HiFiCompass

Viawave SRT-7 ribbon tweeter

What is on the test bench?

Today we are meeting a very non-traditional sound radiator - the ribbon tweeter SRT-7 of the Russian company Viawave. I said "very non-traditional" not occasionally because this is the tweeter in which the first time in the world the technology of sealing air gap between the magnet poles and the ribbon membrane by using U-shaped polymer film have been implemented. In 2019 Viawave company could managed to accuire a Russian patent on its invention.

In some sense a hybrid of dome and ribbon tweeter got out. The first tweeter using such a technology was made in 2017 and called GRT-145. It got very good reviews in German magazine Klang&Ton, from famous Danish speakerbuilder Troels Gravesen and among DIY community all over the world. SRT-7 model is a succesor of GRT-145 and differs of it mostly by the more even frequency response.

You can get familiar with the history of Viawave company here.

Let's start examining the new tweeter and see how the new technology is justified or it is an one more manufacturer's marketing trick.

Why do we test this?

I'm always very curious to discover new tecnologies and interesting speakers, so it is the case. Speaker driver designing is the field where a progress creeps a bit faster than snail comparing to other segments of the audioindustry. It is very difficult to invent something really unique here.

I express my deep gratitude to Dmitriy Malinovskiy, the founder and the main developer of the Viawave company, for the SRT-7 tweeter samples provided for independent testing and the time spent discussing the technical aspects related to the design and application.

What did the manufacturer state?

The datasheet is very detail and contains all the information necessary for a loudspeaker designer. There is also a harmonic distortion diagram at the sound pressure level of 90 dB/1m and that is not often can be seen in manufaturer's data.

Viawave provides its tweeter with very useful data - the curve of the maximum sinusoidal RMS voltage at low frequencies. Ribbon tweeters are good known for their limited overload capacity at the low end of the working frequency range, so that diagram is a "Golden Key" in the designer's hands for the correct and full realization of tweeter potential when designing a crossover.

It is getting clear what the letters in the tweeter model name mean: SRT - (S)ealed (R)ibbon (T)weeter, i.e. a ribbon tweeter with the sealed air gap between the magnet poles and the ribbon membrane using a U-shaped heat-resistant polymer film. According to the manufacturer words the new technlogy made possible to get the folowing:

preventing acoustic short circuiting between the front and back sound waves at low frequencies
extending the operation frequency range down to 1.8 kHz
increasing the sound output at the "low frequencies" by 2-3 dB
significant reducing non-linear distortions which are inherent to "classical" ribbon tweeters

The tweeter has a special sheep wool filling of the back chamber, that allowed to reach very linear frequency response.

Visual inspection

I couldn't believe that there were only a couple of tweeters inside. Scales showed 2,817 kg!

Everything is respectable inside, tweeters are wrapped in plastic wrap and surrounded on all sides by sheets of polyethylene foam. The tweeters feel very comfortable inside and, as it seems to me, such packaging can allow a very high degree of negligence on the part of any parcel delivery company. Fine!

I'm taking the tweeters out of the box and inspecting them.

The workmanship is superior, there’s simply nothing to complain about - all the parts are tightly fitted, no any cracks, skewness, burrs and glue traces. Labels are glued evenly. Weight inspires respect.The flange is diecast aluminum of 4 mm thick and is microtexture powder painted. There are no complaints to the coating. Six mounting holes have pockets for mounting screws heads. The screws, which fix the flange to the tweeter body, are recessed and look very harmonious. The membrane is protected by a very acoustic transparent metal grid. A foam rubber gasket is glued to the flange back side. Useful trifle.

The tweeter body is the main part of a magnetic circuit, covered on the top and bottom sides by aluminum covers, and on the back by a plastic cover, which incorporate a matching transformer filled with epoxy compound and gold-plated screw terminals.

Behind the protective grid wedge-shaped neodymium magnet poles and a ribbon membrane are visible. The ribbon has small embossing and decent transverse corrugation. Sucha a corrugation gives it additional rigidity, which is necessary to improve operation at low frequencies. At very careful examination you can notice at the edges of the membrane the same U-shaped yellowish surround.

In general, the construction is very well thought out and very good realized. Fine!

Impedance frequency response

The impedance curves of the two samples reflect a good selection of speakers in a pair. In the operating frequency range above 2 kHz the impedances are very smooth and almost the same, without any spikes, excepting a small prominence in the region of 3.6 KHz, which can hardly be seen. This is probably an echo of some kind of internal, but well-damped resonance.

The main resonance turned out even slightly lower in frequency - 788 Hz instead of 800 Hz declared.

I don't see any problems with the impedance when working with a crossover.

On-axis frequency response

The measured sensitivity in the range from 4 kHz and higher was on average 98 dB, which is 3 dB (!) higher than the manufacturer (95 dB) specified. This is more than a pleasant surprise. A direct radiating tweeter with an impedance of 8 Ohms and a sensitivity of 98 dB/2.83 Volt is an extremely rare thing and the range of its application in domestic systems is practically unlimited. Estimating the maximum sound pressure, based on the maximum program power of 150 W, turns out to be about 119 dB - this is a lot. Just fantastic!

The measured frequency response is in good agreement with the datasheet one. In the range above 4 kHz the response is exemplary flat. Below 4 kHz the response smoothly declines, going to a shelf of 94 dB, which extends down to 800 Hz, that is just incredibly low for ribbon tweeters of this size. By its width, both down and up in frequency, the SRT-7 frequency response is not inferior to good dome tweeters. The early response falling at low frequencies was always the Achilles heel of ribbon tweeters. As we can see, the technology of sealing the gap really works and the manufacturer’s statement about increasing of low frequency output are indeed confirmed.

The absence of any signs of parasitic resonances in the entire frequency range indicates good damping of the moving system and the rear chamber of the tweeter. Fine!

Off-axis frequency responses

The upper diagram shows the horizontal dispersion of the tweeter at 0-15-30-45-60 degrees to the main axis. All curves are very smooth and monotonically falling with the angle and frequency. No hidden resonances and diffraction artifacts. The maximum response falling at 20 kHz and 60 degrees does not exceed 12 dB and this is the indicator of a very good horizontal dispersion.

The bottom diagram is the same as the upper one but at vertical deviations only. The curves are still smooth and monotonous, but no miracle is happened - the vertical dispersion is noticeably worse than the horizontal one, that is due to the much larger linear vertical membrane size - 50 mm than the horizontal one - 14 mm.

Off-axis performance declared by the manufacturer can be trusted.

Harmonic distortion (315 mm)

So we got to an interesting point. The nonlinear distortion behaviors are given at average sound pressure levels in the range above 4 kHz of 94.5 dB (at the voltage of 2 Volt) and 104 dB (at the voltage of 5.6 Volt). On the diagrams we are analyzing the frequency range from 2 kHz and above - the possible working range of the tweeter.

I would describe the level of distortion as very low for an radiator of such a size, at the level of good dome tweeters. Particularly pleasant surprise is the low distortion level below 4 kHz, which ribbon tweeters had never dreamed of before. One of the best world recognized ribbon tweeter - RAAl 70-20XR, having a membrane area even 2 times larger - 14cm2 and famous for its ability to work down to the frequency of 1.8 kHz, there's nothing to do here. It just should pass the crown of the best ribbon tweeter in the world to SRT-7.

At the frequency range 2 kHz - 5 kHz at sound pressure levels of the order 102-104 dB an increase in the 4th and 5th harmonics begins to be observed, but they are still at a moderately low level of -65-70 dB. It seems the ribbon is already starting to lose a little its stiffness under such loads.

The overall distortion level allows you to safely use the tweeter down to at least 2 kHz without compromising sound quality even at high volume levels.

Measurements of nonlinear distortions confirmed both the values given in the datasheet and the effectiveness of the new technology that brings ribbon tweeters to a completely new qualitative level. Fine!

Step response

The step response shows an excellent slew rate, but the complete calming of the tweeter does not occur so quickly. The response goes to the zero point oscillating and crossing the zero line a couple of times - a consequence of the low resonant frequency and the high quality factor of the main resonance. When used in real systems with a cutoff frequency of 2 kHz, you can ignore it. Very well.

Waterfall

The waterfall shows the same effects as the transient response, in addition exposing hidden resonances which are difficult to see in other types of measurements. In this case an echo of the main resonance is being observed near 800 Hz. In the rest of the range the waterfall is just perfect. There is simply no second slice that follows after 0.17 ms, like all subsequent ones. Such a waterfall is possible due to the lightest membrane weighing only 0.009 grams with minimal inertia and proper damping. Fine!

The 4 kHz ridge does not belong to the tweeter, but is due to the artifacts of the measurement setup. It manifests itself in majority of my measurements.

Listening impressions

I can talk about auditory impressions only after testing the tweeter as part of a real loudspeaker system. By now, I’ll only tell about those impressions that were obtained by briefly listening to the tweeter during the measurements.

The sound signature generally is the same as that of other ribbon tweeters of a similar size. The sounding is very airy and detail, sparkling, very "fast" and abounds with the smallest nuances. Very nice and natural timbre. The main difference from "party members" is that this tweeter has a body and weight at the bottom of the frequency range, here it is much closer in character to dome tweeters, and this is surprising.

"How to use" recomendations

Due to its very low distortion and confident operation at the low frequencies the SRT-7 can be used down to 2 kHz in two and three-way loudspeakers. The main factor which determines the choice of the order and the cutoff frequency of an electrical HF filter is to provide the required maximum sound pressure in the main frequency range above 4 kHz without the risk of overloading the ribbon by an excessive amplitude excursion. The manufacturer provides in the datasheet a graph of the maximum allowable effective low frequency sinusoidal voltage at the tweeter. The long-term excess of these limits can lead to irreversible deformation of the membrane.

When looking at this graph, it is not so simple to determine the maximum permissible filtering modes and associate them with the maximum sound pressure obtained in this case. I tried to model and evaluate the maximum achievable sound pressure in the range above 4 kHz, depending on the order and cutoff frequency of the electric filter.

The diagrams combine the curves of the maximum allowable voltage at the tweeter, taken from the datasheet, and the transfer functions of various electrical high-pass filters, 1st order and 2nd order Linkwitz-Riley. It is assumed that the input signal is a sinusoidal voltage, which is the most hardest test for the excursion limited type of overload. The results are shown in the diagrams and are tabulated.

Choose the sound pressure you want to get and see what type of filtration is the maximum permissible for your requirements. Do not forget that the calculation of maximum SPL was carried out for one tweeter only, so in the stereo system you will get another +3 dB in the total output.

The modeling is evaluative.

Type, order/cutoff of HPF	The test signal	The SPLmax in the range above 4 kHz
1st/2 kHz	Sinusoidal	104.1 dB
1st/2.5 kHz	Sinusoidal	105.5 dB
1st/3 kHz	Sinusoidal	106.8 dB
1st/3.5 kHz	Sinusoidal	107.9 dB
1st/4 kHz	Sinusoidal	108.9 dB
1st/5 kHz	Sinusoidal	110.7 dB
LR 2nd/2 kHz	Sinusoidal	108.6 dB
LR 2nd/2.5 kHz	Sinusoidal	111.3 dB
LR 2nd/3 kHz	Sinusoidal	113.6 dB
LR 2nd/3.5 kHz	Sinusoidal	115.8 dB
LR 2nd/4 kHz	Sinusoidal	117.7 dB
LR 2nd/4.5 kHz	Sinusoidal	120 dB

What is the price and where to purchase it?

You can purchase SRT-7 tweeters, for example, in the following online stores:

The average retail price ranges from 240 to 340 EURO apiece, excluding VAT.

Summary

The testing confirmed a very close correspondence of almost all the measured parameters to the declared ones with the exception of sensitivity. It was 3 dB higher than the datasheet one, that is very good. The new technology of sealing the working gap has proven its real effectiveness, allowing to expand the application boundaries of ribbon tweeters without compromising the quality of sound reproduction. From now, doors for their application in two-way systems are really opened. Before the SRT-7 it was almost impossible and had to enjoy the delights of ribbon tweeters in three and more ways types loudspeakers only. Of course, from time to time there were clumsy examples of marriage of 6 "midwoofers with ribbons, but it's better to keep silent about their sound quality.

I can say with confidence that the Viawave SRT-7 tweeter is currently the best one in the world among the ribbon tweeters of the similar size.

So, what can be noted:

very high sensitivity - 98dB/2.83Volt at 1 m
very even frequency response
low non-linear distortion
excellent building quality
real opportunity for its using in two-way loudspeakers down to the frequency of 1.8 kHz
everything is so good that there’s nothing to complain about

More detailed measurement results can be found here.

Yevgeniy Kozhushko/08.12.2019