Precision Acoustic Resistance

Posted by: Phill82 on 17 January 2004

Do any of you technical people out there know much about Naim's PAR system and the advantages it is meant to bring?

The title of the final year major project that I'm doing is 'Novel loudspeaker designs', and I thought, being a bit of a Naim speaker fan, that it would be interesting to look at this technique. However there seems to be very little information around on it.

Just incase you don't know what I'm on about, the technique involves spliting the loudspeaker enclosure into two, one small box in which the bass driver is mounted and a larger box which the smaller one vents into via an acoustic resistor.

If you look at it from a low frequency modeling point of view, the acoustic resistance effectively just adds resitance to the driver's own mechanical resistance, thus reducing the Q of the driver and allowing it to be used in an overall enclosure of smaller volume (but with a corresponding reduced low frequency extension). So from this point of view you might as well just design a driver with the correct resistance in the first place and mount it in a single large enclosure. Saying that though, I am asuming that the value of the acoustic resistor is not frequency dependant, so that would probably affect exactly what it does.

However, I'm guessing that there are other reasons why the technique is advantageous other than allowing you to mount a high Q driver in a small box. Does it affect the mid range? I know that mounting the bass driver in a small box is meant to have benifits in terms of box panel resonance, but there are other simplar ways of achieving this that don't use an resistor.

If anyone has any comments or could shed any light on this then that would be very interesting and possibly very useful.
Posted on: 18 January 2004 by Phill82
Don't think you need to give a technical comment - does anybody have any views on the difference between Naim speakers and other speakers just from listening to them?
Posted on: 18 January 2004 by Andrew L. Weekes
quote:
does anybody have any views on the difference between Naim speakers and other speakers just from listening to them?


Well, not all Naim speakers use the PAR, only the ones I happen to like (except I've never heard a good sounding set of DBL's yet...)

The IBL and SBL are the ones I'm familiar with and are very different in sound from almost every other speaker I've heard, they have, at first listen, less bass compared to the 'norm' of a box loudspeaker.

What they lack in subjective depth though is more than made up for by quality, they have an uncanny ability to play tunes down there.

The most unusual element to my ears is that even when the fundamnetal is missing, your brain seems to naturally 'fill in' the missing bits. There's no conscious effort to do this, in my view, it does something right that few other speakers do.

You may be interested in the following snippet from the late Julian Vereker, obviously responding to a comment comparing SBL's to another speaker: -

"There is yet another issue, the speakers probably have completely different bass reponse.

For example the SBL has a practically immeasurable bass resonance and a sub-critical Q - both of which are uniquely defined by the PAR and the volumes in the two cabinets.

Other speakers have obvious bass resonances and Qs of varying values and magnitudes - these are defined by the DCR of the voice coil and the amount of copper in the gap - and here is the rub - as soon as one tries to reproduce a note, the coil get hot (DCR increases) and it moves, changing the amount of copper in the gap.

So there you have a system which changes both bass resonance and Q as you play music - a recipe for a lack of fidelity."

I rate my IBL's as the best speaker I've ever heard, in terms of the ability to communicate music (better than the SBL), but the don't do deep bass and don't go loud.

Andy.

[This message was edited by Andrew L. Weekes on SUNDAY 18 January 2004 at 23:15.]
Posted on: 19 January 2004 by JohanR
This is how I see it.

Normally, the Q of the driver (Qts) is dominated by the electrical Q (Qes), and the mechanical Q (Qms) is quite a small part. As stated by JV Qes varies with level booth as the temparature (and therefore the electrical resistance) changes in the voicecoil and by the physical displacement of the voicecoil inside the magnetic gap. Not good.

Making the mechanical Q the dominant one by adding a mechanical resistance is therefore quite clever and simple solution as this mechanical resistance (probably?) is much more linear with level. And, of course, it has been used by others.

Some of the Swedish Stig Carlsson designs used this in the 1960:s together with bass reflex. It was implemented with cloth around the holes in the basket.

Dynaco loudspeakers in the 1960:s used some kind of mechanical damping, I'm not sure exactly how it was implemented. Dynacos loudspeakers was made in Denmark by a company now known as Dynaudio.

JBL L50 used it in the late 1970:s, according to their own sales material. It was by a glass fiber "basket" behind the driver.

It has been used on subwoofers in car stereos, here it looks something like the small bass driver box in a SBL (or probably more like a DBL) that vents out into the luggage space.

The bass driver in my own Royd RR3:s has cloth around the back of the basket.

And a really wild one! In the 1980:s Dynaudio used it together with the Isobarik principle in their big Consequence loudspeaker. The back driver was loaded by a mechanical resistance.

JohanR
Posted on: 19 January 2004 by Laurie Saunders
Andy......someone other than me has not been bowled over by DBLs....

Laurie S