The Sight of Sound: Revealing the Effects of Enclosure Design on Loudspeaker Performance
L-Acoustics crafts sound systems for arenas and concert halls. To accelerate the evolution of their bass reflex speaker designs, they simulated the effects of enclosures and vents on speakers' acoustic output and quality.
By Alan Petrillo
A live event is most memorable when the audience feels like part of the show. Whether it is a symphony, an opera, a football game, or a music festival, the energy of the crowd should meld with the performance into one powerful experience. Connecting with the audience is the goal of every performer, but of course, they cannot do it alone.
What we see and hear in a performance venue is brought to us by many hidden people, from stagehands to set designers. And even as our eyes focus on stages and sets, there is a lot of essential equipment that we may not see — but we sure can hear.
L-Acoustics is a global company dedicated to conveying the aural power of live performance. While the typical concertgoer or sports fan most likely has not heard of L-Acoustics, they have likely heard from the company’s loudspeakers, amplifiers, and signal processing devices. Based in France, L-Acoustics has supplied sound systems to more than 10,000 venues in 80 countries, and their equipment has been used by half of the top 20 music festivals worldwide.
Great performance is rooted in relentless dedication to craft. L-Acoustics must also constantly hone and refine its products to work in harmony with all of these varied performance spaces. For instance, the size and shape of speaker enclosures can have a big impact on sound quality. To ensure that audiences are able to lose themselves in the sound of a performance, L-Acoustics engineers use simulation to help reveal the effect that enclosure and vent design has on acoustical output and linearity.
Just as live performers show us the precise physical movements that create their music, multiphysics simulation now lets L-Acoustics "see" the forces shaping the sound of its bass reflex speaker units.
Like a Laser for Sound: The L-Acoustics Vision
You may be surprised to find that many old and respected venues, from the Hollywood Bowl in California to Sakura Hall in Japan, feature sound systems made by a company that is less than 40 years old. Physicist Christian Heil launched L-Acoustics in 1984, and in 1992, the company introduced its V-DOSC line source array technology, which quickly became the global standard for professional loudspeaker systems. A line source array projects sound in a remarkably focused, controlled way, comparable to the way that a laser directs light. A laser's potential power is inseparable from its precision, which is true of loudspeakers as well.
“We want to create a loudspeaker enclosure that is as linear as possible,” says Yoachim Horyn, head of Acoustics Research at L-Acoustics. “We try to remove any amount of energy that is emitted at higher frequencies or harmonics, rather than the intended frequency. No matter how much we increase input power, we want the resulting sound to be the same, only louder.” Distortion, however, is only part of the problem. Nonlinearities can emerge with a global loss of output at the emitted frequency.
Along with engineering of the loudspeaker driver itself, the design of a speaker’s housing or enclosure plays an important role in its performance. For example, a bass reflex enclosure design incorporates a vented opening called a Helmholtz resonator. By connecting the interior volume of the speaker housing to the outside air, a resonator can help recover part of the energy emitted inside the enclosure that would otherwise be lost. This boosts power, but it also introduces turbulence, which distorts the speaker’s output and can also cause significant acoustic losses of up to several dB. Despite this risk, the potential benefits of a resonator make it an important tool for L-Acoustics, who strive to fill massive spaces with sound.
“Our loudspeakers must perform at a very high level of output,” says Yves Pene, an acoustical engineer on Horyn’s team. “A resonator that is incorrectly designed could lose up to half of its potential output due to turbulence, so it is really important that we design the vent to work efficiently.”
Cutting Wood, Sculpting Clay, Filling Speakers with Smoke
“For several years, designing and testing vents have been a challenge for development teams,” explains Horyn. “We had no way of predicting precisely the amount of loss created by an enclosure with a high level of air displacement.” This meant that the team had to build and test wooden prototypes of every design adjustment to either the enclosure or the resonator vent. In some cases, they applied clay to quickly change the shape of a vent’s opening or interior passages. This could be a slow process — and even a completed physical prototype would not give them all the data they needed.
“Measuring acoustic loss and distortion is interesting, but it does not always give you an insight on where the problems are,” says Horyn. “Sometimes, the problems come from a part of the enclosure, or a part of the vent, that you would not expect. A wooden model does not show you exactly where the problems are." The L-Acoustics team came up with an interesting way to work around this problem. “Something we have done in the past is to make some of the panels in the box transparent. We would blow smoke into the loudspeaker to enable us to see the turbulence,” Horyn says.
While a see-through panel could make a physical model more helpful, the prototyping process remained a major time sink. “When we design and build a mockup, it takes several weeks between drawing it and actual testing,” explains Horyn. “And it would often take multiple iterations to arrive at our design.”
Simulation for Current and Future Projects
To help refocus the L-Acoustics team’s efforts on acoustical engineering, rather than woodworking, Yves Pene turned to multiphysics simulation. His objective, as explained in a 2020 research paper presented to the Audio Engineering Society (Ref. 1), was to model and predict nonlinear acoustic loss in vented port systems for a given loudspeaker, enclosure volume, and port design. The simulation incorporates the coupled effects of the loudspeaker driver’s motion and the resulting fluid motion, including turbulence and related phenomena. Pene developed the model to test the effects of four different speaker vent designs (Figure 4) on a given speaker and enclosure.
To validate the simulation results, they followed up with experimental testing on a speaker enclosure with removable vents. These tests produced results that matched the simulation’s predictions remarkably well: The predicted acoustic losses deviated less than 1 dB from the actual measured values of live testing. “We were quite happy with the results,” says Pene.
Pene’s successful simulation project has revealed insights that could not be achieved with prototyping alone. Introducing simulation into the L-Acoustics R&D workflow also promises further benefits going forward. As explained in the team’s research report, the simulation provided detailed velocity and vorticity mapping throughout the modeled enclosure and vent designs. This provided data on exactly how each part of the modeled surfaces can create turbulence and affect overall sound quality. This granular perspective revealed sources of distortion that the team had not previously considered.
For example, fluid motion mapping showed that a vent’s location in the enclosure had an unexpectedly large impact on overall flow. This suggested that the team should give more attention to a vent’s placement as well as its shape. Simulation helped the team uncover this new direction for further research and refinement.
Before simulation, L-Acoustics engineers would typically wait weeks before seeing each physical model’s test results. This meant that a significant amount of time could be spent modeling designs that, ultimately, would not be used. Now, as Horyn explains, “The simulation is used by our development team to test its ideas day to day, so we can predict the efficiency of a new design before building any prototypes.” Pene adds: “Now we can build prototypes and be confident they will work properly on the first try.”
Empowering More Engineers with Simulation at L-Acoustics
Yves Pene’s use of simulation for bass reflex design brings teamwide benefits beyond this one project. Part of the Acoustics Research job is to make sure that the tools they develop can then be efficiently used by the development team. The Application Builder in COMSOL Multiphysics® enables the team to build specialized user interfaces from their models, which can then be distributed widely throughout the company. “We are using the Application Builder more and more,” says Horyn. “At the end of this project, the Acoustical Engineering team built a simple application based on Yves’s multiphysics model. Users must define only the specific parameters they need for their project, as other necessary values are already in place.”
The application is distributed to other team members through the COMSOL Server™ deployment product, which enables users to access and run simulations on their own. According to Horyn, “The app-building capability is very convenient, as it enables us to have more people using simulation at a reasonable cost.”
Practice, Practice, Practice: The Never-Ending Pursuit of Performance
The best professional sound system is heard, not seen. However, the sensation of being enveloped by live music, as if the listeners and the performers are one, can be achieved through the efforts of many unseen people and their specialized tools — from microphones and amps to signal processors and loudspeakers that envelop the audience with sound. The acoustical engineers of L-Acoustics, like musicians, know that a great performance is built on perpetual practice; there is always more that can be done. Yoachim Horyn and Yves Pene, having implemented simulation for the analysis of their designs, are now using them to explore further refinements: “Great things are coming.”
- Pene, Y. Horyn, and C. Combet, “Non-linear acoustic losses prediction in vented loudspeaker using computational fluid dynamic simulation”, Audio Engineering Society, Paper 10359, 2020. https://www.aes.org/e-lib/browse.cfm?elib=20776