Speaker ageing (not burning-in)
Even I, cautious as a I am, sometime have rosy memories about speakers that caught my eye or ear 20+ years ago. And from time to time, if the price is tolerable, out of curiosity I'll buy them. There are always disappointments, and long-term ageing of their mechanical parts seems to be very common. It is worth commenting that Harbeth-brand speakers - even old ones going back to the Mk1 from 1977 - age well. That is, the performance whilst (perhaps) not completely to the original specification is acceptable and most important, they don't seem to exhibit nasty colourations as time passes.
This good news situation with elderly Harbeths is not the universal situation throughout the speaker industry. If you ask speaker designers to comment on the long-term ageing of designs they've been involved with, I'd guess that most would admit to a problem model. It's not that they deliberately designed-in a latent problem, it's that materials in the consumer's operating environment are not completely predictable, not perfectly stable. What happens in the hot, humid home may be quite different to the long-term behaviour in a temperature controlled lab. Our wider experience in manufacturing drive units for our friendly competitor's service needs has opened our eyes to this whole issue. In my observation, the problems of long-term ageing always related to the soft moving parts of the system - this includes all glue joints (which can be very critical to performance and sonic quality), the surround, the dope applied to the cone, the suspension and the dust cap. Should any one of these gradually slip out of spec. and they may (rubbers and plastics are never completely stable over the long term as they have to be flexible, and this flexibility opens the door to molecular change with time) - you can expect the system performance to change. You may or may not notice the effect on sound quality, but this is over the very long term life of the speaker - perhaps 15 years after manufacture.
The magnet, if metal/ceramic (as all Harbeths are) has a very stable energy (flux) with time. The previous generation of Alnico magnets had a marked flux leakage with time which meant that the bass unit becomes quieter relative to the tweeter and also that the bass response became flabby as there isn't enough restoring force from the magnet. Harbeth have never used Alnico magnets for that reason so I wouldn't expect the bass response of a Harbeth to change with time.
Perhaps the single most problematic part of any speaker system is the bass/mid unit's edge surround, analogous to a cars shock absorber. It looks so deceptively simple but it is super-critical. It has to be flexible to allow the cone to move backwards and forwards but this flexibility carries with it an arthritis-like vulnerability. PVC and PVC-like surrounds as used on BBC-style monitors in the 1960's and 70's have notorious problems in that they have polymer memory and like to return to the flat state over time, hence, weak over-damped bass. The original user simply wouldn't be aware of the gradual change and will describe his speakers in the most glowing heart felt terms new listener's see and hear them for what they really are: in their twilight years. Very sad. What is the operating life of a high quality speaker system? Impossible to say with certainty outside the lab.
General across-the-industry Rule of Thumb then:
Operated regularly (keeps the moving parts flexible), used in 21-24 degrees air conditioned and air filtered environment with low ozone level ... A1* performance (say) about 10 years. Under these ideal conditions the speaker could be perfectly operational at B+ grade at 10-20 years. Replacing the tired bass units at the 12-15 year point (if available) may well extend the life upwards to 25+ years.
Note: none of these degradations will necessarily effect sound quality; to the best of our knowledge Harbeth HL series and Monitor series speakers do not significantly degrade with time if they are treated with respect..
Alan A. Shaw
Harbeth Audio UK