Ultimate Studio Home Theatre

Total Professional Concept

 

Making a loudspeaker boxes

When designing audio systems of class Ultimate Studio, it is necessary to have not so ordinary approach to acoustic treatment of the room and power supply but even more than that, participating directly in process of building that objects. Special design of speaker cabinets for lower tones and mid-woofers is the last special requirement necessary to ensure the highest quality audio reproduction. Let's say you bought TAD speakers or some other professional drivers. How to make their casings that won’t spoil the sound of these speakers?

Full audio Ultimate Studio has one important advantage: their casings do not have restrictions on weight and size, and the correct selection of the number of drivers and split signal points between them in the system, allows more rational approach to this problem. Here, I will not tell you anything particularly interesting, and in some respects I’ll be superficial, because i do not see any reason in repeating the same information that you can easily find not only on the Internet but also in the old acoustic books. Important parts I will specially emphasize.

Tapper form

The tapper form begins to influence the sound only at frequencies above 400-600 Hz. In other words, the fundamental importance it has only at the mid-frequency range. In the transition from rectangular case with a lot of corners to smoother forms it is noticeable improvement in sound reproduction quality. The best tapper form is a ball shape. It does not have any sort of angles and parallel surfaces on which diffraction occurs, or standing waves. But such an exotic design is justified only within home acoustics, where for the midrange frequencies are used ordinary midrange cone drivers. I think that you made a conclusion that in all systems of the Acoustics Ultimate Studio class compression or ribbon midrange driver are applied. They work from 600-1000 Hz and do not require any casing but just acoustic horn. This is also characteristic of all professional speakers, where the range of medium frequencies is 600-8000 Hz and they are also compression drivers. Therefore, the tappers are needed only for subwoofer and mid-woofers, which can only partially cover the range of frequencies above 400-600 Hz, which is not so bad.

To reduce the diffraction of sound wave inside the speaker design, is usually used the construction with a narrow front panel (as permitted by the size of bass drivers). It also increases its resonant frequency and improves the directional characteristics of the speakers by minimizing reflection from the front panel. This approach can also be seen in almost all home acoustic systems that are usually elongated shape. To prevent the formation of standing waves resonant box must have ratio of width, height and depth in the proportions 0, 62:1:1,62.

Materials

The materials for the manufacturing of casings have to apply two basic requirements:

1) To have a maximum loss of mechanical energy in their internal structure when exposed to strains.

2) They must be solid enough to withstand the dynamic driver and not fall apart during the usage.

Vibroabsorbing material properties are characterized by the logarithmic decrement of damping loss factor d or coefficient of loss n. The more these values are, the greater is the amount of vibration absorbed in its structure. There are not many materials used for the production of casings. It’s almost always wood processing products and very rare metals. For example, the damping rate of some materials are: 0,02 pine, beech 0,03, plywood 0,04, MDF 0,08, steel 0,001, aluminum 0,003. Not hard to guess that the materials of wood have a greater preference for these purposes, especially when you consider easy processing and low cost. Although, the pages of glossy audiophile magazines, almost two decades, impose the belief that aluminum is almost a reference material for the manufacture of casings. That suggested opinion is simply silly and has no stand. Its use very rarely goes beyond the sets of multi-channel acoustics of round, egg-shaped design of middle level quality.

Only subwoofer during the work spread vibrations that are transmitted to the body by direct mechanical contact of membrane diffuser and acoustic wave from the back of the speaker. Moreover, then walls become a source of spurious overtones that at some frequencies may even exceed the direct acoustic radiation loudspeaker. The most effective way to reduce vibration is to increase the thickness of the casing. Thus, the resonant frequencies of panels are displaced outside the operating band drivers back into the higher frequency region. Usually increase in wall thickness of more than 2-3 cm does not significantly affect the reduction of vibration, which is typical for most consumer speakers. But this value is highly dependent on the size and power of low-frequency driver. In any case, we should not save on anything. Therefore it is necessary to make the casing wall as thick as possible. For the low-power 15-18" speakers the panel shouldn’t be thinner than 4-6 cm. If you are using dual-driver of 18 or 21" or more, in the high power supplied signal, the greater thickness of the walls is needed. It is usually difficult to find wood panels with thickness greater than 6 cm, so it is necessary to provide the required thickness by twisting two or more panels of material with the screws and glue.

Not hard to guess that the maximum internal audio losses has wood materials, precisely MDF and HDF (Mid Density Fiberboard i High Density Fiberboard). These are pressed remains of wood and glue. Nowadays all professional acoustic casings are made of that material. The use the HDF as a material is still preferable because of higher density, hardness and weight of this brand hardboard. If you have to choose between MDF and HDF is better to choose HDF- High Density Fiberboard (hardboard high density) because of the slightly higher density, strength and weight. Strength can be increased by installation of interior panels. Their application is for the redistribution of vibration between the panels and the overall strengthening of the whole structure. They shall be placed between two parallel walls with a maximum surface area. This is usually the front, back and side walls. If the faceplate is isolated and vibration isolated from the rest of the body with cushioning pad, it should be free from struts. Noticeable result gives at least two or three inside panels.

Absorption of Vibrations

I repeat once again that using even the thickest and most dense fiberboard panels (HDF and MDF), can only shift the resonances of the audio casings in the higher frequency region. The very vibration will remain. For her physical repression of the vibrations on the internal wall of the casings is necessary to put a thick layer of vibration-absorbing material. It is usually made of bitumen. That material has a liquid form like mastics, its solid and leaf shaped, it is more convenient for work and allows appliance to the surface of material of several millimeters. Usually such leafs are often used in auto industry for dampfing of car shelves. The second category of such materials also includes industrial elastomeric (acoustic) plates, which are made of complex polymer compositions. They ensure the removal of noise and vibration by converting mechanical energy into heat. Typically, these plates are used in construction of building foundations for vibration isolation of industrial machines. Loss factor in bitumen pitch and plates varies from n = 0,2 to 0,4. The same quantity of elastomeric plates reaches 0.7 at a thickness of a sheet of 2 cm to obtain tangible results must be applied to the wall a layer of minimum thickness 0,5-1 cm . Maximum performance requires applying a layer of material with a thickness equal to or greater than half the thickness of the wall of acoustic boxes. It follows that the elastomeric panels are optimal solution in terms of features, and in terms of ease of use. Using them the insulating layer of acoustic box increases for a few centimeters. For an excellent result, it takes 2-3 layers of plates.

The O-ring thin elastomeric plate is needed to isolate acoustic driver to prevent the transmission of vibrations from membrane holders to the front panel of the acoustic box. Attaching of the acoustic driver could be done with using screws with the elastomeric undercoat. For a complete prevention of vibration transmission from the front wall to the rest of the casing walls, it could be done by implementing the elastomeric layers between the front wall and the other walls of the acoustic casings by using screws with elastomeric underlay, the same way as attaching the acoustic drivers. Moreover, their thread should not touch the front panel, which is achieved by drilling holes of larger diameter. You can cover the outer side of the acoustic box or even the whole casings with the same pitch. You can also do that with the membrane holder that will decrease its vibrations. The only exception is magnet that has to keep the good cooling effect because the overheating of the coil.

Sound insulation

Even such a powerful design is not able to completely suppress the external vibration of the casings of audio device. But it can be almost completely isolated by applying double-wall construction casings. Moreover, by the above rules the basic acoustic box was made then one additional external box without the front panel with a wall thickness of 2-4 cm. It should be few centimeters larger than the basic because the basic one should enter into additional. Boxes will be next to one another with its lower surfaces of the box and will be under the influence of gravity field. It will take only a few pieces of elastomeric material for the mechanical isolation of the two surfaces. It is not necessary to cover the entire surface, because large area of contact will ensure the transfer of more vibration. Free capacity of such space should be filled with not very dense material that absorbs sound well. The leaking of the sound waves from the main casings through a slit in the front panels of both boxes could be prevented by sealing the cracks with the elastomeric tapes. For these purposes, you should not use the stub of fiberboard, which will ensure the transfer of vibration between the acoustic boxes. Only such a contactless structure is able to provide the highest quality sound insulation.

Sound absorption

Internal resonances and standing waves of body should be suppressed by additional quenching materials that absorb sound. That material is practically ordinary building material, which perfectly does its work. But this is not worth because it can decay. At the slightest vibration from its surface will fly fine particles of material that could fall into the gap of coil thread, which can cause it to overheat and malfunction. They will also fly at high speed from the phase inverter tube and enter the lungs of humans. In that purpose only specially designed materials should be used that consist of a special synthetic fibers in combination with glue which allows them not to decay even under high vibration, and be completely safe for human health.

The standard way of filling the greater part of the low-frequency acoustic box capacity with the materials like wool isn’t correct. This way is useful only for mid-woofers or cone midrange drivers. That way of filling of the subwoofer leads to the nonlinearity of the interior of the acoustic box and the vibrations of the sound absorbers at high volume. Generally, such materials do not absorb the low frequencies. Therefore it is necessary to use special absorbers, which are of the same materials, but very high density. It can be felt or thick acoustic panels made of fiberglass, which can often be found on the walls of the movie theater. The layer of those materials should be thick of 5-10 cm.

The ideal acoustic box design:

Most of the ready made acoustic boxes looks like this:

There are usually vibro absorbing layers under the acoustic driver and the inner capacity of acoustic box is unequally filled with numerous soft absorbers or a thin sheet layer of beige color. Double boxes can be present but there is no problem to present the transition of vibration to all surfaces trough all inner panels at the corners of the box.

Mid-woofer drivers

The casings of the mid-woofer should in fact, look the same, but with the thinner walls, less capacity and the other type of material that covers the inner side of the box. Mid-woofer speakers typically operate at frequencies from 100-150 Hz. There are no practically low-frequency vibrations that allow reducing the thickness of the walls up to 4 cm. The capacity of the box has no influence on sound because membrane pace of those drivers is pretty small. Usually, the internal dimensions of the box are determined on the dimensions of most drivers. Size of the sides of casings should only slightly exceed the size of the speaker. The problems arise already at the upper frequency range of 600-1000 Hz, because at those frequencies quite strongly affects the form of acoustic box and the number of inside material. Even in case when spherical acoustic box is filled with cotton wool, the result will be redistribution and sound absorption that comes from the back side of the sound driver. But if you do not forget about the fact that Ultimate Studio acoustics has no weight and size restrictions, you can do something very simple and most effective: make a very large box and completely fill it with material. Is there something wrong with the box that has capacity of one cubic meter? This will allow you to bypass a laborious process of making spherical structures, but also completely suppress the sound emerging at the back of membrane by a large amount of damped space. In this case, soft materials are needed that absorb sound well in this frequency range. Possible vibration of the material round the acoustic driver you will prevent by rearranging the material on the magnitude of its size.

Acoustic design

An open question on acoustic design of low-frequency drivers still remains. There are many programs that on your parameters of the sound driver can determine not only the size of the box and the bass-reflex, but even construct a rough timetable for future amplitude-frequency response for the subwoofer. Often those programs automatically choose the size of casings that fails to provide maximum efficiency at low frequencies. It needs a simulation model of amplitude-frequency response witch with gradually increase of the volume of the shell will not get the significant increase of sound pressure at low frequencies. Yet we must understand that this is only a calculation that allows exact determination of the inner capacity of the acoustic system and only rough calculation of the size of bass-reflex. After construction of the shell and setting the bass-reflex, it is necessary to check the resulting frequency response settings according to schedule speakers, and be prepared to rework it to find the optimal result. For these measurements you only need fine cheap professional sound card for your computer, microphone and any instrumentation software. We can say nothing more about acoustic box design, because it goes beyond the resource of this site and all the additional information on this issue you can find on the internet, in magazines and books.

A quality audio casing is a complex and very hard for handmade production. It seems that you can avoid this by buying readymade Ultimate Studio acoustic system, but in spite of the high-quality speakers, the body of these casings represents the usual solutions and designs. Therefore the question of making new, high quality casing, even for readymade acoustic systems does not lose its relevance, because it enables us to get even more clear sound with no resonance.

Articles

Basics of studio sound reproduction

Ultimate Studio audio systems

Add-on

Other