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MultiWay Conventional loudspeakers with crossovers 

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11th September 2019, 09:06 AM  #1 
diyAudio Member
Join Date: Aug 2018

Speaker impedance linearity question
Hey guys,
I have a doubt that's been bugging me for a while. By the way sorry if this is the wrong forum to post something like this is, but I couldn't find a more appropriate one. I've noticed that not all amps double their power from 8 to 4 ohms, and some of them even lose power going from an 8 ohm load to a 4 ohm load (I was thinking about some hifimediy chips for example). My question is: given the fact that most loudspeakers have varying impedances at different frequencies, why isn't it that there's enormous differences in frequency response between different speaker + amp systems? I mean, a chip amp that puts out more power at 8 ohms than 4 ohms should sound absurdly different from a very good amp that doubles its power into 4 ohms. What am I missing? Thanks for the help 
11th September 2019, 09:21 AM  #2 
diyAudio Member
Join Date: Apr 2007

Often the amplifier has to adapt the supply upon the Z of the load.
Lower impedance means lower voltage but higher current. Or, a lower Z would provoke a major tension drop than a standard ( 8Ω ) impedance. So it's better to design the amplifier on the desired load, being the amp a voltage source.
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11th September 2019, 09:55 AM  #3 
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Join Date: Aug 2018

Thank you very much, but you've already lost me there
Sorry I have a very basic understanding of electrical stuff, and I find it hard to translate it to real world applications. When the voltage decreases and the current increases because of a lower impedance, shouldn't this correlation be linear in the sense that the total power output stays the same? That would make the most sense to me since then, whatever the load, the Decibel output would be the same. But then again, most amplifiers are rated at a higher wattage when driving 4 ohms. So how does a perfectly flat frequency response speaker (on paper), with a gently sloping impedance towards the higher frequencies, stay exactly linear when driven from different amplifiers? I don't know if I'm explaining myself correctly, I'm sorry 
11th September 2019, 10:08 AM  #4 
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Join Date: Dec 2015
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If the amplifier's output impedance is a lot lower than the speaker, the voltage doesn't change much, the amplifier can be considered a voltage source.

11th September 2019, 10:26 AM  #5 
diyAudio Member
Join Date: Aug 2018

Thank you very much.
I think I'm starting to get it. For example, I know that at resonance frequency a speaker is at its most effecient frequency range. So the impedance goes waaay up, and the power needed goes waaay down; reading it like this, when the impedance goes up, to maintain the same DB output I will need less power (which makes sense, amps output less watts at higher loads), and when the impedance goes down the opposite happens. So we have a flat frequency response. But then I'm not sure about one thing: if I consider two identical speakers (but one is a constant 4 ohms and the other a constant 8 ohms) driven by an amplifier that doubles its power into 4 ohms, does that mean that they both reach the same output level and that consequently the 4ohms speaker is rated 3db less sensitive than the 8 ohms speaker? 
11th September 2019, 10:35 AM  #6 
diyAudio Member
Join Date: Apr 2007

Of course, the sensitivity data is taken with the standard procedure of 1W/1m
With 8Ω to obtain 1W you put 2.83 V and with 4Ω you put 2.04 V or something like that after the zero, do the Decibel calculations!?, which equals to 3 dB
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11th September 2019, 10:40 AM  #7 
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Thank you very much to both, if I have any further doubt I'll make sure to write here again

11th September 2019, 11:20 AM  #8  
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Join Date: Dec 2004
Location: Next door

Quote:
The logical procedure is to inject 2.83 Vrms voltage without taking account of the impedance. Note that the usual frequency reponse of a driver is nothing less than a voltage sensitivity response at each frequency. Each time that, for a given frequency, a sensitivity in SPL for 2.83 Vrms/1 m is converted into sensitivity for 1 W/1 m, it is supposed that the impedance is 8 Ω. Which is often not the case. Or even it can be the case but the impedance is not resistive. So the calculation is wrong. For loudspeaker sensitivity, always think in Volts. Even if the output impedance of the driving amplifier is high, what counts for the level is the voltage across the driver. 

11th September 2019, 12:15 PM  #9 
diyAudio Member

+1  I'll just join the chorus.
A modern audio amp is simply a voltage amplifying device. The ballpark range is like one volt input from the preamp results in maybe 20 volts at the output jacks. Doesn't matter at all what the impedance is, provided you aren't challenging the amp's limits. (That of course works just great for biamp'ed systems where the precise voltage gets to the driver every time at all frequencies. With a passive crossover, sort of a crapshoot what gets through the XO and according to the varying impedance of the driver, inductance, backEMF, and peculiarities of the XO components) The power OP is referring to is the rating of maximum watts into speakers of different impedance. Like a lot of max ratings, kind of more a matter of how much burning smell you can tolerate than any exact figure. In practice, what max watts am amp can deliver to loads of various impedances is limited by a number of parameters internal to the design of the amp.
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11th September 2019, 01:41 PM  #10 
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Join Date: Oct 2008

BackEMF is a component of the impedance.

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