Detailed introduction of the quality characteristics of the speaker unit

Detailed introduction of the quality characteristics of the speaker unit

The speaker unit determines the ultimate potential of the entire speaker, and plays a leading role in the sound performance of the entire HIFI system. Under the current technical conditions, it is still impossible to make a perfect unit. That goal is still decades later, because it requires the unit to have the same density as air, to move completely uniformly at all frequencies, without any kind of distortion.

There is a long road ahead of us, but we should also be full of confidence. At this moment, major progress is taking place in the field of materials science, and many achievements have emerged in the past decade. I am sure that a new breakthrough will be made in two or three years.

We have benefited from significant advances in the study of computer-simulated mechanical behavior, as well as the extensive research carried out by the aviation, automotive, and sports-recreation industries to replace expensive and heavy traditional materials with lightweight, high-performance materials. We now have KEVLAR, carbon fiber composite materials and aluminum diaphragms. In the near future, we will also have artificial diamonds, ultra-low density silica glass, new metal single crystals and carbon single crystals, and new composite materials.

Why do the speaker units have their own different sounds?

The biggest challenge facing designers is how to ensure the uniformity of motion and eliminate the resonance in the middle and high frequencies. This is a compromise that has to be made in various speaker systems (except for massless speakers). Other problems include cavity resonance and nonlinear magnetic field.

Uniform motion

Rigidity means that the acceleration from the voice coil is accurately transformed into the acceleration on the entire surface of the cone or dome; this way you can get a straight frequency response, rapid pulse rise, low intermodulation distortion and a sense of transparency.

Enthusiasts often describe this type of sound as "fast", which surprised those engineers who are based on objective measurements. "How can the mid-bass unit be fast? Because the crossover limits the pulse rise time, it is quite At 1/5 or even 1/10 of the tweeter, this is just as a diplomat often says, "to exchange views comprehensively and honestly", or to exchange misunderstandings.

It can be said that both parties are right and wrong. They actually talk about different things. What the audiophile hears is the uniform cone movement; this phenomenon is manifested in the test: there is no intermodulation distortion, the frequency response is flat, and the pulse response is clean and clear.

Great, so why not make the cone or dome as rigid as possible? How about a metal like bronze. It has good strength and can be processed into almost any shape. The bell is made of bronze. But the problem is resonance, and their echoes last for tens of thousands of weeks.

The answer has two points. First, the metal has high rigidity. Second, the only way for the bell to release mechanical energy is through the air. Due to the difference in density between the air and the bronze, the coupling is poor and the air load damping is small. time. So we expect another performance of the speaker unit:

Self-damping

We also hope that the voice coil can stop the diaphragm in time and prevent them from emitting their own tone. Unfortunately, most rigid materials (such as metals) have little self-damping, causing them to vibrate for a long time. One way to control this problem is to extend the heavy rubber fold ring under the cone, and pay great attention to the damping behavior of the centering support and the fold ring material.

However, the current situation is that even the best KEVLAR, carbon fiber or aluminum diaphragms have a high Q peak at least in the upper section of the working area, which must be corrected with a frequency divider or filter. Unfortunately, this peak generally falls between 3 and 5 kHz, which is precisely the frequency that the human ear is most sensitive to.

Self-damping can eliminate staining and obtain a relaxing, natural, and fatigue-free sound. Many audiophiles and even some commentators are completely unaware of the special sound of cell material resonance, but blame the sensitivity of the amplifier or room.

The 2-way speakers recommended by some magazines use 7 ”KEVLAR and metal dome tweeters. From a technical point of view, the speakers move uniformly in the unit ’s respective working area, but in fact to eliminate the energy of the KEVLAR 3 ~ 5 kHz divided vibration area, It is difficult for the crossover.

The review article about this type of 2-way speaker is introduced in a large amount of space, and through trial and error to select an amplifier that can fully exert the quality of the speaker. In fact, the audio commentator was forced to choose an amplifier with very low distortion at the frequency where the KEVLAR unit split vibration occurred. Because most audiophiles and commentators are very unfamiliar with the direct sound of the speaker unit, they cannot evaluate how many "KEVLAR sounds" or "aluminum film sounds" are retained in the last designed system.

There is also a problem that plagues all 2-way KEVLAR, metal and carbon fiber speakers. At the current state of the art, the 6.5 "or 7" unit has to be played to the edge of its working range in order to interface with the tweeter at frequencies where the distortion is not too large.

If you lower the crossover point, the intermodulation distortion of the tweeter will increase sharply, causing high-frequency degradation when listening at medium and high volumes. If you raise the crossover point, KEVLAR's split vibration occurs again, resulting in the sound rushing forward at a lower volume, and completely broken when the sound pressure is high.

This makes the designer face difficult choices: either coarse sound in the entire treble range; or the typical KEVLAR forwardness, and may bring wild sound to the speaker system. The best way now is to use the 4th order (24dB / Oct) frequency divider to correct KEVLAR resonance.

By the way, I really like KEVLAR and carbon fiber units. But they are difficult to deal with, and they must be controlled by acoustic and electrical methods to control their strong resonance.

As mentioned earlier, rigid cones have some advantages, but damping is very difficult. Another way is to use high-loss materials, traditionally plastic-coated paper cones, but in modern speakers they are gradually replaced by polypropylene. This type of cone can be damped by itself, and the pulse from the voice coil gradually loses energy as it diffuses on the diaphragm surface. Therefore, the requirements for centering pieces and folding rings are not very strict.

This type of material has a fairly flat frequency response during testing, allowing the use of a simple 6dB / Oct divider. I am not very interested in most polypropylene units. They sound a little fuzzy at low and medium volume. Although B & K intermodulation distortion analyzers are not used, I speculate that they have considerable intermodulation distortion due to their softness. In addition, it is extremely difficult to manufacture a material with perfect linear mechanical attenuation. In fact, distortion is always inevitably accompanied by attenuation.

I think that all similar phenomena also appear on the soft dome tweeter; the cone actually divides the vibration across the entire frequency band, and the instrument cannot detect it because it is covered by strong damping, but the human ear can tell. In order to overcome this subjective effect, the best units (Dynaudio, Scan-Speak, Vifa, Seas, Audax, Morel) are made of composite materials. Adding silica, mica or metal powder to the plastic can significantly improve Stiffness can maintain the soft sound characteristics of polypropylene.

Cavity resonance

Although the dust cap of the mid-bass unit or the dome of the tweeter is harmless from the surface, the space between the dust cap and the pole piece of the magnet forms a small resonance cavity. One of the typical examples in this regard is the KEF B110 Bextrene mid-woofer developed in the early 1970s (used in BBC LS3 / 5a).

This unit may be the earliest commercial high-quality midrange unit, but it also has many problems, such as low efficiency and insufficient power endurance, a response peak that is as wide as one octave centered on 1.5kHz (by Frequency divider correction), 3 high Q peaks centered at 4.5kHz (the 3rd order frequency divider designed by BBC can only attenuate it slightly). Acoustic commentators erroneously attributed these peaks to the tweeters. They have a strong directivity and should be caused by dust cap resonance.

Some tweeters popular in the 1970s, including Audax and Peerless 1 "soft domes, also have similar formants between 9 and 16 kHz, which can be partially damped by filling felt pads between the dome and the pole piece. The internal friction of the soft dome is much stronger than the dust cap of the B110, so the resonance is much wider, and the amplitude is only 1 ~ 3dB, but it still exists. Sensitive listeners will perceive the fatigue characteristics .

It is not difficult to imagine that the problems of phenolic plastic, fiberglass and hard paper dome used in the big-seller speakers were very bad. (Hey, does anyone remember BIC Venturis? Cerwin-Vega? RecTIlinear? JBL L100? I have sold these horrible products when I was young, waiting for customers to use them to listen to Pink Floyd's "The Dark Side of the Moon". )

Back to now, high-quality mid-bass and tweeters use two methods to avoid this problem: Nordic manufacturers Dynaudio, Scan-Speak, Vifa and seas use open pole block assemblies; French manufacturers Audax and Focal use bullet-type pole block expansion , Completely replaced the dust cap.

The most well-known products that use an open pole piece to damp the back wave of the dome in the transmission line include: Dynaudio Esotec D-260, Esotec T-330D, Scan-Speak D2905 / 9000 tweeter. Their use in Sonus Faber's Extrema and ProAc (Noble) Response 3 speakers proves that this technology is very successful.

In contrast, Focal T120 and T120K use rigid glass fiber or KEVLAR concave dome on the undamped cavity. The high-frequency end of the working range presents a series of high-Q peaks. This is caused by the resonance cavity and the rigid ball. The first split vibration of the top is generated by coupling with each other. I am puzzled by the general praise that these units received when they started to supply. I do n’t like the sound of them, and there is nothing special about the test data.

However, from all aspects, the new Focal titanium dome T120TI and titanium oxide dome T120TI-O2 are very good, and I recently felt good when listening to the speakers using this unit.

Magnetic field nonlinearity

Most audiophiles know that the speaker unit is an inductive load, and the voice coil is just wound around the ferromagnetic pole piece. But not many people understand the many problems that arise.

If the inductance value is kept constant, like the air core inductance, there will be no problem. Just adjust the divider with the RC network. Unfortunately, it is an iron core inductor, and the inductance value also changes with the position of the voice coil.

The changing inductance value has serious consequences, because the inductance value is an important factor that determines the frequency drop of the upper end of the unit and the acoustic delay. When the inductance value is changed, the frequency drop and acoustic delay also change accordingly. It occurs whenever the unit moves to a considerable proportion of the linear stroke of the voice coil. Take the excellent 8 "unit Vifa P21W0-12-08 as an example, the linear stroke is only 8mm (+ -4mm). The linear stroke of most 8" units is generally 6mm, and the midrange unit is generally 1 ~ 3mm.

Playing some ultra-low frequencies will show the effect of inductive modulation, that is, intermodulation and FM distortion across the entire spectrum. This is a big problem for 2-way and 3-way systems with lower mid-frequency division. In other words, whenever you see the movement of the unit, a lot of intermodulation and FM distortion have appeared. How does this sound feel? You will find that there is a loss of low-frequency resolution, but this may be covered by the problems with the amplifier (such as output transformer saturation, insufficient power supply).

What is the solution? Scan-Speak's SD system and Dynaudio's DTL system use copper-clad pole pieces to short-circuit the eddy currents induced by the voice coil. Careful analysis of voice coil inductance parameters can reveal this secret.

Scan-Speak 21W / 8555, one of the best 8 "units in the world, has an inductance value of 0.1mH, which is much lower than the 0.9mH of Vifa P21W0-20-08. Both units are excellent, but if you want to Simultaneously emit mid-range and low-frequency, Scan-Speak can of course give a more transparent sound.

The inductance value has another meaning. The high-end frequency drop of the unit is determined by the self-inductance of the voice coil and the mechanical drop. If you use the voice coil inductance value and DC resistance to calculate the drop frequency, the value is often much higher than the measured acoustic drop in some units. For other units, the calculated value is lower than the measured value. The reason is that the self-inductance of the voice coil covers the peak of the mechanical system. This is not a good phenomenon. Any change in the mechanical or electrical system will strongly affect the frequency response and transient response.

By the way, the same problem also appeared on the old moving magnet cartridge. Not surprisingly, such cartridges are much less transparent than advanced dynamic cartridges.

The following will explain how audiophiles find their favorite speakers, draw their own conclusions, and even guess the audio preferences of manufacturers, commentators, and your friends.

Type of unit

Familiarity with the basic characteristics of the unit is very helpful for listening and comparison. You can determine whether it is a good thing in the same unit. By carefully listening to and studying all relevant parameters, you can find out how well designers are doing in solving problems.

1 Paper cone unit

It first appeared in the patent application documents of Rice and Kolog in the late 1920s. The quality of the paper is quite different. The worst can be found in cheap radios. Excellent ones are Scan-Speak 5 "midrange for Thiel speakers and SEAS 6.5" midrange for Wilson WATT. This ancient material is actually a composite structure, and its properties will change significantly when a suitable plastic coating is used (the choice of coating is a trade secret of cell manufacturers). Because the characteristics of paper change significantly with humidity and time, the coating is indispensable, both stabilizing the material and improving self-damping.

advantage:

Good or even excellent self-damping, excellent resolution and detail, flat response, and gradually began to split the vibration. Relatively easy to cooperate with low-order linear phase frequency divider. The sound of the paper diaphragm is better than predicted by its measurement data.

Disadvantages:

The stiffness is not as good as KEVLAR, carbon fiber and metal film, so it lacks electrostatic details. The sound pressure level is not as good as other materials.

The consistency of paper is not as good as synthetic substances, so the pairing is not very accurate, which may affect the image forming force, of course, depending on the accuracy and quality of production. Even after being coated, the performance may change over time.

High-quality products:

Scan-Speak 8640 5 "cone / dome midrange, linear response up to 13kHz, low distortion, excellent impulse response, and rich details.

SEAS 6.5 "mid-bass (used in Wilson's WATT, but may have been improved).

Audax PR170M0 6.5 "high efficiency (100dB / m) midrange.

It is said that the paper cones and folding rings produced by Kutmüller are of the highest quality and are used by Scan-Speak, SEAS, Vifa and other manufacturers.

2 BEXTENE cone

This is a plastic synthesized from wood pulp, and it is always necessary to use a coated damping material to control its first resonance at 15kHz. It was first developed by the BBC in 1967 as a material with better consistency and predictability to replace paper to suit monitoring applications. It was widely used in the early 1970s. The typical fever speaker at that time was often an 8 "KEF or Audax BEXTENE mid-bass with Audax 1" soft dome tweeter.

The design from the BBC always uses equalization to keep the BEXTENE unit flat in the mid-band. The most famous unit is probably the KEF B110 for the LS3 / 5a monitor box.

Now BEXTENE has been replaced by polypropylene first developed by the BBC. The frequency response of the polypropylene unit is flatter, no coating is needed, and the efficiency is improved by 3 ~ 4dB due to the reduction in quality. BEXTENE has withdrawn from the stage of history.

advantage:

Good structure, better resolution than most paper cones.

Disadvantages: very low efficiency (85dB / m), strong staining, sudden split vibration at not too large sound pressure.

High-quality products:

Que Ru.

3 Soft dome treble

After the appearance of Peerless 1 "soft dome in the early 1970s, it gradually became popular. Subsequently, the Audax 1" treble was adopted by many designers in Britain and the United States in the 1970s and early 1980s.

When titanium, aluminum domes and Focal glass fiber concave domes appeared in the mid-1980s, these designs fell out of favor, and Audax soft dome units were pushed out of the fever-level market.

In the past few years, the soft dome tweeters represented by Dynaudio and Scan-Speak have regained their enthusiasm. They use new dome molding, new coating materials and new designs, and their performance is comparable to any metal dome. The resolution and details of the sound are comparable to the best metal dome, but there is no typical 22kHz ~ 27kHz resonance of the metal dome.

advantage:

Inherent self-damping and extremely flat response, first-class impulse response. Natural, open, fatigue-free sound, this is undoubtedly the most valuable quality when listening to digital recordings.

Disadvantages:

The sound of the old-fashioned soft dome is dull. The power endurance is quite limited, and an 18dB / Oct crossover is needed to reduce intermodulation distortion. Compared with metal domes, high frequency divergence is worse.

Except for divergence, the latest design has no other defects.

High-quality products:

Dynaudio Esotec D-260, Esotec T-330D, Scan-Speak D2905 / 9500.

4 Soft dome midrange unit

After listening to AR-3, AR LST, ADS, Audax 2 ″, Dynaudio D-52 soft dome midrange, I found that they made a mess of the speaker system. The measurement was very flat, but the sound was opaque and seriously stained, making People fatigue.

One of the problems is that the linear displacement of the soft dome midrange unit is very limited (generally 1 ~ 2 mm), resulting in a limited bandwidth, and even 500Hz frequency division can not be well matched, but only between 800 ~ 3200 Hz The range works best.

The second problem is that they are prone to lateral deflection, because there is no centering piece to assist the folding ring to maintain linear back and forth movement.

The third problem is that the stiffness of the silk film dome is insufficient to complete the strong power conversion task in the mid-band.

A new generation of cone-ball domes, such as the 5 "Scan-Speak 13M / 8636, 13M / 8640, Dynaudio 15W-75 are completely different. These three units are actually high-precision cones, not midrange Domes. The only similarity between them and soft domes is that they have a large dust cap that can also play the role of dome at high frequencies.

They obviously have a larger stroke, lower distortion, and a much wider frequency response. Such units can obtain real and transparent sound. Because they use KEVLAR, paper and polypropylene, they will be described in detail below.

Another special case is the professional-grade ATC 3 "dome (with a horn). It uses double centering pieces to significantly reduce intermodulation distortion. Its performance is the best, but it is very expensive (about 300 US dollars a ), And need to be manually selected to match the resonant frequency of the left and right channels.

advantage;

no. Maybe metal dome midrange still has potential, but they have very strict requirements for crossovers. Note: ATC unit and cone-ball dome are not listed here.

Disadvantages:

The distortion is large, the sound is tiring, the crossover frequency is high, and the frequency band and power endurance are limited. Only laser holography can detect their defects.

High-quality products:

The ATC 3 "professional series is very different from the general soft dome, but the price is about 4 times more expensive. Some people think that Dynaudio's D54 is the best midrange.

5 Polypropylene unit

In 1976, the BBC developed this material to replace BEXTRENE. Because it has strong self-damping, a properly designed polypropylene unit can obtain a flat response in the work area without any balancing. In addition, its efficiency generally reaches 88 ~ 91dB, which is also a big improvement.

Polypropylene has become a universal material in the world because it requires the least manual handling when assembling the loudspeaker-the only difficulty is finding the right adhesive. This problem was solved in the early 1980s.

Nowadays, polypropylene units are used for all kinds of loudspeakers ranging from cheap combination speakers to the first-class ProAc Response 3 and Hales System 2 signature versions. The final quality of such units depends mainly on the shape of the cone and the additive materials in the polypropylene formulation.

advantage:

If the design is correct, you can get a flat response, very low acoustic staining, good impulse response, the frequency divider can be simple, high efficiency, and the division vibration appears slowly. High-quality products can achieve transparency comparable to the best paper cones.

Disadvantages:

The transparency standard set by the rigid cone unit and the electrostatic unit has not yet been reached. Due to the difference in resolution, many polypropylene midrange bass cannot match well with the popular metal dome tweeter. Not suitable for 10 inch or larger woofer, carbon fiber should be more competent in this regard.

High-quality products:

Scan-Speak 18W / 8543 7 ″ mid-bass (for ProAc Response 3), may be the best polypropylene unit in the world.

Dynaudio 17W-75 EXT 7 ″ mid-bass (for the signature version of Hales System 2).

The Vifa P13WH-00-08 5.5 "unit is another winner, especially suitable for midrange or mini monitor boxes. Its extremely flat midrange and smooth second-order drop are unique.

6 Metal dome tweeter

In the mid-1980s, Germany's progress in metallurgical technology (ELAC and MB) led to the birth of thin titanium and aluminum domes. Now there are many manufacturers in Germany, Norway and France that can supply such units. Their sound can be very transparent, and if designed properly, their performance is comparable to electrostatic speakers.

The disadvantage is the lack of self-damping, but the performance of the aluminum film in the ultrasonic frequency band is slightly better than that of the titanium film. At this stage, all metal dome tweeters have significant peaks in the ultrasonic section, with amplitudes ranging from 3dB (excellent) to 12dB (general).

However, the impact of these peaks does not seem to be large, because "sophisticated" SONY / PHILIPS has already ensured in the CD Red Book standard that CD records will never contain any music information above 20kHz. Maybe when the super CD with the concept of HI-FI is commercialized, we can obtain a recording with a frequency limit of at least 32 kHz and a resolution of 20 to 24 bits.

advantage:

Uniform piston movement and proper design can produce extremely high-resolution and transparent sound. The divergence is very good, because the radius of curvature of the metal dome is larger than that of the soft dome.

Disadvantages:

"Metal" staining may occur due to the intermodulation of UHF peaks and sound in the audible band. Some early products have limited power endurance. When there is a strong overload, significant split vibration distortion occurs across the entire frequency band.

High-quality products:

Vifa D25AG-35-06 1 ″ aluminum dome, with open pole piece, strong power endurance, even if the phaser is removed, the peak value of UHF is only 3dB. Focal's new T122TI-O2 is also very good.

7 Rigid unit

Aluminum cone. The first rigid units for HI-FI speakers are Jordan Watts 4 "aluminum cones. They are hand-produced. They are of high price, low efficiency, and cannot be popularized. They are almost invisible in the US market. Now some British speakers Using 5 "and 7" aluminum basin mid-bass unit, its sensitivity is very low, but also need to be corrected by the frequency divider.

Foam basin. Another type of foam woofer is represented by KEF B139, but its efficiency and power endurance are very low, and the intermediate frequency has serious high Q resonance. The peak value of B139 at 1100Hz is 12dB. They were commonly used in the 3-way and 4-way transmission line speakers in the 1970s.

carbon fiber. The next generation is carbon fiber developed by the Japanese. The earliest appeared is the professional studio monitor box 12 "TAD, which is very efficient and expensive (one in 1980 was about 300 US dollars). Now, the price of carbon fiber has been reduced, Vifa Both Audax and Audax have very good products of this type. Of course, the production in Japan is much larger.

The carbon fiber unit has true piston movement, and the response at low and mid-low frequencies is outstanding, but the split vibration at the upper end of the frequency is annoying and must be corrected by a complex frequency divider.

Although I do n’t like units that require complex filters, I have to admit that Vifa 8 "and 10" carbon fiber units are the only direct radiators that really make me feel low frequencies.

KEVLAR. The KEVLAR unit appeared in the product lines of Focal in France and Eton in Germany in the mid-1980s. The unit of Eton has excellent damping characteristics due to the addition of a high-loss honeycomb structure between the two layers of KEVLAR. Eton and the updated Scan-Speak KEVLAR unit share the reputation of the world's most outstanding high-tech speaker unit.

On the new Scan-Speak KEVLAR unit, a unique and desirable feature, smooth landing, can be found. All other KEVLAR units have chaotic split vibrations. Scan-Speak is the only one that is well controlled, so it has a significant improvement in smoothness and transparency.

Compound pot. Audax re-enters the High-End market with a special composite technology HD-A. It is made by mixing granular carbon fiber and KEVLAR fiber in a certain proportion in acrylic colloid. The factory test results show that it combines good piston movement with the lowest high-frequency peak and smooth high-frequency drop.

Recently, Russian scientists have realized a low-cost diamond vapor coating that can be used on computer disks. Hope that Scan-Speak and other manufacturers can quickly adopt this technology.

Overall advantages and disadvantages of rigid elements

Advantages: The best transparency, imaging power and sound field reproducibility, if carefully designed, can reach or exceed the level of electrostatic speakers. High efficiency, large sound pressure level, and low intermodulation distortion. This type of unit is considered by many designers to be the most advanced, and as material technology advances, there may be rapid progress.

Disadvantages: The old design has severe peaks at the upper end of the operating frequency band, and almost all units have uncontrollable split vibration regions above the high-frequency peaks. This will cause fatigue for long-time listening and a sense of compression of sound field perspective.

Because the high-frequency peaks of KEVLAR and carbon fiber cannot be corrected with ordinary low-pass filters, speakers using these units must properly design appropriate frequency dividers.

Although they can have large sound pressure levels, split vibrations often occur suddenly, very similar to amplifier clipping. Some KEVLAR and carbon fiber units require a very long "burner" time (more than 100 hours) in order to soften the fibers in the cone; this is a disadvantage, indicating that the mechanical properties of the material are unstable.

High-quality products: Scan-Speak 13M / 8636 5 "midrange, 18W / 8544 7" mid-bass, 21W / 8554 8 "bass. Only the KEVLAR units show a good drop zone above the high-frequency peak. These units are in The pinnacle of rigid cell technology.

The Audax HD-A series' HM130Z0 5.25 "midrange, HM170G4 6.5" midrange and HM210Z0 8 "midrange are all good.

German Eton is also worthy of close attention. The manufacturer has been devoted to the process research of improving self-damping characteristics while maintaining the rigidity of the cone.

Evaluation Unit Method

My method of selecting speakers seems to be somewhat primitive. I put the unit on the IEC baffle for trial listening. No crossover or cabinet is required. Listen to pink noise to assess the severity of the peaks that appear in the sine wave and FFT waterfall measurements, and listen to music to feel how much potential the unit has to analyze. This does require your ears to be well trained. This listening process can make you realize how complicated the crossover needs to be.

Then, I will carefully analyze the results of the MLSSA computer test system (using the same IEC baffle) and examine the following: impulse response; group delay relative to frequency response; cumulative attenuation spectrum waterfall; degree.

Listening and testing can be said to be equally important, and both can only reveal part of the true characteristics of the unit. Even today's best audiophile system may be found to be seriously flawed after 5 years. Through testing, you can find the problem of sound staining, and it is precisely that today's audio equipment cannot be exposed. The MLSSA system can help you solve these problems.

A well-thought-out designer should treat his work as well as a famous artisan, even if he treats hidden parts that are not visible from the outside, he should not hesitate to devote all his attention.

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