why do they build hard to drive speakers? (1 Viewer)

[Arthur S]

Actually, Advent designed its Smaller Bookshelf speaker as a 4 ohm (25 years ago), for no particular receiver, just knowing that virtually all receivers will deliver all they can into a 4 ohm load. Both the original Advent and the Smaller Advent were smashing successes, and I used both of them with ordinary receivers at very high volume, with no problems at all. You just had to be there.

 

[John Garcia]

Besides impedance, there are a host of other factors that ultimately determine how much current a speaker draws. Just because speaker A is 4 Ohm and speaker B is 8 Ohm doesn't necessarily mean speaker A will draw more current than B.

I understand what Andrew is saying, and I agree that the terminology is a little backwards. Because a lower impedance speaker allows more current to flow it is harder to drive than a lower impedance one simply because you are using more current. As long as you have enough available current it isn't a problem.

 

[Holadem]

If what you are saying is that the spec called "impedance " is just a gross approximation of the electrical property of the same name then yes, I agree. The actual impedance of a reactive device like a speaker is a mathematical expression which has no place on a consumer product. The approximation will have to do.

If you are saying that there is a property other than impedance which determines the current response to an input voltage, then I have to disagree with you.

--
H

 

[JeremyErwin]

In its speaker reviews, Stereophile includes graphs of phase and impedence versus frequency.


source

so, might someone explain "phase angle?"

 

[Phil_O]

"Resistance" usually implies a DC network. For our HT's we are using of course AC networks. Impedance is the AC network term used that is made up of resistance, inductive reactance, and capacitive reactance. The phase angle of an impedance has to do with the inductive and capacitive properties of the load (speaker in this case). At lower frequencies, such as 59 Hz, the capacitive reactance has the most effect on the total impedance at that frequency. (source: my college Intro to Circuit Analysis text). It must be that at that frequency of 59 Hz, the extra capacitance is what makes the speaker harder to drive.

 

[John Garcia]

Yep, that is what I'm getting at. That is why they qualify speakers as "nominal" impedance, more or less giving you an very vague clue to the fact that give two speakers withsimilar specs, the lower nominal impedance one will almost certainly draw more current. If you take two speakers that have different designs though, say a small 4 ohm bookshelf and a 8 Ohm large multidriver floorstander, the 4 Ohm speaker shouldn't automatically be assumed to be the more difficult load to drive.

 

[ChrisWiggles]

In terms of being hard to drive, we should also not forget the sensitivity spec which is of paramount importance as well.

While an amplifier with some gain will be under more strain with a lower impedance load, that isn't the whole picture either. Because one speaker may be a great deal more sensitive, one speaker may play a lot louder at that gain setting than another.

So we should not confuse the speaker as an electrical *load* alone (basically the impedance characteristics and the amp) and what SPL you're actually getting out of the speaker, which is that and then also the sensitivity.

 

[JeremyErwin]

this page looks like it covers the whole reactance, inductance and resistance relationship pretty well-- it's aimed at amplifier builders.

 

[Phil_O]

good site, thx for that link

 

[JeremyErwin]

and in simple language:

source
Wow. With all these new terms I'm learning, I might just be swayed by the claims of biwireists....

 

[Holadem]

At the input terminals of the purely resistive load, the voltage and current waveform are in phase, i.e., they peak and dip together. But if the load is reactive (like most loads), meaning it's has capacitive and/or inductive properties like a speaker, one waveform will lead or lag the other. The phase angle is a measure of that lead/lag. It is expressed in degrees, up to 90 deg or half a period. A basic understanding of a sine waveform is needed to see how the phase angle relates to time.

Why does the phase angle matter? Because in a nutshell, the maximum power both waveforms represent is delivered to the load when they are in phase. The more the waveforms are out of step with each other (i.e., the greater the phase angle), the less actual work they do in the speaker. Hence the drive difficulty in your quote: not only does the magnitude of the impedance dip down to 4ohms @ 59Hz, a good part of the power the amplifier is straining to provide at that frequency is not used, but reflected back to the amp by the speaker's reactance.

Note that the phase angle is part of the impedance, not a separate property.



The graph in your link (reproduced here) shows the magnitude and phase angle of the impedance accross the entire audible frequency range, thereby providing a complete picture of the device's impedance.

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H

 

[Arthur S]

Holadem

That is one impressive explanation and presentation of phase angle!

 

[Holadem]

Thanks for the kind words Arthur. I hope it's not condescending to say that it's really basic stuff for an EE and there are quite a few around. Any of them could explain this in their sleep.

--
H

 

[Arthur S]

And Holadem, thanks for answering the question I was wondering about, but didn't ask

 

[Parker Clack]

One reason that speaker impedance has changed over the years too is that years ago amplifier power was hard to come by (aka expensive) and speakers were designed so they were easier to drive (aka known as requiring less current to drive them). Then amplifier power became relatively cheap so speaker designers didn't have to worry about lower impedance that much any more in their designs. As has been stated impedance is a mean or average of a load to what the circuit sees. The speaker may have a nominal load of 8 ohms but in effect the load can go from low to high depending on the sine wave it is sent. I like to think of it as a variable resistance.