Nelson Pass: The Slot Loaded Open Baffle Project

[Nelson Pass]

Originally Posted by bocama

No that is wrong. It the of preservation of energy law that makes your reasoning improbable. If the pressure is higher it is only possible because the volume displacement is smaller if the volume displacement was the same there will not be a higher pressure. The speaker puts the same pressure on the front air as it puts on the back because the area and force are the same.

1) I don't see anything that violates conservation of energy here. I am not
claiming that the energy that comes out is in excess of what I put in.

2) Your second sentence actually does not make sense to me. Pressure is
higher = volume displacement is lower. I don't know any speaker that
behaves that way.

3) If that were the case, I would not be measuring greater SPL in the front
than in the back.

I say that clearly that the acoustic load is asymmetric front vs back. There
are other examples of such things - How about a horn without an enclosure
behind the driver? Clearly it works, and it works because the acoustic
load on the diaphragm is not symmetric.

[sreten]

Originally Posted by Nelson Pass

1)

I say that clearly that the acoustic load is asymmetricfront vs back. There
are other examples of such things - How about a horn without an enclosure
behind the driver? Clearly it works, and it works because the acoustic
load on the diaphragm is not symmetric.

Hi,

No you can't. Your measurements are flawed in some respect.
There is no sensible reason you have real asymmetric loading.

Slot loading drivers, which is used commercially regarding
dispersion issues, would be ubiquitous in professional
systems if it had the advantages claimed, it doesn't.

rgds, sreten.

[Nelson Pass]

Originally Posted by sreten

For (C) :

I don't see Heil's AMT principles being relevant. As first base
Air Motion Transformer - Wikipedia, the free encyclopedia indicates
its all about moving more air, with its relevant pressure increase.
The air speed increase related to diaphragm velocity is inevitable.
It does not imply increased efficiency for a volume displacement.

The point of the Heil is to use magnetic fields more efficiently, it
has essentially the same efficiency as a stretched out diaphragm
with a relatively huge magnetic structure, i.e. its a cheaper way
of moving more air, and being smaller less dispersion issues.

Simply replace every pleat in the AMT with a driver, use a zigzag
baffle and work out what happens at low frequencies, nothing
special. The Heil is special because it uses the fore and aft field
far more efficiently than a planar diaphragm, and that leakage
field will always exist wasted in any planar design, best used.

I quote from the Heil patent:

"Such diaphragm arrangement moves more air with less kinetic energy
than conventional diaphragms. The vibratory diaphragm portions may be
directly driven by applying an audio current, respectively an audio voltage
to conductors attached to the diaphragm portions and located in a strong
magnetic field, or the diaphragm portions may be indirectly driven by a pair
of voice coils alternatingly attached to adjacent vibratory diaphragm
portions to move the latter toward and away from each other."

(Italics mine)

[Salas]

Why the argument is on velocity and slot pressure when the differences observed are more likely matters of baffle step diffraction, line source near field directivity, and wavelength? Its a different acoustics projection front to back, not a matter of different enough pressure IMHO.

[bocama]

Originally Posted by Nelson Pass

1) I don't see anything that violates conservation of energy here. I am not
claiming that the energy that comes out is in excess of what I put in.

2) Your second sentence actually does not make sense to me. Pressure is
higher = volume displacement is lower. I don't know any speaker that
behaves that way.

3) If that were the case, I would not be measuring greater SPL in the front
than in the back.

I say that clearly that the acoustic load is asymmetric front vs back. There
are other examples of such things - How about a horn without an enclosure
behind the driver? Clearly it works, and it works because the acoustic
load on the diaphragm is not symmetric.

But you are saying that the same energy that the speaker puts out of the front gets bigger than the energy that it puts out of the back. If the pressure and the volume displacement are bigger in the front (the potential and kinetic energy) as you seem to think it is. Also the small slot has a worse coupling to get better acoustic coupling you need a bigger area like a horn. I do not see the exact procedure described in your article but maybe the explanation for the difference can be explained by the way you measured.

[sreten]

Hi,

You can claim anything you like in a patent, it is not checked,
and they are generally very poor sources of information usually.

The quoted italics part is highly debatable, IMO simply
not true, and the rest of the argument falls apart.

As it is the quoted part makes no sensible sense.

rgds, sreten.

[GRollins]

Oh for cryin' out loud...
I read Nelson's article, saw that the slot thing might prove of use for something I've been meaning to try, and started trying to weigh trade-offs. None of the books I have on hand treat open baffles seriously and the little I could find on slots could be written on the head of a pin.
So I went to the web.
And in trying to Google up some info on slots, I discovered that there ain't no *******' info to be had and the 8th or 9th hit was this thread.
Oh, great!
Those of you who know some of the backstory can imagine how that made my day.
And, no, I haven't had a change of heart regarding some of the members here. And, no, I most certainly haven't had a change of heart regarding at least three of the moderators. (I offered to start a thread way back when, wherein I would post the complete sequence of e-mails between a certain moderator and yours truly. No way! Can't have that, Grey! [Well, duh, can't have the moderators behind-the-scenes behavior exposed.])
Then I went and looked at this thread, only to discover that Nelson was being hounded by a rather hostile member. It got my dander up.
Damn.
All right, I'll wade into the fray.
Here's my take, with the caveat that I haven't built a slot critter yet, so this is arm chair theorizing for the moment.
I think we can also agree on some particulars, to wit:
--The air mass in front is equal, or at least very, very close to the air mass moved in back. (You might, for instance, make a case that the slot restricts air flow into the slot chamber such that it has difficulty taking in a full charge of air during the rarefaction part of the cycle, whereas the backs of the drivers face little or no restriction.)
--The energy imparted to the front air mass and the back air mass is equal.
--There's no violation of the laws of physics involved. There's equilibrium between the back and front, even if it's not immediately evident how to square this away.
Etc.
And yet, something seems to be going on here. So which fundamental assumption should we reexamine?
When you're trying to "break" a mathematical formula, your go-to numbers are negative infinity, negative one, zero, positive one, and positive infinity. That's not to say that there aren't other numbers that can do the job, but those are high-probability numbers for giving formulas fits. So let's try it here.
Let's take the underlying assumption that you're dealing with, say, a 20Hz tone. Let's set the frequency to zero. Yes, zero. Let's assume the air flow is DC.
Say what?
How the hell are you going to set the frequency to DC?
It's easy. Elwood does it every day, though I doubt he's ever thought about it in quite this way.
Take an air compressor and charge it. Press the valve on the end of the line. What comes out is an even flow of air, moving in one direction only. Presto! DC air.
That's the setup. Here's the experiment. Do as indicated and hold the air source in one hand, whilst moving your other hand away from the source. It's perceptible for quite a distance. Now hold your hand beside the source. More than an inch or two away, there's no perceptible movement at all.
Thus, I propose that one of your fundamental assumptions is flawed. In particular, the assumption that low frequencies are inherently omnidirectional.
What if the slot and chamber cause the low frequencies to be more cardioid? If we assume that the back radiation pattern is omni, or at least half-space to a first approximation (throw in floors, walls, cabinet wings, etc. later), and the front is in a more lobed pattern, then that would explain the reduced cancellation between back and front waves. The rear wave compression/rarefaction comes around the edge of the baffle only to discover...nothing. The front wave that it would have cancelled is already half-way across the room.
My hypothesis also addresses the raised, but falling dB at Nelson's listening position, in that the plume of onrushing air will spread as it moves away from the panel, weakening. At a distance, it would approach the 'normal' levels that you would expect from a more ordinary speaker. Nearfield, it would be quite a bit higher than normal.
Crikey, Sreten, don't act like the guys in the Vendetta thread. Don't just sit there and proclaim that something's impossible and act all snotty about it. Actually engage your mind and try to figure out what might be going on. We've got enough "no" people in Washington. What we need are people who do more than say "no" to everything. We need people who suggest solutions. If you're not going to suggest a solution--just sit on the sidelines and snipe--then say nothing at all. If you're going to use "There is nothing so practical as a really good theory" as your signature line, how about putting it into practice and coming up with a theory as to what's going on, eh? Either that or change your signature to "I never met a theory I couldn't hate on sight."
Now, mind you, if my hypothesis is correct, there are implications. Suppose we use an ordinary box fan as our DC air source. It's well known that a box fan can push air at you from all the way across the room. What most people have not thought about is the fact that there is not a corresponding column of rarefaction extending behind the fan. Compression acts through a distance. Rarefaction is an entirely local effect. Think of it in terms of momentum.
Extend this concept to Nelson's slot loaded baffle and you will find (I propose) that the compression jets out quite a ways, while the rarefaction will be a more localized effect, leading to a relative flattening of the negative portion of the cycles if you're listening at a distance.
One listening consequence would be that a woofer of this design would be more sensitive to absolute phase in a recording. If the drum head moves at you, you're going to perceive a kick in the gut. If the recording is out of phase and the drum head moves away from you, you're going to get an unusually weak wavefront.
Note that there are other implications to my hypothesis. One is that it would explain why--other flaws notwithstanding--nearfield listening is so enjoyable. The ultimate nearfield--headphones (or earbuds)--would couple the ear more closely to both compression and rarefaction phases.
There are other implications as well, but I'm dismayed to find that nothing has changed in the years I've been away and I'd really rather not be here, so I'm going to let it go at that. Perhaps my hypothesis will allow sreten to find common ground with Nelson and not be so abrasive. Perhaps not. But I felt the need to give it a shot.

Grey

[vacuphile]

Just stumbled accross this thread and sorry to jump in late.

Air flow behaves differently being sucked (1) in from being blown out (2). In case (1), a partial vacuum is created, that is replenished from all directions outside the slit. In case (2), the air being blown out will have gained momentum, which will cause a directional airflow.

Blown out, the air has been accelerated, and in that case I can see Nelson's point that more sound energy has been imparted on it (by having a better loading on the transducers). Being sucked in, however, it is not immediately obvious where the extra acoustic energy generated would have to come from.

In other words, I expect this set-up to be rather assymetrical in its workings, potentially generating even order distortion components. Which we like in bass, by the way, as it increases apparent loudness, just to mention one thing. Would be interesting to see some distortion figures.

vac

[kropf]

Originally Posted by bocama

But you are saying that the same energy that the speaker puts out of the front gets bigger than the energy that it puts out of the back. If the pressure and the volume displacement are bigger in the front (the potential and kinetic energy) as you seem to think it is. Also the small slot has a worse coupling to get better acoustic coupling you need a bigger area like a horn.

Let me propose a different way of looking at the physics of the situation. The energy (or intensity) in a wave is proportional to the square of its amplitude. Or, in mechanics, the kinetic energy is mass times the square of the velocity.

Let's say the ratio of the area of the driver cone to the area of the slot mouth is R. If the driver displaces a volume (or mass) of air, M, at a velocity, V, then the mass of air moving through the slot mouth is also M, but the velocity is R*V.(*) The mass transport is the same, but the velocity is R times as large, so the kinetic energy, M*(VR)^2 is, in fact, R^2 larger at any instant than energy from the open rear face of the driver (M*V^2). Note that this is not a case of something for nothing, since the air pressure changes in the slot between the drivers (and therefore SPL between the drivers) are significantly higher than it is for the driver in free space for the identical cone motion. The energy between the drivers is primarily potential energy from increasing the pressure inside the box.(**)

The slot and slot mouth is the mechanism to convert the potential energy to kinetic energy. So, yes, in the mouth of the slot, you would have an R^2 increase in SPL compared to the driver in free space, or in this case the back side of the driver.(***)

This analysis doesn't say anything about how the slot loaded speaker behaves at the listening position in a real room or at what pressure a driver cone will no longer support the pressure increase inside the slot. Furthermore, at some point, the fluid dynamics characteristics will become increasingly important, so smaller and smaller slots will not continue to increase the gain. Fluid dynamics also may be one reason why this would not work well, in practice, for the small exit port in a big box.

Discussion?

Jeremy

(*) For low enough frequencies and velocities any air compression inside the speaker will be reversed as the air exits the slot mouth and expands into free space

(**) Let's say inside the slot between the drivers, the SPL is 140dB. The pressure change is about +/-4,000 Pa vs. atmospheric pressure (100,000 Pa). That's only a 4% compression/expansion of the air (volume change).

(***) ignoring similar effects from the driver's frame

[sharpi31]

Loving the thread, but mighty annoyed as I can't access Nelson's article: The Slot Loaded Open Baffle Project Article By Nelson Pass

Is this a problem at my end or is the website down?