Toccata Grande, Part 1

By Lars Mytting
20 Jan 1998

Cabinet questions

Well, the situation looks bad -- together with our two speakers, forty 15" drivers have suddenly showed up in our listening room. The solution was to spend some hours in the basement together with test equipment, trying to find the nature of the resonances of different materials, and to find methods of vibration damping and the effect of decoupling the driver from the materials.

I mounted different drivers in different materials, with different bolts, with different bracing, decoupled them with rubber materials of different viscosity, ran them at high level with different frequencies, and a lot other obscure procedures. All the time I logged the sound output that was transmitted through the material. The proper tool for this is a accelerometer and equipment to record and process the data from it. I had only a oscilloscope and a SPL meter, but I got a good idea of the situation anyway. However, without the proper equipment, it is of no use to quantify the differences I found. Instead, you have to accept this list with a summary of the discoveries I found most vital:

• All materials have one or more frequencies where they enter resonance and start vibrating. The intensity, decay time and frequency range of the vibration depends on size, weight, density and stiffness.
  
• The higher mass of the material, the higher SPL is needed to excite vibration. A extremely heavy and rigid cabinet is universally the best cabinet.
  
• MDF and chipboard resonate at averagely 150-400 Hz, with the strongest resonances usually at 250-300 Hz. Nearly all speakers suffers heavily from cabinet distortion in this frequency range.
  
• MDF have shorter decay time than chipboard, and the vibration peak is more damped. Still, MDF have substantial vibration, and is far from the optimum cabinet material. You can easily find the resonance frequencies of a speaker cabinet with a sinus generator(7) (or test-CD) and a amplifier. Play a sinus tone at loud level and change the frequency slowly. Hold your hand on the cabinet side to find the frequencies where it vibrates. When you hit the resonance frequency, the sound will also change, and you will hear the distortion generated by the cabinet. Maybe you recognise some irritating sounds that you thought belonged to a weak part of the system! You can also check the resonance properties of a given material by mounting a driver to it and run the previous procedure.
  
• The most effective means of reducing vibration decay time is to brace the cabinet. A single-piece cabinet, like a concrete tube, (see http://www.speakerbuilding.com/content/1019/) will usually have less vibration than a cabinet that is assembled of separate parts.
  
• Vibration is easily transmitted to other materials by mechanical coupling.
  
• Using materials of different resonance frequencies may reduce some of the output of the vibrations. The layers should have as little mechanical coupling between each other as possible, or the cavity should contain a material whose damping properties corresponds to the resonance frequency of the inner wall.
  
• Sand is a good agent of dissipating vibrations of a wide frequency range into heat.
  
• Air will also dissipate vibrations, because no mechanical coupling is the best decoupling. This can be done by making a second box outside the cabinet with as little mechanical coupling to the inner cabinet as possible. The outer wall must have properties so that it does not "let out" the sound from the inner wall. This is a exciting method that may be investigated further.
  
• Decoupling the drivers with elastic materials like thick gaskets or rubber strings is difficult. I had great hopes for this method, and a interesting idea would be to decouple midranges that are crossed below the resonance frequency of the cabinet. But practical tests showed that the decoupling had to be very soft to work. With soft coupling, the driver would wobble and partly cancel its own output. With stiff material the vibrations would be coupled, achieving nothing. A 5" midrange with a 10 gram cone generates at 104 dB SPL a oscillating force of 44 Newton(8) -- equal to 4,5 kg. The weight of the driver and the decoupling must withstand these forces, or the driver will wobble. A 25 mm tweeter at 106 dB will have a acceleration of 7000 g (6 g is regarded lethal for humans!), but assuming the dome weighs a typical 0,35 gram, it will generate just 24 Newton, and the weight ratio of the dome and the rest of the driver is so large that a softer decoupling can be used -- but the only effect would be to isolate the tweeter from vibrations in the cabinet. Another problem is that the frequencies where it is interesting to decouple drivers from the cabinet (200-500 Hz) often corresponds to the lower crossover frequency in 3-way systems!
  
• It is possible to get very low cabinet vibration by using separate cabinets with different resonance frequencies for bass and midrange/tweeter. The crossover frequency should be the median of the two resonance frequencies of the cabinets.
  
• A pure bass cabinet should be extremely stiff and rigid with high resonance frequency. The walls must not flex when excited with the large air pressure caused by the big air displacement in the bass region.
  
• The midrange cabinet should be "thick, lazy and dizzy", with a low resonance frequency. A solution may be to use two thin walls with a thick sand layer and no bracing. Flexing walls is far less critical than for bass drivers. Making the midrange a dipole may be another solution.
  
• Fastening the driver magnet to the cabinet has a very good effect on the sound quality of both woofers and midranges. The driver will stay very firmly in place this way, and yield a far more detailed midrange, and a tighter, harder bass. The best condition for a driver is that only the cone is to move -- and nothing, nothing, nothing else. Fastening drivers this way will do more for precise sound reproduction than any cable investment.
  
• Using bolts instead of screws to fasten drivers may give a slight reduction in cabinet vibration. I believe this is because the vibration from the cone is transmitted to a smaller area instead of directly into the wood.
  
• Make your woodwork strong. If a wall joint is not well fit, the forces inside the cabinet will attack it and excite a resonance. Use every chance you find to brace as many sides of the speaker to each other.
  
• Bracing should run parallel with the largest (longest) side of the cabinet -- so that the largest area is divided. A floor-standing cabinet should have a brace from floor to bottom; not a horizontal shelf. Then continue bracing with the next largest area, and so on.
  
• A common, but not optimum formula for sand damping is to use two walls of MDF of the same thickness, bolted and glued together over spacers that form a cavity where a layer of sand can be filled. This will couple vibrations from the inner wall to the outer, which has the same resonance frequency, and the potential of the sand is not fully utilised.
  
• The viscosity of the sand can be far better employed to absorb vibrations if the outer wall is very thin, and with as little contact with the inner wall as possible. The sand layer should be as thick as possible. The only job of the outer wall is to keep the sand from leaking out. There is no need to make it rigid, because if the inner wall is properly constructed, it will withstand all the internal air pressure. The outer wall can be veneered board, and you can therefore get a nice wood finish in one step.
  
• Tests showed that even a very modest layer of 9 mm sand towards a outer wall of 6 mm will dissipate heavy vibration. A 6 mm outer wall were more than enough to keep 9 mm sand safely in place in a big cabinet. Even better would be a 3 mm wall and a sand layer of 15-20 mm, but some evenly distributed fastening points is then necessary to prevent the outer wall from cracking up!

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