6. Reflections and Flutter Echoes
Having dealt with the most influential area of furniture, carpets and curtains, we can turn our attention to the fine-tuning details. The walls and ceiling of a room are large reflective surfaces and the strength and frequency band of reflection greatly influence things like spaciousness, depth of image, image stability, width, etc.
Where sound bounces repeatedly back and forth between reflective surfaces, the result is 'flutter echos' and even more sound degradation.
Reflections
In your listening seat you hear sound from two sources. You hear the direct sound from the speakers first, followed by the reflected sound from the floor, ceiling and walls. The strength of the reflections and the time delay after the direct sound govern the 'spaciousness' of the room sound. This room sound overlays the recorded acoustic and so must be very carefully controlled. It is all too easy to overdamp a room and produce a closed-in claustrophobic sound.
You can avoid this pitfall by being very selective about the placement of absorbers and by creating a relatively 'dead' end where the speakers stand and a relatively 'live' end where you sit; though strong reflections from the walls near your seat are very distracting and uncomfortable. If the speaker end is 'live', and you sit in the 'dead' end, the sound is too distant and echoing. If the speaker end is 'dead' you hear a more forward direct sound with the room acoustic added - a much more natural and satisfying sound. Of course, this only applies to a two-channel system.
Diagram 5.15: A listening room, showing the Dead End, Live End and Absorbers
Before using any absorbent wall treatment however, it is essential that you place the loudspeakers and your listening chair in the best positions. Many reflection problems are simply eliminated this way.
'Dead End'
Conventional advice on the subject of speaker placement, "loudspeakers should be located as far away from reflecting surfaces as practicable", (F. Alton Everest - The Master Handbook of Acoustics, 3rd Edition, page 341) is guaranteed to increase the problems of reflected sound (amongst others!). This technique increases the distance the sound must travel and so increases the time delay. It may, superficially, enhance spaciousness in a system where the speakers are too close together; but two wrongs don't make a right.
Everest (and others) attempt to solve this problem by placing absorbers on all reflecting wall and ceiling surfaces. This modifies the reflections, producing numerous audible anomalies and puts the 'dead' end at the wrong end of the room. In my experience it also 'overdamps' the room and tends to make it look and sound like a recording studio. I feel strongly that a domestic listening room should remain looking 'domestic' and that acoustic treatment should be modest and 'invisible' as such.
A better solution to the reflection problem is achieved by placing the speakers as wide apart as is practically possible - closer to the rear wall and corners than is 'fashionable'. This technique places the sound source close to the first reflective surface, shortening the reflection path to the point where it is indistinguishable from the source itself.
It also moves the main reflective area further down the room where it is easily dealt with by the room furnishings (chairs, curtains, bookcases, record cabinets, etc.) and the absorptive treatments. 'Toed-in' to fire directly at the listening seat which should be as far back as is practically possible.
The role of room furnishings to absorb and diffuse reflections
Bookcases and record storage cabinets make excellent sound diffusors and absorbers. A diffusor breaks up reflected sound waves by being an uneven surface. An absorber breaks up reflected sounds by absorbing them in the material it is made of. A bookcase (without doors!) combines both types because the books themselves absorb sound and their arrangement and uneven size diffuses the sound. Record storage works similarly but not quite as effectively.
Diagram 5.16: View of a bookcase from above
Your record storage unit could, quite conveniently, be placed to the side of the listening chair and a bookcase (or extra record storage) placed on the wall behind. Unfortunately, CD storage won't be much help here, another good reason for owning a record collection! Reflections from behind are clearly as distracting as those from the side. You will find it quite easy to listen for the reflections and then deal with each in turn.
Coffee tables in front of the listening chair are often very audible and degrading. The top forms a reflective surface in just the wrong place! Better to put it to the side of your chair in the room.
Flutter Echoes
'Flutter echoes' occur between hard parallel walls where the sound repeatedly bounces back and forth. They sound very nasty indeed and must be cured. It is very easy to identify where they occur by simply clapping your hands. Walk up and down the room to locate flutter echo points and mark them for treatment.
You only need to treat one wall to cure a 'flutter' so you can choose which one to suit the decor or furnishing arrangement. Flutter echoes can occur in just one place, or can extend right down the whole length of the room.
Diagram 5.17: 'Flutter Echo' in a modern plastered room
Curing 'Flutter echos' caused by modern plastering
If your room has been completely replastered with hard, modern plaster, the best solution to this sort of replastering is with the careful selection of wallpaper. The idea is to put as thick an absorbent layer onto the plaster as possible.
First use a good lining paper and in severe cases use several layers to build up a good depth. Then choose a good thick expensive paper with a 'flock' pattern. This is rather old fashioned and may not be your first choice but aesthetics must come second to sound quality in the music/Hi-Fi room! An alternative is an 'Anaglypta' paper which is thick and heavy but with an embossed design that leaves a pattern of air bubbles that is very effective in high frequency absorption and the control of reflections. The Anaglypta paper can then be painted with emulsion paint.
I tell you this story as an object lesson in how important an apparently small difference can be. The reflective difference between matt and vinyl silk emulsion is easy to see but not easy to equate to the difference in sound it can produce. If you consider though that the small difference is multiplied over the four walls and ceiling of the room you can appreciate how influential it might be. I say 'might be' because it is only a part of a complex relationship involving the room structure, decoration, furnishing and loudspeaker (to name only the principal ones!). Remember that the objective is to achieve a natural balanced sound, rather than pseudo-scientific absolute acoustic values.
Using hangings to cure 'flutter echo'
If redecoration is not an option, Oriental rugs, carpets and tapestries make excellent 'unobtrustive' acoustic absorbers in listening rooms. They are easily hung from rods or specialist supports. If your chosen hanging is really too thin for good acoustic effect you could have a thick backing sewn on. Shops are quite experienced with this and will often ask if the hanging is wanted for its visual or acoustic effect. Where an attached backing is not possible, a sheet of insulation board cut to the right size can easily be screwed to the wall behind the hanging. The insulation board will be invisible but very effective indeed.
Insulation board is a soft, fibrous board used in the building trade. It often has what is called a 'painted' or hard side and a soft fibrous side, allowing some fine tuning of the effect, but normally place the hard side to the wall. Commercial acoustic tiles are made of insulation board and the large panels of shop/office suspended ceilings use a thick version. Insulation board is 1/2" (12.5mm) thick and available in 8'x4' (12.4m x 1.2m) sheets from builders merchants or wood merchants for about £10 a sheet. It can be cut very easily with a Stanley knife.
Remember that the balance you need to achieve is of a slightly 'live' overall character whilst the listening end is a little livelier compared to the speaker end. A very useful benefit of buying some sheets of insulation board is to use them initially as temporary moveable absorbers. That way you can quickly get a 'feel' for the effect of treatment in different positions.
