[ThumperSD]

Speaking in general, would a 5" driver produce better highs and mids compared to a larger 8" driver?

 

[mcmalden]

If you upscale a 5" driver to an identical 8" driver you basically got the same sound I think. Weither a tweeter is good depends on it's weight and friction compared to it's driving power.

 

[mark2410]

essentially, yes
 
this is why tweeters are small.  of course the same result could be achieved by throwing more power at the problem

 

[mcmalden]

Fr = m x a

 
 

 

[Ikarios]

In general, smaller dynamic drivers have faster diaphragms in that they return to the "resting state" faster than larger diaphragms - think about a giant circle of rubber vs a tiny circle of rubber... if you pull away the center of the rubber, the smaller one will "snap' much faster than the large one. Same principle applies to headphone drivers.

 

[mcmalden]

In general, smaller dynamic drivers have faster diaphragms in that they return to the "resting state" faster than larger diaphragms - think about a giant circle of rubber vs a tiny circle of rubber... if you pull away the center of the rubber, the smaller one will "snap' much faster than the large one. Same principle applies to headphone drivers.

t = (s:1/2a)^(1/2)
a = Fr : m
The relation between the force the rubber applies on itself and it's size/mass should be linear I think. The relation between pulling height 's' and the force isn't though so the accelleration should be about equal at relative pulling length. Though the larger rubber cirkel has got more friction caused by air so that will make it slower. But, if you pull both rubber cirkels to the same absolute height, the larger should be just as fast or even faster:s.
To eliminate most air friction tweeters don't produce bass and mids which cause most diaphragm movement.

 

[High_Q]

I think it depends on how much power required on the big driver.  The sound wave perceived is based on frequency of the physical wave.  Tweeter is smaller which means less force required means less current or voltage(power dependency on both, see RMS equations below) required to drive it to high frequency, so therefore easy to drive.  The Subwoofer sized speakers, it better to have for low frequencies since it take alot of power in RMS to drive them.  
 
 

Average electrical power

Electrical engineers often need to know the power, P, dissipated by an electrical resistance, R. It is easy to do the calculation when there is a constant current, I, through the resistance. For a load of R ohms, power is defined simply as:

However, if the current is a time-varying function, I(t), this formula must be extended to reflect the fact that the current (and thus the instantaneous power) is varying over time. If the function is periodic (such as household AC power), it is nonetheless still meaningful to talk about the averagepower dissipated over time, which we calculate by taking the simple average of the power at each instant in the waveform or, equivalently, the squared current. That is,

	(where   denotes the mean of a function)
	(as R does not vary over time, it can be factored out)
	(by definition of RMS)

So, the RMS value, I_RMS, of the function I(t) is the constant signal that yields the same power dissipation as the time-averaged power dissipation of the current I(t).

We can also show by the same method that for a time-varying voltage, V(t), with RMS value V_RMS,

This equation can be used for any periodic waveform, such as a sinusoidal or sawtooth waveform, allowing us to calculate the mean power delivered into a specified load.

By taking the square root of both these equations and multiplying them together, we get the equation

 
By the way, there is a equation relating frequency to power(we engineers know there are millions of power equations

, everything relates to energy or power since we deal with transfer of energy).
 
Parseval's theorem(power from frequency perspective):
 
 
In physics and engineering, Parseval's theorem is often written as:

where 

 represents the continuous Fourier transform (in normalized, unitary form) of x(t) and f represents the frequency component (not angular frequency) of x.

The interpretation of this form of the theorem is that the total energy contained in a waveform x(t) summed across all of time t is equal to the total energy of the waveform's Fourier Transform X(f) summed across all of its frequency components f.

 
 
Basically what mcmalden is saying above is air friction area is proportional to the diaphram size since it moves back and forth through air, and more friction means more resistance force, means more force required to move the diaphram, means more power, which is proportional to force, means more current or voltage in RMS of power.  
 
Conclusion.  Its more efficient to have tweeter to be smaller sized because of the low power draw.  I'm a EE, Fourier is our king(king of signals), not to menion Maxwell(electromagnetic guru that unified theory of light).  Like Newton(the physics of above poster) is King of physics. 

 

[Jack C]

Speaking in general, would a 5" driver produce better highs and mids compared to a larger 8" driver?

In general yes. There are some good 8" drivers that produce good midrange, and there are also good 8" full range drivers. But generally speaking, a it's easier to design a 5" driver that has good midrange and high frequency performance versus a 8" diver. Neither are optimal, however, and will have to resort to specialized driver design.

 

[Uncle Erik]

We're talking about dynamic drivers, right?

I'll take a 6' ribbon over a 1" soft dome any day.

Then there's that matter of efficiency.

 

[Jack C]

We're talking about dynamic drivers, right?

I'll take a 6' ribbon over a 1" soft dome any day.

Then there's that matter of efficiency.

Yea, I think so. Once you start talking about other more esoteric designs, the dimensions do not neatly transfer over. And there's the issue of how popular a design is, which brings in the conversation of what's possible vs what has been implemented by manufacturers.

Jack

 

[mulveling]

Not sure I'm fully buying the efficiency/power argument here. The displacement of a diaphragm on high frequencies is much, much smaller than for low frequencies, and this must be factored into any attempted calculations.
 
I thought the main reason for small (dynamic) tweeters was dispersion. Once the moving surface becomes large relative to the wavelength it's producing, you have to worry about off-axis cancellations. So you can either have a small surface that disperses the higher frequencies reasonably (e.g. a dome tweeter), or a really LARGE surface that projects a usably large "on axis" beam (e.g. electrostats, maggies...). You don't want a sweet spot that's literally inches wide (if that).

 

[RexAeterna]

high frequency move much faster than low frequencies. the smaller diapham helps when the tweeter is experiencing excursions when playing. then you got on and off axis dispersion. smaller drivers do this better than bigger drivers. usually a dome tweeter is considered the best kind of tweeter for this job. due to it's size it can have a very large on axis dispersion meaning you can get a bigger soundfield/soundstage if reflections aren't clashing due to room acoustics and it can handle very high excursions very quickly compared to other tweeters meaning more improved high frequency performance. you can improve/control the tweeters dispersion by using a wave guide,basically meaning a tweeter horn. horns can be form of any desired tweeter from paper cone,compression driver,domes,ect. it can be endless experimentation.

mid-woofers/midranges do have good dispersion on and off axis but not as big as a tweeters would. then you have woofers and subwoofers. woofers and subwoofers usually just basically beam straight and have very little dispersion at all since low frequencies reflect very slowly. it's actually the tweeters dispersion to help relocate the woofers/subwoofers dispersion on and off axis believe it or not. then you have other factors that can affect everything from room,position of the speaker,crossover,cab resounance,ect.

 

[leeperry]

the smaller the driver, the more bloated and "one note" the bass IME...for dynamics at least.