses".

Fixing treble can be accomplished using two general methods. The first is to absorb some of the sound that you don't want to be hearing. Damping the area halfway between you and the speaker as well as the back wall behind you can work well in reducing unwanted treble energy. Wall hangings or commercial acoustic panels are extremely affective. In fact, people usually go too far in absorbing treble in some areas so that they become dead and areas nearby are still highly reflective. Use low-absorption in an larger area as opposed to high absorption in a smaller area. A good test is to play a radio station with an announcer. They should be easily understood and the voice should come from directly between the speakers. The scale should be similar to if the person was standing in the room. The brain is very sensitive to voice so you will know quickly how close you are to the proper treble absorption.

The second way to fix treble reflections is to scatter them to a high degree so that they collide with themselves (they cancel just like bass does, but on a much smaller scale). A bookcase filled with many different sized books makes an excellent diffuser. Think of anything with random angles and different depths of 1/4" to 2". The sound will be diffused in a more harmless way than direct reflections. Diffusor ideas don't need to cost a lot of money if you think in practical terms. We used hundreds of 2x2 wood end cuts with 45 degree angles to create a random wall behind our listening area. Any treble energy that hits them is scattered harmlessly in all directions instead of coming back directly.

Another cheap way of fixing treble is to sit closer to your speakers. The brain prioritizes sound it hears first. If you sit closer to the speakers the reflections from the room will always arrive after. Toeing in the speakers also creates less off axis energy and can work wonders. Too far and your sound will become closed in.

The key in all treble tweaks is to cut down on unwanted reflections. Absorbing and diffusing can work equally well if done correctly. 

COMMERCIAL ROOM TREATMENTS AND "GIZMOS"

There are many excellent commercial room treatments, however many audiophile products are grossly overpriced. Look to pro sound products as well which all have technical measurements (some are accurate) and be careful not to buy a product (especially when absorbing) that is too effective. A dead room is in many ways more unnatural than a "live" room. There are some effective bass traps, but the vast majority do not have the size to absorb enough bass energy to overcome the inherent flaw of the room. Be very wary about technical claims with bass traps. There is no way to cheat physics in this regard and there are very few domestically acceptable options that actually work. Our prior recommendation of speaker choice and placement is far preferable and practical for the vast majority of people. Create the right amount of bass with the right speaker choice and you will have to tune less. Lastly, be extremely cautious of gizmos which are small and purport to drastically smooth bass response. This is impossible. Bass waves are big and powerful. Small things cannot affect them. In the case of treble, "focussing" wood statues or small metal cones placed on wall centres generally do not have any effect. Invest in a $5 beach towel to use as a cheap "test" absorber and compare the effect to $1000 "miracle" room treatments. 

EQUALIZATION AND DSP

After many years of electronics research we are finally seeing intelligent equalizers and computer processing which can address bass modes and tune treble. These options are still very expensive and most purists agree they introduce at least some artifacts into the rest of the music, but for more casual listening or in very bad rooms they are becoming viable options. In the case of digital sources such as CD or computer they make more sense since they can operate entirely in the digital domain, whereas they need to convert analog to digital to be able to process the sound which almost always destroys information. In the next 5 years we should see affordable room correction devices that actually work.

SUBWOOFERS

Since many people are moving towards smaller speakers in smaller spaces, subwoofers are increasingly used to maintain a realistic bass response. By separating the bass of the traditionally large speaker into an isolated box, it is often possible to more effectively place the sub where bass output is optimized. This might be in the middle, back or side of the room where it would be impossible to put standard full sized speakers. Using the phase control on the sub, or even using two subwoofers in a complicated in-phase out of phase relationship can work well for difficult rooms. Placing a sub is a trial and error affair but if you are willing to put in the time, there are now good quality music-specific subwoofers available such as those from REL. Most sub-$1000 home theatre subs are atrocious quality.

CONCLUSION

When dealing with room acoustics, follow common sense. 

1. Choose the right size speaker for your room.
2. Place the speaker in the optimal position for most even bass.
3. Ensure symmetry between the speakers and the listening position.
4. Subtly absorb and/or diffuse treble energy using wall treatments.
5. Don't spend a lot of money trying to fix something that can't be fixed.

We offer free advice on room setup and treatment when buying a new piece of equipment. Just ask!

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