Mounting Drivers

[Welder]

I’ve built a number of speakers over the years and one problem I rarely see addressed on any forum is driver mounting and sealing for reflex designs.
In general the tendency seems to be to bolt the drive units very tightly to the baffle often using a thin rubber gasket to maintain the seal :scratch:
One problem with this method apart from the risk of distorting the chassis is vibration is transmitted to the front baffle. My current speakers have a 1 ¾ inch thick baffle and thicker baffles certainly help to reduce unwanted resonances in the box.
I tried this and found it works quite well and is reasonably cheap.
You need a rubber grommet per mounting bolt with an ID as close to the OD of the bolt as possible. You also need a tin of a product called Dum Dum.
http://www.frost.co.uk/item_Detail.asp?productID=8317&frostProductName=DUM%20DUM%20Sealing%20Compound%20 %28400g%29&gclid=CMPfrPPWp6MCFeT92AodHG8uvQ.
Finally you’ll need a fairly sensitive adjustable torque driver :mental:

All the below is carried out with the speaker enclosure on its back.

First find a washer that covers the OD of the grommet plus a bit; insert bolt through grommet and thread it into one of the drive mounting holes. Using the torque driver find a torque that deforms the grommet but doesn’t squash the life out of it.

Most speakers’ baffles are rebated to accept drivers so they fit flush. Any rebate will have to be adjusted to allow for the thickness of the grommets at the torque you discovered above.

Next comes the messy bit, rolling out the Dum Dum. Actually, it’s not that bad. A bit of ethyl alcohol cleans it up nicely.
Dig out a lump of Dum Dum half a golf ball size and roll it on a suitable flat surface into a snake. The diameter of the Dum Dum snake is determined by the grommet thickness.
You want to be looking for a consistent diameter of about 25% larger than the thickness od your grommet and a length of about 10”; anything longer gets hard to handle without it stretching. Line the rebate with the “snakes” but joining each new length. Where the threaded mounting holes are, go around both sides trying to from a circle rather than an eye the same size as the OD of the grommet. The idea is when you start to tighten up the drive unit the Dum Dum squeezes into the slot running around the grommet.
Once the entire circumference is done, carefully place the drive unit so the holes all line up and rest (not push) onto the Dum Dum ring (don’t forget to place all the grommets!) You will only get one shot at this so get it right.
Now tighten up the bolts to the torque you found with the washer earlier.

Dum Dum forms a skin and providing you have got the diameter of the “snakes” right and tightened to the right torque, the skin will prevent sag if left for 24 hours to form before you stand the enclosure upright.

I’m not big on the Hi Fi descriptive jargon so all I can say is I found it improved the sound from my speakers.

I hope you can follow my attempts at explaining this. If anyone tries this I would appreciate any comments you have, good or bad ;) :(

[Reid Malenfant]

Interesting ;) I certainly doubt it'd aid in the bass department but my guess would be it'll help isolate the driver from putting too much vibration in the midrange frequencies into the baffle :scratch:

Frankly 1 3/4" MDF or Plywood is so thick that it'll likely not flex at bass frequencies if sensibly braced, however it'll act as a great soundboard for midrange as this would be the frequencies it'd now resonate (flex) at due to being so stiff.

Just out of interest where are you finding better definition from the speakers?

[Welder]

My understanding is the baffle should not transmit any energy to any driver in ideal circumstances
The ideal baffle is there to mount the drivers; nothing else. It’s the box volume behind the driver that’s important with sealed enclosures.
I believe the principle of de-coupling the drivers from the box (even bass drivers) has been fairly well documented on various speaker building sites. Have a look at the Linkwitz site for example. I think you’ll find the physics is sound enough for drivers at any frequency :scratch:
The method I outlined above is far from perfect but as a cheap attempt you may be surprised at the results should you give it a try ;)
It’s not really a case of the baffle flexing, it’s vibration/energy transmitted by the through the driver chassis.
I’m not sure why you don’t believe it would be of any benefit to a bass driver. The same principles apply throughout a speaker’s frequency range. Provided the chassis doesn’t move, the cone which moves the air doesn’t care how the chassis is mounted :trust:

I was quite careful to state I wanted to avoid jargon in any attempts to describe the improvement I found. Jargon is very misleading.
All I can say is I found I preferred the sound of my speakers built in this fashion. For £10 or so if you’re a DIY’er it’s not a big deal to try out. Even if you don’t like the results Dum Dum makes a great sealant and anti resonance fix for most enclosures so really you’re talking about the price of a few grommets.
Like all mods, the only way to find out if they work for you or not is to try them

[Reid Malenfant]

I'll get back to you on this tomorrow ;) While i'm not in total agreement with you i think it's much more of a case of 'getting the words right' so we both know where we are coming from.

Linkwitz is an incredibly clever chap, i'll never deny that simply because i'll be using one of his networks to correct for sealed box response on a new set of enclosures i'll be building. His math & circuit design is beyond reproach & damn handy to boot :lol:

It's that time 2419 Bests to you, until tomorrow (if i remember what i was doing this evening :lolsign:).

[DSJR]

My speaker cabinets are designed to vibrate, thereby keeping midrange colourations to a minimum.......

Once upon a time, KEF decoupled their bass units from the baffle in the misguided attempt at preventing resonances getting into the baffles. they all sounded washed out and boring, as all the attack seemed to disappear as the driver vibrated out of phase with the cone.

Linn's approach was to all but glue the drivers in, which was a total nightmare come service time, the only way to remove the bass units being to un-peel the surrounds, yank what was left of the cone out and physically heave the driver chassis out, taking care not to take the wood veneer with it.

[Welder]

Reid Malenfant
I’ll definitely agree with you concerning Linkwitz. I’ve used a few of his designs myself.
I’m not saying (and I believe I’ve attempted to make this clear) that the above is the way all drivers should be mounted in every case, just suggesting it’s worth a try for a few quid.
(I knew there was an advantage to not drinking alcohol ;)

DSJR
That’s a new one on me (vibrating cabinets by design). What speakers are these?
Most of us try really hard to build cabinets that vibrate as little as possible. How do you go about tuning vibrating board to minimize midrange coloration only; separate enclosures? :scratch:
It sounds rather complicated.
I’ve not owned any KEF speakers so I can’t really comment.
I have owned a pair of Linn Tukans but, I didn’t try removing the drivers so I’ll take your word for Linn “all but gluing” their drivers in.

In general, I’ve found very few mods that work for all, and many that don’t work at all and cost a fair amount of money.
Suck it and see I reckon :)

[hifi_dave]

DSJR is tal;king about the thin wall BBC designs. These use relatively thin wood for the cabinets, often Birch ply and these are damped internally. The idea is that the cabinet walls resonate at frequencies out of the important mid-band and thereby give a cleaner sound than thicker cabinets.

[Welder]

Thank you Dave.
I’ve not come across speakers designed in this fashion before.
I used birchwood ply and roofing felt laminate for my speaker enclosures but they ain’t thin ;)
Originally I had high density flooring grade chipboard cabinets. I can’t say if the material change made a sonic improvement because I made a number of changes in one go :scratch:

Good to see Radlet Audio still going. I bought a couple of items from you many years ago :)

[Reid Malenfant]

My speaker cabinets are designed to vibrate, thereby keeping midrange colourations to a minimum.......

Once upon a time, KEF decoupled their bass units from the baffle in the misguided attempt at preventing resonances getting into the baffles. they all sounded washed out and boring, as all the attack seemed to disappear as the driver vibrated out of phase with the cone.
The Kef debacle is exactly what i was thinking of :lol:

I’m not saying (and I believe I’ve attempted to make this clear) that the above is the way all drivers should be mounted in every case, just suggesting it’s worth a try for a few quid.

Most of us try really hard to build cabinets that vibrate as little as possible. How do you go about tuning vibrating board to minimize midrange coloration only; separate enclosures? :scratch:
All things are now clear :) I'm in 100% agreement, however i'd tend to go in another direction as well.

Decoupling the bass driver from the enclosure is a bit of a bad idea as has been mentioned. There is no harm in decoupling any midrange or tweeter from the bass cabinet though ;) Kef went this route with there Reference 105/3 & used ac totally seperate mid & treble module bolted to but decoupled from the bass enclosure & it worked rather well. Roksan went one further with the Darius & suspended the tweeter on springs :scratch: Those speakers have quite a following.

Assuming a 3 way speaker is going to be built this would be my solution. Bolt the bass unit to the enclosure with no decoupling. Make an inner enclosure out of say 18mm MDF & brace it so as to avoid flexing in the bass end. Unfortunately this raises the enclosure panels resonant frequencies into the midrange, but i have a plan swiped from Wharfedale ;) Build an outer enclosure with a 4-6mm gap between the two & fill this with dry sand. Any mid frequencies that excite the inner enlosure walls won't get through to the outer enclosure walls as the sand partical movement will dissipate it very well indeed.

Midrange & tweeter units could be decoupled as you mention :)