Basic information about monitors

  • Thread starter Thread starter Xipe
  • Start date Start date
Ok, let's do some math.

I found this handy homework problem to help us. http://faculty.millikin.edu/~jaskill.nsm.faculty.mu/exp54.html

The useful equations are:

1. P = epsilon*sigma*Area*Temperature^4

and

2. (T/Trt)=(R/Rrt)^0.83

Solving for T in eqn2 we get:

T=(P/(e*sig*A))^0.25

assuming a filament with emissivity of 1 and surface area of 8e-5 m (5cm long by 0.5mm dia) we can obtain a ballpark temperature at 100 Watts of:

T=(100/5.7e-8*8e-5)^0.25 = 2200 K.

We also know P=IV and I=V/R which yields:

R=V^2/P

The RMS voltage of 110 VAC is about 78V so the operating resistance of 100W filament is:

R=78^2/100 = 61 Ohms

Now let's put it all together.

Solving eqn2 for the room temperature resistance we get:

Rrt = R/(T/Trt)^1.2

And we know room temperature Trt = 300K, T = 2200K, and R = 61 Ohms.

Therefore:

Rrt = 5.6 Ohms

or

R/Rrt = 10.9

So what have we learned from our back of the envelope calculation?

1. A light bulb has a SIGNIFICANT room temperature resistance.

2. The resistance changes by a factor of 10 at its operating temperature – well before it's ready to "pop".

Crunch these numbers on any bulb you choose, home, car, or panel light, and they'll turn out similarly. If you're still not convinced, take an ohmmeter to your bulb. Then please don't tell us putting a light bulb in series with your speaker has no effect on the sound.

barefoot
 
are you talking about the same bulbs / fuses as we are barefoot?
Or -- are you just spouting textbook tech crap again?

Lets see what we can conclude.... from this one and your posts about monitors:

Using fuses / bulbs has been done for decades by some of the world's largest studio monitor and speaker manufacturers, as it protects the (expensive) speakers with minimum audible effect on audio.
Of cause they are all wrong, have been wrong for years.

There is no difference between hi-fi "home" speakers and studio monitors, apart from studio monitors being in many cases better than monitors

You know what? You should be rich! Everyone is wrong and you are right!

Its a shame that you cannot combine your unquestionable technical know-how with some real-life experience.
 
Last edited:
barefoot said:
1. A light bulb has a SIGNIFICANT room temperature resistance.

2. The resistance changes by a factor of 10 at its operating temperature – well before it's ready to "pop".
Click to expand...

I love maths... But this wouldn't affect the sound. The resistance will affect the output level. But not the sound. To affect the sound, it would have to be frequency dependent, which; since the freq is not in your formula's, it isn't. Or not significantly. (Maybe more precise formula's, for dynamic behaviour?)
Or the resistance would have to be amplitude dependent. But since the 'mean power' of what you mix should be at a certain fixed level, the bulb will heat to an equilibrium temp, and it's resistance will stay the same (the changes in power will be fast enough so the bulb will not cool down...), thus not affecting the sound.

However. Nice try. :D
Give me more...
 
I always wondered what the hell that funky light show was coming out of the bass ports on my JBL Control 1's when I would push them hard. I am enlightened!
 
The force on the speaker cone F = i*B*L = Current*Magnetic Flux*Lenght of voice coil

In our series filament i = V/R(i) where R is a function of T which is a function of i. So R is a function of i.

This is a NONLINEAR EQUATION.

Let's think of what happens when we run a simple sine wave through our speaker-bulb. We'll assume the speaker impedence is constant.

The voltage starts a zero, the current is zero, the filament is at room temperature and the resistance is constant. The voltage starts rising, the current is still low, the filament is close to room temperature and the resistance is still pretty much the same. All if fine. The speaker cone is tracking the input voltage closely. But now as we continue our way up the sine wave, the higher voltage causes the current to rise. The rising current instantaneously heats up the filament which causes the resistance to rise. (Yes there is an overall equilibrium temperature, but instantaneous fluctuations are still occurring) Suddenly each incremental step in voltage no longer produces the same increase in current, and the force per volt on our speaker cone starts dropping. This has the effect of flattening the top of the sine wave.

We haven't just changed the amplitude of the wave. We've altered its shape. Changing the shape is tantamount to adding frequencies – frequencies that weren't there to begin with. There is a name for this. It's called distortion. Maybe you've have heard of it?

Even if this dynamic compression weren't the case, simply adding a large resistor in series with your speaker reduces damping, as I have discussed earlier. Less damping results in more intermodulation distortion.

I stand uncorrected.

barefoot

P.S. - As for my "unquestionable technical know-how" combined with "real-life experience":

(Sorry folks. I hate this sort of thing. But, when challenged, I feel the need to defend myself.)

I've been designing and building loudspeakers for over 10 years. 5 of those years I ran my own company. Currently I engineer laser/acoustic systems for doing atomic resolution thickness measurements on computer chips. I also designed and built experimental ultra-high-vacuum equipment in graduate school while studying physics. I published a paper ["Low-energy broad beam electron gun", T.W. Barefoot, H.D. Ebinger, J.T. Yates, Journal of Vacuum Science and Technology A, 15(5), Sept/Oct 1997] regarding one of my devices which just happened to contain a sophisticated resistively heated thermionic emitter - i.e. a filament.
 
wonderful barefoot, congratulations

I'm a mere recording engineer and producer, I just work with my ears, and if you have more then 20 CD's in your collection, there is a fair chance that I have worked on at least one of those.
Oh by the way ......... I think I ought to warn you. It will sound horrible, and so do all the other CD's you've got. They have all been recorded using distorting reference monitors. If only I'd known you before.
 
Last edited:
sjoko2,

I didn't realize you were Rod Elliot of Elliot Sound Products. Did you recently move from Australia to California?

Perhaps you should give credit to the person who's work you are posting? In fact, it's easier to just give the URL . http://www.sound.au.com/project33.htm

barefoot
 
don't tell me that was something you actually agreed with?
I was trying to make a point - pretty futile waste of time
 
Great comeback barefoot! ;)

Should have known that. hehe. Still. If the bulb gets to its equilibrium temp, the temp changes will never reach their full range. The warming and cooling needs far more time than the voltage and current changes (which will be at least a few Hz, and above 10kHz in normal music covering the full spectrum). But there will indeed be changes.

I still doubt that they will affect the sound audibly after reaching eq temp anyway :D . (In fact... even less when using bad DAC's :D )

Don't go calling names again, now. You've got a very nice reference, and I learned alot from your posts, but get off your horse... You're smart enough not to feel attacked that fast. Peace...
 
Roel,

I'll concede that in the days of magnetic tape and vinyl (not so long ago) these effects might have been incidental. But now we have 24 bit 192 kHz recordings and media to play it through. The bar is continuously being raised. Loudspeakers are the weakest link in the entire audio chain, even well beyond microphones. They always have been. Every bit of improvement counts.

You're still off the mark about the transient heating mechanisms in resistors, but I'll let it rest.

It was good debating with you all. :) I'm over and out on this thread.

Thanks,
barefoot
 
You must log in or register to reply here.
Back
Top