There are countless ways to upgrade an audio system or recording setup. Investments in equipment often result in subtle improvements in audio quality. Despite the common tendency for continuous upgrading, the extent to which the room itself affects the sound is often overlooked.
Consider the dramatic change that occurs when you remove soft furnishings from a room to redecorate. If the effect of removing the curtains is so clearly audible, surely room acoustics are a fundamental influence on the audio quality we so actively pursue?
With a sound system we are often looking for a ‘faithful reproduction’. The listener wants to experience the sound as the producer intended it to be heard. The audio professional wants to hear ‘accurate’ sound so they can be confident their production will translate well to listeners’ audio systems.
With any system we hear the original sound combined with resonances and reflections from boundaries of the room. What are the implications of the room’s influence on our sound?
For many, the acoustics of a room are characterised by reverb. It is the most noticeable attribute of a room’s acoustic, particularly when we move between a large and small room. In fact reverb only accounts for the mid and high frequencies. Bass frequencies also contribute to the acoustic but these are less directional in nature so we’ll look at bass separately.
Reverb consists of countless reflections caused by sound bouncing off hard surfaces. The length of time it takes for these sound reflections to subside is known as the Reverb Time (RT). The RT of a room is a product of its size and the type of materials present in the space.
A long reverb can be desirable when recording acoustic instruments. An orchestra rarely sounds as imposing without reverb and as such many concert halls have an RT in excess of 2 seconds. A recording engineer will use reverb as a creative tool and understands how different RTs affect the listener’s perception of the music.
In a listening environment, such as a control room, a short reverb is desirable – normally around 0.5 seconds. We don’t want the room to be completely “dead” as it would be an uncomfortable space to occupy and it would not be representative of a normal room so translation would be poor.
In a small room, reflections can arrive at our ears in very quick succession. As such the early reflections in these spaces are interpreted as being combined with the original sound. This causes certain 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. This is known as flutter echo and it can be very disruptive at the listening position.
Whilst 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.
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 us to minimise acoustic 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% absorption). Absorption is normally tested between 125Hz and 4KHz with a separate coefficient at 6 different frequencies.
When buying absorbers, check they perform adequately at the frequencies you need to address. 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 – especially in a domestic room.
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.
1. Choose your sitting position. You should be centred between the left and right walls just inside the front half of the room.
2. 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.
3. Install as many corner bass traps as you can fit / afford. You can’t have too many and the more you have, the more even your bass response will be.
4. 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.
5. Add additional tiles to your taste or to treat specific problems. For example you may put some tiles on the wall behind you if the room is still too live.
The manufacturer will be happy to help you get the most out of your room. We always ask for a drawing with measurements and some photos if possible. This allows us to get the most out of your room and available budget.
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.
• Acoustics is not sound proofing! 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!