Glues, and their importance
All mechanical design is about compromise. You can build a fast car (an Aston) or one that is extremely economical (a Micra), or has a super-smooth ride (a Bentley). But you cannot design a car that is simultaneously fast and economical and smooth. At best, as a designer you can aim to satisfy two of the three objectives. A better all-round plan may be to aim to balance the three objectives into a working engineering compromise providing that you can find customers who seek that particular compromise. That's what inspired the BBC monitor speaker: regardless of whether it was used to record rock music, or drama, or jazz or classical music, it performed well within it's entirely adequate power capabilities. But if the user needs head-banging loudness and extreme durability under stress, then as we know, the BBC monitor was never engineered for that workload, and the user would be best advised to look elsewhere.
Originally Posted by Sebastien
About glues .... this is a really interesting and important subject.
When we at Harbeth started making drive units some fifteen years ago, we used 'soft' glues like Uhu, Bostik or similar. In fact, the BBC's pioneering drive unit design work (in the 1960s) would surely have used such adhesives. Considering that the drive unit is very much 'alive' and is not a rigid, static structure, you can imagine that different elements of the assembly are under vibrational stress. The problems are in the interface areas where one part abuts another: for example, the edge of the cone bonding onto the rubber surround. You can imagine that this joint is under real stress as the drive unit moves in and out at high power levels. If you look at old BBC monitors made 20+ years ago, you will see the tell-tale problem of 'glue creep' where the surround has slowly pulled away from the cone leaving a horrible brown ring around the cone edge. Eventually the surround will completely separate from the cone and that will be the end of the driver's life.
It is almost certainly to be expected that changing glues will influence the measurable response and/or the sound to a small degree. You may be aware that during the early/mid 2000s we made many thousands of drive units for KEF UK from kits of components that they free-issued to us on the same production line that we made our own Harbeth drivers. Included in this technology transfer was the most superb documentation covering in minute detail (for their own internal training) how to make these drivers, and what glues to use and where. I'm not sure that we made any money out of this venture, but what we learned was priceless: it opened our eyes up to a whole world of specialist adhesives. As a small company, Harbeth wasn't able to engage with the major international adhesives suppliers, but using the relationship with KEF we were very soon in direct contact with the majors, including the Loctite parent company, Henkel. And, they have the laboratory facilities to examine every glue joint in a drive unit and recommend the best adhesive. And in their view the optimum glue joint means the longest lasting, most secure bond regardless of the cost of the adhesive. If a standard product is not quite right, they'll compound one specifically for your needs. The downside is that one speciality glue we buy from them costs GBP500 for one litre!
So, whilst we still use some contact adhesives, we have moved towards superglues over the years. That way we can be very confident that providing you don't overload your woofer, it should outlast you.
BTW: there are two downsides of using superglue to assemble drive units. These hard glues are completely unforgiving: if we make a tiny assembly error on a RADIAL woofer which we could easily rework, we can't. We can't separate the permanent glue bonds no matter how hard we pull. We have to throw the whole drive unit away. Also, this means that we can't (as some speaker companies do) supply recone-kits because you too won't be able to disassemble the driver.
Alan A. Shaw
Harbeth Audio UK