Matt's Dipole Theory

[m@]

Here's my dipole theory. I haven't seen anything similar written anywhere, so I can't quote anyone. But I haven't scanned the AES archive for it yet.

Dipoles (planars and dynamic drivers) seem to be incredibly detailed. Drums in particular have a very lifelike quality. At first, I thought it was a lack of dampning - increased distortion. But then I tried different crossovers to change that. It didn't make any difference.

So it occured to me: dipole radiation is dependent on two pressure zones cancelling one another (in the null). This gives the characteristic radiation pattern and off-axis repsonse. When you picture a sine wave, it makes sense - you have alternating (in time and rearwards and forwards from the driver) peaks and nulls in the waveform. They cancel one another dictated by wavelength and distance.

But that only holds true for steady state sounds. Sine waves (or bass lines, for example) are highly symmetrical and would cancel accordingly.

But the leading edges of notes, pick attacks, drum strikes, transients all of kind, are usually highly asymmetrical. They would not cancel (in the null) to the same degree as steady state sounds.

The overall effect would be a power response that is dependant upon the sound being played - overall power output would be higher for many (most) transient sounds than for everything else.

I could see that as leading to a more dynamic, textured and detailed sound. Which is exactly how I would characterize dipoles.

Two more things that always pop out at me are sax and trumpet. Watch those on a scope and you'll notice one thing - they're highly asymmetrical as well.

Anyone's thoughts are welcome.

 

[leadbelly]

But that only holds true for steady state sounds.

I think this is the hole in your logic. It's something I've struggled with myself. There really is no such thing as steady state when talking about signals.

 

[richie00boy]

You're confusing the offset/symmetry of a waveform, with the way in which a dynamic driver moves back and forth.

Sorry but I'm afraid your theory is wrong.

 

[nunayafb]

What you are forgetting is that the null is caused by out of phase pressure waves, in dipole they are usually referred to as velocity waves but the end result is the same.
When the cone/diaphragm moves outward by some dist,vel,accel it displaces some air molecules in front of it, simultaneously a small gap is created between the air molecules and the back of the diaphragm. This gap would require alot of energy to maintain and considering the free flowing nature of air molecules they close the gap instantly(on a macroscopic level anyway). Youll notice symmetry is inherent in the physics here, regardless of the input the output is the same- cone/air motion.

Why do dipoles sound better? Dipoles and IB and Ob all share one common attribute, they dont attempt to contain the backwave, which will undoubtedly reflect back onto the cone, colouring the sound. Dipoles also reduce room reflections which is desirable in most standard room layouts.
The reason dipoles sound cleaner on transient signals is probably due to the internal reflections of most monopoles that are incident on the cone after some delay. Reflections/echos are then easier to hear because there is silence afterwards enabling the ear to differentiate between what should be there and should not.

 

[m@]

think this is the hole in your logic. It's something I've struggled with myself. There really is no such thing as steady state when talking about signals.

I'm not sure if steady state is the right way to say it. Let's just say symmetrical. There are symmetrical waveforms. That's what's important in this context. Hole in my vocab, maybe, not my logic.

You're confusing the offset/symmetry of a waveform, with the way in which a dynamic driver moves back and forth.

Are you suggesting that what's on the scope does not correlate with how the cone moves? If our voltage (music) doesn't dictate the motion of the drivers, what does?

What you are forgetting is that the null is caused by out of phase pressure waves, in dipole they are usually referred to as velocity waves but the end result is the same.

I think a dipole is referred to as a velocity source, yes. Low and high pressure areas created by that are very much the same low and high pressure as created by anything making a sound.

When the cone/diaphragm moves outward by some dist,vel,accel it displaces some air molecules in front of it, simultaneously a small gap is created between the air molecules and the back of the diaphragm. This gap would require alot of energy to maintain and considering the free flowing nature of air molecules they close the gap instantly(on a macroscopic level anyway). Youll notice symmetry is inherent in the physics here, regardless of the input the output is the same- cone/air motion.

There is not a gap at the back of the diaphragm (which would be a vacuum) there is a low pressure area. On a symmetrical waveform, that low pressure area would be equal and opposite to the high pressure area on the front of the waveform (albiet offset by a finite amount of time). Those opposite pressure areas cancel to some predictable degree.

An assymetrical waveform would give a net surplus of pressure on one of two sides when compared to the symmetrical waveform - hence, more sound.

Why do dipoles sound better? ...

Yeah, I think those are true as well.

Thanks gents.

 

[richie00boy]

Are you suggesting that what's on the scope does not correlate with how the cone moves? If our voltage (music) doesn't dictate the motion of the drivers, what does?

Not at all. What I am suggesting is that you are looking at it from the wrong perspective. The input signal is irrelevant to a degree and whether the speaker is a boxed or open type the input stimulus produces the same kind of effect on the speaker and the same kind of output from the speaker as a net result.

What the OB does is have front and rear waves that cancel at certain points and this is what causes the 'OB effect'. You need to stop concentrating on how the cone moves/input signal, and look at how the waves from each type of speaker are emanated and interact.

 

[nunayafb]

Low and high pressure areas created by that are very much the same low and high pressure as created by anything making a sound.

No, actually monopoles generate pressure waves that actually compress the room air volume and increase the pressure.

An assymetrical waveform would give a net surplus of pressure on one of two sides when compared to the symmetrical waveform - hence, more sound.

You need to get past this symmetry thing, it has nothing to do with anything. It doesnt matter if a speaker plays a positive first assymetric transient signal or a positive constant value sine wave, the diaphragm will create compression on the front of the cone and rarefaction on the backside and the time averaged values will be equal and opposite.

 

[m@]

nunayafb, as you so aptly put it, symmetry has nothing to do with anything. A single cone acting as a dipole cannot create a net pressure increase or decrease. Every movement is 'matched' on either side and so the pressure created is always created with an equal and opposite.

Thanks!

 

[m@]

Wait. Actually, no, this must be true to some extent. At least for dynamic drivers.

The basket of a dynamic driver acts as a lowpass filter. That means our two pressure areas would never be exactly equal and opposite across all frequencies.

But this is still linear, and is not related to what I was originally suggesting.

(Shakes fist at the sky, damning the gods.)