Designing a loudspeaker: The SBS.1 story
This is the story behind the Sigberg Audio SBS.1 active speakers, the design choices made and the reasoning behind them. Hopefully it is a good read to those interested in loudspeaker design in general and specifically the design choices made to come up with this somewhat unconventional design. There are three things that sets them apart from most traditional hifi speakers. The first is that they are active. There are three built-in mono amplifiers in each speaker, one for each driver. Second, we employ a coax driver. The third and perhaps most unusual, is the fact that they were designed to be accompanied by at least one subwoofer. They have a -3dB point at 90hz. In plain English, that means that they won't play deep bass.
So..Speakers that require a subwoofer? That doesn't make any sense, does it? Actually it does. Let's look into why.
What the movie nerds got right
The surround sound crowd has been using subwoofers for ages. You'd be hard pressed to find a home cinema without it. And most of them run active high pass on their main speakers, typically crossing them over at around 80hz. So they have speakers that were designed to play deep bass, but in practice they don't. The're connected to an AVR that actively routes the bass to the subwoofer. And it works. The result is increased capacity for all speakers, less load on the amplifier, and better bass. So why not take it one step further? Why not design the speaker for subwofers to begin with?
But what about the hifi enthusiasts?
However, the main target audience for the SBS.1 speakers aren't surround and movie nerds. The main target audience for both our subwoofers and subsequently the SBS.1 speakers are hifi enthusiasts and music lovers. People who use their system for music first and movies second, if at all. And hifi enthusiasts are a somewhat conservative crowd. To the point that things like subwoofers and advanced DSP (both of which have been natural parts of home cinema setups for a couple of decades) are frowned upon.
Selling the idea of a DSP enabled speaker which required a subwoofer to hifi enthusiasts might be difficult. At the same time, we knew in our hearts that it was a fundamendally good concept. This concept had the potential to provide better bass and better room integration than almost any pure stereo system out there. And we already sold subwoofers, so it was a natural next step from a marketing perspective as well.
The development process and early design choices
Since our initial subwoofer offering, the Inkognito subwoofer, had an unconventional design, we decided to develop a more traditional subwoofer in parallell with the development of the SBS.1 speakers. That became our 10D dual 10" subwoofer.
We also decided early on to be transparent and share with the community throughout the development process. We had a couple of reasons for doing this. First and foremost, sharing is both fun and valuable in terms of being able to get early feedback on design choices. Second, it would provide essentially free marketing of our upcoming products.
The development threads are still available:
SBS.1 development thread on audiosciencereview.com
10D development thread on audiosciencereview.com
SBS.1 development thread on hifisentralen.no (Norwegian)
10D development thread on hifisentralen.no (Norwegian)
Early design choices: Coax driver
To be able to provide a compact speaker with a lot of capacity, a coax driver was a natural choice. This enabled us to employ dual midbass drivers on the same baffle size as a small 2-way speaker. A 2.5-way design meant the coax would play both midrange and midbass, while the second driver was limited to midbass duty (rolling off at 400hz). Some worry about the doppler effect (frequency distortion caused by the driver moving back and forth) when employing coax drivers. In theory this would be even worse with a 2.5-way design, where the coax played all the way down to 90hz. However, this was mitigated with the dual midbass driver design, cutting the required excursion (how far the driver had to move) in half. The fact that deep bass was handled by the subwoofer, also meant excursion would be moderate even when playing very loud.
Another important aspect of a competent coax driver is accurate, pinpoint imaging and a wide stereo perspective. It also has even dispersion characteristics off-axis, meaning it sounds the same (and good!) across a wide listening area. This choice had a significant impact towards our goal of creating a small speaker with big sound. Listeners auditioning the setup early on frequently compared the sound to large, expensive floorstanders.
The speakers was also designed to play with zero toe-in, meaning the speakers could be placed parallell with the wall, as opposed to angeled towards the listener. The wide, even dispersion combined with the accurate imaging of the coax meant you get the best of both worlds if you set up the speakers with little to no toe-in. A very wide soundstage but still accurate imaging.
Early design choices: Active speakers
We had already partnered up with Hypex to power our subwoofers. Hypex produces some of the best amplifier modules on the planet, so we knew they would work very well in a speaker application as well. Active, DSP enabled speakers also meant we would be able to prototype and experiment with crossovers way faster than with a passive setup. It also meant we would have extreme control over frequency response. Finally, powerful and dedicated amplifier modules to each driver ensured we could utilize the full potential of the very capable drivers. In fact each SBS.1 speaker has a total of 350W of continuous power available. This translates to loud, uncompressed playback in any normal sized listening space. Again, listeners auditioning the speakers during the development process was a good indicator that we had something special on our hands. They were literally blown away by the wall of crystal clear sound provided by these small speakers.
Early design choices: Building speakers that required a subwoofer
This is the major choice that enabled high capacity in a small package, but at the same time meant the customer simply HAD to have at least one subwoofer. This meant the product would not be for everyone. At the same time it is a major advantage. The speakers themselves take less visual space, and using a high quality subwoofer means you have access to bass that surpasses most floorstanders.
This also meant we could choose drivers for the speakers that were optimized for midbass duty. The built-in DSP in each speaker are actively limiting them from playing deep bass. No matter how loud you play, the drivers will have limited excursion and significantly lower distortion than a traditional 2-way speaker that are struggling to produce deep bass.
As a customer this means you can take a modular approach to how you build your system. Start with the speakers, and then add one or multiple subwoofers that have the capacity suitable for your needs and/or wallet. The system can also be upgraded later by adding another subwoofer or replacing your current subwoofer with a more powerful one. Even combining the SBS.1 speakers with a single Inkognito 10 subwoofer gives you deep, impactful bass that will be plenty for most situations. If you just can't get enough, two 10D subwoofers will give you a system that has four 10" drivers in the bass, and a total of 8 individual amplifiers with the combined power of 1700 watts..
With the built-in DSP to our disposal, we were able to both experiment quickly and make design choices that would be difficult to implement in a traditional, passive crossover. The crossovers in the SBS.1 speakers are assymetrical 1st order crossovers. This means they are primarily 1st order, ensuring crossovers with perfect frequency response and no phase shift. Further down (or up) in the frequency range, a second, 2nd order crossover is activated to ensure the roll-off is steep enough.
In addition to the natural roll-off provided by the small cabinet, there is also an active high pass filter at the low-end to reduce bass output. This limits driver excursion and distortion even when playing extremely loud.
The loudspeaker cabinet
The SBS.1 cabinet gradually evolved throughout the development process and several prototypes were built. The cabinet consists of three separate chambers: One for each driver, and one from the amplifier mounted in the back. The cabinet also has a 4 degree tilt. This allows us to tune the speaker to provide slightly more high frequency energy in the room, since the on-axis is tilted every so slightly away from the listener in the vertical plane, and also in the horizontal plane since they're designed to play without toe-in. The drivers are flush mounted, and the chambers are generously padded with felt on all surfaces, with an extra thick layer on the back wall of each chamber.
From a technical perspective the cabinet didn't need to be as deep as it is. The depth was chosen partly because it looks better, and partly because it was hard to find suitable loudspeaker stands for the initial, more shallow design. We are also all used to seeing quite deep loudspeakers, since the internal volume is needed to produce deep bass. But as we know, the SBS.1 doesn't have that problem.
The small footprint means the cabinet is inherently rigid, and the internal walls provide additional bracing. The fact that bass is reproduced by separate subwoofers means the speakers are completely unaffected by the vibrations generated by the deepest bass notes.
Many have tried and failed to properly integrate a subwoofer with their stereo setup. We wanted to reduce this problem as much as possible. Since the listening room has a major impact on how bass is reproduced, it's almost impossible to guarantee a perfect result. But we could at least do our best to minimize potential problems.
Both the speakers and all our subwoofers are sealed. This eliminates the phase shift inherently present in loudspeakers and/or subwoofers that have bass reflex ports. Second, the crossover frequency between the SBS.1 speakers and our subwoofers are perfectly matched out of the box. This essentially means all that is left is to volume match the subwoofers to the speakers, compensating for room gain and/or personal taste.
For the advanced user it's also possible to use the onboard DSP to calibrate the speakers and/or subwoofers frequency response. This makes it possible to fix problems introduced by the room. We also decided to partner up with DSPeaker and provide their Antimode products for sale to our customers. The Antimode range of products provide automatic room calibration with no prior knowledge necessary, as well as powerful manual options for those who'd like to tweak further. The sum of all this means anyone could get near perfect bass with a Sigberg Audio setup, and with far less effort.
The most important part: Tuning the sound
What did we want these speakers to sound like? The goal was that the speakers should give the experience of a neutral, balanced sound. But we knew that to many hifi enthusiasts, "neutral" translated to boring. And we did NOT want to design boring speakers. We also did not want the speakers to be "revealing" in the sense that bad recordings sounded really bad, or even worse, unlistenable. Some high end speakers sound great on really good recordings, but piercing and unforgiving on everything else. Not the SBS.1. Finally, we also wanted them to have a full and punchy sound, with an element of sounding "larger than life".
So to sum up:
- Neutral and even frequency response
- Open, clear midrange and top end
- Must sound good on a wide range of music genres and recording styles
- Should NOT sound piercing or give listeners fatigue
- Dynamic and punchy midbass, complementing our subwoofers
We spent countless hours over the course of almost 8 months before finalizing the configuration for the production speakers. Technical measurements done both indoors and outdoors and in a number of different rooms large and small. Tuning and tweaking the frequency response and crossovers. Endless listening sessions, listening to hundreds of different tracks across all genres of music in an attempt to provide a balanced sound that worked across different rooms and taste in music.
We are extremely happy to report that all the hard work has paid off. The sound of the final product (available for purchase at the Sigberg Audio SBS.1 product page) can be summed up in the bullet points above.
Finally, the built-in DSP in all our products provides the customer with a manual 9-band EQ. This means you may further tailor the sound to your taste, and/or correct for the impact of the listening room.
The end result is a breathtaking listening experience, both when it comes to sound quality and capacity. Mission acomplished.
We hope this glimpse into how we have worked to design and build the SBS.1 loudspeakers was an interesting journey. Thank you for reading!