More Obnoxious Impedance Questions

GreyBeard · Dec 20, 2002

Originally posted by Bob Lee (QSC)

I'd say go with the bridged operation. You'll have more headroom, which should give you better punch. Damping factor is not an issue; it's determined more by the speaker cabling (a fact that amplifier marketing specs conveniently ignore) than by the amp. And if you use good low-resistance cabling, you'll have both good damping and negligible power loss.

Damping factor is a thing that has always puzzled me and I hope you can explain it to me in a way that makes sense. As I understand it, damping factor is the ratio between the internal amp resistance and the speaker/cable resistance. Some papers read that it is real important and others dismiss it as you have. Crown for instance seems to think it is a big determining factor in a amps performance. And, get this, my tube amp even has a switch to turn damping up or down.

geshel · Dec 20, 2002

Originally posted by GreyBeard
As I understand it, damping factor is the ratio between the internal amp resistance and the speaker/cable resistance

Herin lies the rub.

It's actually the ratio between the (internal amp + cable resistance) and the speaker resistance! An amp with a rated damping factor of over 100 has less output resistance than the cables will have, so anything much above that isn't really any better.

The difference between tube and SS amps is noticeable, though.

Solid-Body · Dec 22, 2002

Part I. 
Why the 180? First I'm a moron who needs to read more. Now you agree with me. 
Originally posted by Bob 
In a linear load, such as a loudspeaker, the current through it is exactly proportional to the voltage applied across it. Current is the rate of electron flow, and voltage--or potential--is the pressure forcing the flow. In order to double the current in the load, you must double the voltage. If 2 amperes of current is flowing through the voice coil, you can be assured that the voltage across it is twice what it takes to push 1 ampere through it.
 
Not true. You can double the current by halving the resistance.
 
My reply 
If you have 1 amp through the coil at x voltage and y resistance, then y/2 resistance and x voltage will induce 2 amps current. Thats roughly what we have when we put the speakers into a 2 ohm load, but its not exactly true as not all of the current is flowing through both speakers. 
This is where I explain that fact.
 
Bob's Counter-Reply 
Hey, man, you still don't get it. I think you should stop now because your explanations keep going off into uncharted territory. If you're interested in learning how amplifiers and speakers work, there are some good books on the topic. I would be happy to recommend any.
 
What unchartered territory? Does that mean territory you don't understand, or territory that has not been explored before? As Greybeard later said: The damping factor (having the speaker to stop moving quickly after a loud attack, like a shock obsorber on a car) is much higher at 8 ohms than it is at 2 ohms. My argument is an explanation as to why this occurs.
 
A later post by Bob 
In bridged mono, driving a load of X ohms is electrically equivalent to driving X/2 ohms on each channel, so that's something to keep in mind.
 
This is exactly what I was saying. If I need lessons, then you do too.
 
Yet, a later post by Bob 
I'd say go with the bridged operation. You'll have more headroom, which should give you better punch.
 
You said it. I didnt. Again, I was simply giving the reason why that button exists in the first place. If there is no difference beteen a 4ohm load and a 2ohm load, that button might as well be labeled Do Not Press or Self-Destruct.
 
Joris hit it right on the head by saying: A lower impedance means more power. There's really nothing more to it (the fact that voice coils respond to current is not important in this context). More power means a higher sound pressure level. Period. 
This is true when you bridge two channels (thereby combining their power) and placing that across the speakers in parrallel. This puts the combined power across each speaker. Assuming both channels are equal power, this doubles the power across each speaker. It also increases the current, but not the voltage. But, if you place the speakers in parrallel, both speakers drop half of the power and act nearly the same as before, but without the stereo effect.
 
Part II. 
SPL does NOT equal moving air. <a href=http://physics.mtsu.edu/~wmr/log_3.htm" target="_blank">SPL is the logarithmic ratio of pressure amplitude</a>, where pressure amplitude is a measure of the size in variation in air pressure caused by a sound wave. The formula for SPL:
 
SPL = 20 Log ( P / Po), where P is the pressure amplitude (in rms, average, whatever) and Po is an understood constant of ambient pressure, 2x10e-5 N/m^2.
 
The human ear measures pressure differences, therefore, SPL is the percieved level of sound (commonly refered to as volume, but it is not cubic volume).
 
If you still dont believe me, then look closely at the figures from that site. A difference in pressure of about 0.03% corresponds to a painfully loud sound. I bet you would return your oscillating fan if it only made a difference of 0.03% in moving the air around your house. In fact, fans are rated in feet per minute, understood as cubic feet per minute, or CFM; they are NOT rated dB (understood as SPL). Here is a table of actual ratings of various 12V DC motors used to cool electronic equipment: 

<table cellspacing=3>
<td>CFM<td>dB<td>Webpage
<tr><td>5.1<td>26<td><a target=_blank
href=http://www.circuitspecialists.com/level.itml/icOid/3265>Circuit Specialists</a>
<tr><td>13.7<td>28
<tr><td>37.8<td>31
<tr><td>42<td>37
<tr><td>62<td>42
<tr>
<tr><td>6.9<td>30.5<td><a target=_blank href=http://www.pclincs.co.uk/acatalog/PCLincs_Online_Store_Fans__Fan_Adaptors__Filters__Grills___Gaskets_10.html>PCLinks</a>
<tr><td>4.8<td>23.5
<tr><td>10.5<td>32
<tr><td>19.53<td>33
<tr><td>37<td>46.5
<tr><td>20.5<td>33
<tr><td>18<td>27.6
</table> 
Did you notice 37.8 cfm @ 31dB and 19.53cfm @ 33dB? This proves that it is possible to have a larger volume of air moving at a lower SPL rating. The numbers also suggest that volume and SPL are proportional, but does not prove it.

[Edited because colors did not come out correclty.]

geshel · Dec 22, 2002

Solid-Body, I'll say it again: the things you say indicate, to me at least, that you don't know what you're talking about - you've found this information from various sources and are assembling it hodge-podge in a manner that best suits the way you think things work. But, you're wrong. I've tried to explain this several times, but it's all such a jumble inside your head that for now at least I'm giving up. You can believe what you want about current and voltage and speakers, I won't argue with you anymore (unless you start giving advice to people about such topics).

Best of luck.

throbbinnut · Dec 22, 2002

Solid,

Buddy, I'll just point out 2 things that are in error.

The decibel system is used to describe the relationship between any 2 quantities. That Po you reference refers to the reference Power, not some fixed quantity. Also, for power, I do believe it's 10 * Log ..... , not 20 * Log....

And that SPL / dB spread you show is the flow rate for the fans, and the noise they produce at that flow rate. That's all. It's a dealer's chart that shows you what your choices are. That's not a fundamental relation between SPL and CFM. You can have a crappy cooling fan with a low SPL and high dB, or you can have a really nice fan with High SPL and low dB.

Chris

geshel · Dec 22, 2002

Yes, a cooling fan is hardly a good thing to compare to a speaker.

But, throbbinnut, the equation Solid-Body gave is for dB SPL, and Po is an actual constant. That, at least, is correct.

If somebody could explain to me how the same size speaker could move more air at 50Hz and 90dB SPL, that another one does at 50Hz and 90dB SPL, I'd greatly appreciate it.

Mark Reccord · Dec 22, 2002

Originally posted by geshel

If somebody could explain to me how the same size speaker could move more air at 50Hz and 90dB SPL, that another one does at 50Hz and 90dB SPL, I'd greatly appreciate it.

I've been in the midst of a move and a 2000 mile drive so I've stayed out of this so far, although I've been keeping an eye on it.

That being said, I think we both know the answer to your statement there. I suspect Bob knows too...

Solid-Body; No offense intended, but you're a bit out of your league here. Bob, Throb, Joris and Geshel are all EE's, and Bob actually works at QSC Audio. That might suggest he has some idea what he's talking about

You're missing some key concepts. Any of these guys (me too, but not as well as the other guys) would be happy to talk at length about the subject, but I hardly think treating these guys like they don't know what they are talking about will result in a productive exchange.

throbbinnut · Dec 24, 2002

Originally posted by geshel
But, throbbinnut, the equation Solid-Body gave is for dB SPL, and Po is an actual constant. That, at least, is correct.

Okey dokey. Point taken. I've gotten so used to drilling theoretical dB's into peoples' heads that I wasn't thinking about SPL dB.

Nowadays I mainly use dBm for optical power with fiber optic transmission systems, and in that case, it's referenced to 1 milliWatt, hence the dBm notation instead of just dB.

Chris

Solid-Body · Dec 28, 2002

{You is being used in the collective sense, except where specified.}
Congratulations on your BSEE, for those who have them. I know how much work is involved in obtaining one. I am also glad to hear Bob has a good job at x. Good jobs are hard to come by these days.

But, with all due respect, I would expect better arguments from EEs. I have stated my claim and backed them up with respected theories from multiple disciplines. I would welcome defeat to my argument if I was shown the error of my argument, but so far I have only heard personal insults and misdirections. It would be a good learning experience for all.

In fact, I thank you for the compliment. From someone you believe to be so ignorant on the subject, you seem to have trouble pointing out where my foundation is crumbling. Yet I have pointed out the cracks myself. I might suggest going back to college to obtain your Masters so you can become a professor. You seem to have just the right amount of disdain and condescension that make typical professors so endearing to students.

I have come with the argument of reason, and have been assulted with the stones of "Trust me", "You are ignorant", and "I am smarter than thou". It is quite obvious I have committed the crime of heresy in the "Kingdom of 8ohm Linear Loads". I meant no harm, only discussion. But banishment and censorship are the punishments for a poor peasant like myself. I shall retire to the land of Australia to practice my witchcraft of circuit repair and design. Let all EE Bishops strike the records of my heresy.

And remember, do not even think about the 2ohm load switch. It is the work of the Devil! (Actually, countless EE hours and thousands of dollars were pumped into R&D to make it work. But, obviously, the Devil made them do it.)

Mark Reccord · Dec 28, 2002

Ok. In one of your earlier posts you insinuated that 2 speakers, one with a nominal impedence of 8 Ohms and one at 2 (but otherwise identical) dissipating the same amount of power will sound different, in that the 2 Ohm speaker will move more air. This is not the case. If they're dissipating the same amount of power, the acoustic outputs will be identical. In the end, the voltage and current don't really matter, their product does (ie power).
The only real difference is that you'd be able to get more ultimate power from the amplifier (assuming SS amp) with the 2 ohm speaker. If you run the power amp out to its maximum power, the 2 Ohm speaker will be louder than the 8 by virtue of more power being dissipated in it. But if the two speakers are set up to be dissipating the same amount of power, I can assure you that they'll produce the same acoustic output regardless of the currents and voltages being different.

You also seem to be saying that in a system like a speaker, you can change voltage, current and impedence without the change in each variable affecting the others. If you have a particular driver, if you want to double the current through it, you need to double the voltage across it. It's true that you can get the same result by halving the impedence, but you can't just arbitrarily do that, you need to change the speaker.

SPL=moving air and it's especially noticeable at low frequency. What we feel as air moving from a speaker is the compression and rarefaction of the air due to the movement of the speaker at low frequencies, ie the propogation of a sound wave. The more the speaker moves, the more intense the compression (or pressure). 100dB at 40Hz will feel like it moves more air than 80dB at 40Hz because it does. Power makes speakers move, not just voltage or current. You can't have one without the other.

Also, you can't really compare cooling fans or DC motors to speakers. That's like comparing apples to codfish.

Again, I still think you're out of your league here. You haven't backed anything up in any kind of systematic or reasonable way and your defensive attitude suggests that you don't really know what you're talking about.

Solid-Body · Dec 28, 2002

Eureka. Heretic has seen the light.

I'm not out of my league, but I'm wrong ... at least in my supporting reasoning. And now I see why. That's what I wanted to know. Why. I see what Bob, Geshel, and Mark (and everyone else) were driving at. Yes, in the fixed world of the speaker and amplifiers that exist, power is the key. I went into a tangent of wondering how current affects the speaker, disregarding what we actually have to work with. And I really don't want to get into that tangent again.

But my original statement of running a 2ohm load creating more ohmph is still correct because, as Joris pointed out, it DOES increase the power, nearly twice I believe, when channels are bridged. {Again, assuming the amp and speakers can actually handle the load.}

Anyway, sorry for the confusion. I'll still stay exiled, if you wish.

Joris · Dec 28, 2002

Solid-Body, we're really not dissing you. And I really can't discern a personal insult in any of the lines below:

Originally posted by Mark Reccord
Solid-Body; No offense intended, but you're a bit out of your league here. Bob, Throb, Joris and Geshel are all EE's, and Bob actually works at QSC Audio. That might suggest he has some idea what he's talking about You're missing some key concepts. Any of these guys (me too, but not as well as the other guys) would be happy to talk at length about the subject, but I hardly think treating these guys like they don't know what they are talking about will result in a productive exchange.

Your 2nd last post, however, is fairly rude. You might want to edit that one.

I still think you're missing a few EE key concepts, because you have posted more inaccuracies than you corrected in your last post.

Regards,
Joris.

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