How Does Listening Room Acoustics Affect Sound Quality?

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, < 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
• Early reflections (0 to 15 ms): -10 dB or less relative to direct sound

Source : Wolfgang Hoeg, Listening Conditions for Subjective Assessment of Sound Quality; the Status of International Standardisation, Audio Engineering Society preprint 3788.

What are the main acoustical problems? How do they affect the sound quality?

Reverberation
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 listened 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 second depending on the frequency, while the recommended values are between 0,2 and 0,4 second for good listening conditions. Much more definition and details are obtained when excessive reverberation is controlled.

One’s room RT60 can be calculated with the following tool ( Sabine’s theory):

http://www.saecollege.de/reference_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, depending on the off-axis behavior of the loudspeaker can create phase problems by combining to the direct one or it can be beneficial to the overall listening experience. The result, as measured with a microphone produces numerous dips in the frequency response. This phenomenon is called comb filter. 

However, this is something that is measured by a microphone but note something that is necessarily badly processed by our ears.  The human brain/ear processing of sound is much more complex and forgiving than what a microphone measures at one point in space.

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 )

The best way both aesthetically and efficiency to solve standing wave problems in small room acoustics is through modal manipulation using multiple subwoofers.  Once you reduce the seat-seat variance at bass frequencies, any remaining peaks can be EQ'ed out of the response to eliminate room resonances.  Low frequency bass traps can be used to supplement this solution if they are deemed to be necessary and practical.

For more information see:  History of Multi-Sub and Sound Field Management

Conclusion