Integrators are struggling to find the peak audio performance with multiple subwoofers. San Williams finds out how RP22 solves these issues along with what defines a ‘perfect sound’ with bass in home entertainment.

Home entertainment rooms have changed greatly in just the past ten years, particularly in the custom installation (CI) space, when rooms were almost always purpose-built for home theatres along with dedicated audio systems. But today, living rooms are larger and architecture and aesthetic ideals have changed. Given this move out of the dedicated theatre and into the living room, audio has had to find new ways to create immersive experiences.

Newer homes have fewer carpeted rooms, leaving more hard surfaces for sound to bounce off from and increasing sound variation. As a result, the industry has countered by opting for multiple subwoofer setups to potentially improve the situation, and the increasing awareness and acceptance of adding more subwoofers to audio systems continues in the industry. However, the solution stems beyond simply installing the latest and greatest audio devices.

Recognised as an official guideline for immersive audio standards, CEDIA/CTA – RP22 Immersive Audio Design Recommended Practice is the first recommended practice to define an objective set of performance criteria for home audio, which CEDIA chief executive officer and global president Daryl Friedman says will “benefit integrators, manufacturers and homeowners who can be assured of the same high audio standards no matter the room”.

In common situations like ‘phantom imaging’, which is mostly ineffective in the bass region and more applicable in higher frequency ranges (such as in the location of a voice between two regular full-range speakers), adding an extra subwoofer can boost bass in its optimum position, while reducing seat-to-seat variation. Hence, everything comes down to the number of speakers and where they’re specifically placed to achieve peak sound performance. This isn’t easy to do when you’re working in a crowded room like a living room, especially with bass sound devices.

“No matter how good your gear is, it takes engineering to produce the performance levels defined by this recommended practice. Audio interacts with its physical surroundings: The same speaker will sound completely different in a different room or position,” CEDIA senior director of technology and standards Walt Zerbe says.

“RP22 goes well beyond setting benchmarks to include rich technical guidance for audio system design professionals.”

Dirac, a European audio software company, believes that RP22 serves as a good start for those hoping to achieve peak performance with a multiple subwoofer sound system.

“RP22 is a really important step because audio is so filled with emotions and different opinions. There’s not one simple solution to designing a great-sounding audio experience, but RP22 shows the key problem areas to work on to get better sound. CEDIA and CTA did a good job of summarising the collected wisdom of the industry and academia,” Dirac chief technology officer Mathias Johansson says.

RP22 identifies 21 different performance parameters that can be predicted during system design, tested then verified before system sign-off. Using these metrics, it sets criteria for four different levels of system performance:

• Level 1 – conveys basic artistic intent.
• Level 2 – a higher level of performance that more accurately conveys artistic intent.
• Level 3 – meets or exceeds reference commercial cinema standards.
• Level 4 – achieves the maximum level of performance across every parameter.

With these defined levels, home technology professionals can use RP22 to define the difference between off-the-shelf speakers and a state-of-the-art immersive home audio system. This is all well and good, but then what constitutes ‘good sound’ or bass? After all, if we’re going to accomplish peak audio performance and a perfect bass in an ecosystem, we need to know what it is or what it means.

We can currently revert to three defining areas in constituting good sound, according to Mathias. The most obvious is the timbre, the relative levels of different frequencies, at the listening positions. Then, there’s spatial resolution which is how we can make sense of the space and measure directions and localisation. Finally, the dynamic range (or just dynamics) is how sound can be played both extremely loud and soft and how a speaker or amplifier can hold a significant shift in voltage.

The importance of bass to the overall listener experience is explained extensively in RP22, holding consistent with research that shows overall user rating preferences of bass in sound systems as high as 50%. RP22 also delves into what defines good bass, with areas Mathias says his team has been recently embracing in audio systems.

“RP22 talks about the importance of optimisation, equalisation and bass response. It is important to realise the huge impact that the listening room itself has on your sound experience,” he says.

In many cases, the room is where the biggest colouration of the sound happens. There are problems like swings in bass response due to speaker placement and/or differences in listener experience, from seat-to-seat variations and other factors like video and lighting, if it’s a home cinema and struggling to differentiate.

Furthermore, there will be different reflection patterns depending on the placement of loudspeakers. Sometimes a bass frequency is completely cancelled out at one location, whereas in another location, a strong bass build-up (a resonance) occurs.

Dirac provides a handy tool for this with its Dirac Live Bass Control, a measurement-based bass optimisation scheme that can be applied to whatever subwoofer placements you have. This technology is useful since it is very difficult to know how, say, two subwoofers are going to interact with one another as sound bounces off the walls and roof of a listening space.

“What we do with Dirac Live Bass Control is adjust the phase response of each speaker in the bass region with all-pass filters, and by this method we make the speakers sum up in phase and thereby create a better, more even, and powerful bass response throughout the listening space,” Mathias says.

“In doing so, we can time the output from one subwoofer at a certain frequency to that of another one in the room and ensure that throughout your listening area, you’re getting the same bass response, they all sum up coherently.”

Audio isn’t one-dimensional. In the task of acquiring pitch-perfect sound, there is a lot to consider with room variations and budget playing factors in an audio system build. At a general level, RP22 seems to be solid educational material, defining what makes a good speaker, what a well-functioning room is and providing an idea of what should be achieved.

As multiple subwoofer audio systems are becoming more common, and because it is difficult to predict the optimal positioning of the subwoofers, advanced bass management schemes are gaining traction.  Mathias further points out that even if you could predict the best speaker position, it might not be possible to position the speaker there, which makes advanced digital room correction and bass control schemes an attractive solution.

“Using a microphone and audio room correction technology like Dirac Live, you can remove a lot of the negative effects of the room acoustic interaction with the speakers, and enhance the loudspeaker performance,” he says.