Unfortunately when home audio systems are set up, the acoustics of the listening room are rarely taken into account. Indeed, most people opt for expensive, top of the range sound systems in an attempt to reach the best-possible sound quality. But they often ignore one essential thing: the acoustics of the listening room itself. As a sound system is used in an enclosed space (the listening room), the acoustical conditions of the room will inevitably take control of the sound quality.
This first article focuses on the main acoustical problems of the listening room and on how they deteriorate the perceived sound. In upcoming issues of CANADA HiFi, each of those topics (acoustical phenomena) will be explored and accompanied with practical advice to improve the acoustical conditions of a listening room.
Acoustics: An integral part of the reproduction system
In a typically furnished, medium-sized room which is not acoustically treated, the critical distance is small – usually only about 2 meters. The ‘critical distance’, measured from the speaker, is reached when the reverberant sound level equals the direct sound level. The direct sound is the sound that comes directly from the speakers, without any reflections. So, at that distance, 50 percent of the sound is the result of the reflections on the room’s boundaries. These reflections cause various problems such as: phase, frequency response, definition, aggressive highs, blurred image and low frequencies that are resonant, have holes in them and lack tightness. Going beyond the critical distance means reducing the direct to reverberant ratio, thus the quality of the sound deteriorates even more.
In other words, the acoustical environment should be considered an integral part of the reproduction system.
Quality and price
Contrary to what one may think, acoustical improvement constitutes one of the most efficient and economical ways to enhance the performance of a sound system, since all of the various listening parameters can be improved in a very significant way. For the DIY-ers, it is possible to treat a room with affordable semi-rigid fibreglass panels. From these panels, we can build everything we need to control acoustical problems in the listening room: acoustical panels to absorb mid and high frequencies, distant absorbers to control low-mids, lower frequency absorbers that reflect high frequencies, pressure gradient traps and bass traps.
For aesthetics, the panels can be covered with sound transparent fabrics such as the Guilford of Maine FR701. For most rooms, the acoustical materials needed to treat the room will cost somewhere between $300 and $600. Of course, fees for an acoustical consultant will vary.
What are the acoustical specifications for a good listening room?
Researchers, audiophiles and sound engineers have conducted many tests to establish listening room criteria. The International Telecommunication Union, the European Broadcasting Union and the British Broadcasting Corporation publish some recommendations about this. The most important listening room criteria include: speaker positioning, room frequency response, space between axial standing waves, RT60 (reverberation time), background noise level and early reflections. See box below for details.
Recommended room specifications
• Speaker positioning: according to ITU BS-775-2 recommendation
• Room frequency response: +- 3 dB, from 250 Hz to 2 kHz (partial)
• Space between axial standing waves: > 5 Hz, < 20 Hz
• RT60: 0.3 x (V/100 m3)^1/3 +- 50 ms from 200 Hz to 4 kHz, typically 200 to 400 ms
• Background noise level : NC-15 or near 20 dBA (demanding)
• Early reflections (0 to 15 ms): -10 dB or less relative to direct sound
What are the main acoustical problems and wow do they affect the sound quality?
Reverberation (or decay rate)
Reverberation is the product of multiple closely spaced reflections on the room’s boundaries. Reverberation extends sound and causes a mask effect on the details of the audio content. Reverberation time (RT60) is defined as the time it takes for a sound to decrease by 60 dB after its emission. In an untreated room, RT60 can vary from 0.5 to 1.5 seconds depending on the frequency, while the recommended values are between 0.2 and 0.4 seconds for good listening conditions. Substantially increased definition and details are obtained when excessive reverberation is controlled.
A room’s RT60 can be calculated with the following tool (Sabine’s theory): http://www.saecollege.de/reference_material/pages/Reverberation%20Time%20Calculator.htm
Lateral reflections create phantom sources outside the speakers, enlarging the stereo image. By doing so, they also contribute to enlarging every sound element distributed between the speakers. The result is a blurred image that lacks precision.
Direct to reverberant ratio
Direct to reverberant ratio is the difference between the direct and reverberated sound levels. Obviously, a poor ratio will contribute to mask the perceived details of the sound. What’s more, the stereo image will suffer if the reverberant sound is too strong. Let’s not forget that only the direct sound delivers the information about the position of the sound elements in the image.
Early reflections are those which reach the listener within a delay of 15 ms relative to the direct signal. The retarded sound creates phase problems by combining with the direct sound. The result is numerous dips in the frequency response. This phenomenon is called a comb filter.
Standing waves (also called room modes) are low frequency resonances that take place between two parallel reflecting wall surfaces. They result from the interaction of wavelengths (?) and the distance between the surfaces. So in the low register, standing waves cause: a level boost at some frequencies, a prolonged duration of sound at those same frequencies (resonance) and some profound dips at other frequencies. The phenomenon does not present itself the same way in all the spots in the room. This is why the low frequency response varies from spot to spot in the room. Three methods are used to solve standing waves problems. These will be discussed in the next article.
Standing waves can be calculated with the following tool: http://www.bobgolds.com/Mode/RoomModes.htm
As illustrated above, a listening room’s acoustical conditions are key contributors to the final sound quality because they affect several parameters in a very significant way. The objective here is not to have a ‘dead’ room, but a neutral room. Some people falsely believe that the listening room should have the same acoustics as a small concert hall. This is a mistake because the recordings already contain the reverberation and the acoustical ambiance of the room where the concert took place. We don’t want to change this. I’ve recorded sound reproduced in an untreated listening room in the past with the aid of very high quality microphones. Believe me, the sound signature of such a room brings nothing positive and considerably degrades the perceived content. Stay tuned for more on this topic in upcoming issues of CANADA HiFi.
Michel Leduc is an acoustics professor (Cégep of Drummondville), researcher (Musilab, CCTT sound technologies) and acoustical consultant (Sonart Acoustique, www.sonartacoustique.com). He can be contacted by email at email@example.com.