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frequencies to be cancelled out altogether – an effect know as comb filtering.

Where two parallel flat surfaces face each other, reflections can bounce back and forth like a ping-pong ball. Known as flutter echo, it can be very disruptive at the listening position.


While mid and high frequencies bounce around the room, low frequencies are less chaotic. Bass has the ability to excite the natural resonances of the room itself. This results in some notes being louder than others. At different points in the room the bass frequency response can vary considerably. Resonance, like reverb, continues to ring on after the original sound has finished. This causes a lack of definition in the bass.

In a purpose built listening room the dimensions are chosen very carefully. The relationship between the length, width and height of the room determines how evenly spaced the modes or resonances are. This makes for a flatter and more uniform bass response at different listening positions.

ACOUSTIC ABSORBERS All materials, including air, absorb sound to some degree. Porous materials tend to be most efficient; hence acoustic absorbers are often made from compressed fibres or open cell-foams. Adding acoustic absorption to a room reduces the RT time, but strategic placement allows minimised distortion. The performance of an absorber is known as the absorption coefficient. This is a value between 0 and 1 (0 meaning no absorption; 1 meaning 100 per cent absorption). Absorption is normally tested between 125Hz and 4KHz with a separate coefficient at six different frequencies.

When buying absorbers, check that they perform adequately at the frequencies that need to be addressed. Fire ratings vary between materials so check they comply with your needs. Foam is generally the most affordable option, but fabric wrapped fibre products tend to last longer and can be relocated easily. The look of the product is often a key consideration.

The thickness of a material determines the range of frequencies it can absorb. A balanced room has an even decay time from low to high frequencies. If we cover the walls in thin acoustic tiles, it may appear dead when we clap our hands but in fact the bass and low- mid frequencies will still be untreated. Bass absorbers are much thicker so they absorb lower frequencies. Often known as corner traps, they are designed to sit in the corners of a room, ideally in a stack of more than one trap.


There are various approaches to acoustically treating a listening room. The ideal scenario is a purpose built room professionally designed for optimal acoustics. For those of us without this luxury there is a simple process, which works well in many cases:

Choose your sitting position. You should be centred between the left and right walls just inside the front half of the room. Set up your speakers according to the manufacturer’s specification. For monitor speakers this is normally an equilateral triangle formed by the two speaker positions and a point just behind your head. Install as many corner traps as you can fit and afford. You can’t have too many and the more you have, the more even your bass response will be. Treat your first reflection points. These can be identified by having someone move a mirror around the walls and ceiling. If you can see one of your monitors in the mirror, note the location and treat it with an acoustic tile. The common locations are behind the speakers; either side of the sitting position and above the sitting position. Add additional tiles to your taste or to treat specific problems. You may put some tiles on the wall behind you if the room is still too live. Acoustic treatment is essential for accuracy with any listening system. A properly treated room will have a clear stereo image, lots of detail and even sounding bass. With the right products, good acoustics are easily achievable and could well be the best investment you make.

THINGS TO REMEMBER Acoustics is not soundproofing. Many acoustics companies also sell sound reduction materials but this must be integrated during the building phase; eq and room correction systems can help your speakers, but won’t improve your room as most acoustic problems occur after the sound has left the speakers; egg boxes and carpet are too thin to absorb anything other than very high frequencies and can be a fire risk; decent speakers in a well treated room sound better than fantastic speakers

in a bad room. THE COMPANIES RPG

Inspired by the research of Manfred Schroeder, Dr Peter D’Antonio built his first Quadratic Residue Diffusers and in 1983 set up RPG Diffusor Systems in Maryland, USA. RPG developed a complete range of diffusors, absorbers and combination products, many of which have shaped the industry as well as influenced a new direction of studio design ideas, such as the ‘reflection free zone’. RPG have played a crucial role in the development of many of the practices and designs in use today as well as helping to develop standards for the measurement of diffusion and scattering. Acoustic GRG Products began working with RPG Diffusor Systems some 20 years ago, becoming RPG Europe, manufacturing the complete range of RPG Diffusor Systems in the UK for the UK and European acoustic market.

RPG has a wide range of acoustic devices for studios, both pro- audio and home, from simple foam tiles to bass traps that are effective at 35Hz. Rather than just producing another generic foam panel, RPG developed a new foam panel that would allow users to


Eq and room correction systems can help your speakers but won’t improve your room as most acoustic problems occur after the sound has left the

speakers Will Benger EQ Acoustics

audioPRO October 2010 33

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