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A question from an EXTREME novice!!

 

New member
Username: Fixedrider

Post Number: 1
Registered: Dec-04
Hello All,
First let me start of by saying that this forum is top notch! There's alot to be learn here and I'm glad that all of you are here!
Now to my question...
1. I know all of you are rolling your eyes but what is speaker impedance? What advantages are there to setting up your system with 4 ohms as opposed to 8 ohms, or vice-a-versa?
Thanks
 

Silver Member
Username: Frank_abela

Berkshire UK

Post Number: 244
Registered: Sep-04
[Rolling eyes lordy lordy lordy where to start? :-) ]

Welcome to the forum Bob. Impedance is a description of the amount of load a speaker presents to an amplifier. It's the AC equivalent of resistance. Now as you'll probably know, if you short circuit a plug, it will go bang. The reason it goes bang is that there is zero resistance so, since Volts = Current * Resistance, to maintain the 110v (or 240v if you're in the UK) across the terminals, it would have to supply an infinite amount of current - hence bang! Impedance works in a vaguely similar way. The lower the impedance, the more AC current you have to send into the speaker to keep the same amount of volts across its terminals. Now this is where it gets tricky...

If I send a 1khz tone into a speaker, I can measure the impedance at, say, 8 ohms. Now if I send a 10khz tone into the speaker, the impedance might change to 30 ohms. If we plot a graph of the impedance against frequency (typically 20hz to 20khz which is our hearing range), you'll get a strange line which looks a little like a couple of mountains next to each other. Every speaker is designed differently and so has a different impedance curve. This is why the impedance of a speaker is often referred to as 'nominal' 8 ohms. The impedance figure that is quoted is a guide rather than any true figure, since it changes with frequency.

Now, when I send a 1khz tone into the speaker, it'll generate a sound. The loudness of this sound is measured in decibels. These are logarithmic in scale, so 87db is half as loud as 90db. If I use 1 watt of power to send a 1khz tone into a speaker and this generates 87db of noise at 1 metre, then you have a speaker of average sensitivity. However, it may be that you find speakers which are still harder to drive than most with this result, simply because their impedance curves are all different!

Broadly speaking, the world comes down to three main bands, being 4, 6 and 8 ohm speakers. 4 ohm speakers tend to be more difficult to drive since for that same 1 khz tone, the amplifier has to drive twice the amount of current into the 4 ohm speaker than the 8 ohm speaker to generate the same number of decibels at 1 metre away.

For your information, the average numbers are a sensitivity of 87 to 88 db/w/m. Impedance of the typical speaker is 8 ohms. However, manufacturers like to make people think their speakers are easy to drive. If the manufacturer quotes his speaker as a 4 ohm speaker, the measurement can appear that the speaker is 90db when it should be 87. If people just look at the sensitivity, they get the misleading impression that the speaker is an easy load when in fact it's an average load, so always keep an eye on both values, impedance and sensitivity. The combined are the true guide of the speaker's characteristics.

Some speaker manufacturers such as Living Voice, JBL and Cerwin Vega are renowned for building sensitive speakers. These guys make 8-ohm speakers that are in the 94 - 104db/w/m bracket! Others, such as ATC and Totem are renowned for making speakers that are a hefty load being of the order of 84db - 88db/w/m with 4 - 6 ohm speakers. Typically, speakers that are hard to drive give a well controlled, clean sound. The easier to drive speakers tend to have more rough edges, but this is a generalisation and there are exceptions such as Living Voice above.

So there is no true advantage with setting up a system with one type of speaker or another. The important thing is to take these points into account when matching the amplifier to the system. If you're going with a set of 4 ohm speakers, you need to ensure the amp can drive them and is well within its operating envelope. If you go with the sensitive speaker range, then you have to bear in mind that even a low volume setting may be loud and you're now on the other edge of the operating envelope of the amp, which can be just as bad! I personally have a crazy situation where I have a super powerful 350 watt/ch amplifier feeding a pair of 94db/w/m speakers. It just worked out that way! I have to be careful not to turn it up too far since I'll end up catching the drivers, otherwise there's no problem. On the other hand if you try to feed a pair of ATC 10's with a weedy little 30w/ch amp, then it'll begin to distort very quickly and this will either break the speaker or fry the amp.

So good luck in your quest - you've worked out now that it's a quest right? :-) Try to find a dealer who has a name for good advice. When s/he makes recommendations, ask why he chooses certain combinations in the shop and then you'll get a feel for how the products he has in the shop work together.

Regards,
Frank.
 

J. Vigne
Unregistered guest

Here's a site that might give you some answers to your questions:

https://www.ecoustics.com/electronics/forum/home-audio/67821.html


 

New member
Username: Honk

Post Number: 9
Registered: Nov-04
to keep it simple...the lower your impedance the harder your amp has to work to give you, basically, the same speaker volume...as far as i know, the main problem occurs when you drive more than one set of speakers from one set of amplifier out-puts.(i.e. A+B front speakers or paralleling into other rooms (bathroom,kitchen etc.)).each time you add speakers, the impedance goes down...8 ohms + 8 ohms = 4 ohms, + 8 = 2. most receivers can't handle this.they either (hopefully)shut down, or,(in my case) go on to receiver heaven....i hope this was the answer you were looking for.
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