Author Heißmann-Acoustics

In this article, the working method, the wide-ranging possibilities and ultimately the reliability of software-supported crossover development are to be considered. It will exclusively the use of the computer as virtual crossover considered. No housing simulation, no frequency deviation simulation according to data sheets, TSP or any other parameters that are not based on real measurements on the object ⇒ All relevant data is obtained from real measurements!

The concrete approach in software-based crossover development is usually the following

• Concept creation (driver / housing / baffle geometry) taking into account all possible factors, which will not be discussed here
• Structure of the speaker: Case construction, absorption / insulation, cabling, bass tuning (BR / CB etc.) ⇒ Without crossover
• Measurement of the individual, unfiltered drivers: Amplitude frequency response including relative acoustic phase under all desired angles and impedance response
• Import of measurement data into the software, in this case Xover 2.04A

At this point a note: The measurement of a 2-way speaker under eg. 22 angles (+ -90 ° horizontal in 15 ° steps and + -20 ° vertical in 5 ° steps) including impedance measurements takes ~ 15 minutes

Let's imagine the following measurement setup

• A cascade of "n" identical measurement microphones in a horizontal and vertical plane around the loudspeaker, which do not influence each other
• A measuring device for detecting the impedance paths of the individual drivers and the whole loudspeaker simultaneouslywithout affecting the results
• Hardware that can process and output all measurement results in real time
• Software that can process and output all measurement results in real time
• All required components without deviating from the target value ... or Components that can be set to any value using a controller
• A transparent translator that changes / adjusts parts on instructions 😉

Everything is not really feasible, but should illustrate what is possible if we collect all the data one after the other and then let the software work with it.

Here is an example of the course of a crossover development with Xover 2.04a

The program is on the pages of the interest group DIY Hifi (IGDH eV) as Download is available for storage, management and analysis.

Top left: 0-90 ° Horizontal, sum / branches and phase
Top right: 0 ° and + -15 ° vertical, sum / branches and phase
Middle bottom: Impedazgänge branches and total
Bottom right: Voltage frequency response

Quiz question: What's going on at 1kHz? Use the comment function :)

We have a virtual crossover board on the screen, and see the effects of every component change on the amplitude frequency response (horizontal and vertical), phase, impedance response & voltage frequency response in real time. We can simultaneously Things that the on-foot developer thinks if any, only one after the other.

"On foot" is usually there. a Microphone, Impedance measuring stripe and a Component that is changed. Then you can an Zweig or the sums, or look at the impedance frequency response. Everything under one Corner. The effects of a component change can basically only one aspect to be viewed as. The effects on other aspects must be "guessed" ...

If I change the slope of the tweeter in favor of a better phase angle, how does this affect the radiation behavior horizontally and - or vertically?

Of course, even very good speakers can be created. Direct-HA is the last loudspeaker that I developed ... The results are very good, but it took hours to do things that now take minutes or seconds. In the meantime I consider it very questionable whether the last bit of detail can really be achieved in a walking development.

The question of accuracy still arises ...

In order not to anticipate it quite unprovocatively: The virtual one - is more precise than the real development switch 😉 ...

We have real measured values ​​of the drivers (amplitude frequency response including rel. Phase and impedance response) and put them in a (virtual) AC circuit with frequency-dependent (L / C) and frequency-independent (R) impedances. The mode of action of capacitors, coils and resistors can be detected by formula. There are no relevant unknowns.

In contrast to the “real” development switch, there are practically no sources of error. The components do not have any deviations, there are no increased contact resistances, no wiring errors, no component mix-ups, no cold soldering points.

So that Reality In the end, there are really only a few possibilities:

• Measurement conditions have changed (measurement setup, temperature, ...)
• Components deviate from the standard value
• Errors in the construction (crossover, cabling, faulty components ...)

If we trust our measurements, we can look at differences between reality and simulation, the error on the real object. Often the simulation can help here as well. We carry out targeted changes as long as the results correspond to the error. So I have often found out which component differs from the norm, where a contact resistance is too large, or where two components were confused. It will not be published until the simulation and final measurements of the speaker are practically the same. The virtual crossover as a reliable fault diagnosis tool!

Here's an example of the match between simulation and measurement on the finished speaker:

In Jumping simulation and final measurements of a speaker at horizontal angles of 0-90 ° in 15 ° steps

It can be practically talked of congruence at all angles. The minimal deviations are due to slight deviations in the final measurement setup and possibly tolerances in the components.

The advantages of a software-supported crossover development summarized once again

• Time
• Material: No cabinet with components needed.
• More efficient crossovers: Both “on foot” and in the simulation, a “filter finding” takes place as follows, for example: You start with a, say, 2nd order high-pass filter and a voltage divider. Then there is an increase in the air, which is then ironed out with a blocking or suction circuit. As soon as everything looks good you are finished "on foot". In the simulation you can now try to find a filter function that delivers an identical result with fewer and / or cheaper components ... often works!
• The simulation as an error diagnosis tool. If the simulation and the final measurements agree, it can be assumed with a probability bordering on certainty that the structure is error-free. Very good in terms of "replica security"
• Steep learning curve when it comes to filter design, even with experienced developers ...
• Variability: Variations of a loudspeaker can also be created in retrospect because a complete and verified simulation model exists.
• "Trial & error switch" until it fits => pre-defined acoustic filter edges
• The cascade of "n" identical measurement microphones ⇒ an extremely practical way to "controlled directivity" loudspeakers

In conclusion:

The art of developing a good loudspeaker should not primarily lie in the crossover design, but rather in finding an overall concept that is coherent for the intended work area. Find very good drivers and define a meaningful work area. A good housing design (inside and outside) and the baffle geometry that is so often ignored. How the drivers ultimately want to be filtered is, in my opinion, largely predetermined by the overall concept. Theoretically, we could leave the development of the crossover completely to the computer ... A good crossover optimizer, fed with 360 ° measurements horizontally, vertically and diagonally, to which we make specifications regarding the energy behavior of the loudspeaker and the area of ​​application of the drivers, will find better / more effective crossovers than we can . An exception is of course "the first litter of the experienced developer" 😉

It is to be examined how the design of the baffle and the arrangement of the drivers, affect the reproduction quality of loudspeakers. In particular, the attention is paid to the directional behavior, of the tweeter. In addition to the frequency response on axis (0 °), a balanced and uniform directional behavior has a considerable, and in the author's perception sometimes (too) neglected, influence on the playback quality, also and especially in terms of spatial representation of the sound, of speakers.

Imbalances between on-axis, and off-axis frequency responses very often result from an inappropriate baffle design including missplacement of the drivers. The tweeter "sees" the baffle edges in the intended frequency range, where secondary sound sources are created.

Those who value a loudspeaker not only by its tone, but also by a good stage presentation, should take a trained look at the baffle of the speaker in addition to reading impressive sound descriptions. To train this view is the concern of this article ...

In the following, the directional behavior of 25mm tweeters in different installation situations is examined. The influence of midrange speakers is usually less problematic and should not be considered here.

The first four examples are based on (reliable) simulations, assuming a tweeter linearized on the axis and separated at 2kHz. The simulations and measurements show the angles 0,30 & 60 ° horizontally.

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First, two installation situations without or with only minimal influence on the housing:

The infinite baffle

Uniform, flawless directional behavior, starting from the frequency given by the tweeter surface of the tweeter. No edge diffraction and / or secondary sound sources.

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Tweeter with very small front panel, without baffle

The "housing edges", in this case the edges of the tweeter itself, are so close that the associated frequency is in a range in which the tweeter no longer works as a half-space radiator. He practically does not "see" them. The system can be viewed as a system without a baffle.

Uniform, impeccable directional behavior without problems due to edge diffraction.

The non-existent baffle also stabilizes the directional behavior towards the woofer, even below the frequency specified by the membrane surface of the tweeter. Ideal conditions for a seamless connection to the directional behavior of the woofer.

This concept is used in the Kit Center-HQ.

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"Worst case" scenario

... in the author's point of view

Baffle rectangular, narrow, tweeter in the middle, no waveguide, no chamfers

The tweeter "sees" the baffle edges in the intended working area. => Edge diffraction. The result is an on-axis dip with a peak at angles. In the simulation, the tweeter is linearized, => the dip on the axis has been compensated, which makes the peaks at angles more revealing accordingly. Regardless of how the driver filtered to the axis, the discrepancy between on-axis and off-axis frequency responses remains.

Follow us:

Tonality: Imbalance under different listening positions.

space: The space suffers from the imbalance of axis and angular frequency responses

Usually such concepts are coordinated with a sink on the axis, which is then "filled up" again at angles. In terms of tonality, that can sound very good. In an area that is sensitive to the ear there is a depression (I like to call this "ear flatterer"), but nothing is missing energetically, as it is compensated at angles.

A second approach would be to separate very high (in this case> 3,5kHz). However, this causes other problems. The mid-bass driver already directs clearly in this area and is then "replaced" by the tweeter, which still works as a half-space radiator => The energy frequency response is unbalanced. In the area of ​​the crossover frequency, too little and too much energy is released into the room. Furthermore, it becomes more and more unbalanced in the vertical direction as the crossover frequency increases. The distance between the drivers must always be seen in relation to the crossover frequency.

This concept finds application in very many, vA speakers developed under time, and or selling pressure 😉

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... with 15mm bevels

Baffle rectangular, narrow, tweeter centered, 15mm bevels on both sides

The chamfers weaken the interference, but the fundamental problem persists. The above written remains valid.

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How to deal with the problem?

In addition to the infinite baffle and the "practically non-existent" one, there are other, promising approaches to avoid or significantly reduce edge diffraction.

The following examples are all based on real measurements from speakers I have published. There is no claim to completeness.

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Wide baffle with generous rounding on the sides

In this case there is no 60 ° measurement, so 0,30 & 45 ° are shown.

No edge diffraction in the intended work area. The (rounded) edges are far enough away that they cannot be "seen". Extremely even straightening behavior.

This concept was used in the construction proposal Wave Wall 182, Another very appealing speaker that follows a similar approach is eg. the Grimm LS1.

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Tweeter positioned off-center with 12mm chamfer sideways and up

Low influence by edge diffraction at 2,2kHz, which can be easily compensated for by a suitable filter from the midrange driver. In addition, uniform and trouble-free omnidirectional behavior.

This concept was used in the construction proposal Samuel HQ

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Trapezoidal chamfers with a steepness> = 20 °, brought as close as possible to the tweeter

Slight expansion around 5kHz, otherwise no problems due to edge diffraction. The bundling behavior is, characterized in that the tweeter sees only very little baffle, to the directional behavior of the mid-bass, which allows a seamless connection to this only.

The upper chamfer is, depending on the concept, not necessarily needed. Under certain circumstances, the result could even be (even) better without this.

This concept is used, for example, in the building proposals Samira & Samira-HWG.

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Tweeter with sound guide / waveguide and maximum narrow baffle

No edge diffraction at all. Extremely even directional behavior in the entire transmission range. In contrast to normal dome tweeters, even in the super high range (> 10kHz) wide and even radiation Concerning. horizontal bundling an almost perfect solution.

However, the distance to the mid-bass driver is greater than without sound conduction, which adversely affects the vertical radiation behavior. This problem is avoided by a correspondingly deeper separation, which, thanks to the sound pressure increasing and thus driver-relieving effect of the waveguide usually. is also feasible.

This concept is used in the building proposals Cinetor & Cinetor-HWG

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Sound guide (DXT) & trapezoidal chamfers

The same applies as in the previous example, but without the problem of the large driver gap. The tweeter used here is the Seas-DXT. The size of a "normal" tweeter (D104mm), with the effect of a real waveguide. Fascinating!

The trapezoidal phases do the rest, especially in terms of stabilization of the bundling behavior towards the mid-bass driver. In wider baffles there would be a widening in the transmission range, usually. to 3kHz which would be compensated only by a correspondingly high crossover frequency.

The horizontal straightening comes very close to the theoretical ideal perceived by the author!

This concept was used in the construction proposal DXT-Mon

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And finally ...

There are very meaningful and simple approaches to tackle the topic of edge diffraction in tweeters:

• Give it a lot of baffle: The case edges are far enough away so that the tweeter will not see you in the intended work area
• Unsymmetry: If possible, no identical distances between the edges and the tweeter. This can be achieved by the position of the driver (off-center) and / or the design of the baffle. As an example, the Rocket of Hifi DIY it was already mentioned.
• ... accompanied by as little baffle as possible => the directional behavior is steadied towards the bass-midrange driver, which can make the seamless connection to the bass-midrange driver considerably easier (=> e.g. trapezoidal chamfers).
• Use of sound guides (waveguides, horns) which have a clear directivity in the, for edge diffraction relevant frequency range. Usually. Waveguide solutions should aim for a maximally narrow baffle in order to stabilize its emission towards the mid-bass driver.

... there is actually only one problematic case

• A rectangular baffle, approx. The width of the bass-midrange speaker, with a central tweeter and without sound guide ...

In my experience, uniform and largely unbroken bundling behavior is one of the most important, if not the least The most important quality feature of a good speaker.

With this article, the dear reader has (hopefully) learned to be able to judge this, at least partially, by looking at a loudspeaker. Nevertheless, being able to read a little measurements is definitely helpful ...

Finally:

It is not sensible to provide existing loudspeakers with chamfers, new baffles, sound guides and / or driver arrangements. In the vast majority of cases, this requires a metrological check and a corresponding adaptation of the crossover.

For questions, suggestions & criticism: feel invited to use the comment function!