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Room for improvement

Tuesday, 31st March 2020

As we are confined to our homes during this unsettling period, Simon Dalton decides to revisit a selection of Russ’s thoughts on room acoustics for some practical advice.

What we intend to do here is offer some broad advice - practical steps you can take to improve your room. Much of this will be trial and error as room acoustics are very specific so it’s impossible to say what the issues are affecting your own situation. Nevertheless, these simple suggestions should be helpful so long as you are willing to take the time to experiment. Bear in mind that this is by no means an exhaustive list of actions you can take – for Russ’s full article on this subject, click here or phone us for a printed version.
Sound Absorption

room acoustics sound absorption

This is the amount of sound 'echo' damping within a room. In order to achieve an acceptable reverberation time (i.e. good sounding), a room needs to contain a certain amount of absorptive materials. Fortunately, a normal sitting room with a carpet, curtains, upholstered suite and other furnishings usually achieves an acceptable reverberation time.
Aim to use materials and techniques that achieve smooth broad band absorption rather than narrow band 'problem' curing solutions. Manmade materials tend to have narrow-band effects but natural materials – such as cotton, wool, feather and leather - have broadband responses.
Carpet and carpet underlay

Room Acoustics Carpets and underlay

Carpets can have a profound effect on the sound of a room. A fully fitted carpet covers a large proportion of the surface area of a room and so exerts a marked effect. In general terms a thick pile carpet (especially of high wool content) is much better sounding as it absorbs over a broader band.

Carpet underlays are clearly audible in their effects too, so avoid the ubiquitous rubber underlays that shops now expect you to want, and choose good old-fashioned underfelt… and buy the best quality.

You can vary or tune the effect of carpeting by using rugs on top of a fitted carpet, especially in the area between the listener and
the speakers.

Curtains and Windows

room acoustics curtains and windows

Curtains are very useful indeed in controlling delay time and absorbing mid and high frequencies. They are more effective if they go right down to the floor and are lined.
A thick material like velvet or velour is most effective, and you can make them more effective still if you allow extra widths of material so that the folds are deep.
Windows absorb quite large amounts of bass because both the glass itself and the flexing of the glass absorb bass. When you draw a curtain across a window you create a complicated kind of diaphragm absorber using the window recess as a ‘cavity absorber’ and the window glass as a ‘resonant absorber’. It's no wonder that the room sound can change dramatically when you draw the curtains so consider this when listening during daylight!

room acoustics 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. Be careful though - 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. This will create a much more natural and satisfying sound.
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.
Conventional advice on the subject of speaker placement states that, "loudspeakers should be located as far away from reflecting surfaces as practicable". However, 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.

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. 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.
Flutter Echoes

room acoustics 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.
If your room has been completely re-plastered with hard, modern plaster, the best solution is with the careful selection of wallpaper. The idea is to put as thick an absorbent layer onto the plaster as possible. A heavy flock type is most effective but if this is too old-fashioned use an Anaglypta paper painted with matt emulsion - matt emulsion is far more effective than silk!
If redecoration is not an option, Oriental rugs, carpets and tapestries make excellent 'unobtrusive' acoustic absorbers in listening rooms. 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.
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.
The role of room furnishings to absorb and diffuse reflections

room acoustics furnishings

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.
Your record storage unit could, for example, be placed to the side of the listening chair and a bookcase (or extra record storage) placed on the wall behind. 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.

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