Diagonal chamfers ... effect, steepness and positioning

The topic of baffle geometry, and its influence on the playback quality of loudspeakers, has already been described in the article "About baffle design, edge diffraction, secondary sound sources, ..."Detailed. If you are still in the belief that designing a baffle and the arrangement of the drivers on Selbiger is primarily a question of optics, may the article like (again) read.

In the process of finding a suitable baffle, I often ended up with the variant "slanted chamfering". Again and again, the question arose how steep these really have to be to be effective. So far I have relied on secondary knowledge in this regard ... Read in books, forums, magazines. When the question arose recently, I decided to investigate the matter metrologically.

To accomplish this, a baffle with adjustable, beveled chamfers had to be made. After a lot of headache, how to build this, I finally came to the following conclusion:

schraege_fase_variabelIt was milled from behind a V-groove, just less deep than plate thickness, the whole thing with tape from the front "secured" and a 2mm strong copper wire festgetackert from above, which serves as an adjustment mechanism. The "wings" can be set as desired by 0 ° -55 °.

baffle_xt25_back

As a tweeter, I chose one that by itself shows no disturbances in the straightening behavior, in the area relevant for edge diffraction, which could falsify the results. The choice a lot on the very much appreciated by me XT25TG-30 / 04.

In 55 ° steps for chamfer angles of o-5 °, the respective radiation behavior of 0-90 ° in 10 ° steps was measured. Altogether 120 measurements.

The initial situation: Bevel angle 0 °

See also "Worst case" scenario

The left picture shows the radiation behavior in a sonogram normalized to 0 °. The right-hand example shows the angles 0, 30 and 60 °.

In the sonogram it can be seen that the loudspeaker at 3kHz is far too broad, and too narrow at it. Axis, and angular frequency responses are unbalanced to each other. Where there is a sink on the axis, there is an elevation under angles. The speaker changes its character with the listening position. Desirable would be a steady and evenly narrowing course.

Measurement results

Sonogram for chamfers with a slope of 0-55 ° in 5 ° steps

fasen_deg_var

0,30 and 60 ° for bevel angles of 0-50 ° in 10 ° steps

winkel_fasen_deg_var

The main disturbance (widening by 3kHz along with the constriction below) decreases with larger chamfer angle. Chamfers with a steepness <20 ° do not bring satisfactory results yet.

The chamfers themselves form a new edge, which comes to bear with increasing steepness. Recognizable by the arising expansion around 5Khz. This is less pronounced because the tweeter in this area is no longer working as a half-space radiator.

This is also one of the main tasks of the oblique chamfer, in addition to the resolution of the "rectangular geometry": to bring the case edges so close to the tweeter that it no longer "sees" them or only weakens them. Less distance ⇒ Smaller wavelength higher frequency ⇒ more directivity ⇒ less influence of the edge

The concept "oblique chamfer" works particularly well when the tweeter in the corresponding frequency band already bundles clearly and thus this is practically no longer "seen". Of the Seas DXT 27TBCD / GB be mentioned here by way of example. How well this driver works with oblique chamfers is an example DXT-Mon to see.

In summary:

  • Bevel <20 ° are not sufficiently effective
  • The distance of the chamfers to the place of sound creation must be sufficiently low, otherwise one reaches only one, at least partial, displacement of the problem
  • Depending on the situation, the chamfers can also zu steep be!

The concept of slanted chamfers can not be very effective to solve the problem of secondary sound sources by edge diffraction. But not unconditionally. Slope and position must be chosen carefully and according to the concept. If you asked me for a universal recommendation I would say:

"Make the chamfer 25-35 ° steep and position it as close to the tweeter as possible"


Finally, the measurements in single images, for those who want to take a closer look. The steepness of the chamfer is in the lower left corner of the picture.

Note: The picture change is also possible with the mouse wheel

CNC adapter plate for various Scan-Speak and Peerless tweeters on WG-300 and PCT-300

From the houses Scan-Speak and Tymphany, there are some tweeters, all sharing the same front panel. One or the other of them is also outstanding on the P-Audio (PCT-300), or Monacor (WG-300) waveguide. In order to connect the drivers cleanly with the sound conduction, a CNC machined adapter plate was created after many test series, which allows a simple and very precise fitting.

If you have access to an 3-D printer, you may like to download the following file and print out the adapter plate.

⇒ Adapter plate as STL file⇐

The following tweeters are suitable for the conversion

The first ones were rebuilt by me and measured with very good, partly outstanding results. The following all share the same front panel and are, at least extremely similar to one of the measured drivers ... .and there are a few more potential candidates.

Construction of the adapter plate with "home remedies"

Only a few of us own a CNC milling machine. To build such a simple construction as this adapter plate, there is no need for anyone! With jigsaw, cordless screwdriver, router and the following step-by-step instructions, the construction of the adapter plate should not be a problem. It does not even have to be measured 🙂 Let's go ...

  • Download PDF (click on the picture)

  • Print out with Acrobat Reader (select option "actual size"!)
  • Size check: Outer diameter 120mm
  • Stick on a 9mm strong piece of MPX / MDF / screen printing plate (spray glue / glue stick ...)
  • The drilling holes => drill press is an advantage!
  • With the circular milling device (each speaker should have one) 3mm deep to appropriate mark milling (D67mm)
  • Use the jigsaw to saw the inner hole and saw off the plate in the last step

From the practice with the "home remedy method" another supplementary tip from Thorsten (www.felbi.at)

If you glue your artwork directly onto the substrate (eg MPX), you will have the difficulty to get it down again later. For visual reasons, that bothered me and came up with the following idea: Before you glue your artwork on the MPX, just past the MPX with tape. Now the artwork will be cut to the size of the MPX board. Spray glue is then sprayed onto the package tape and allowed to dry for about 5 minutes (Note the spray adhesive is not on the paper spray!). Now the print template can be glued on.

Once all drilling and milling work has been completed, the package strap can easily be pulled down again from the MPX board.


Assembly

First, the front panel of the tweeter must be removed. On some models, this is easily fixed with a silicone-like adhesive. In this case, with a large flat-head screwdriver discreetly "lever" as shown in the following picture. Work slowly and evenly. In advance, of course, the screws of the front panel should have been solved (TX10).

Dressing the waveguide neck

The adapter plate is placed on the sound guide and the neck is ground flat with circular movements (⇒ dust mask).

The following video shows the final montage

Contrary to the indication in the video no M3x16mm, but M3x12mm Screws used! The M4x10mm are welcome, but they do not have to be longer.

Finally, two shots of the Scan-Speak D2604 / 833000 or 830000 & the Peerless XT25TG30-04 with waveguide

Disclaimer

You do the conversion on your own responsibility! I accept no liability. Possibly. you lose your warranty claims during the conversion.

I did a lot of conversions and nothing ever happened. I think the manual is pretty foolproof 🙂

... coffee is available from 1 € 😉

Test Peerless / Tymphany XT25TG-30 / 04 (XT-300 / K4) at the waveguide WG-300 / PCT-300 4 Ω ring emitter with waveguide

Datasheet © www.heissmann-acoustics.de
Measurements in 20x40cm baffle, 10mm rounded, in 70cm distance at 2,83V. Level scaled to 1m.

The Waveguide was milled with a CNC adapter plate mounted on the tweeter.

XT25TG30_04_WG

XT25TG-30 / 04 Sound pressure at angles

Vifa XT-300 (XT25TG-30 / 04) WG300 Sound pressure at angles

System-induced overshoot in the frequency band of 1-7kHz. This can be due to their symmetry very easy to equalize with the crossover. It is the purpose of a sound guide to act as an acoustic amplifier. This results in a more uniform bundling and higher load capacity. The Vifa XT-300 (XT25TG-30 / 04) fits perfectly with the waveguide. Extremely uniform bundling without significant disturbances. Due to the large dome (30mm voice coil) slightly stronger directivity from 8kHz.Recommendation: On the axis a slight increase> 8kHz in the tweeter schedule.

XT25TG-30 / 04 Sound pressure standardized at angles to 0 °

Vifa XT-300 (XT25TG-30 / 04) WG300 Sound pressure normalized at angles

Vifa XT-300 (XT25TG-30 / 04) WG300 Sound pressure normalized at angles

Here, the frequency response was linearized on axis. The 2. Picture (sonogram) clearly shows the excellent uniform straightening behavior of the combination of XT-300 and the waveguide.

XT25TG-30 / 04 decay behavior

Vifa XT-300 at Waveguide WG-300 decay behavior

Minimal delayed decay by 2kHz. Otherwise perfectly.

XT25TG-30 / 04 harmonic distortions

Vifa XT-300 at the waveguide WG-300 harmonic distortion

As expected, the distortion decreases compared to the variant without waveguide. The XT-300 already showed very good distortion even without waveguide, but with a significant increase of K2 under 3kHz. With Waveguide the effect is also present but attenuated by about 5dB. In sum, very low distortion. Even with 100dB, everything from 2kHz remains below 1%. K3 moves at the measurable limit.

Rating


Frequency response / directional behavior:4.5 5 out of stars
Range:5 5 out of stars
Power handling:5 5 out of stars
Constance / Without Rating /:0 5 out of stars
Haptics:4 5 out of stars
Price-performance:5 5 out of stars

Summary


The Peerless XT25TG-30 / 04 and the Waveguide WG-300 are a highly attractive combination for extremely low-level installations, eg in home cinema. Can be used already from 1,5kHz. I think I'll test that in Cinetor.