Crank up the Volume [II]



continuation from [i]…

Big drivers are also prone to cone breakup if the frequency is not with the limits of the loudspeaker spec (inner part of the cone moves rapidly while the outer part can’t keep up because of mass, this can and will destroy the cone if not managed properly which of course manifests itself in terrible distortion. One needs to be totally deaf not to hear it).  And then you also have the power limits of most tweeters – they cannot handle the huge amplitudes needed to drive bass speakers.  Often listeners aren’t even aware that the tweeters have been damaged.
Filters: High Pass and Low Pass
So, the manufacturer of your loudspeaker enclosure uses a cross-over filter which uses capacitors and inductances (a capacitor displays reverse properties to that of an inductance i.e. the ‘resistance’ or reactance drops as the frequency rises).  So, by applying a capacitor in series to the high frequency driver (the tweeter) as the frequency rises more power is fed to this driver and by adding an inductance in series to the woofer (the low frequency driver), less power will be fed to the loudspeaker as the frequency goes up. Of course some loudspeaker enclosures carry more than two drivers, sometimes there is a squawker as well (mid range driver) – here the manufacturer would apply a filter comprising of both inductances and capacitors configured to allow only a certain range of frequencies through.

How good is this cross-over system?
Manufacturers have built some outstanding loudspeaker systems using the passive crossover network but they use extremely expensive components as opposed to the entry level sound systems one would get off the shelf.  These more expensive units are also very much designed around a specific speaker type, with specific dynamics in mind, so don’t fiddle.  In all likelihood you may think you have done a wonderful job by swapping bass drivers with a more expensive unit but in all probability if you haven’t consulted an audio engineer your final result may be worse.  Of course passive crossovers have one very important drawback – power loss.

In simplistic terms passive crossover networks are popular and the sound quality leveraged from even an entry level system is quite good but the enthusiast should be looking at an ‘active’ system – one which has crossover points that can be changed by the user, and an amplifier for each range of frequencies. The subwoofer is a classic case. Their cut-off frequency is very low but the entire system is designed around the loudspeaker driver.

Although I could drone on and on about this subject, readers do have access to some excellent reading material on the web with regards to filters, one of course is www.wikipedia.com.

Next article: Bipolars versus MOS devices (and radio valves/tubes as well). – please note these articles aren’t meant for the hard-core electronics engineer.  In time we are hoping that readers will start submitting their own ideas and gripes on to the forum about the various topics covered.

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