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Old 13th February 2004, 09:08 AM   #1
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Join Date: Dec 2003
Location: Mumbai
Default Assessing driver power rating / procedures

HOW THE POWER OUTPUT IS RATED FOR A DRIVER

AND THE CRITERIAS THAT NEEDS TO BE CHECKED BY USERS TO DETERMINE THE RATING

PLUS

WHAT ALL PARAMETERS DOES A RATING DEPEND ON

hi

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


FIRST SECTION

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

------------------------------------------------------------------------------------

SECOND SECTION

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 :

a)
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)

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


thanks Amp Man , hope this comes of use ,


Suranjan Das Gupta

Transducer design engineer
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