Tuesday, April 14, 2009

Great Article on Listening Room Acoustics

From Audioholics...

How Does Listening Room Acoustics Affect Sound Quality?
by Michel Leduc — last modified April 12, 2009 19:15

Unfortunately, where sound quality is concerned, the acoustics of the listening room is 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 ‘a listening room’, the acoustical conditions of that room will inevitably take control over 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 the next articles, each of those topics (acoustical phenomena) will be developed and will be accompanied with practical advice to improve the acoustical conditions of a listening room.

Acoustics: an integral part of the reproduction system

In a medium-sized room, furnished in a usual way, but which is not acoustically treated, the critical distance is of a few meters only – generally 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% of the sound is the result of the reflections on the room’s boundaries.

These reflections cause various problems such as: phase, frequency response irregularity, loss of 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 the various listening parameters can be improved in a very significant way. For the DIY, 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 the aesthetics, we cover the panels with sound transparent fabric, like Guilford of Maine FR-701. All the acoustical material needed to treat a room will cost between 300$ and 600$. Fees for an acoustical consultant vary.

What are the acoustical specifications for a good listening room?

Researchers, audiophiles and sound engineers have conducted many experiences to establish listening room criteria. The ITU[1], the l’EBU[2] and the BBC[3] publish some recommendations about this. Here are some of them, with some of well known acoustician’s. Those requirements are precise and are not less important than the audio system’s specifications.

Speaker positioning: according to ITU BS-775-2 recommendation

Room frequency response: +- 3 dB, from 250 Hz to 2 kHz.

Space between axial standing waves : > 5 Hz, < href="http://www.saecollege.de/reference_material/pages/Reverberation%20Time%20Calculator.htm">http://www.saecollege.de/reference_material/pages/Reverberation%20Time%20Calculator.htme_material/pages/Reverberation%20Time%20Calculator.htm

Lateral reflections

Lateral reflections create phantom sources outside the speakers, enlarging the stereo image. By doing so, they also contribute to enlarge 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 weak 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

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 to the direct one. The result is numerous dips in the frequency response. This phenomenon is called comb filter.

Frequency response with treated and untreated first reflection.

The red line shows a comb filter.

Standing waves

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 (λ) and the distance between the surfaces. So in the low register, standing waves cause: a level boost at some frequencies, an extent of the duration of sound at those same frequencies (resonance) and some profound dips at other frequencies.

A typical listening room low frequency response.

The phenomenon does not produce itself the same way at different locations in the place. This is why low frequency response vary from place to place in a room.

Three methods are used to solve standing waves problems. They will be discussed in the next article.

Standing waves can be calculated with the following tool:http://www.bobgolds.com/Mode/RoomModes.htm )


A listening room’s acoustical conditions are therefore key factors of 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 falsy believe it should be necessary to have a listening room whose acoustics have some characteristics of that of a small concert hall. It would be a mistake because the recording already contains the reverberation and the acoustical ambiance of the room where the concert took place. We don’t want to change this. I occasionally recorded the sound reproduced in an untreated listening room 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.

Graphs courtesy of Ethan Winer of Real Traps

For more information search for “acoustic AND room” on these websites:

Audio Engineering Society: http://www.aes.org/e-lib/
Acoustical Society of America: http://asa.aip.org/map_publications.html#0
Michel LeducAcoustics professor, Cégep of DrummondvilleResearcher, Musilab, CCTT sound technologies Acoustical consultant, SONART ACOUSTIQUEListening room and recording studio design