Re-foam Your Old Speakers




About: I'm an Italian freelance structural engineer, graphic designer and photographer, and I'm teaching physics in Waldorf high-schools. I always investigate electronics, robotics and science in general, I'm a pas...

In last years it happened to me to run across ruined speakers more often than we would think. I love those old heavy and huge black speakers typical of '90 years. I obtained this pair for free, I decided to repair it and this time I supported the process with a full photographic documentation and a detailed instructable for your pleasure ;-)
The re-foam process is the same for squawkers and subwoofers, but since I fixed both you can see the entire repairing dynamic for both cases. I also documented myself before planning my first speaker refurbishing attempt, and I found a good guide on After some successful realizations I can tell that's a useful reading.

Step 1: Is It Really to Trash?

As you noticed from the first image the speakers appeared in a very bad situation. Actually you wouldn't tell, but this is an ordinary situation for a 20 years old woofer, indeed the edges (made of foam or rubber), after many years crumble. The tweeter's cone doesn't need to move as much as the woofers so there is no edge which could break. A speaker in this state usually emits a very low sound, and it's practically useless, many people throw it away, since a refurbishing professional service could be expensive. I will explain how to bring them back to life with a 15$ investment. 

Step 2: The Replacements

All you need are the substitutes for your speaker's surrounds. You can choose either foam or rubber edges from eBay. To choose the right size unfortunately is not so simple as to choose between S, M or L. You have to measure the external diameter of the cardboard cone, and the inner diameter of the steel frame. Practically you need the exact inner and outer diameters of the bumping central stripe of the surrounds. The inner and outer diameters of the full rubber/foam piece could vary a little bit. Measures are usually specified in inches, so if your meter uses metric system, I suggest you to convert it in imperial units. Some surrounds sellers let you choosing between flat and sloping cone border, so choose that feature after inspecting your speakers, that will affect the slope of the surround's inner stripe. I bought these and these replacements for my speakers.

Step 3: Cleaning Process

Once obtained the replacements, first action is to remove old surrounds and clean cone and frame, so to obtain a smooth surface. As tool you can use a flat blade cutter, scratch out the foam and glue remains, ant pay attention to not cut the cardboard cone. 

Step 4: Removing Big Domes

To center the cone during the gluing, wee need to obtain access to the central steel core. Since it's usually covered by a convex cardboard plate which protect the core from the dust, we have to take it off. 
Dust covers have to be removed AFTER cleaning the surfaces from any dust and especially any iron filing remained after scratching the frame. Indeed magnet of the speaker will attract those filing, and they will obstruct the gap around the magnet, contrasting the cone movement.
With a cutter, paying attention to NOT CUT THE CONE, detach the dust cap from the cone. In the case of the subwoofer this parts are glued to the cone and with a small pressure they could be detached. We'll see that it's not so simple for the midrange speakers. You can try to leave a small hinge attached so to insert caps in the exact initial position.

Step 5: And Little Ones

Indeed for these smaller speakers, the dust caps are glued much more tighter, and I learned that trying to unglue them could ruin the cardboard cone surface. So I decided to cut them at an inner circumference, as you can see in pictures. But please try not to cut the power wires which lay at the bottom of the cone.

Step 6: Centering the Cone

The paper sheet is needed to form some paper stripes, so that they can act as spacer to isolate the cylindrical core from the metal tube connected to the cone. This thickness is not constant for every speakers, and you have to try with more sheets and lengths.
I made a mistake here since with my 80 g/m2 sheets I had to make one and half turns to reach the exact thickness. That results in a not centered cone, because the spacer on one side is double than on the opposite side. By the way this mistake doesn't affect the speaker functioning in my case, but choose a different sheet thickness if you encounter in a similar circumstance, or use four shims, trying different thickness.

Step 7: Pour Glue

As glue any vinylic glue is good, but I suggest you to check that it will become completely transparent, some of them remain white after drying. With the paper cylinder well into position place the foam surround over the cone, so to see if it fits right. You can also remove the speaker from their wood case, but I find useful to have a so handy stand.
Add glue all around both frame and cone, try to pour the right amount of glue and to diffuse it uniformly.

Step 8: Gluing Edges

Now add the foam edge pushing it gently over the glue. Avoid forcing the cone on one side or the other, since it has to remain centered. With your finger spread the excess glue, don't worry if it appears to be too much.

Step 9: ...glued

Indeed when the glue will dry any excess will magically disappear. You can now remove the inner paper spiral. Now make a constant pressure on the cone with your hand and check that there is no friction at all between core and metal ring. If you feel friction, and you are sure it's not due to an unbalanced pressure from your hand,  you can unglue the external stripe of the surround, and glue it again letting it free to position.

Step 10: Gaskets

Now set up all the foam gaskets cleaning them from any glue remains, by the way if your work is not accurate nobody will notice that ;-)
These gaskets are supposed to keep the front cover far from the surrounds, and to protect these from any accidental bump, anyway they are not essential.

Step 11: Gaskets Glued on Subwoofers

Add some vinylic glue on the outer stripe of the surrounds and also on the metal frame circumference, then place the four parts of the gaskets so that they match with the holes and between themselves. Push them over the glue and let them drying. 

Step 12: ...and on Squawkers

As you notice the foam gaskets for the midrange speakers are too tight and the make an interference with the new surrounds. This could be easily fixed cutting away a stripe of gasket from the inside circumference. 

Step 13: Dust Caps...

If you arrived to this step with no mistakes you can be proud of yourself, since the worst is left behind.
Gluing back in place the dust caps is not easy but could not affect the behavior of the speaker, so proceed serenely ;-)
I used a pair of adhesive tape to handle the domes better, but use paper tape, since in my case this ruined a little bit the cardboard.
Place some glue all around the disc border, don't leave any interruption along the glue trail.

Step 14: ..glued on Subwoofers

Place the cone in his original position, push it gently trying not to leave big lacks of glue, and leave it drying. Any small split  which appears without glue will be filled spreading glue with your finger. Place a weight (not too much of course!) over the cardboard dome until it's dry. Any further split in the gluing will be filled after.

Step 15: ...and Glued on Squawkers

The process to glue back in place the cover caps of the midrange speakers is identical to what you did for the subwoofers. Also in this case I suggest to use paper tape, you will have less damage to your caps when you'll take it off.

Step 16: Refinishing

When the glue around the caps is dry you can check there are no splits, and you can add some more glue just to be sure. Spread it with your finger and let it dry. The white will disappear if the glue is transparent enough, if not you can paint it with a black marker. Try not to add too much glue to keep low the weight of the cone and consequent inertia.

Step 17: Final Breaking In

Assemble everything back and screw tighten each screw.
Your speakers are not ready to be placed around your hi-fi... you have to wire them but also to wait some more time before hearing them in all their new shape... anyway don't worry, all the exertion is done.
Indeed when you'll turn the hi-fi on, the sound will be horrific, that's because the new surrounds need to work for some time to loose their rigidity.
To do this process in the shorter time possible place speakers one face to the other, connect one speaker's wires reversed (positive wire on negative connection, and negative on positive), and let both speakers working with music at high volume... not too much high depending on your neighborhood ;-) By the way one speaker will cancel the sound of the other, and you will hear a low music.
After one hour, more or less, you can connect both speakers with right polarity and place them at your hi-fi sides.

Step 18: Ready to Play!

Place also the two covers back in position, obviously after washing them. I had also to glue four new velcro pieces at their corners, because old velcro didn't work well anymore.
Now I can enjoy my new stereo system! Do you recognize another interesting project in the picture? ;-)



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108 Discussions


Question 5 months ago

Hi, great instructable! I will probably try it out myself, but out of curiosity, is it possible to simply change the woofer?


2 years ago

Nice job.

I have a set of very old Technics speakers, from back in the day when Technics was Panasonic's professional label. The problem is the speakers are metric not inch sizes and the available kits don't fit. I did do the mid-range drivers, what I guess you call "squeakers", but they requires making surrounds from another set of "inch' parts. One day I might do the woofers if i can find a kit that will work.

1 reply
andrea biffiLotus14

Reply 1 year ago

maybe you can buy a kit a bit bigger and cut it... just thinking...


5 years ago on Introduction

Many years ago I threw out my blown Boston Acoustics A70's. I'm sorry I did that... I didn't know this process was so cheap. Oh well. The replacement CR9's are much better and still going strong!

1 reply

4 years ago

Very cool! Thanks for sharing.


4 years ago on Step 18

I agree - excellent Instructable. I only replace the foam rubber surround on my 8inch SEAS woofers. These are priceless (in South Africa anyway) and they sound good as new. i got the surrounds from in the US. Good job ..

1 reply
Dr. Science

5 years ago on Step 17

MIGHT I SUGGEST? This step is optional.... Paint the new surrounds with a black paint (brush or spray). By covering the surrounds with paint, they will last much longer. The reason they disintegrate is because of ultraviolet light. Paint protects them from this.

11 replies
psychotronDr. Science

Reply 5 years ago on Introduction

Surrounds disintegrate due to many things - theories abound, ultraviolet light, ozone and fungus and others. None of them stand up to much scrutiny. Ultraviolet light the least of them. I have speakers that never saw much if any light - still crumbled surrounds after 15 years. Time and material is the main factor. Painting a surround will change the frequency response - especially of a midrange or tweeter. The surround has two functions - cone centering and terminating the resonances of the cone. Changing the mass or flexibility of the surround by paining will change how this happens.

Dr. Sciencepsychotron

Reply 5 years ago on Introduction

True story... but visible light (which contains ultraviolet light) is their worst enemy, much like PVC pipe.

erugerDr. Science

Reply 4 years ago on Introduction

Yeah, no. These materials degrade over time no matter what conditions you keep them in, though lots of factors can speed it up. UV, however, can hardly be a major factor when they are enclosed behind opaque cloth, as older speakers generally would be. To confirm that conclusion: old cloth-faced speakers that lost that cloth often degraded rather quickly. After that. Modern speakers and cabinets with grids or silicone surrounds are another story.

I don't suggest it, since they are made by rubber or foam to have a very soft flexibility, if you cover them with a paint you'll change their behavior. You can paint the cone, which is rigid, anyway it has to be as much light as possible to reduce inertia...

Honestly, Andrea, that is a valid concern if the waveform were shorter. Do NOT use a thick paint like latex house paint. Woofers have a much longer (lower) wave form. I'm not saying that there is not a difference, but it would take a signal noise generator and a fine mic hooked to an oscilloscope to notice the contrast. And please do not take offense, but those Phillips in the pix aren't exactly super accurate anyway. Speakers such as A/D/S/ use the solid rubber surrounds. I've even used vasoline and smeared the foam surrounds with that to prolong the life. Find an old pair and experiment... listen for a week. Use the fine paint like they use on models (its more like ink). PS: you can really improve those Phillips by ordering some soft domes tweeters from Parts Express in Ohio. Great company.

psychotronDr. Science

Reply 5 years ago on Introduction

Yeah, of course you have looked at them (not listened, or measured) and believe you can improve them with a cheap tweeter - good grief!

Dr. Sciencepsychotron

Reply 5 years ago on Introduction

Paper cone tweeters cannot respond to super high frequencies as well as the soft domes, plus the dispersion characteristics are totally different from a cone vs. a dome. AR made a pretty decent one in the early 80's that had a lubricant applied to the voice coil, allowing higher levels of voltage to pass through (like the AR-18's). But, the dome type tweeter provides an even higher frequency response out beyond 22,500 cycles while the paper cone starts dropping off at around 13,500. A fine microphone and a real time analyzer with a pink noise generator hooked up to a scope will prove that. Some folks cannot hear beyond 10k and for them, its a moot point. Finding decent soft domes for that price is a good deal. Of course, all of this is subject to one's own opinion, but the real time analyzer does not lie.

psychotronDr. Science

Reply 5 years ago on Introduction

I have a fine microphone and I have measurement equipment and I have measured several paper tweeters that go beyond 20k. I have also seen many cheap "dome" tweeters like you are suggesting (if you take it apart it is actually a cone, so LOL! ;) ) that are complete garbage. Dispersion between cones and domes of the same diameter is comparable, and a well engineered cone may actually have better dispersion because the voice coil is smaller. An RTA is useful for measuring noise, but it is not useful for designing or evaluating speakers. I have been designing speakers for 25 years and most of what you say in this thread is VERY misleading, or misinformed opinion.

Dr. Sciencepsychotron

Reply 5 years ago on Introduction

A paper cone can produce beyond 20,000 hz but it won't be flat as most start to drop off at 15,000. There might be exceptions but none that I'm aware of. I've seen "cheap" dome tweeters as well that do not produce flat also, but I've also had "inexpensive" ones that do an outstanding job. A good example is the Radio Shack Minimus 7, no longer produced, but still sought after. That little 2-way was very accurate to within the range it could produce. A/D/S/ also produced extremely (almost perfect) response. Comparing speaker designs is like comparing the colors of paint for your living room, what sounds great to you might not sound good to the next person. A RTA and a good flat mike with a pink noise generator does not lie. I've done the same tests of which you speak. If you've been designing loudspeakers for 25 years, which ones have you designed? Which mike are you using? Are you using an anechoic chamber? How far away do you place your mike? I do know of what I speak as I currently own over 200 different pair of "classic" loudspeakers and many different amps. I've also been in the business of selling different major name brands since 1975. There is much I do know, and much I do not know, but I'm not green to this. I've literally sold $millions in audio equipment.

Read "tweeters" Wikipedia: "It was typical of 1960s/1970s-era cone tweeters, exhibiting flat response from 2,000 to 15,000 Hz, low distortion and fast transient response. The CTS "phenolic ring" tweeter was used in many makes and models of well-regarded vintage speakers, and was a mid-priced unit. (see photo there)

Cone tweeters have a narrower dispersion characteristic that is the same as a cone woofer's. Many designers therefore believed this made them a good match to cone midranges and woofers, allowing for superb stereo imaging. However, the "sweet spot" created by the narrow dispersion of cone tweeters is narrow. Speakers with cone tweeters offered the best stereo imaging when positioned in the room's corners, a common practice in the 1950s, 1960s and 1970s.

During the 1970s, the introduction of higher quality audiophile discs and the advent of the CD caused the cone tweeter to fall out of popularity because cone tweeters seldom extend past 15 kHz. Audiophiles felt that cone tweeters lacked the "airiness" of dome tweeters or other types. Nevertheless, many high-end cone tweeters remained in limited production by Audax and SEAS until the mid-1980s."