Speaker Drivers

`Tween Electricity and Air

By Roy Johnson, loudspeaker designer, Green Mountain Audio, Inc.

Send in the juice and hear sound come out. At first glance, this is a one-way path-- electrical energy goes in and leaves as sound and heat. Following that simplest path, it took decades of work by brilliant professionals to create the mathematical expressions for making sound from electricity. Rather simple equations emerged by mid-century, since they were intended as tools for measuring, for verifying what might make better sound.

We are now perhaps beyond the research of basics. If so, then we should be exploring more deeply 'what goes wrong' in drivers, crossovers, cabinets, listening rooms... Nowhere are problems more prevalent than in our raw drivers, because they have the distinction of distorting in two worlds: electrical and mechanical. As the most troublesome areas of driver design are described below, it becomes obvious how sending in the signal's energy can never be one-way.

Energy flows because Time passes, so Energy has Time to bounce back and forth between the electrical and mechanical worlds. And that happens when there are 'flaws' in either world.  It is interesting to know these two worlds always communicate, that a problem in one will be seen in the other. To recognize in which world a flaw actually resides, we must know first where and what kind of flaws are most likely and what would cause each. This article is not about the flaws themselves (since there are so many), but what questions must be asked in order to find them.

Choose your Battle

Decades back, improvements in raw drivers were often large from one generation to the next, sometimes spurred by unexpected advancements in materials, technology or math. Today, improvements come from choosing the best materials for a raw driver. With all of our modern choices, this process requires a lifetime of experience, and so has become essentially an art, definitely leaving no time for the same person to also master designing a complete speaker-system.

Speaker-system design now seems to require its own lifetime of experience, to select from all the drivers and examine all our cabinetry and radiation-pattern options. While driver-design and speaker-design require different skills, educations and experience, they must remain connected in order to produce the best-sounding outcome. The best designers always learn as much as possible about the others' field.

As a side note, it was quite surprising to realize most raw-driver designers never know how good their new tweeter actually sounds- as its true performance is being concealed by problems from the woofer, midrange, crossover and/or cabinet. Then again, most speaker-system designers cannot determine by ear what the best tweeter might actually be, for the same reasons.

So, if one chooses to focus on designing a complete speaker system, it is important to correctly narrow down the choices of raw drivers. For this, each must be examined from certain mechanical and electrical viewpoints, beyond looking at how each appears to a microphone.

To know what to examine, a speaker-designer must first know 'what goes wrong', since all drivers are imperfect. This comes only from knowing what could go wrong in a given driver, and that comes only from knowing why that 'something' could go wrong.

And finally, that comes only from learning about the physical and electrical flaws in the imperfect materials that make up any driver- why they are there and how those produce certain distortions which can be seen in the driver's measurements, before you purchase it. This is a far different approach for selecting drivers than simply choosing one for how it measures, how much power it can handle, how it looks, or even how much it costs. All of the latter dominates in designing mass-market speakers, as it produces new and 'different' models far more quickly.

Pick your Poison

How do you want to move the air? The simplest method has turned out to be the push-pull piston, like a cone woofer or a dome tweeter. You could make the air expand and contract directly by heating and cooling it. One could make a large panel move back and forth. No matter what technique one chooses, each has distinct limitations and possible superiorities. To make the best choices for moving air from the bass to the treble, one must know the limitations of each choice. And that comes (again) from recognizing the imperfections of each.

Ask all the Right Questions

Oscillatory FlowTo find imperfections in how the air is being moved by a diaphragm, one must first know how that diaphragm 'grabs' the air on each stroke. But air is a fluid- difficult to 'grip'. So, when air next to a cone is first put under a bit of pressure from the cone's initial motion, that pressure flows away at the speed of sound (340m/s), while the cone is still moving out at <1m/s. This means the cone is essentially pushing on air always at 'normal' pressure-- it never has the chance to build it up full-pressure, as if the cone was an air compressor, and why speakers are inefficient, only ~1%.

That 'initial' pressure escapes more and more easily as we go lower and lower down the scale, since those notes have slower and slower frequencies of vibration-- the air pressure has more time to escape, and we finally lose the lowest bass. That 'initial' pressure is more retained however, when we use a larger woofer rather than a small one--to produce the same amount of pressure change in the room (to move the same volume of air), a large cone strokes less than a small woofer. Thus, the 'initial' air pressure rise located at the center of its cone cannot escape the outer edges as quickly as from a small woofer. This is why we hear more 'impact' from the larger woofer, even if the small woofer went just as low on steady-state test tones.

Part of how to balance size versus stroke is found in the basic concepts behind 'Radiation Resistance'-- the resistance to any such flow-- a good thing, as we would like to keep the initial air pressure near the cone while building it on up to full pressure before 'releasing' it into the room.

One would also ask how well is that diaphragm being 'grabbed' by the electric or magnetic field. Do those incoming E/M fields distort the signal by changing in shape or intensity? Do they excite mechanical resonances directly inside their metallic paths? Do they momentarily create heat, which reduces their strength? Finding E/M problems comes from knowing how E/M fields misbehave in real materials.

Finally, does that diaphragm being pushed or pulled have its own mechanical issues which prevent it from moving correctly, from changing direction when it is told? Finding mechanical problems means understanding why something flexes (since everything does) or fails to flex if it's supposed to, like the suspension around a cone.

Change is good

All conductive materials physically change when electric and magnetic fields move through them. We know our always-imperfect diaphragms change in shape. In both realms, it is the demand for change that creates our problems.

Music and sound are all about change. Change takes time and occurs in space, which turns out to be important to remember.

To get a handle on Change, physicists often start by applying the concept of energy. Energy is a concept, not a 'thing' to which one can point. It is a very useful concept as it is a very convenient 'bookkeeping system', literally, forcing us to ask where the energy came from and where did it go? The energy in a moving cone or dome is defined as coming from its mass moving at a speed. Energy, as a concept, has units of mass and of speed. Kilograms and meters per second are the easiest units to use, compared to pounds and feet.

If we accelerate an object, we must be changing its speed or direction or both, by applying force or reducing its mass, or both. Therefore (for bookkeeping purposes), we are changing its energy, by so much every second because it is accelerating-- changing its speed and/or direction every second. Its mass and its speed are things we can point to. Its 'energy' is just a calculation.

The units that can change are mass, direction, and speed. Anything going wrong is somehow changing the mass, direction, and/or speed.

What goes wrong with 'mass'? A cone or dome that is 'breaking up', no longer moving as a perfect piston, has some mass that is moving with the signal and some that is standing still--the mass varies.

What goes wrong with 'direction' or 'speed'? Something failed in the moving system, electrically and/or magnetically. Perhaps the suspension failed to let the cone or dome move properly. Maybe it was the air trapped behind the dome. Maybe the frame of the woofer flexed or the cabinet did. The adhesive between the voice coil and cone could be softening.

Stay tuned!