Article: Loudspeaker Damping Materials.
An Overview of Damping Materials.
The selective use of foams and damping materials within a loudspeaker cabinet can significantly alter / enhance it's acoustic performance.
These materials fall into two basic types,those that can selectively absorb energy over a limited range and those that can shift / move the energy elsewhere. All vibrating objects produce sound pressure waves. These can act as a form of airborne interference and spoil the overall listening experience. For best cabinet acoustics we require a speaker cabinet to be acoustically dead and a number of designs exist to meet this criteria with varying degrees of success. These range from using very heavy panelling materials, e.g. concrete or marble slabs to sand-filled double-walled panels. A variety of materials attached to the interior of the cabinet can also modify it's response. The fact that there are so many options suggests no one approach is optimum for all occasions, disregarding the cost considerations of course.
Vibrating panels effectively act as large speaker cones with a small peak amplitude. These have selective modes of vibration, (resonances), and can be of sufficient amplitude to colour the overall sound. In addition the rear radiation from the speaker cone may be reflected around the inside of the cabinet and then back out through the speaker cone. This all contributes to a background noise which tends to distort the listener's sound experience and give rise to what's known as a 'box sound'. Low priced manufactured items are more prone to this problem as it's an easy area in which to make economies. Increasing the signal volume does little to help as the cabinet noise is increased also. So ----- what to do?
At this stage, it is important to realise that light weight materials can only be effective at high frequencies and that the heavier damping materials progressively take effect at the lower end of the frequency spectrum. So, lets look at the options.
Long Haired Sheep's Wool
Traditionally used to stuff transmission lines, at a density of around 450gms wool per 28L of enclosure volume, the wool is used as a moderator to the line velocity and thereby tune the line to some desired end result. Historically it was based on Dr Baileys processing prescription and was called Dr Baileys Long Haired Wool. Also used to stuff loudspeaker cabinets in which the acoustic properties are applied to provide a small dampening effect on medium frequency output, (HF Tweeters often sit in their own enclosure and are therefore unaffected). In addition, the propagation delay through the material is used, to some effect, in closed box designs to acoustically increase the box volume and hence change the characteristics of the box. The mathematics of this is complicated and is not investigated here.
Herdwick Sheep's wool
Because of the relatively small usage for this application it is now difficult to obtain such material and consequently has to a large extent, been replaced by BAF, (Bonded Acetate Fibre), although the performance of this material is not as good.
The loose wool fibres settle in volume after a period and a number of ways
have been invented by DIYer's to keep it suspended within the
cabinet and hence maintain performance. These range from
using wooden dowel rods as a suspension medium, small stuffed parcels of modified fine denier tights and
trapping the wool using plastic mesh.
QTA can supply wool from Herdwick Sheep which has been scoured / cleaned but not mothproofed. This is a close equivalent to the original, although the fibres are not quite as long.
VIEW WOOL PRODUCTS
Bonded Acetate Fibre (BAF)
A white, man-made synthetic material commonly known as 'BAF' , Bonded Acetate Fibre, and now often used as a replacement for wool-based acoustic applications. The fibres are all of a similar diameter and therefore have a different sonic performance to the more randomly distributed fibre diameters of wool. In this respect it is inferior to sheep's wool. Available in a number of weights, gms per sq metre and fibre thickness, (denier), the material is easy to cut, self supporting and moderately cheap. Available from good quality stores where it is often used as stuffing (insulation), for outdoor clothing. However, from most outlets there is no choice in specification.
Sound Absorbent Foam
Generally available in two shapes / profiles, flat and egg
box, the foam family are usually opaque in colour and have a closed
cell construction, The foam cells are chosen to be a specific size /
diameter in order to provide a bulk material characteristic suitable for
acoustic damping, i.e. the cells are mostly airtight and provide
resistance to the passage of air if you try to blow through the material. This
closed cell structure absorbs energy when the cell content, air, is compressed and
rarefied due to the sound pressure wave. This type of damping is used to
alter the high frequency response of cabinets and transmission lines. It does not work well for low frequencies
as the amount of damping / energy absorption which the foam can support
is relatively small.
VIEW FOAM PRODUCTS
Felts have similar absorption characteristics
to foam but, due to their heavier structure, alter the sound quality at lower
frequencies. There are no closed cells present but the felt fibres
adsorb energy, raising their temperature. The technical study of this is
complicated but the degree of effect, is
dependant on the fibre thickness, material and the felt density.
VIEW ASSORTED FELTS
Dual Density Speaker Felt.
Usually specified in weight oz per sq yard e.g. 35oz or 50oz etc. Some felts are manufactured to a dual density whereby the inner section has a lower density than the outer walls. This material is however hard to source.
High Mass Materials. Bitumastic Damping Sheet.
The second variety of damping uses high mass
materials e.g. Bitumastic sheet, (self adhesive), to physically alter the
mass of a panel. Other materials such as laminated hardboard panels are
also effective. The volume of these materials should be considered when
constructing cabinets as they will modify the free volume box
calculations. Heavier panels vibrate at lower frequencies and hence
their noise spectrum is shifted.
In this they do a good job but such damping will not be effective at high
frequencies. A mixture of the two types of approach (high mass and low
mass), is often used in the more expensive cabinets along with strategically placed bracing struts.
One should note however, it is possible to move Bass Bin out-band resonances, which occur by way
of their design, into their 'working' sound
spectrum, hence making the problem worse so.......some care is needed.
VIEW BITUMASTIC SHEETS
In order to further moderate / tune a cabinet's response, loose volume stuffing may be used. For a closed box design the design often relies on the stuffing to moderate the cabinet volume to produce a specific acoustic response. The cabinet volume is effectively altered / increased by some (5 -10)%.
Stuffing's generally fall into two types, man-made and natural fibres. They have different characteristics but the most commonly used is man made, of which Bonded Acetate Fibre, BAF Wadding for short is the front runner. Polyamide fibre as used in jacket stuffing's is also practical. The second type is Sheep's wool. This has superior characteristics but is more difficult to use.
The application of....Foams and Felts.
Mesh Trapped Damping
Unwanted sounds are generally called colorations
and these should be minimised to an acceptable level. There is no
specific definition of coloration limits so... Different
cabinet designs and shapes, have differing colorations, some leading to
what is well known as 'the box sound'. In this respect small cabinets can be
worse. Its actually more difficult to make a 'good', small, speaker than a
To prevent / reduce internal cabinet reflections from emerging out through the speaker cone, creating coloration, a thick wad of felt, say (20 - 25) mm, may be placed on the rear cabinet wall, behind the drivers. In addition, a similar thickness may be glued to the rear of the driver magnet assembly. This reduces internal reflections. If you have vented rear pole assemblies don't forget to cut a hole in the material to allow the vent to breathe. This should be augmented by foam damping on the other interior cabinet walls of say (10 - 16)mm. Small cabinet boxes may need to be slightly larger in volume to offset the lost volume due to the damping materials. There is no easy way to guess or measure the actual felt / foam volume, some experimentation and listening trials are required The use of Bitumastic Damping in small cabinets is not often required, as small boxes are very stiff and hence flex much less than larger panels The picture shows a typical assembly with part peeled backing paper and fixing tacks.
Loudspeaker Grille Foam.
Grille foams are, by nature, acoustically transparent. Also known as reticulated foam, the cell-like structure is of an open weave arrangement and easily permits the passage of air. Each cell is around 0.7mm in diameter and the number of cells per linear inch specifies the structure. For this application 35ppi, (particles per inch). Usually black in colour the open cell structure has little intrinsic strength and the material has to be supported on some type of frame. The material will not take to stretching as the restoring force is minimal and once stretched the original dimensions cannot be recovered. The minimum usable thickness is around 10mm. At 20 / 25mm the material becomes self supporting, (depending on area and hence avoids the effects of grille frames). Susceptible to UV light, sunlight, the material slowly crumbles and has a usable lifetime of 10 / 15 years before requiring replacement. Often used on designs of the 70's and 80's.
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