Waveguide WG104-XX/34 for BlieSMa T34B-4 (T34D-4) tweeter

  What's it for?

This article will introduce you the results of my experiments on developing a compact correction waveguide for one of my favorite beryllium diaphragm tweeters BlieSMa T34B-4 (detailed review of the tweeter here). At the moment, the point is not yet reached, but I think good results have already been achieved.

The T34B-4 tweeter is already good on its own, as it is. It draws a very wide and detached from the speakers sound picture, usually located behind them. And that's very cool, but sometimes you want a little different sound presentation, bigger, closer and more expressive sound images. Besides, acoustic properties of rooms, which are always and everywhere different, require a respectful attitude to themselves. Since the characteristics of the formed sound picture are directly related to the ratio of direct to diffuse sound, I decided to correct them with a small waveguide.

  Goal

The developed waveguide must meet two main requirements:

1. To reduce the imbalance of spatial sound dispersion between the frequency ranges of 3-7 kHz and 10-20 kHz
This will make it possible to noticeably change the character of the sound picture, increase the expressiveness of sound images and reduce the unpleasant emphasizing of sibilant sounds.

2. To be as compact as possible

Increasing of sensitivity, reducing distortion in the lower treble range and decreasing of the time offset between a tweeter and cone midrange drivers by bringing their acoustic centers closer together in depth is not a priority for this design, but will be a pleasant bonus.

  Construction

The development of the WG104-XX/34 waveguide (the "34" at the end of the name refers to the 34 mm diameter diaphragm) was carried out iteratively, based on measurements of prototypes, analysis of their behavior and corrections in subsequent versions. The prototypes were manufactured very precisely from MDF by CNC milling.

The size of the waveguide was chosen to be as compact as possible - 104 mm in diameter, which coincides with the diameter of the flange of the tweeter itself.

The waveguide has a completely simple design, as its back side is completely plain. It is used as an overlay on the tweeter flange with the protective grille removed beforehand. It is also available with mounting holes.

The waveguide can be made of any hard material (fiberglass, SRBP-Synthetic Resin Bonded Paper, acrylic, polyamide, wood, metal) by CNC milling or turning. 3D printing is also possible.

The waveguides fit both the beryllium T34B-4 and diamond T34D-4 tweeters due to the identical geometry of their diaphragms.

Measurements were taken in a 1650x850mm shield at a distance of 315mm to the microphone. The measurement signal was a logarithmic sweep tone with a voltage of 2.83 Volts.

  T34B-4 without a wavegude

Below are diagrams of off-axis frequency responses of SPL - normalized and normalized, where the axial response is taken as a reference, and the off-axis ones reflect only the difference with it. Measurements were taken with the protective grille removed. To reduce the influence of parasitic diffraction from any inhomogeneities of the tweeter flange (pockets of mounting holes, mounting groove of the grille), the space around the diaphragm was covered with sheets of paper.

Very flat and smooth on-axis response, unevenness in the range 1.5 - 26 kHz does not exceed +/-0.5 dB.

Very wide spatial dispersion up to the 6 kHz knee point.

Below are the results of simulating the behavior of a tweeter with an high pass filter in VituixCAD software. The target power response curve is assumed to be a straight line with a slope of minus 1 dB/octave towards high frequencies. Such slope is not a dogma, but from my personal experience, in most cases it gives good results in living rooms.

First diagram. The purpose of modeling is to obtain a smooth axial sound pressure response and to analyze the frequency response of radiated power and DI directivity index: 

We see that if we take the horizontality of the axial frequency response as a design criterion, there is excessive radiated power in the band up to 10 kHz in relation to the band above 10 kHz. DI grows very slowly up to 6 kHz, after which there is a bend and uniform growth occurs at a noticeably higher rate.

Second diagram. The purpose of modeling is to obtain the characteristic of the radiated power with a slope corresponding to the slope of the target curve minus 1dB/octave:

Since any manipulation in the electrical domain does not affect the tweeter's spatial dispersion, the DI curve remained unchanged. But equalization of the energy balance resulted in a strong scoring of the axial frequency response above 7 kHz.

The third diagram is the same as the second, except that the level of the signal applied to the tweeter is reduced so that the actual power response matches the target curve in both slope and level. This is done for ease of comparison with subsequent diagrams of tweeter operation with a waveguide:

  Waveguide for T34B-4

Below are the off-axis responses and simulation results for the versions of the waveguide that I found interesting for some reason. Each version has its own characteristics that determine their behavior and application in speakers. The simulation was performed according to the criterion of coincidence of the radiated power response with the target curve. The high-pass filter is completely passive. The input signal level is unchanged for all versions of the waveguide - 2.83 Volts:

A distinctive feature of the effect of the influence of all waveguides is:

• an increase in axial sensitivity in the region below 7.5 - 11 kHz by a minimum of 1 dB, up to a maximum of 3 - 4 dB in the region of 4 - 4.5 kHz
• average 2 dB decrease in output above 7.5 - 11 kHz
• the radiated power response coincides with the target curve, while the axial response retains a smooth horizontal character
• the starting point of dispersion narrowing (DI rise) decreased from 6 kHz on average to 3.5 kHz
• waveguide versions WG104-7/34, WG104-8/34, WG104-9/34 have a uniform DI growth
• waveguide versions WG104-3/34, WG104-4/34 are characterized by a directivity index reaching a more or less flat area starting from 6 and 7 kHz, respectively.

Click to show

  All-inclusive

And here is the most important and most illustrative diagram summarizing the results of all the experiments. I expect a certain level of competence of my readers, so I will leave it without comments. I hope that many people will find it very useful.

  What's on offer and what's the price?

At this point, a set of documentation is offered that is necessary for self-manufacturing a waveguide of any of the versions discussed.

The documentation includes:

1. Graphic drawing file in PDF format (only for one version of the waveguide)
2. STL file for printing on a 3D printer (only for one version of the waveguide)
3. STEP file for printing on a 3D printer (only for one version of the waveguide)
4. IGES file for printing on a 3D printer (only for one version of the waveguide)
5. Waveguide profile files in DXF and DWG vector formats

The cost of documentation for one version of the waveguide is 50 EURO. Payment is accepted only through the PayPal payment system. The documentation is sent to the buyer by e-mail within 24 hours of payment.

Send purchase requests to hificompass@gmail.com