Performance, Precision and Money: Compression Drivers Exposed

While big subwoofers tend to get the attention of both sound companies and audiences, the compression driver keeps its intricacies hidden from view, as shown in Fig. 1. This month, we’ll tease out some of the components within compression drivers and help explain why these small devices are every bit as complex — and often more expensive — than their booming big brothers. Along the way, we’ll talk a little bit about how the way something is made influences the cost of production.

The Early Days

The compression driver’s genesis starts with Albert Thuras, who with Edward C. Wente was granted a 1929 patent on an “Electrodynamic Device” — see Fig. 2. Other than its use of electromagnets, the Wente/Thuras creation is recognizable to modern pro sound practitioners as a compression driver. Equipped with a throat, dome diaphragm, voice coil and phase plug and was the genesis of the now-classic Western Electric Model 555w.

Some Basics

The intervening years have seen compression drivers split into two major design categories. The first are the dome-based drivers and the second instead use ring-shaped diaphragms. At a high level, though, both types of compression drivers have similar internal complexities. Both have a magnet assembly, diaphragm(s), and a phase plug tasked with bringing sound to the throat of an attached horn.

The compression driver phase plug’s base purpose is reducing the open area the driver diaphragm radiates into and then gradually transitioning that smaller area to the horn’s cross section. The decrease in radiating area increases the local pressure and creates a better impedance match to the diaphragm, which increases output. Increasing output was the central consideration in the early days of loudspeaker drivers.

Modern drivers are now less concerned with how much the phase plug can increase efficiency, and more with improving compression driver extension and response smoothness. They are also concerned with the wave shape that enters the throat of the loudspeaker horn. Horns, especially the waveguides used for line array systems, make assumptions about the nature of the sound waves that enter them. For instance, is the wave front from the compression driver flat (i.e., a plane) or does it have some curvature? Path length differences to the driver mouth from the various phase plug openings will influence the curvature of the wave at the driver mouth.

Modal anomalies from the diaphragm are also transmitted through the phase plug and change the sound wave front entering the horn. This can result in changes to the coverage angle or errant reflections bouncing their way down the horn flare. Improving the modal behavior of compression drivers’ diaphragms is one of the central aims of loudspeaker manufacturers, as well as central consideration in the cost versus performance equation.

Project from the Diaphragm

The production of sound by a compression driver begins when the voice coil moves and transfers this movement to the compression driver’s diaphragm. This is true for both dome and ring diaphragm drivers. As the voice coil moves, it sets up vibrations in the diaphragm and those vibrations couple to produce sound in the air.

It is important to understand that vibrations in the dome take time to travel across the dome’s surface, just as sound vibrations in air take time to travel to the listener. When the voice coil moves, the diaphragm does not instantaneously follow, but rather vibration from the voice coil travels in waves across the diaphragm. There is a speed of sound in solid materials, just as there is a speed of sound in air. When the voice coil moves, the speed of sound in the diaphragm material has a profound influence on how sound waves get from the voice coil into the world.

The speed of sound in a solid material depends on the nature of several materials parameters:

• Density (p): The mass (m) of a material per unit volume (V):  p = m ÷ V

• Elastic Modulus (E) — how much a material deflects (strain, ε) when you apply a force (stress, σ): E = σ / ε

• Poisson Ratio (v) — how does a change in strain (∆ε₁), caused by deflecting a material in one direction, induce strain (∆ε₂) in another, perpendicular direction: v ∆v₁ / ∆v₂

Poisson’s ratio is why a piece of chewing gum gets thinner in the middle when you stretch it out.

It can be shown that the speed of sound (c) in a block of material is equal to the square root of the elastic modulus divided by the density:

This shows that either increasing the modulus, or lowering the density, will raise the speed of sound in the material. Typically the compression driver designer desires the speed of sound in the dome to be as high as possible, as this means sound vibrations will reach the center of the dome in the shortest possible time.

A reasonable question to ponder is why sound traveling through the dome quickly is desirable. To understand the benefit, it is helpful to imagine water rippling in a bathtub. The waves travel across the surface at a defined speed, reflect off the walls of the tub, and soon form a pattern of standing peaks and valleys across the surface. One would see a similar effect if they could visually observe the vibrations in the driver diaphragm as sound bounces across the dome, especially when the dome is larger than the wavelength of sound being produced. These various peaks and valleys are mathematically known as the “modes” of vibration in the diaphragm and represent deviations from perfectly uniform diaphragm movement. They are analogous to the peaks and valleys in bass response one experiences in a venue due to “standing waves.” As the speed of sound is fast in solid materials, modal behavior only occurs at very high frequencies where wavelength is small.

Consider the behavior of a thin plate, which is similar to a dome diaphragm, but easier to define mathematically. The onset of the first mode in said plate is determined by:

Now, the realities of modes within a dome are more complicated than this equation, but can be simulated on the computer using numerical methods. As a simple guideline, the modes of vibration in the compression driver diaphragm only start to occur after a sufficient time difference arises between the original and reflected wave. This difference depends on the size of diaphragm and the speed of sound in the diaphragm. Ultimately, if you raise the frequency high enough, all domes and ring diaphragms will exhibit modal behavior. Another way to think of this is that the faster the speed of sound in the diaphragm, the more even the diaphragm’s movement can be.

Our thin, stiff, light dome diaphragms eventually succumb to modal behavior, no longer moving like rigid pistons, but more like a rippling drum head. The modal behavior causes local variations in the production of sound within the diaphragm. In the same manner that low frequency modes in a room can tremendously influence the evenness of the bass response based on location, so to can the modes in the dome influence the sound that leaves the dome and enters the phase plug from different points on the dome. To complicate this effect even further, there is modal behavior of the air between the compression driver dome and the phase plug!

This led to the selection of stiff (i.e., high elastic modulus, ) and lightweight dome materials. The most common material is titanium, followed by aluminum and certain plastics. There is also the exotic choice of beryllium. Beryllium has a very high value of “specific modulus,” which means it creates a diaphragm with a high internal speed of sound that is also very light.

Manufacturing and Cost

If beryllium has these manifest advantages for compression driver diaphragms, then why is it not ubiquitous? The engineering answer would start with the toxicity of processing beryllium, followed by the difficulty of forming it into diaphragms. The geopolitical answer would discuss beryllium’s sourcing and importance as a material for nuclear energy. But of course, the “in the trenches” answer is: beryllium diaphragms are very, very expensive. How expensive you might ask? Try more than 20 times costlier than a state of the art titanium diaphragm.

Compression drivers are intricate devices, even without a beryllium diaphragm. Compared to their low-frequency brothers, they have more parts that require expensive manufacturing operations. A modern compression driver will have a mixture of cast, machined, and injection-molded components, in addition to the diaphragm. These components must then be assembled with precision jigging, typically to receive a diaphragm also placed with precision jigging and/or robotic techniques.

A compression driver typically has a back assembly that is either injection molded or die-cast. Die-casting is in some ways similar to injection molding, but instead, the die filling material is a molten aluminum alloy. Both of these techniques incur substantial up front costs in the form of non-recurring engineering (NRE). NRE costs include the substantial amount of money required to make the injection dies. Die costs start in low five figures USD, and the sky is the limit from there.

If you have ever wondered why your favorite speaker driver company doesn’t release a new aluminum horn design every year, the answer lies partly with the high up-front costs of tooling the dies. For consumer products filling the shelves of a big box retailer, the tooling costs get lost in the noise against millions of units. But for smaller industries like pro audio, the NRE is a substantial part of the baked in cost of every device. And in compression drivers, that NRE is everywhere. Tooling costs for the diaphragm forming equipment, dies for back plates and phase plugs, machining of magnetic steels, and secondary machining operations on parts like phase plugs. And further NRE for the development cost for these secondary items necessary to enable production of the compression driver.

Due to their high efficiency and output, any weaknesses in compression driver design are clearly on display for the entire audience. The engineering design team needs to be very sure of the potential performance and consistency of their compression drivers heading into production. No one wants to have to discard an expensive die because it doesn’t form diaphragms with the desired performance. All of this is money spent, and potentially re-spent, before anything is ever presented for sale to potential end customers.

The Bottom Line

This development cost dynamic plays out on both the final product cycle and the product cost. Visit the booths of loudspeaker driver manufacturers at a trade show and you might see half a dozen new cone drivers for every new compression driver. The market profitably supports myriad new woofers, but manufacturers tread very carefully when introducing a comparatively complicated and expensive compression driver. There’s a lot of money already spent for the product to not achieve market penetration.

At the end of the day, capital facilitates business. As anyone who runs a pro audio business knows, life is a constant race between accounts receivable and accounts payable. Lose that race for any length of time at your own peril. The same is true for engineering physical goods. The loudspeaker driver company either has to have sufficient R&D reserves or access to capital to fund that engineering activity, and then the product has to sell. The next time you get sticker shock at the price of an (outwardly) simple compression driver, remember the hidden complexity and cost packed away inside these marvels.

Phil Graham is FOH’s regular technical contributor and resident scientist.