Kantendiffraktion

About baffle design, edge diffraction, secondary sound sources, ... ... and their influence on the sound quality of loudspeakers

KantendiffraktionIt is to be examined how the design of the baffle and the arrangement of the drivers on self, affect the reproduction quality of loudspeakers. In particular, the attention is paid to the directional behavior, that is the radiation at angles, of the tweeter. In addition to the frequency response on the axis (0 °), a balanced and uniform straightening 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 axis, and angular frequency responses very often result from an inappropriate baffle design and installation situation of the driver. The tweeter "sees" the housing edges in the intended working area, where secondary sound sources are created.

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

In the following, the straightening 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 an on-axis linearized and 2kHz-separated tweeter. The simulations and measurements show the angles 0,30 & 60 ° horizontal.


First of all, two installation situations without, or with only minimal housing influence:

The infinite baffle

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


Tweeter with very small front panel, without baffle

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

Uniform, impeccable straightening without problems due to edge diffraction.

The non-existent baffle further stabilizes the directional response to the mid-bass driver, even below the frequency specified by the memranface of the tweeter. Ideal conditions for a seamless connection to the directional behavior of the mid-woofer.

This concept is used in the construction proposal Center-HQ.


"Worst case" scenario

... in the author's point of view

Baffle rectangular, narrow, tweeter in the middle, no sound conduction, no chamfers

The tweeter "sees" the case edges in the intended working area. => Edge diffraction. The result is an on-axis collapse with an elevation at angles. In the simulation, the tweeter is linearized, that is, the break-in on the axis has been compensated, which makes the elevation at angles more revealing accordingly. Regardless of how the driver is connected to the axis, the discrepancy between axis and angular frequency responses remains.

Consequences:

Tonality: Imbalance under different listening positions.

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

Usually. such concepts are tuned with a sink on axis, which is then "refilled" under angles. This can sound very good in terms of tonality. In a sensitive area for the ear there is a sink (I like to call it "ear flatterer"), energetically still nothing is missing, because it is compensated under angles.

A second approach would be to separate very high (in this case> 3,5kHz). However, you are dealing with other problems. The bass-midrange already straightens in this area, and is then "replaced" by the tweeter, which is still working as a half-space heater => The energy frequency response is unbalanced. In the area of ​​the cut-off frequency too little energy is released into the room. Furthermore, it becomes more and more unbalanced in the vertical along with an increase of the cut-off frequency. The distance between the drivers must always be in relation to the crossover frequency.

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


... 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.


How to deal with the problem?

In addition to the infinite baffle, and the "virtually nonexistent", 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.


Wide baffle with generous rounding on the sides

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

No edge diffraction in the intended work area. The (rounded) edges are far enough away that you will not be "seen". Extremely consistent 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.


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


Trapezoidal chamfers with a slope> = 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.


Tweeter with sound guide / waveguide and maximum narrow baffle

No edge diffraction. Extremely uniform straightening behavior in the entire transmission range. In contrast to normal dome tweeters, even in the super high tone (> 10kHz) wide and even radiation. Concerning. horizontal bundling a nearly 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


Sound guide (DXT) & trapezoidal chamfers

The same applies as in the example previously written, but without the problem of the large driver spacing. The tweeter used here is the Seas-DXT. The size of a "normal" tweeter (D104mm), with the effect of a true waveguide. Fascinating, and not fully understood by the author.

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


Conclusion:

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 mentioned.
  • ... along with as little as possible baffle => The straightening behavior is reinforced towards the mid-bass driver, which can greatly facilitate the seamless connection to the midrange woofer (=> eg 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 only one problematic case

  • A rectangular baffle, approximately in the width of the woofer, with 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.

The good reader has (hopefully) learned with this article to be able to judge this by looking at a loudspeaker, at least somewhat. A little reading-reading skills is still very 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 free to use the comment function!

 

 

6 thoughts on "About baffle design, edge diffraction, secondary sound sources, ... ... and their influence on the sound quality of loudspeakers"

  1. Hi Alexander,

    Thanks for this very informative article. He helps me a lot already. But I have one more question 🙂

    I would like to know how the baffles of the Harwood reference, or CelLine, line up here. Especially the baffle of the CelLine is very interesting and I would like to use it for my own speaker.

    Theoretically, this baffle should come closest to the optimum of a narrow baffle and also allow a central positioning of the tweeter.

    I am still too inexperienced to form a meaningful opinion, which is why I hope for your expertise.

    Regards
    Jacek

      1. Hey Alexander, many thanks for your answer. The case is now at the top of my list of favorites 🙂

        I also found the following description in a forum from the developer (Peak LSV) of the case:
        "My approach to this front is that the sound must be diffracted by more than 180 degrees before" seeing "the baffle ... and then it encounters irregular surfaces that scatter the incident sound in many directions. The influence of the baffle through reflections on the direct sound is thus reduced. "

        Would that mean that the sound on the axis very well, but the omnidirectional behavior suffers?

  2. Hi, very insightful work, above all clearly stated, Have already read a lot of LSP literature, but never in this clear, to the point brought language (German ....) Had not thought that it has such a strong impact ... .. a good option would then be a separate HT case on the box set, just in HT size as you see it in some LSP as a ball o. similar? Greetings Norbert

  3. Hi Alexander,

    Thank you for the interesting article. For a guy like me very informative, will certainly be considered in the next case planning.

    Regards,
    Lars

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