Audio qualia

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turntable plinth building

Where to start? Well, not in the garden shed/garage, that's for sure. The best place to start for a project like this is on the computer, looking at data which will help in deciding on a design, and the materials with which to build.

Let’s review the functions of a plinth:

 to support the turntable mechanicals

 to reduce vibrations from the turntable mechanicals

 to reduce the affect from aerial vibrations, such as loudspeaker sounds

 to reduce seismic events, such as footfall.


Let’s think about a simple plinth, a panel of wood. Why would this make a good plinth, and why does it make a bad one? The key to this conundrum is resonance. Let’s state what is meant by resonance. 

A resonance is a vibration, but one that occurs at a certain frequency, and usually, if not damped, with high amplitude (sometimes referred to as Q). The frequency of the resonance is dependent on the material, and its dimensions. That is, the material's Young's Modulus, density and Poisson Ratio, its breadth, width and depth. All contribute to a range of resonances, according the mode. The first mode or fundamental, as it is the lowest frequency, is when standing waves occur along the length and breadth of the panel, dividing each into two regions each. [The panel is thought of as having free edges]. This is usually referred to as F1,1. This resonance frequency is usually the highest in amplitude, too. There will be resonances at F1,2, F2,1, F2,2, etc.

Now, the resonances will only be a problem if they are excited by other vibrations at, or very near to the resonance frequency. The vibrations cannot be eliminated, only reduced in amplitude, or shifted to a different frequency by changing the material and/or its dimensions. The reduction in amplitude is effected by damping, the change in frequency by adding (or subtracting) mass i.e. changing dimensions of the panel.

Adding damping is difficult, and a bit hit and miss. The best way to have damping is to use a material that has intrinsic damping. This characteristic is a function of the material itself, and has to be measured; it's similar to density in this respect. And like density, it does not rely on dimensions of the test piece, although the test piece must be of sufficient mass to overcome any damping of the measuring device, and so it must be above the Heisenberg limit.

A list of materials and their measured damping factors is given here:

http://qualia.webs.com/dampingfactorvalues.htm

There are three other factors [apart from damping, they are stiffness, mass and critical frequency] which must be considered when choosing a material, and its dimensions. For a given panel, there are three main regions which are controlled by different properties of the material. The main regions are:

a) the stiffness controlled region, this occurs in the bass region, up to the fundamental resonance frequency

b) the mass controlled region, which occurs between the fundamental resonance and the critical frequency

c) the damping controlled region, which is at and above the critical frequency.

To tick all the boxes [stiffness, mass and damping] the material chosen must be quite stiff, but not too stiff, as this lowers the critical frequency, have sufficient mass, but not be too thick, which also lowers the critical frequency, and intrinsic damping, which should have a value above 0.1 and ideally approaching 0.4 or above.

The most important property here is damping, as many materials are stiff and will have sufficient mass, but only a few have the right amount of intrinsic damping. One can choose the material that has the above properties, and be able to be made into a plinth using a person's own skills and tools.

What is the critical frequency (Fc) ?

It is that frequency, which is between the mass and damping controlled regions, where the panel becomes transparent to vibrations around the Fc. For a chosen material, the critical frequency is only dependent on the panel thickness, the thicker the panel, the lower the critical frequency. Here are some values of common materials, for a 1mm thick panel, for a 10mm thick panel, divide by10:

  material            Fc, kHz

  acrylic                 30.8

  aluminium            12.9

  glass                  15.2

  lead                    55.0

  plywood             16.0

  steel                  12.7


So making the plinth thicker (to add mass) is going to lower the Fc, not a good idea, because above Fc, damping controls the vibrations, and if the plinth material has little intrinsic damping, the plinth is going to be lively! So a plinth made from plywood of say 100mm thickness will have an Fc of about 160 Hz.

However, all is not lost if damping is not applied.

Mass can be added, and according to Newton's 2nd law of motion:

force = mass x acceleration

If the mass is increased, and the force remains the same, then the accelerations (vibrations) will be reduced pro rata. But the total energy will remain the same, and if damping is not significant, the magnitude of the vibrations can increase over time, and become a problem. It is, perhaps, ironic that in some instances of poor plinth material choice, the record deck actually damps the plinth, rather than the other way around!

 

So the choices are:

choose a stiff material (although if it is too stiff, this will lower the Fc)

choose materials to have enough mass, but not be too thick

choose materials to have significant intrinsic damping

 

If appropriate materials are chosen, then a high mass plinth is not necessary; a low mass damped plinth will be better than a high mass undamped plinth because the resonance peaks will be much less significant, as their Q (their height compared to their width) is proportional to the reciprocal of the damping factor. So with high damping factors, resonance peaks will be broad and low; low damping (high Q) means narrow high resonance peaks, not a good thing in this case.

 

so choose a material which is stiff, and has mass, and intrinsic damping.These materials include:

 material             damping factor

bamboo block           0.200

chipboard                 0.228

oiled olive wood       0.273

panzerholz               0.6-0.9

aluminium composites

  with polyalkenes    0.240

  with mdf                 0.218

acrylic/mdf                0.315

polyester resin filled with

bentonite clay          0.618

 

The two stand out materials are panzerholz and polyester resin filled with bentonite clay. However, composites also look quite good, as does the bamboo block.

Another property of materials is the acoustic impedance, that is, how easily sound passes from one material to another. Nearly all the data available is concerned with supersonic frequencies of sound, typically 5 MHz, so not really useful in the audio range. However, it does suggest that getting sound from one material to another seems to be dependent on their respective acoustic impedances, easily calculated as:

 

  Z0 = p x c    where Z0 is the acoustic impedance,
                                                   p is rho, for density,
                                                   c is the speed of sound through the material [longitudinal wave velocity]
 
         Some acoustic impedances of common materials: Z0 in N·s/m³ /106
 
 
material         Z0   [MRayls]       Material            Zo [MRayls]          material            Zo {MRayls]
 
aluminium                  17            araldite                     4-13             all plastics                 1-4     
brass                         37            brick                          15                all wood                    1-4
copper                       42            concrete                    7-10             except cork               0.1
cast iron                    37            glass                         10-15          carbon fibre               30
lead                           25            granite                      27
magnesium               10             marble                      10
steel                         45             slate                         12
tin                             24
tungsten                  101
 
So to transfer sound from one material to another effectively may mean using materials with similar acoustic impedances, or, to hinder transfer, (as in isolation), choosing materials with very different acoustic impedances.
 
It seems as though a number of companies are using modern materials and constructions to advance plinth building. Of note are  Clearaudio who use an aluminium/panzerholz/aluminium sandwich construction. Another company is Artemis Labs who use a resinated bamboo/ebony/resinated bamboo sandwich to do the same job. 
It's good to see professional companies using modern thinking with materials for plinths.


 

 

new blog

Sorry for these problems

The powers that overlord this website have threatened to deactivate this site for being too popular, without them getting their pound of flesh. Over the last several weeks they have withdrawn the stats counter, so that I cannot monitor the bandwidth, which is now over their specified limit. They have now stopped any change to any pages except this side panel.

So I have had to set up a new forum so that I can continue to help the hifi community. The new forum will be here:

http://

audioqualia.prophpbb.com

All the pages on this site, and many more, to found on the new site.

Hope to 'see' you all on the new forum, opening soon. Dated 21 Sept 2012


the second audio qualia site continues, here:

http://audioqualia2.webs.com/apps/blog/


script change history

there have been problems with the editor for webs.com, which has resulted in no new material being able to be added. So this site will remain open until the problem is fixed, but another site will be constructed to host a more practically based one. Hope to see you there, details as soon as I have found a stable platform (sounds like hifi!) kindest regards   Cats.


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