Old speakers - less distortion?

[Firebottle]

Here's a query aimed primarily at Paul.

Given a well used set of speakers/drive units, with the suspension and spider components well past any 'new' stiffness, is there likely to be less distortion from said units than when they were new?

[Haselsh1]

Trying to keep to subject here, I have often wondered about how loudspeakers change their sound over time. My current KEF Q500's were brand new in 2013 and took about nine months to sound totally free and relaxed. I wonder how they may change over the next ten years or so.

[Reffc]

Here's a query aimed primarily at Paul.

Given a well used set of speakers/drive units, with the suspension and spider components well past any 'new' stiffness, is there likely to be less distortion from said units than when they were new?

Hi Alan

it's not possible to offer general advice on distortion as there are so many variables involved, not least of which is the drive unit spec and cab/crossover design in the first place. However, as drive units age, and depending on the material used for the magnets, several issues to crop up to varying degrees.

Firstly, as suspension, and in some cases, surrounds become more compliant, the T-S parameters for the speakers will in fact change, so tuning points or the speaker Q (for sealed boxes) may well alter. This may not necessarily result in increased distortion on its own, but in some cases, power response or efficiency can be reduced from what is optimum. Magnets which use ALNiCO in particular can have changes to BL force factor due to weakening of magnetic strength which can occur as a result of over-driving (ie too much power driving them), knocks or lots of temperature variations and this can in some cases result in increased distortion but magnets can be sent off for re-energising to bring up to as good as new performance. The other issue is that variations can occur between drive units throwing speaker matching out.

Where Tannoys are concerned, I usually advise that for HPDs in particular, they are reconed rather than refoamed as this ensures that they are restored to a matched pair, and that suspension and Fs are restored to factory spec, or close enough to it. Depends how fussy people want to be and some aren't too worried as long as things sound ok, so little point in them pursuing real or perceived in those cases.

With some speakers having a high value for Qms (mechanical Q), this usually indicates the amount of control, openess and dymanics a drive unit may exhibit, and is purely a function of things like mechanical suspension control and control offered by the surrounds. It is a measure or an indicator anyway of how much energy might be stored. Drivers with high values for Qms tend to have fewer losses. As suspension wears so losses increase, and performance and efficiency can decrease. However, speaker drive units with high BL values, tend to have powerful motors which can control their cones better, so slight loss of suspension efficiency may impact less in those cases....or so you might think. I have measured 30 year old drive units with very high BL values which have still exhibited a loss or drop in Fs of 10 Hz or more due to the effects of suspension softening altering free air resonance but I suspect that some magnet strength loss may have also been a factor. You can also have issue with over driving of increased hysteresis depending on magnet material, strength and power applied.

It really does depend entirely on the driver design, cabinet design, loading, and variation from factory spec. In general though, speakers do start to drift from optimal design values from around a few hundred hours on new although the rate of drift is usually very low/slow and takles may years.

If they sound ok, then chances are they are ok, but for any loudspeaker used and abused over several decades, drive unit suspension will have softened up and depending on the design, crossovers may also require attention. The latter can be very economical to address and my own findings are that older crossovers are the main culprits for increased distortion, which is easily heard and easily measured (I have the test gear here to test drive units).

By using speakers with crossovers which have either dried up electrolytics in them, or loose hand wound inductors, distortion can be appreciable but easily remedied.

[danilo]

Small addendum :-)
Ferrite Magnets Lose Flux over time.. Starting to be measurable by their 5th Birthday and absolutely by their 10th.
Simple nature of the material.
Is this a problem?
Dunno, but it certainly was/is in motors requiring precision where deviations are quickly recognised.

[walpurgis]

Ferrite Magnets Lose Flux over time.. Starting to be measurable by their 5th Birthday and absolutely by their 10th.

It was my understanding that Ferrite magnets were pretty stable and that magnet alloys like Alnico lost flux more readily. Not sure where we stand with Neodumium items though.

[danilo]

Unfortunately that information is inaccurate /reversed.
Alnico has a projected/estimated Flux stability/Half life of 2 millenia !
Longer than we needs concern ourselves with.
Albeit a small setting down of alnico's magnetic domains occurs in the first day or two following initial Magnetisation.
Also has a Currie point ~ 850 C In case house fires are a concern :-)
Stable flux in Spades. Some myths claim that alnico needs 'a remag' periodically.
Ain't so in real life, despite babbles to the contrary.
But stories persist, misinformation is ubiquitous in our culture.. no news there
Ferrite flux losses are measurable..replicably so.. by it's 10 th Bday. Again; no news there either.
Ferrite Does generate a shaper edged Bass /impact sound though.
Alnico softens a bit on those transients as the domains deform under mag loads.. and quickly reform.

All this is researchable.. but best to limit such efforts to Magnetic materials suppliers, or ideally Magnetic materials research areas..
Not Audio sources ;)

[walpurgis]

That's not exactly what it says here:

http://www.rare-earth-magnets.com/content/pdf/Permanent-Magnet-Selection-and-Design-Handbook.pdf

[struth]

Alnico's can and do go down, and can be rejuvenated. Never had any others do this in same way. Nicest sounding speakers Ive heard if made well.

My Telefunken ones are suublime

[Reffc]

Alnico's can and do go down, and can be rejuvenated. Never had any others do this in same way. Nicest sounding speakers Ive heard if made well.

My Telefunken ones are suublime

Agreed Grant. Some of the finest 'speakers I've heard have used ALNiCo magnets. Tannoy went over to ceramic and more efficient magnets but have since reverted back to ALNiCo. Some of the sweetest sounding mid and HF drivers I've used have also had ALNiCo magnets.

[shane]

Presumably all this apply to cartridges as well?

[walpurgis]

Presumably all this apply to cartridges as well?

Some of it certainly. In fact I'd say the effects of some of the break in, loosening, age effects and varying magnet characteristics must apply to all transducers seen in Hi-Fi to an extent.

[Firebottle]

Many thanks for a most detailed response Paul :thumbsup:

[ScottL]

Just adding a few thoughts to the above -depends what you mean by 'distortion' since there are a whole range of factors there. Depending on suspension design / materials, it usually doesn't take very long to bring a driver from factory fresh to the baseline operating window, and there it will remain for quite a long period assuming no other external force is involved. Eventually it will deteriorate, when again depending on the design & materials involved. Cones vary also; in themselves it's rare for them to deteriorate significantly, although some will eventually start to break down; those intended to flex will probably let-go sooner as they're more vulnerable to stress fractures over an extended period. Rarely an issue though -they're more likely to be damaged via an external mechanical force than as a result of normal use. If you hammer them constantly to extreme excursions of course, that will accelerate the deterioration for obvious reasons. I sometimes lost the will to live on another site (which will remain nameless) when a handful clambered onto their soap-box to proclaim that 'drivers should be able to be taken to Xmax indefinitely', which is a little like saying 'I expect my car to last as long, if I flog the nuts off it, as it would if it was driven normally'. Madness. Especially when you consider that Xmax doesn't actually mean a lot since there is no industry-standard definition for what it actually is. Last I checked, there were more than half a dozen different ways of generating a figure you could call Xmax, they all give different numbers if applied to the same driver, and very few manufacturers state which they used anyway. ;)

Be that as it may. Driver distortion in terms of normal measurements is typically referring to frequency / amplitude response deviation from what it was designed to have, and harmonic distortion, which is affected by cone / suspension resonances and the motor design. THD, although popularly noted, is generally worthless without the individual plots since without those, you've no way of knowing what order the distortion is, where it is, how much it changes with drive level etc., all of which are significant elements in the basic character. Some of these will be affected with driver age -either through the moving components moving out of spec through deterioration with age / use / other environmental effects, or the magnet losing some strength. Depends on how well it's been handled / treated, but as a rule, it's more likely than not that an AlNiCo magnet will have lost some strength over time & require re-magnetising to bring back spec. Ferrite tends to be quite stable, likewise the rare-earth types. Something to look out for with some vintage speakers (as in speakers rather than raw drive units) is damping material from the box getting into the driver basket, particles in the magnetic gap etc. It's more of an issue in the US, since it was more common there to staple thin layers of damping material across the rear of the driver in reflex boxes, adding more layers until a basic click test indicated the alignment was properly damped, without [excessive] ringing. A very sound policy on the whole, but it does mean that over the years, fiberglass strands (the most popular type of material used) had more opportunity of getting into the driver, which doesn't do it many favours.

That apart -as was noted, if electrolytic caps were used in the crossover, they're likely to have dried out & moved away from the design value; generally its better to replace like-for-like if you don't know what the original transfer functions / acoustic slopes were and want to keep it reasonably 'original' as it's quite possible that the ESR of the cap was accounted for in the filter design -it almost certainly will have been in a quality speaker. Otherwise, you either have to keep your fingers crossed, or measure the impedance, phase & frequency responses of the drivers in the cabinet & design a new filter. Cabinets, if they haven't been subjected to abuse, are usually OK & rarely have any issues other than possibly some leakage if a glue-joint has failed, which is easy enough to fix. If it is a type with a screw-on baffle, I'd be wary of making any major changes unless a gasket is obviously perished, as a small amount of leakage may have been accounted for (or even in a handful of cases intended).

[awkwardbydesign]

That's not exactly what it says here:

http://www.rare-earth-magnets.com/content/pdf/Permanent-Magnet-Selection-and-Design-Handbook.pdf

That's useful, thanks.

[Barry]

I have to confess I've never understood the desirability of Alnico magnets over rare earth types. Surely a magnetic field is a magnetic field regardless of source? Can someone enlighten me - and whilst we're at it, explain why field coils are thought by some to be the best of all?

[struth]

Supposedly greater immunity to flux modulation, greater heat stability and greater suitability to shielded applications..personally i like em as they sound good.

[Barry]

Supposedly greater immunity to flux modulation, greater heat stability and greater suitability to shielded applications..personally i like em as they sound good.

Can you explain what you mean by flux modulation Grant? Surely all permanent magnets are saturated. Are you saying the flux created by the speaker coil tends to demagnetise the magnet?

[struth]

I think ferrite mags had a copper shorting ring around the pole to make them respond better. Ferrite without these doesnt fair so well flux stability wise, or so i,m told. Ferrite is crap stuff but because of all the work done on them they are pretty much as good now..also there is lots more ferrite around..think the availability of alnico magnets stopped for a while( maybe due to a African war but not sure)
If you put too much current through alnico it can demagnatise. More of a problem now although this too can be enginnered out, but its very expensive so ferrite wins... Doesnt sound so good imo though.