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Old 19th July 2004, 12:53 AM   #1
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Location: Toronto, ON, Canada
Default Between the underhung and XBL^2 topology...

...which has the least distortion for low excursions, given that everything else is equal? Am I right in assuming that an underhung motor will distort slightly less, or will they, in practice, be equal?
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Old 19th July 2004, 01:21 AM   #2
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Default Underhung with Faraday ring for low excursion

For low excursion drivers I believe an underhung motor with a thick Faraday ring to keep inductance both low and flat would be the lowest distortion design. A radial magnetic field is sometimes also used, including approximations like the Seas Hexdym(sp?)

For high excursion drivers I believe the JBL Differential Drive motor which uses counter wound voice coils to nulify inductance would be the lowest distortion design. Anyone know when this motor patent expires?
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Old 19th July 2004, 01:39 AM   #3
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Where would the Faraday ring be placed, and why would it lower inductance in that position?

What exactly is a "radial" magnetic field? Aren't all fields in the gap "radial" as in symmetrical when rotated around the pole piece axis?

Also, please check out the "Best Sounding Magnet" thread, below this one on the first page. I've some questions in the last page.
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Old 19th July 2004, 06:20 AM   #4
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Default Aura Radial Motor

attached picture of Aura radial motor.

Center pole surrounded by copper Faraday ring,
voice coil gap
NdFeb magnet circular segments
Iron return path
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File Type: jpg aaaa.jpg (27.4 KB, 164 views)
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Old 19th July 2004, 06:28 AM   #5
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Default JBL Differential Drive

top and bottom magnetic fields are opposite. This allows top and bottom voice coil wire to be wound in opposite direction, and hence cancel out much of the coil inductance.
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File Type: jpg jbl_differential_drive_motor.jpg (75.2 KB, 152 views)
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Old 19th July 2004, 08:30 AM   #6
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Distortion :-

this statement can be said as correct that :

a power rating given to a driver , means that at all input levels of the driver rating within its specified limit , the driver :

1 ) should perform within the rating in long term use , robustness

2) should perform within very acceptable limits of distortion

3) and should show a certain limit of linearity in relation with the , actual accoustic power output increase to the input power supplied from the amps

without compression

the distortion is now adays mostly the deciding factor of the power rating including long term use

Here it needs to be devided in two sections


in a broad context

distortion / you will find some speakers distorting _ much before the voice coil actually burns out

for eg - considering a driver in which this takes place :

a driver or if they are pro audio or drivers with simple pole vent cooling with the coils set in a tight magnetic gap - the coil can be on a paper former , oridinary wires used in the vc s

the vc s temperature will not rise easily normally 60 deg max , at this time it will distort

then this distortion is not from the vc temperature

(though in most books vc temp is one of the main source of distortion , why ? read on ....)

there are 3 basic types of non linearity in a transducer , these generate distortion , due to this distortion the voice coil heats up / at this moment no amount of cooling can help ,

the distortion effect from driver non linearities are such that it may lead to vc temp rise exponentially leading to vc burn out

the non linearities are

1. non linearity in the suspension mechanism

2. non linearity in the motor sturucture

3. non linearity caused when a coil moves in a permanent magnet motor structure (this is bound to happen even if the above 2 non linearities are addressed well )

to check for these non linearities while purchasing drivers

1. suspension non linearity

for the x - max of the driver you would like to purchase (or the one specified by the manufacturer) meaning you either know the x-max or have one in mind , then :

step a
push the driver cone downwards by that amount , it should go down linearly (the stiffness of the suspension increasing linearly) to the amount of the x-max

step b
pull the cone upwards towards you from beneath and it should come up to the specified x-max , again stiffness increasing linearly

if this does not take place the driver will distort -

unless the transducer manufacturer identifies specifically that such a non - linearity has been addressed , by making suitable compensations in the vc itself

2. motorstructure non linearity

fringe fields exists at the magnetic gap - the motor structure should be - sfg a symmetrical field geometry _ topology

therefore while purchasing a driver look if the motorstructure is - sfg or not

since expenses goes up in a sfg structure it will be specifically mentioned mentioned by the driver manufacturer , to inform customers

3. Non linearity caused when a coil moves in a permanent magnet motor structure

explaining this nature of non linearity is complex , anyone interested can see the sections on shorting rings in my thread

what is important

is that : -
has this been addressed by the transducer manufacturer

it is addressed by shorting rings placed in the motorstructure

since this too involves expence , and a considerable slow down in the manufacturing process this too will be specified by the driver manufacturer , to inform customers


the above are the main sources of distortion that determines the safe operating power of a driver , and are not the only sources of driver distortion (other sources of distortion are eddys , bottoming to name a few)

due to such complex stuff good driver manufacturers all over the world can be counted on finger tips



in good drivers context

good drivers are in which all the above criterias have been addressed

then the rise in vc temperature over long time use / the cooling efficiency comes in , and mostly all speaker failures of reputated manufacturers result from vc burn out (as mentioned by Navin)

mostly these limits are tested thoroughly and drivers tested to distruction and safe regions specified

i have found driver malfunction due to negligence of owners to go in for a service contract when pro audio drivers are used eg - movie halls , discs

in home audio , sources of reputated driver burn outs are :

amp malfunction and dc component generation in the drive signal or signal clipping

speacially when high power speakers are used by under powered amps , many people do not even consider that a 35 w amp can damage a 100 w speaker system _ plain simple signal clipping can distroy well made drivers

if you have an under powered amp please ask a sound engineer to determine the clipping levels of your amp and restrict from running at clipping levels (in high end audio amps from manufacturers like Nad and onwards this is built in as standard , in the circuitry , therefore the prices)

or the suspension parts have outlived _ may be the owner !

Suranjan Das Gupta

Transducer design engineer
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Old 19th July 2004, 08:41 AM   #7
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as for low excursions - an underhung will be the best - provided the 3 non - linearites i have mentioned above have been carefully addressed

for extreme excursions - XBL - 2 is very good / but it will lack the active correction - provided by the differential coil ,

Differential coil drive is very good too - as it activly corrects the voice coil position resulting it never to go into undesired over - excursion there-by reducing - distortion

Line source

seeing the way JBL is upgrading the Eons etc to DCD i really dont thinl the patent will be running out soon - Yamaha - car audio subs used a double mag DCD - I hope u have seen the info on it

suranjan das gupta

transducer design engineer
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