Blog Post

Expanding point-source, the Story of S-series

  • by Mika Isotalo
  • •
  • 28 Sept, 2024
  • •

Sum of all parts and virtual enclosure width

It was a rainy night in Glasgow, early 2018. Two years prior we had applied for a patent for Passive End Fire Technology. First applied to subwoofers by this time it was apparent that the passive cardioid solution we had invented was even better suited to upper bass or low mids depending on how you look at it. Going from 5m average wavelengths to 2m wavelengths, approx 160Hz band-pass complies well with 50cm cube enclosures which again are the most common compact touring "top" speaker cabinet size. This all made sense.

When combining the Passive End Fire philosophy to a coaxial multiway loudspeaker the virtual size of the enclosure can be expanded almost three times the actual size, in other words the real size reduced to a third. Reason being the radiating surface consisting not just the front baffle but sides and in some cases, like with S1, the rear side too radically expand the wavefront created at the low frequencies and hence creating an uniform radiating surface large enough to control the wave front much larger than the single driver could.
Then adding up M6 mid range combined with the newly developed HF section and ”voila!” we have created a 4-way beast with all four bands and individual drivers having their acoustical centers right at the back of the enclosure, a true point-source!
This is further defined and refined by a passive cross-over with increasing order filters going up from first order for LF to 5th order for HF.

20dB reduction of 3rd harmonic at 500-5kHz ”vocal range” and why it matters?

Let’s go back to tech talk part 1 see where were at ten years later. Weirdly enough pretty much at the same place where we were back then with M6 being just developed. The only major update we did was to create a coaxial HF section and that took a good three years to finalize. Over the last decade there’s been some major developments in terms of refining mid range drivers as well as coaxial annular diaphragm drivers. Although there have been some improvements with frequency response linearity and overal power handling, there has not been major leaps in harmonic distortion reduction, especially in third harmonics. This is mainly due to high compression ratios used to create the higher sensitivity.

A clear advantage of using a mid range driver such as M6 with high crossover point instead lowering the crossover below 3kHz with any type of high frequency driver is the ability to take advantage of the acoustical low-pass behaviour of horn-loading. This is mainly accomplished by the front chamber of the horn and the spring like force created by the air volume in it. Reason why this is so effective is because the horn or waveguide is last in line after all non-linearities caused by or within the driver before the wave hits the air and eventually our ears.
The way we designed M6 was to reach the 4kHz mark that in our philosophy represent the critical point when reproducing vocals, distorted guitars and keyboards. 5kHz and upwards makes less damage in terms of third and upper harmonics as those harmonics are above our hearing and in my opinion in that sense less relevant.

So why do we think this is important? First of all as previously stated our ears are particularly sensitive in the higher mid range and while this is mainly part of human evolution and eventually comes down to survival of the species, the other more relevant factor is clarity, transparency and less fatigue in your ears after a long show. All harmonics while they might sound ”interesting” still ocupy our brains a lot while the brain tries to find the right sounds from the chaos it is receiving. Think about being in a noisy party and trying to follow a conversation for example. Afterwords you feel tired and for a good reasion.

Another way of looking at it is to think about what happens to the harmonic content of let’s say tube amped guitar sound mainly consisting of second harmonic. That second harmonic is multiplied by the second and particularly third harmonic when coming out of the speakers creating 4th and 6th harmonics but also the multiples of the main harmonic.

Come rain or shine, but rain particularly...

Active speakers vs passive multiway speakers

Do we want to get our electrinics wet, while some might argue it’s not a problem, we believe most of us would not prefer that if given a choice. In that case the passive speaker option is really the only way to go. Within that frame work the next challenge we would face is the complicated amplification and heavy cabling to accompany that.
The solution is simply passive crossed over passive end fire multiway loudspeaker. One amplifier channel required and much less wiring!

to be continued...

Technical insight #2

by Mika Isotalo • 25 Mar, 2019

Directional character and noise cancellation is of increasing importance in open-air events and is also known to help produce sound more accurately in reverberant environments such as concert halls and sport arenas. The key feature of Passive End Fire Technology compared to prior ways of obtaining directional low frequency reproduction is considerably higher acoustic output in front of the speaker system with exceptional transient capability. A single enclosure bass loudspeaker systems typically have a very low directivity because of the large wavelength which is to be radiated to produce sound in bass region which equals wavelengths from 10m to 3m. To have a directional character comparable to these wavelengths the enclosure and particularly the radiating surfaces need to be around the same size which would make them difficult to carry or install in most environments such as indoors or limited space outdoor facilities.

There are several approaches already known which can be used to improve the directivity of sound sources. One option is to use open back enclosure or have two separate sound sources in which case one sound source radiates to listening direction and other sound source to the opposite direction. When inverting the polarity of the sound radiating opposite direction with addition of delay one can create directional radiating character. It is also known to use to sound sources (two or three units) in a row and delay the front one(s) to match the propagation time difference due to distance. When used in relation to quarter wavelength compared to sound wave radiated by the speaker system this arrangement is known to cancel the sound wave by half wavelength distance when observing behind the row of speakers, ie. opposite direction of radiation pattern.

The revolutionary Passive End Fire Technology is based on providing a loudspeaker system in which loading in front radiating sound source with internal propagation time difference compensates the time difference of the sound wave radiated from opposite direction radiating sound source arriving to the front. This is achieved with horn-like loading at the front side which has the same phase behavior as the reflex-loaded enclosure facing back. No additional delays or processing is needed so only amplifier channel is enough to drive the whole subwoofer!

Technical insight #1

by Mika Isotalo • 25 Mar, 2019

1#   Maximise efficiency and linearity using acoustic innovations.
Our primary philosophy is to concentrate on the acoustical design first and foremost. The tools available for correcting problems in a speaker design have evolved radically, thanks to the increases in DSP power. But we strongly believe that concentrating on developing the natural qualities of the loudspeaker design and making them as linear and efficient as possible, before applying DSP techniques, allows our products to exceed expectations.


2#   A system approach; speakers + controller amplifiers + accessories.
An optimised systems approach is the foundation of our philosophy. It enables us to offer simple to use, flexible but powerful systems no matter what the size or application. We take care that each loudspeaker we produce is as acoustically accurate as possible. By ensuring every detail of the design is performing optimally there is a solid foundation from which we can process and power the system. The need for complex DSP is therefore decreased, meaning our approach brings many benefits. Some of these include; increased headroom and dynamics, giving more impact to your mix, improved linearity and reduced need for compensation EQ mean improved coverage across the audience space and these are just a few examples. Our controller amplifiers build on our core acoustic innovations to deliver the energy, accuracy and reliability that brings the best out of our designs.


3#   Focus on what happens in the last 10dB; operation at full power and beyond.
This ties the two previous concepts together and refines them to focus on what is happening at the limits of what a system can deliver. When loudspeakers are reproducing linear sound continually throughout their range, without complex limiting and protection being required, the result is a significant increase in dynamic range. The combination of higher band sensitivity, our ultra efficient class-D controller amplifiers and the expanded dynamic range can easily increase system efficiency by more than ten times.

 

Last 10dB available?

Sound system is like a sports team; the weakest link defines the overall performance but the peak performer defines the overall marketing.

As sound engineers we all have encountered various sound systems with colourful spec sheets stating max SPL levels that end of the day turn out to be completely something else when used in real life situations. This white paper explains the acoustical approach to minimize these variations through driver loading techniques developed by Aura Audio.

To do this we divide the audio band in hi's and low's from about 300Hz mainly concentrating on most commonly used acoustical solutions such as large compression drivers in mid range and reflex-tuned subwoofers and comparing them to technologies found in A-series and XD-series.

 

The Hi's

Human hearing is most sensitive in range of 1kHz - 4kHz and although reasons for this is beyond this white paper we can agree that our own vocal range and speech intelligibility plays a big role in human evolution here. This also means that nonlinearities such as harmonic distortion and decay variations are easier to spot by anyone at this frequency range.

It was decided early in A-series designing process that harmonic distortion levels over 3% are not acceptable between 400Hz to 4kHz. This might seem not to be a common problem but apart from a few reviews there are no industry standard for (max)SPL vs. Frequency vs. THD (total harmonic distortion) which could state a linear sound reproduction of speaker system.

Then there's the common argument regarding a musical tone of second harmonic distortion. Although used in music production (tubes started it a century ago) for lots of reasons it's no excuse for poor speaker performance. In fact it's easy to understand that when you have a sound with added second harmonics like an electric guitar for example adding another second harmonic actually turns the original second harmonic into fourth harmonic and so on. This is why we feel that all the harmonics not just third and above should be kept as low as possible and below 3% in any level.

M6 mid range driver used in A-series was developed to answer this issue. Horn-loading was obvious choice as it improves the efficiency and the air lock in horns compression chamber acts as a low pass filter canceling the harmonics. To increase this effect from common 12dB/oct to >18dB/oct an internationally patented multi-path waveguide was developed. This technology also has two additional features: waveform shaping for planar wave front creation and wider bandwidth. Planar wave front contributes to array ability when used in large columns for long through applications.

 

Wider bandwidth is achieved by low compression ratio for lower high pass frequency and small path length deviations in sound channels contribute to higher cut of frequency in upper frequency range. Higher cut off and crossover frequency of 4kHz enables the use of light moving mass small size compression drivers at highest frequencies with less critical THD performance as these harmonics are for most parts out of human hearing range (for example 3rd harmonic for 6kHz is 18kHz).

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