From a Resource: Speaker Design Course Notes + video materials
In the past, speaker repair services were scarce, often expensive, and replacement surrounds for DIY nearly impossible to source. The web has improved this, surrounds are widely available, but of variable quality. Even so, for a few vintage Hi Fi drivers of unusual size, suitably sized good quality generic surrounds or reasonably priced OEM parts aren't available.
Loudspeaker surrounds made of foam, decay eventually, especially in hot humid conditions. Less often, rubber ones can perish or split. Recent surrounds are sometimes made resistant to UV and tropical climates. When faced with high labor costs, many are prepared to tackle DIY replacement, and OEM or generic surrounds, as well as how to guides, are available on the web. If doing a full restoration, the cost of an OEM part (surround, recone kit, or driver) can be justified, for highly prized vintage models. Exotic full range drivers and Sound reinforcement speakers generally require specialists.
For otherwise good systems, of no particular worth, and where a suitable replacement, can't be found, making the surround itself, can be considered. It can't be justified in $ terms but it can be approached as a one off learning experience, useful recycling and a new skill set, or a potential supply of beer from others if word gets around that you've become good at it.
The driver may not perform exactly as the original, but in the cases tried here and most others the resonant frequency and response were close, due to some experience in what to look out for. The surround is material impregnated from one side (the non gluing side) with a small amount of silicone rubber just sufficient to be air tight and formed in a mould with a release agent so it doesn't stick. Making the mould requires access to some machinery and takes the most time but having done it once it can be reused for the same type of driver. The drivers shown here are 80's vintage; a Technics SBX model and a Dynaudio 21W54, good drivers in their time. Some drivers eg vintage JBL have quite small voice coil clearance, that may require dust cap removal and use of shims for precise centering.
If a system has an unknown history or hasn't been used for a while it's worth checking crossover components, and other system drivers initially. Establish there is no damage to the voice coils before proceeding. Practicing on a non critical driver, and making a new surround when a previous attempt hasn't turned out as expected, before assembly is worthwhile. Removing all remnants of the old surround and glue is worth giving close attention to, for a good looking result.
Step 1: Preliminaries
If the old fragile surround is mostly intact, you can estimate its thickness and measure its 4 dimensions, typically A B C D on a web guide, to set up the mould inner and outer roll diameters as well as the width of the flat parts. The width of the roll is important but If the surround is absent, estimate C and B as accurately as possible with an adequate A and D for attachment purposes. Some web research or enquiry to audio forums, might turn up images or dimensions to go by. Heavy cones from a 3 Way with longer cone travel (Xmax) might have rubber surrounds or of somewhat thicker foam. with a wider roll, compared to narrower rolls on lighter cones with an extended response and of lower Xmax from say 2 way vented systems. Exotic / vintage full range drivers are best left to specialists or dedicated DIY online forums
C sometimes has a small deliberate overhang beyond the cone edge for performance reasons. Note the polarity of wires before removal from box.
For those with the equip, and curious about before and after comparisons, measuring the driver res freq out of the box and response in the box, with and without the crossover in the mid and upper range notwithstanding the likely air leaks can be done. But isn't mandatory just to get a system going again.
In most instances if the inner flat is not angled it can still be satisfactorily be glued to the upper or underside of the cone when the glue (PVA from speaker repairers) is in the tacky stage.
Step 2: Mould Making Part 1
The mould is made from a 25 mm thick MDF circular blank rotated at low RPM in a wood turning lathe or suitable other equip. DUST precautions apply.
Be careful you have a sturdy support for the carving tool and with larger dia blanks. If a novice using a a local cabinet making business or facilities at a trade college may be advisable. After final sanding a couple of coats of gloss acrylic will do.
Step 3: Plaster Part of Mould Part 2
The MDF part is fenced with wide packaging tape on the outer edge, the central hole taped over, and the entire surface smeared with vaseline with no lumps or ridges. Moulding plaster is poured in (creamy texture to about 20 mm thick) tapped to encourage bubbles to surface, and allowed to set on a level surface, so it's not moist and white in colour, possibly over a day or 2 to be sure, the latter part of drying out can be with the tape removed . Mark where the 2 parts are aligned. Gently tap the mdf all round to release the 2 parts. Smooth / fill any air bubbles Can also be given a thin coat of gloss acrylic.
Step 4: Surround Finishing and Attachment
The coating of the stretch cotton interlock material can use med viscosity 2 part silicone thinly applied with a brush, this works the best, as it has a longer pot life, and flows somewhat, but costs more. If you want to economize try plumbing air set silicone (in 300 gr cartridge or tube) applied very sparingly with a putty knife but you have to work quite quickly and the surface finish tends to be uneven in early attempts.
VENTILATION and Plastic gloves are useful precautions.
Consult the web for suitable release agents, or can try cling wrap on each side of the mould. While setting align the 2 parts of the mould as they were separated originally. Apply enough pressure evenly, but not so much as to risk cracking the plaster. A No of large spring clips can suffice, or heavy weight (10's of KG for a 10 - 12 in) on a flat surface. Leave overnight or longer for 2 part, trim to size with sharp scissors, or trimming blade. Assess the outcome for weight, (as light as possible generally, mass similar to orig foam is achievable) appearance and no later leaks when held up to light, a minute dab of silicone will do if there's the odd one. PVA won't adhere to silicone hence coating material on non gluing side only. Silicone and a few special glues will adhere to silicone adequately but future removal from paper cones is problematic. If you coated the wrong side of the material, Silicone could be used on driver frame for the outer surround after centering has been fully checked. Cardboard or cork gaskets on the frame need case by case planning from the outset re salvage in one piece and later re-attachment.
The surround below was rejected and shows what happens when too much is applied it permeates to the gluing side and had excessive mass compared to a foam one. Very old heavier rubber surrounds can split eg Philips 10100 and 12100 so this could be as a replacement if the gluing aspect has been taken into account. Full range drivers with very light paper cones like Philips AD4200M or 9710 with a corrugated paper surround, also have a small Xmax, use of a roll surround isn't recommended. But it has been done by cutting off the old surround and using one with a narrow roll.
Another instructable "How to refoam your woofer" gives extra detail of surround attachment.
Proceed as per web guides for driver attachment, noting correct orientation, ie roll up or down, glued to upper or lower edge of cone etc. Ensure adequate cone travel with no rubbing. Recheck driver ohms with a multimeter and Fres if required, before system re-assembly.
Sit back and enjoy or collect beer dues.