HiFiCompass
BlieSMa M142P-6 7" paper dome midrange
We continue our review of the 7-inch dome midrange speakers from the German company BlieSMa, and now it's the turn of the M142P-6 model with a paper diaphragm. For a proper understanding of today's material, I strongly recommend reading the previous review of the aluminum version of the M142A-6, or at least the first part of it.
In the meantime, let me remind you of the basic ideas behind the concept of the aluminum M142A-6 and beryllium M142B-6 dome midranges:
- Increasing the size of the diaphragm makes it possible to reduce the amplitude of its excursion at a constant sound pressure, which helps reduce the Doppler effect and all types of nonlinear distortions compared to smaller midrange drivers
- A large diaphragm size always has the opposite effect of narrowing the radiation pattern and lowering the maximum upper operating frequency, so the convex dome diaphragm! Thanks to this, the directivity pattern is noticeably broadened and even outperforms "cone" competitors with smaller diaphragms
- One more conceptual idea behind of BlieSMa M142A-6 midrange is working together BlieSMa T34A-4 and T34B-4 tweeters. The convex dome shape of the diaphragms of the midrange and tweeter contributes to a significant convergence of their acoustic centers in the direction of the main axis of radiation and approximation to a time-coherent loudspeaker
Looking ahead, I will say that the paper version of M142P-6 can't boast such a wide sound dispersion as the aluminum or beryllium ones, therefore, the very important second point from the above mentioned ones falls out. Nevertheless, this did not prevent the M142P-6 from seeing the light of day and thank goodness! According to the manufacturer himself, the M142P-6, as well as the M74P-6 and the T34P-4 tweeter, were created not for the sake of impressive performance, but for the sake of a huge army of paper speaker fans all over the world (and therefore for me too
).
I would like to express my great gratitude to Stanislav Malikov, the BlieSMa's designer and owner, for providing for testing the samples of M142A-6 and M142P-6 midrange speakers.
Here you can get familiar with the BlieSMa history.
What did the manufacturer state?
Getting familiar with the technical data sheet:
Of the numerical parameters it is worth to note the low mass of the moving system, only 11.7 grams at the radiating area of 180 cm2, very low inductance - 0.03 mH, high mechanical quality factor Qm=7.3 and huge diameter of the voice coil - 137 mm.
The sensitivity is quite high, 94 dB/2.83 volts. Linear excursion is not 6 mm, as in a bass driver, but only 1.85 mm per side, but this is compensated to some extent by the large diaphragm area, and don't forget that this is a midrange, it doesn't need a large excursion like a bass driver. We will come back to this issue in the "How to use" recommendations section.
Among the design features, I was pleased with a natural silk surround - for a true midrange speaker this is important, a huge single-layer voice coil with a ribbed wire, and an underhung-type motor with a copper sleeve.
The datasheet provides additional frequency response graphs for the rear radiation. These may indeed be useful for those considering open-back applications, although the "rear" characteristics here are markedly different from the "front" compared to the aluminum version of the M142A-6.
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The ordinary but quite reliable package made of sturdy glossy corrugated cardboard. Inside, the speaker is fixed with cardboard inserts with shaped cutouts.
But the speaker itself is very far from an ordinary one. For the first time I took it in my hands and examined it from all sides, from different angles and even armed with a magnifying glass. What kind of a beast is this? A huge dome diaphragm behind a grille at the front, and a massive metal ring and a huge donut hole at the back - that's the whole speaker! I think the above pictures will make your brain work a little harder too.
After 10-15 minutes of close examination, the puzzles of this design gradually begin to form in my head. The familiar basket has not quite the usual look - it is a 180-millimeter aluminum ring with a height of 7 mm with four thick, but very short legs, on which the underhung magnetic system is held. On the back side, the basket has a foam rubber gasket around the entire perimeter.
The magnetic system is a massive circular steel magnet core, encompassing a huge circular neodymium magnet on the outside, of which only the inner side surface is visible. There are 12 small holes around the perimeter of the magnetic core to provide ventilation for the voice coil, which is completely hidden in the magnetic gap and completely inaccessible to the eye. However, the 137(!) mm diameter titanium voice coil former, which is densely perforated, is clearly visible through the gap between the basket and the magnetic core.
The four-wave spider is made of natural cotton by the German company Dr. Kurt Muller. Such spiders are characterized by low mechanical losses and are particularly good for use in midrange speakers.
The half-roll inverted surround is made of natural silk, also by Dr. Kurt Muller. From the rear, it is very clearly visible in its entire size, but from the front it appears much narrower because it is partially covered by a thick, about 9 mm, aluminum ring to which the grille is attached. This ring has three main functions: decorative - it covers the joint between the surround and the basket, functional - it reduces the outward protrusion of the dome profile, and load-bearing - the grille is attached to it. It also has six holes with pockets for the heads of mounting screws.
A black hole gapes through the huge opening in the magnet system - yes, yes, that's what the black matte paper diaphragm looks from behind. It is smooth on the front and lightly embossed on the back. The convex profile of the diaphragm is quite high and protrudes about 25 mm from the basket.
The diaphragm is protected by a metal grille with a high degree of transparency, at least 70%. The grille and all aluminum parts are protected by a durable semi-matte powder coating.
The workmanship is of the highest quality. No glue marks, scratches, dirt and dust, stains or dents on the diaphragm as well as no chips, gaps or dents on the diaphragm. Absolutely nothing to complain about.
In the hands, the M142P-6 gives the impression of a solid monolith. All components are absolutely inert when tapped, and there is not the slightest hint of any extraneous sound.
Overall, the speaker made a pleasant impression with its unconventional design and excellent workmanship.
The diagrams below show the frequency responses of the impedance at different scales:
The measured value of the resonant frequency Fs=57.2 Hz was lower than the declared value of 63 Hz, and the total quality factor Qts=0.3366 is slightly higher than 0.31 in the datasheet, although, for a midrange, these parameters are not particularly important. Quite a narrow impedance peak reaches 137 ohms at the resonant frequency, which at low mass of the moving system indicates very low mechanical losses. And so it is - the measured mechanical quality factor Qms was as low as 8.412, which is even slightly higher than the declared 7.3.
The impedance curve above 200 Hz is virtually flat, with a rise to 20 kHz relative to 500 Hz of less than 45%, reflecting the extremely low inductance of the voice coil. This low inductance (0.03 mH) is due to the short (2 mm) single-layer voice coil and the massive copper sleeve in the magnetic gap.
A series of bursts above 2.5 kHz accompanies the paper diaphragm breakups - quite natural and inevitable phenomenon. But the hump at 1 kHz is quite unexpected. Usually in this range there is always an echo of the usual resonance of the surround and the edge of the soft cone diaphragm, but in the case of M142P-6 this effect should be absent at all, because the interface between the surround and diaphragm is supported by a rigid titanium voice coil former. In fact, it is caused by an acoustic effect due to a parasitic Helmholtz resonator formed by the elasticity of the air in the diaphragm subdome space and the mass of air in the short "tube" of the magnet system. This is a very unpleasant effect, but we will return to it a little later.
So, the impedance measurements told us about a moving system with very low mechanical losses, a very good motor that promises low nonlinear distortion, and the presence of parasitic resonance in the subdome space.
On-axis frequency response (at 315 mm)
Below is the axial smoothed (1/12 octave) frequency response magnitude of the M142P-6 measured in a test baffle at a distance of 315 mm to the microphone at 2.83 volts:
The frequency response is very close to the datasheet, but looks better in my opinion, because the step at 1 kHz has a more gentle character and lower height.
The measured sensitivity in the range from 200 Hz - 1 kHz averaged 93 dB, which is 1 dB below the claimed 94 dB.
Up to 2.2 kHz the frequency response is quite smooth, but it can't be called flat because of an unpleasant 3-3.5 dB step in the 1 kHz region. The first resonance of the diaphragm occurs at 3.7 kHz and is manifested by a spike of about 16 dB. Here it is no longer as narrow as that of the M142A-6 aluminum diaphragm, but rises as a wide mountain in the range from 2.3 to 7 kHz. This behavior is explained by the lower stiffness and higher internal damping of the paper diaphragm.
The first resonance is followed by a descending series of bursts and dips accompanying the breakup mode of diaphragm operation.
As you have already noticed, the M142P-6 midrange is not like any other. Its original design, while offering a number of advantages, faces problems that classic concave cone diaphragm designs never do. It is, of course, about the sub-dome resonance and its manifestation in the form of a step in the frequency response in the region of 1 kHz. This problem can be solved in the traditional way, indirectly, namely to fight the symptoms of the disease and elementary correct the step with the help of DSP in systems with active filtering, or do the same in a passive crossover, even if it is a little more complicated. Fortunately, the manufacturer does not leave us with the problem alone and for a non-standard speaker offers a non-standard approach, fundamentally eliminating the problem acoustically. The essence of the method, detailed in the document "M142 Series Application Note AN1", is to create a special acoustic load for the rear diaphragm radiation, neutralizing the parasitic sub-dome resonance.
As they say, trust but verify, so I decided to test the proposed method. The speaker was mounted in the test baffle, and a rear chamber was constructed from plywood and filled with sound-absorbing material in strict accordance with the apnote. In effect, the open-back speaker became a huge 7" dome midrange with a rear chamber:
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Below are measurements of the speaker in the test baffle as is - the black curve, and with the rear chamber - the red curve:
Wow, that's a completely different thing! The frequency response is magically transformed and became very smooth and even. The step has completely disappeared and the average sensitivity in the 300 Hz - 2 kHz range is already equal to the datasheet and even slightly exceeds it. I can say that this uncomplicated method demonstrated an excellent result and allowed the speaker to reach its full potential. Excellent!
Off-axis frequency responses (at 315 mm)
Below are graphs of off-axis frerquency responses - conventional and normalized, in which the axial response is taken as a reference, and the off-axis ones reflect only the difference with it:
Off-axis responses monotonically decrease with frequency and angle of deviation. The dome shape of the diaphragm did not give anything special here, unlike the aluminum version. We can say that the width of the sound radiation is quite adequate to the size of the driver and is between 6" and 8" drivers with traditional cone diaphragms.
Harmonic distortion (at 315 mm)
Above are the dependences of harmonic distortion at voltages of 2 and 8 volts, which corresponds to average sound pressure levels of 90 and 102 dB, respectively. The measurements were taken on axis at a distance of 315 mm to the microphone. To limit overloading of the speaker in terms of diaphragm displacement amplitude, a second-order digital high-pass filter with cutoff frequencies of 50 Hz at 2 Volts and 100 Hz at 8 Volts was used when measuring harmonic distortion. In these graphs, we analyze the frequency range only above 150 Hz.
As the frequency decreases below 250 Hz, the harmonics of all orders begin to rise, due to the increasing amplitude of diaphragm excursion.
Between 2.3 and 4.2 kHz there is a spike in all harmonics, which can be explained by the main diaphragm resonance. It is not a big problem as it is outside the potential operating frequency range (150 Hz - 2.2 kHz) and will be suppressed in the process of magnitude frequency response shaping in the stop band.
There is also a spike in all harmonics in the 600 Hz - 1.2 kHz range, for which I do not have a precise explanation. It is likely that this effect has something to do with aforementioned underdome acoustic resonance. Nevertheless, even despite this spike, I would characterize the overall level of harmonic distortion in the 150 Hz - 2.2 kHz range as "low".
Voice coil current harmonic distortion
This type of measurement, although simple, is a good tool for evaluating the linearity of a speaker motor. The graphs above show the frequency dependencies of the 2nd, 3rd, 4th and 5th harmonics of the voice coil current at voltages of 2 and 5.6 volts (with the second-order high-pass filter on with cutoff frequencies of 50 and 80 Hz, respectively).
Voice coil current nonlinearity is the direct nonlinearity of the mechanical force driving the speaker cone, since this force is related to the current by a simple relationship F=B*L*I, where B is the magnetic field strength, L is the length of the voice coil wire inside the magnetic gap and I is the current. So, it is practically impossible to obtain sound pressure distortion lower than current distortion in the frequency range where the contribution of the motional nonlinearity becomes insignificant.
As the frequency decreases, harmonics of all orders increase due to the increasing amplitude of diaphragm excursion. Spikes in the region of 600 Hz - 1.2 kHz correspond to similar behavior of harmonics in sound pressure. Above 2.2 kHz all harmonics show a very nervous and growing behavior, which is due to the diaphragm entering the breakup mode of operation.
The overall level of current harmonics of all orders in the possible operating frequency range is very low.
The intermodulation distortion measurement is one of way of to analyze the device non-linearity. It is not an alternative, but an additional method and allows you to identify the spectral components of the inharmonious structure, which are much more harmful for high-quality sound reproduction and to which our hearing is more sensitive.
For testing I chose the frequencies with ratio (1:8.5). In this case the Doppler distortion is not yet dominant and the contribution of amplitude modulation can still be observed. The fractional coefficient eliminates the superposition of harmonic and intermodulation components on each other:
- 125 Hz and 1063 Hz
- 500 Hz and 4250 Hz
The graphs below show the intermodulation distortion spectra for voltages of 2 and 5.6 Volts.
However, in these plots products of the motor nonlinearity are mixed with products of inevitable frequency modulation due to the Doppler effect. How to determine who is who? We can analytically estimate the level of the first pair of the side Doppler components using the following formula [http://www.linkwitzlab.com/frontiers.htm#J]:
As(dB) = 20*log10(pi*A1*f2/c), where pi=3.14, A1- the modulating signal amplitude in meters, c=343 m/s, f2 - carrier frequency. In our case f2 =1063 Hz or 4250 Hz.
The amplitudes of the modulating frequencies (obtained by calculation) as well as the estimated maximum level of the second-order spectral components (IMA2Doppler) corresponding to Doppler distortions are:
If the measured side spectral components are higher than these values, then the speaker nonlinearity and the amplitude modulation are predominant, if lower, then the Doppler distortion and the frequency modulation caused by them is dominant.
- 125 Hz and 1063 Hz - these frequencies correspond to a quite real and most difficult mode of operation of the midrange speaker in terms of intermodulation distortion. 125 Hz signal stimulates maximum cone amplitude:
For voltages of 2 and 5.6 Volts, the measured IMA2 values were -47 dB and -37 dB, which is slightly higher than the Doppler components of -50.7 dB and -41.7 dB, meaning that in the intermodulation distortion spectrum for the 125 and 1063 Hz frequency pair, amplitude modulation dominates over frequency modulation.
- 500 Hz and 4250 Hz
For voltages of 2 and 5.6 Volts, the measured IMA2 values were -58 dB and -49 dB, which are slightly higher than the Doppler components of -59.57 dB and -50.57 dB, meaning that in the intermodulation distortion spectrum for the 500 and 4250 Hz frequency pair amplitude modulation is almost equal to frequency modulation.
Everything in this world is relative and in order to adequately assess the level of intermodulation distortion, it is necessary to have the correct reference point. The speakers of the Danish company Purifi are characterized by a very advanced design and, in my opinion, have the lowest nonlinear distortion to date. Therefore, as a reference I chose Purifi PTT6.5M08-NFA-01 6" midrange. The table shows comparative values of the second and third order intermodulation components IMA2 and IMA3 measured at the same sound pressure levels of the modulating signal:
Wow! For all intermodulation components, except only IMA3 for the 125+1063 Hz frequency pair, the M142P-6 performed significantly better and from now on it becomes my reference for evaluating the intermodulation distortion level of midrange speakers.
The step response rises up quickly and quickly returns to the rest state. It is quite spoiled by the oscillating process of the main diaphragm resonance of 3.7 kHz sitting on it. The picture is very far from nice looking, but after suppressing the resonance peak with the help of DSP with one biquad only, whose parameters are clear from the the graph, the situation improves and only ripples from smaller breakups remain:
The waterfall shows the same effects as the step or freequency responses, in addition to exposing hidden resonances that are difficult to see on other types of measurements. In this case, you can clearly see the main diaphragm resonance at 3.7 kHz with a long tail and a series of subsequent small but also long breakups. The line between “good” and “evil” can be confidently drawn at approximately 2 kHz - below this frequency everything is absolutely clear, above iy there is an area of utter darkness, completely dotted with breakups:
And this is already a waterfall, but with the same notch filter, as in the case of measuring the step response. Everything is the same as before, but the first, most powerful crest is gone:
After finishing all measurements, I took some time to listen to the M142P-6 mounted in the test baffle with the optimal enclosure according to the recommendations from the manufacturer's application note. The crossover schematic and pictures of the setup are shown above in the review.
The overall timbre is very characteristic of paper diaphragm speakers. Natural, rich in tonal nuances and aftertones, with a pleasant velvety quality and a slight veil. The sound is deep, thick and slightly dark, detailed and dynamic, with well conveyed energy. There is no dryness and emptiness. Musical instruments have relief, fullness and are well separated in the sound canvas.
Parasitic manifestations of high-frequency diaphragm breakups were practically not noticeable, and when this range will be filled by a tweeter in a real loudspeaker, there is nothing to talk about. One can forget about any overloading of the speaker, even at very high volume level - everything is very clean.
The most intriguing question for me was whether the M142P-6 had some special character of sound due to its convex dome diaphragm shape, because until then I had never heard such speakers. Friends, I must say that it is there! It seemed to me that in comparison with speakers with usual cone diaphragms, the M142P-6 sounded more open, more relaxed, more spacious, less clamped in the upper midrange and less tied to the speaker itself.
With a very open and acoustically transparent magnet system at the rear, the driver has excellent prospects for open acoustic design applications.
Compared to the aluminum version M142A-6, the paper version loses in hardness, speed, attack and transparency, but wins in timbre naturalness, colours and richness of aftertones. Comparing the M142P-6 and M142A-6 7" midrange dome speakers reminded me a lot of comparing their smaller 3" counterparts - the M74P-6 and M74A-6 - the feeling is exactly the same.
So far, that's all I can say about my first subjective impressions after listening to the M142P-6 midrange.
Based on these measurements, I would recommend using the M142P-6 as the midrange of the highest quality loudspeakers in the 200Hz to 2kHz frequency range, preferably in the manufacturer's recommended design. In this application, the sensitivity of the driver is increased to 94.5 dB, the frequency response is equalized, and the low frequency roll-off is almost completely formed in a natural (acoustic) way, all that remains is to add the first electrical order to limit power and amplitude overload.
Since the M142P-6 has a convex dome diaphragm and was conceptually designed to be used in conjunction with BlieSMa T34A(B) tweeters, it is best used in conjunction with these tweeters, especially with passive filtering. In this case, it is much easier to ensure time matching of the drivers. With active filtering, with the possibility to adjust the delay time using DSP, your hands are completely free in terms of tweeter selection.
The choice of the lower limit frequency of the application largely determines the maximum achievable SPL limited by the maximum linear excursion of the M142P-6 moving system. I simulated three possible applications of the M142P-6. Shaping of the low frequency response was performed using a 2nd order digital high pass filter with such parameters to obtain the lower cutoff frequencies of 200, 250 and 300 Hz by sound pressure in the optimal enclosure. Below are the frequency responses of diaphragm excursion and corresponding to them frequency responses of the maximum SPL at which the diaphragm excursion does not exceed its maximum linear value of 1.85mm.
For a lower cutoff frequency of 200 Hz:
For a lower cutoff frequency of 250 Hz:
For a lower cutoff frequency of 300 Hz:
- for F3=200 Hz - SPL(max)@1m=113 dB
- for F3=250 Hz - SPL(max)@1m=117 dB
- for F3=300 Hz - SPL(max)@1m=120 dB
If we take into account that there are two channels in a stereo system, this gives an additional gain to the total SPL of 3 to 6 dB. It seems to me that the obtained results satisfy 99.9% of cases when M142P-6 is used in home audio applications.
What is the price and where to purchase it?
The M142P-6 midrange is already on sale. The average retail price for the M142P-6 is €882/piece excluding VAT. You can purchase it in the following online stores:
- https://audio-hi.fi/en/bliesma_m142p-6-p-5376.html
- https://www.compacbel.be/boutique/#!/products/bliesma---m142p-6
- https://loudspeakershop.eu/glosnik-bliesma-m142p-6-papierowy-sredniotonowy-p-8332.html
- https://www.soundimports.eu/en/bliesma-m142p-6.html
- https://solen.ca/en/products/m142p-6
We got acquainted with a new, very interesting model of 7" dome midrange speaker by BlieSMa. The speaker is characterized by a very original design and a number of high technical characteristics. This is not my first review of BlieSMa products and I am once again convinced that each new product is a reflection of the company's firm philosophy in the matter of speaker design, namely minimizing the weight of the moving system, minimizing nonlinear distortion and maximizing the efficiency of the transducer. The M142P-6 midrange is another example of this.
So, what can be noted:
- dedicated midrange speaker
- high sensitivity - 93 dB/2.83 Volt*1 m (in the optimal enclosure up to 94.5 dB)
- titanium voice coil former
- huge 142mm paper convex dome diaphragm
- very flat and smooth response in the optimum acoustic design
- low harmonic distortion in the range 150 Hz - 2.2 kHz
- very low intermodulation distortion
- excellent workmanship
More detailed measurement results can be found here
Yevgeniy Kozhushko/02.05.2024
CONTACTS
- Ukraine
- (+380) 95 904 7827
- hificompass@gmail.com
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