Why Synergy horns?

schlager

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In another thread I was asked, if I would provide more details about my speakers, so I thought why not?

I have played on active 4 way horn systems since 2016. First iteration was front loaded bass horn, midbass horn, tractrix midrange horn and tractrix tweeter horn. I worked nicely, with all the attributes associated with well implemented horns. Clarity, dynamics, realistic live sound etc.

However some problems will arise, with such horns. First of all, the center to center distance between the different horns is big, compared to the crossover frequencies. We need to be within 1/4 wave in distance at x-over for a seamless transition. For instance if you x-over from the midrange horn to the tweeter horn at 3 KHz the c-to-c distance would have to be 340/3000/4= 2.83 cm (1.11 inch). This is virtually impossible with "normal" horn configurations. This problem rears its ugly head, at every x-over throughout the audio frequency range. As frequency decreases, the wavelengths gets bigger, but so does the horns in the specific bandpass and then c-t-c also increases. It is a linear problem, that can't be solved with the regular approach, aka stacking horns on top of each other. This creates interference problems and lobing in the vertical response curves, that will color the reflection from floor and ceiling. Secondly a large column of vertically stacked horns, will push the sweet spot (SS) further back, for the horns to be perceived as more coherent and integrated, with one another.

But the biggest problem is that almost all horns beam with increasing frequency, it's their way of nature so to speak. What that means, is that the off-axis FR will not be similar to the on-axis FR. This translate into a poor power response, which is not considered a good thing, in terms of best sound quality.

Luckily we can circumvent all these problems with clever engineering and have our cake and eat it too, so to speak. Enter the Synergy horn.
 

Keith_W

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But the biggest problem is that almost all horns beam with increasing frequency, it's their way of nature so to speak. What that means, is that the off-axis FR will not be similar to the on-axis FR. This translate into a poor power response, which is not considered a good thing, in terms of best sound quality.

Just so I understand you correctly, the power response refers to the total energy output of a loudspeaker as measured in a sphere around the speaker vs. frequency. Horns direct most of the energy forward. If you had said, "narrow listening window" and "poor off-axis spectral response" I would have understood you and agreed, but I am not so sure about the total power response. Can you elaborate please?

Luckily we can circumvent all these problems with clever engineering and have our cake and eat it too, so to speak. Enter the Synergy horn.

It looks as if you cut your post short before it started to get interesting! Please continue.
 
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schlager

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Keith, you basically told it yourself. On-axis and ALL off-axis responses will translate into the total power response, so they are essentially the same thing.
 

schlager

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Technical aspect of Synergy horns.

The Synergy horn are a way to combine the outputs of multiple drivers, over multiple frequency ranges and then “knit” them together seamlessly acoustically, in both amplitude and phase. By properly combine multiple numbers of drivers into a single coherent acoustic source; it acts as if it were one source in time and space.

There is no magic to synergy horns, only clever design and ingenuity, but if there is one rule to rule them all, so to speak, it would be the the rule of 1/4 wavelength.

For instance, if you have a pair of subwoofers and you put them 1/4 wavelength or less apart, they combine into one new source and feel each other’s radiation pressure, which increases their efficiency.

Same thing applies to synergy horns. By placing the different drivers, that handles their own specific band pass, within 1/4 wavelength or less apart, they also combine into one new source.
 
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schlager

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....continued.
So why is the 1/4 wavelength rule important. It is because at that specific distance at that specific frequency, two different drivers with overlapping bandpass, will seamlessly combine into one. The problem is how do we place relatively big drivers, close enough, so they will be with in 1/4 wavelength? In a normal horn setup and a ordinary baffle speaker, we can't.

Let's take at a normal D'Appolito design

Udklip1.JPG

The c-t-c distance between the 2 midranges and the tweeter will be bigger than 1/4 wavelength, resulting in lobes and thus impairing the total power response. But if we bend the baffle around the tweeter, we get something like this.
2 way.JPG
and we can continue this idea with more drivers and go from a large baffle speaker

Udklip.JPG
to a smaller design with the same driver configuration

3 way.JPG
so by placing the different drivers, each playing in their specific bandpass in a pyramid-shaped, multi-band waveguide, we properly combine the amplitude and phase from a multiple number of drivers into a single coherent acoustic output. Really clever, but it gets even better :)
 
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schlager

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No I made my own, following the "recipe". I'll come to that in a future post.
 

schlager

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In a synergy horn we put the tweeter at the horn apex, easy enough, but how do we know where to tap in the midrange and bass drivers. We use the rule of 1/4 wavelength. So if we want to cross over from the midrange to the tweeter at 1200 hz, we would have to tap in at 340/1200/4= 7 cm (2,8"). At the same time the cross sectional area (CSA) at the tap in point with in the horn. should be no bigger than in circumference, than the highest frequency being used in the bandpass. So at 1200 Hz the wavelength is 28,3 cm, so CSA can be no bigger than 28,3 cm, otherwise the hornwalls will no support the frequency. Same thing applies for the bass drivers. If we want to x-over from bass to midrange at 400 hz, then the axial distance from the apex to tap in is 340/400/4= 21,2 cm (8,3"). This 1/4 wave rule makes sure that the drivers bandpass, is cut off and basically acoustically self terminate. What happens is that the frequency at play, for instance at 1200 Hz, travel toward the apex of the horn and then back again, but this time, 180 degrees out of phase, so a cancellation notch occur. In this way we can acoustically short circuit the bandpass, smart. What this cancellation notch will also do, is to acoustically lower the harmonic distortion above the cancellation notch. This effect can NOT be done electrically ONLY acoustically. This cancellation notch can be as big as -30 dB, so harmonic distortion is also lowered -30 dB. The result is a much cleaner sound, compared to other speaker designs.


This is a measurement I did on the midrange, on one of my synergy horn. It is easy to see how the FR is dropping like a rock, at the cancellation notch. Only a high pass filter was on and no low pass.
1679784869448.png
 

schlager

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That measurement is ONLY the midrange driver, with a high pass filter at around 500 hz, just to show how the driver "turns off" at the notch frequency above 1000 hz. So there is no low pass filter on the midrange. The driver in combination with the horn, achieves this. It's all a balancing act of 1/4 wave relations, horn geometry and using the right size of drivers. If the midrange drivers are to big, let's say 8", then you can't physically place it close enough to the apex, for a higher x-over to the tweeter, maybe only up to 700-800 hz. So the tweeter has to reach that low to get a seamless transition. But no many 1" horn drivers are working well down to 800 hz. But you could use 1,4" driver, but then you get more beaming above 10 KHz, so again it's about balancing things.

Basically the midrange functions as a "filler driver" between the bass and the tweeter and will often only cover 1-2 octaves.
 

schlager

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The horn itself is conical, aka straight sided. It can be square like a pyramid or rectangular, depending on the situation and needs. A conical horn has controlled/constant directivity. What that means, is that we can control the sound radiation pattern, specified within the angles of the horn, in both the horizontal and vertical axis. This cannot be done with other horn profiles, such as tractrix, exponentially, spheric, LeCleac'h etc. These horn profiles will beam, when frequency goes up, unable to retain a smooth off-axis frequency response.

A narrow radiation horn would be 50 by 50 degree angle and will give a high part of direct sound, compared to the reflected sound energy. In a typically home situation, that will be translated into a dry sound, with little room ambience, almost like headphones. For a bit more spatial sound, the horizontal horn wall angle should be at least 60 degree and up to 90 degrees.

The straight sided horn walls also enable us to mount the drivers to the horn side, where they tap into the horn. We have established, where to tap into the horn using the 1/4 wavelength rule, but we don't just cut a big hole in the horn, a big as the driver itself. No, what we want to do is hornload the driver through small holes. This push up the efficiency, by up to 10 dB and if we place more drivers, to play in a specific bandpass, that will further raise the efficiency and powerhandling also goes up. Furthermore the horn loading, creates a natural band pass and creates a low pass filter for the driver(s) with rising frequency, that ads to the harmonic distortion filtration, the before mentioned notch filter creates. Again this is a balancing act, where different variables should be in sync, to make the horn/driver combo work correctly.
 

schlager

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Regarding how to tap into the horn, as explained earlier, we want to horn load the driver through portholes, placed on the straight horn sides. The trapped volume under the cone comprise an acoustic low pass filter. The smaller we make the holes, the lower that corner frequency is, the shorter we can make the duct part of that hole, the higher the corner frequency is, this also gives the compression needed to increase efficiency and lower distortion. Like normal horns, this create a high efficient bandpass. since the midrange drivers are a bandpass, we need to have them playing high enough to mate with the compression driver. Moreover, being a bandpass there is a natural roll off on the top end. We want to have that roll off frequency at least as high as the compression driver plays, and ideally have the ports within 1/4 wavelength away from the compression driver cut off. Two things are happening with that, the natural xo in the bandpass and the null notch created by the midrange bouncing back up the horn. Increasing the number of midranges makes the FR flatter and makes the top end roll off steeper. And off course more drivers results in higher efficiency and power handling.

In normal horns, we can expect a 2-3 octaves bandpass span, but because of the reflection notch, that is created with the ¼ wavelength rule, the midrange reflecting back from the apex, we only get 1-2 octaves in a synergy horn.

The portholes should be about 1/10 to 1/20 the area of driver and be placed in the corners of the horn as the pressure is lower in those areas and thus the tweeter compression driver doesn't really "see" the holes. Another thing to take into consideration is that we don’t want port air velocity above 17 meter/sec, because that will create port noise. The port length also play a part in that equation. We can use software like Hornresponse to simulate this.

All these balancing rules, off course also applies for the bass drivers, just at different frequencies and thus have to be placed further out the horn, away from the apex. Portholes also needs to be comparatively larger, to keep air velocity below 17 m/sec.

An example of how it could look like, the tweeter at the apex of the horn, then the midrange holes and further out the horn, the tap holes for the bass drivers.
1679950592544.png
 
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schlager

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Yes, if you do an analog filter, you have your work cut out for you :) With DSP and FIR filter it is very easy, to sew the drivers on the horn, into a seamless integrated unit.
 
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Solypsa

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Can you time align the various drivers with a passive network?
 

schlager

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Can you time align the various drivers with a passive network?
The drivers are in fact closed to time aligned, due to the horn loading coupling. What that does, is to create a group delay, that would be equivalent to physically moving the midrange and bass drivers back, toward the apex of the horn, where the tweeter is situated. Again, by using the physics of law, the synergy horn provides acoustic solutions due design, that more conventional designs can't achieve in the same manner. But for this to work correctly, in the analog domain, one would have to know how to apply filter technics. At the end of the day, this is much easier done in the digital domain.
 
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schlager

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CD horn advantages.

So we have established, that a constant/controlled directivity (CD) horn, has a better power response, compared to more conventional horns, such as tractrix, exponential etc. But CD horns has more advantages over its cousins.

Most horns has problems doing the “disappearing act”, that is often associated with small 2-way speakers.

Big speakers with a big baffle will have more problems due to more diffraction problems and often have bigger drivers, which are spread out on the baffle, creating lobing in the vertical plane. Baffle/cabinet diffraction causes reflections, which arrive later in time vs the direct sound. This create a second sound source, smearing the image and degrading the sound quality in general.

A well-made synergy horn, with a proper termination of the horn at the mouth, will not suffer from these problems, as it is essential baffle-less. Other advantages that the CD horn has over conventional horns, is that they load better in the base, when placed closed to room boundaries. Something called the horn expansion rate, will dictate how the horn couples to the driver and ultimately to the surroundings (room). An exponential horn has constant expansion rate of 1. In a CD horn (straight sided), the expansion rate is not constant, thus it changes out through the horn.

At the apex, expansion rate is very rapid, as in higher than 1. That is good for loading high frequencies. Further, down the horn, the expansion gets slower (less than 1) providing better loading for bass frequencies. What that gives us is a horn profile that provide us with a useful expansion rate, for the specific bandpass. So by placing the tweeter at the apex, the midrange a little bit further out the horn and the bass drivers out toward the mouth, we provide the best loading for that particular bandpass, that is indeed very smart audio engineering, using the laws of physics.

Fig. 5.9 shows the impedance loading of an exponential horn measuring 25 cm in length and with a mouth of 1210 cm2. We see that the loading drops off rapidly below 700 Hz and the driver starts to behave like a direct radiating driver.

1680033920978.png

A conical horn with the same dimensions behaves differently regarding its loading. Its acoustic impedance back to the driver doesn't go to zero below the nominal cutoff frequency. Instead, the horn continues to provide real useful impedance back to the driver. That happens because the mouth dimensions of the horn contains the exiting wavelengths directivity. This can been seen in the impedance plot below for a straight-sided horn; fig. 5.8
1680033999314.png

What this means is that the horn loading falls off, at a slower rate and we still have useful loading at 300 Hz. We do not have more loading in total, per se, over the exponential horn, but because the real part of the acoustic impedance at the horn's throat does not go to zero below the same-size exponential horn's cutoff frequency, the mouth located bass driver still feels the acoustic loading of the horn. This also lines up perfectly with the slower expansion rate at the horn mouth, which is well suited for loading bass frequencies.

This difference in loading means that the impedance match is more evenly distributed. We can utilize that by placing the conical horn within ¼ wavelength to near bound room surfaces, or better, in the corner and retain more loading in the base. The surface boundaries will keep the impedance loading outside the horn mouth and essentially become a part of the horn as an extension of the horn.

Because the real part of the acoustic impedance at the horn's throat doesn't go to zero below the same-size exponential horn's cutoff frequency, the mouth located bass driver still feels the acoustic loading of the horn

Conventional horns typically provide 15-20 dB of SPL gain, over using the same drivers in direct radiating mode, Then we have the -6 dB loss of using a straight-sided conical horn instead of exponential and using boundary gain below 1/4 wavelength axially, we still have 9-14 dB of horn loaded gain using a straight sided horn in boundary gain, than using the same woofers in direct radiating mode. So by placing synergy horns in a corner configuration, they behave just like corner horns, like we known them from the old classic Klipsch speakers.
 

schlager

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How to EQ constant/controlled directivity horn

The constant/controlled directivity allows the tweeter to obtain uniform high frequency response with the angle of coverage. In others word, the CD horns are able to direct the high frequencies off axis. This means that the amount of high frequency energy formally available directly on axis is less, so the high frequencies will drop off at -6 dB per octave, starting at about 2.5kHz to 3.5kHz, where the mass roll of the driver itself begins. Therefore, the CD horn no longer measures flat directly on axis without EQ.

On conventional horns, this does not happen, because the horn will beam with rising frequency, so the on-axis response will be flat, at the expense of a dropping high frequency response off-axis thus its power response, which translate into the speakers total sum of the radiated acoustic output, as measured in a sphere around the speaker, will be impaired.

For the CD horn to work correctly, we need to EQ it to counteract the -6 dB drop off in the high frequencies. A normal horn loaded compression tweeter driver has a very high efficiency. 110 dB/1w at 1 meter is not unusual. That means there is usually power to spare, allowing the frequency compensation to be added. There are 2 ways of doing that. In the analog domain using passive circuits to create a low cut, a capacitor in parallel with a resistor, in the combination series with the driver. But in this modern day and age we don’t use passive components for signal shaping, we use digital EQ. The power requirements above ~2 KHz fall at around 6dB/ octave (20 dB/decade). Therefore, we can easily boost the top end above 3 KHz, without running into trouble, power wise. As long as the DSP do not distort the signal, which means that the DSP output reader does not go above 0 dB, all is fine.
 

schlager

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What drivers to use.

We want a tweeter compression driver, which can reach low enough to meet the midrange drivers. The bigger the midrange drivers are, the lower their top end FR will be, according to the ¼ wavelength rule, because driver size determine how close to the apex you can put them. For home use, where high SPL above 120 dB usually are not needed, we can get away with a small 1” CD tweeter. What type of CD tweeter will be up to the user. Your mileage may vary.

For midrange, 5” drivers or less are typically used, as they can be placed close enough to the apex of the horn, to reach high enough in FR, to mate with the tweeter. The midrange drives requires a closed back, to work correctly. You can get pre made closed back drivers or you would have to close the driver yourself. That could be done with a tube that fits the driver.

For choosing a midrange driver, you would have to simulate it in Hornresponse or similar program. Alternatively, you could just follow “the recipe” of one of the examples of synergy horns that are published online.

As for the quality of the midrange driver(s), one would think that if you used an expensive and well-regarded driver, the sound quality would be better. However, this is not necessary the case. Expensive drivers are often linear over several octaves and made to have a flat frequency response, if placed on a baffle. In a synergy horn, we do not need this wide band FR as it will be low passed by the horn design and the horn loading will change the native FR. The best driver for a synergy horn is the driver that allows the horn/driver combo to work as intended.

For bass drivers we can use different sized drivers, as there is enough room for placing the driver(s) further out the horn. Anything from 5” to 21” can work, as long as it can cover the intended bandpass and reach requirements for SPL. The bass driver should not be a closed back type, but would need an enclosed box to play in the lower bandpass. Some synergy horns also incorporate reflex ported enclosure, to reach further down into the base. Again, simulation is your friend.
 
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