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 paneling 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.
Using Sheep's Wool for Loudspeaker Damping
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
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.
Herdwick sheep's wool.
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. See picture below.
QTA can supply wool from Herdwick Sheep which has been both scoured and moth proofed. This is a close
equivalent to the original, although the fibres are not quite as long.
See damping materials.
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 for Loudspeaker Damping.
Close-up of acoustic 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.
Fibre Felts for Loudspeakers
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.
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
Pure wool felt.
Similar in construction to the Mixed material felt shown above this felt has a Jute central base upon which is
woven / needle stitched one or two layers of pure wool yarn. The basic material thickness determines the layer construction.
Available in similar weights as above, measured in oz per sq yard. This equates very roughly to felts ranging from 7.5mm to around 19mm thick.
The material has a more uniform performance but is very price sensitive to the cost of the available raw wool material.
Dual density speaker felt.
High Mass Bitumastic Sheet.
Speaker 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. The picture shows a typical assembly
with part peeled backing paper and fixing tacks.
Loudspeaker sheep's wool: Internal support with netting.
Light mass damping materials require both trapping and support if they are not to
ultimately settle inside the cabinet, with a subsequent reduction in performance. Some mention of this has been
made regarding the use of wool fibres. For containment purposes simple lightweight plastic mesh may be used, commonly
purchased from hardware stores as mesh for garden use and moulded into appropriate shapes / volumes. Alternatively,
the nylon mesh used to package fruit and vegetables is also suitable, the contents being healthy too.
Loudspeaker Grille Foam.
Reticulated 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. Grille Foam OFFCUTS available.
How to Use Sound-Damping and Acoustic Materials in DIY Loudspeakers.
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.
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.