Originally Published: May 29th, 2012



In this continuing series on defining Legitimately High Fidelity Loudspeakers, we have discussed the following topics:

This article is dedicated to the actual loudspeaker drivers themselves (ie. woofers, tweeters, midranges, etc). Let us first define what a loudspeaker driver is and what its primary role is. A loudspeaker driver is an electromechanical device that converts electrical waves (music is more than impulses) into mechanical vibrations which in turn produces audible soundwaves. The size of the diaphragm will determine the bandwidth in which the loudspeaker will be most efficient at reproducing sound. A bass driver (usually 8” or larger) is designed to reproduce low frequencies (20-500Hz), a midrange driver (usually 3” to 7”) is designed to reproduce frequencies above 100Hz to 4kHz and a tweeter (typically 1” or smaller) is designed to be most efficient at reproducing high frequencies (ie. 1kHz to 20kHz).





Do Exotic Driver Materials Make Speakers Sound Better YouTube Videos Discussion?



The Basics in Loudspeaker Driver Mechanics



It’s important to understand how a basic loudspeaker works so we’ve included a basic diagram of a typical woofer with all the major components labeled.

Loudspeaker Diagram – Courtesy of Polk Audio

The loudspeaker is broken into several parts:

The Diaphragm (aka cone or dome) - the actual physical surface area of the driver that moves up and down from the voice coil interaction with the permanent magnet, thus producing sound by compressing and rarefying air, thus creating sound waves. (sound is not “moving air” it is propagating pressure fluctuations.)

(aka cone or dome) - the actual physical surface area of the driver that moves up and down from the voice coil interaction with the permanent magnet, thus producing sound by compressing and rarefying air, thus creating sound waves. (sound is not “moving air” it is propagating pressure fluctuations.) The Dustcap (optional) – located at the center of the cone and covers the voice coil and can increase rigidity of the diaphragm and most importantly in seals the diaphragm.

(optional) – located at the center of the cone and covers the voice coil and can increase rigidity of the diaphragm and most importantly in seals the diaphragm. The Basket (aka. Frame)– the physical structure that holds the entire driver together

(aka. Frame)– the physical structure that holds the entire driver together The Voice Coil – basically a wire coiled around a bobbin (electromagnet) at the center of the speaker and attached to the spider. When current flows through the coil supplied by the amplifier. it produces a temporary magnetic field when a signal is applied to interact with the permanent magnet (++ oppose ; +- attract). Where extra bobbin strength is needed, a collar can be added to that portion of the bobbin not covered by the coil.

– basically a wire coiled around a bobbin (electromagnet) at the center of the speaker and attached to the spider. When current flows through the coil supplied by the amplifier. it produces a temporary magnetic field when a signal is applied to interact with the permanent magnet (++ oppose ; +- attract). Where extra bobbin strength is needed, a collar can be added to that portion of the bobbin not covered by the coil. The Spider (aka. Damper) – Two prime functions of the spider, in conjunction with the surround, are to center the diaphragm, and force the forward and back movements of the voice coil to be linear, so that the coil does not scrape the narrow magnetic gap. The Spider provides the primary restorative force in the suspension being ideally placed close to the motive force, the voice coil. Linearity of a drivers suspension (and hence compliance) is critical and a lot of research has gone into designing spiders with the necessary stiffness and linearity, and designed not to add structural resonances of their own. The compliance of the spider & surround helps determine the low frequency limit of the drivers useful bandwidth, specified by the well-known free-air resonance (fs) parameter

(aka. Damper) – Two prime functions of the spider, in conjunction with the surround, are to center the diaphragm, and force the forward and back movements of the voice coil to be linear, so that the coil does not scrape the narrow magnetic gap. The Spider provides the primary restorative force in the suspension being ideally placed close to the motive force, the voice coil. Linearity of a drivers suspension (and hence compliance) is critical and a lot of research has gone into designing spiders with the necessary stiffness and linearity, and designed not to add structural resonances of their own. The compliance of the spider & surround helps determine the low frequency limit of the drivers useful bandwidth, specified by the well-known free-air resonance (fs) parameter The Suspension (aka. Surround) – typically made of cloth, butyl rubber or foam, attaches the diaphragm to the driver basket and in conjunction with the Spider, helps to control and dampen cone movement. In some cases there may be more than one spider and in some cases (tweeters) there is no spider. In woofers, the surround is necessary to simply hold the edge of the cone in place and, along with the spider, to ensure linear in and out movement of the voice coil. It also functions as a mechanical termination for the radial traveling waves in the cone (cones do flex!) where it can dampen standing waves that will occur. The stiffness of the cone material is a factor in determining the frequencies at which resonances occur - higher being better, but at least outside the intended operating frequency range.

(aka. Surround) – typically made of cloth, butyl rubber or foam, attaches the diaphragm to the driver basket and in conjunction with the Spider, helps to control and dampen cone movement. In some cases there may be more than one spider and in some cases (tweeters) there is no spider. In woofers, the surround is necessary to simply hold the edge of the cone in place and, along with the spider, to ensure linear in and out movement of the voice coil. It also functions as a mechanical termination for the radial traveling waves in the cone (cones do flex!) where it can dampen standing waves that will occur. The stiffness of the cone material is a factor in determining the frequencies at which resonances occur - higher being better, but at least outside the intended operating frequency range. The Permanent Magnet – a fixed DC magnet which is part of the motor structure affixed to the basket of the driver creating a stable magnetic flux across the annular gap which the voice coil sits in.

– a fixed DC magnet which is part of the motor structure affixed to the basket of the driver creating a stable magnetic flux across the annular gap which the voice coil sits in. The Phase Plug (optional) – similar to the cone but typically looks like a bullet. Its purpose is three-fold, to reduce moving mass and on-axis beaming and also serve as a heat sink and to a smaller extent offering venting.

(optional) – similar to the cone but typically looks like a bullet. Its purpose is three-fold, to reduce moving mass and on-axis beaming and also serve as a heat sink and to a smaller extent offering venting. The Vented Pole Piece (optional) - this is a hole located in the center at the back of the permanent magnet to help reduce air pressure under the dust cap and cool the voice coil.

To generalize a bit more, the magnet assembly of a loudspeaker consisting of a top plate, bottom plate and pole piece, voice coil and magnet can be considered the "motor structure".

This Polk example appears to be quite a heavy duty driver as evident by how much venting and free air-flow they are incorporating into the design. This is the type of bass or midrange driver you typically see in more expensive speaker systems that are able to sustain high output levels with low distortion and low compression. More budget-oriented designs typically won’t have vented pole pieces, which will limit how much thermal energy they are able to dissipate at high output levels, thus potentially causing more compression.

Check out our recently added YouTube Video discussion on Loudspeaker Driver Science with Shane Rich, Technical Director of RBH Sound to supplement this article.

YouTube Video Discussion: Loudspeaker Science: Identifying Good Speaker Design



Cast vs Steel Stamped Driver Baskets

Loudspeaker driver baskets are typically constructed either of stamped steel, plastic or cast aluminum. Plastic baskets are generally used solely for cost savings over cast, or heavy gauge stamped frames. They are fine for smaller driver applications (6” or less) but are usually not used for larger and heavier bass drivers where greater durability and rigidity is needed. Though there are exceptions. A well designed molded plastic basket with glass fiber can provide better acoustics than both stamped and cast aluminum. The plastic can be designed by geometry and with fiber fill to have adequate rigidity, and it offers the benefit of damping any residual parasitic vibrations. Since stamped and cast baskets are more commonplace in the industry, we will focus most of the comparisons on these two type of frame options.

the only reason to use stamped baskets is for cost savings

Here are the facts about cast aluminum vs. stamped steel frame speakers. Aluminum is paramagnetic. That means it is not entirely NON magnetic, as its permeability (let’s call it ‘desire to carry magnetic lines of force’) is about 5% higher than air. Steel on the other hand, is hundreds of times more willing than air to carry magnetic lines of force, the same way copper carries electrical currents, with ease. The result is that the very attachment of the steel to the top plate (part of the magnetic circuit into which the voice coil is immersed) guarantees the amount of magnetic force in the gap will be reduced by the parasitic flow of magnetic lines of force into the steel frame. However, much of this can be alleviated by moving the basket away from the frontplate by using a plastic, paper or other non-magnetic spacer. Good driver designers know and do this when dealing with stamped frames. In reality, flux leakage though the basket typically isn’t a huge deal unless you are designing pro drivers that need every bit of efficiency possible. But we included this point in this article to be as thorough as possible when comparing various basket types.

Still, I can imagine some of you asking, why not use stainless steel instead of regular steel? Good question. Two reasons: 1. It is much harder to form it by pressing (stamping) and 2. It is much more expensive than soft steel.

Stamped woofer basket (left pic); Cast basket woofer (right pic)

A significant advantage of aluminum over steel is the way in which it effectively draws off heat from those parts of the driver that surrounds the voice coil that get heated most quickly, such as the top plate. (remember, the top plate is directly attached to the speaker frame, so this physical coupling will result in the transmission of heat between the parts.) Since most cast aluminum frames are several times thicker than the steel ones, and since aluminum is an ideal material for heat sinking (it is what most heat sinks are made of), it will also let the speaker run cooler for a given amount of current feed into the voice coil. In addition, it will allow a larger amount of current to be put into the voice coil by drawing away and dissipating the heat more efficiently than steel can do. The ONLY reason to use steel instead of aluminum for a speaker frame (and it’s a legitimate reason) is cost. With the proper tool, a single punch can stake and swage (attach) a steel frame to a steel top plate in a second or less. No drilling costly threaded through holes for screws in the top plate is necessary. There is no need to buy the screws, or make sure they are glued in place or screwed down with the right amount of torque. Stamped steel is a far less expensive route for a big mechanized factory than cast frames are.

Cast aluminum frames, if properly designed, are always going to offer an advantage over steel frames, especially with respect to rigidity. That being said, you can still have well designed stamped steel frames. The basket material alone does not define the quality of the driver. Manufacturers building legitimately well-engineered stamped frames typically use thicker gauge steel than the one pictured above left. This increases rigidity and reduces susceptibility to ringing. You will find many of the high output budget subwoofers offered in the $1k or less price range employ thick stamped steel baskets. When trying to hit a target price point, while still offering respectable output and performance, this is a design compromise that often makes sense.

Editorial Note about Cast Aluminum vs Steel Frames by David Waratuke The issue of stiffness in a driver basket frame is not one of steel versus aluminum per se, it is one of geometry and manufacturing. Steel as a material is actually three times stiffer than aluminum as indicated by a property call the Modulus of Elasticity, which for steel is around 29,000,000 pounds per square inch (psi) while aluminum is only about 10,000,000 psi; a modular ratio of 2.9. The lower stiffness in a steel basket comes from using stamped manufacturing that limits thickness of resulting basket as compared to what can be cast. This difference in thickness creates an exponential differences in the geometric component of the stiffness for the resulting baskets. It all comes down to the fact that a cast aluminum basket typically contains more aluminum, with the correspondingly increased thickness, than there is steel present in a typical stamped basket and that more than makes up for the aluminum being a softer material.

Additional Information about Cast Frames

Cast aluminum frames are made by pouring molten aluminum into a block of steel that has been hollowed out using a very precise computer-cutting (CNC) program. This process is known as tooling, and it is a central process in all consumer products made today. Whenever you see a nicely molded, shaped piece of plastic or metal, chances are it has been formed by some sort of tooling process. Tooling is very expensive, so much so that smaller manufacturers jokingly refer to the need to “tool” something as a four-letter word.

There are two basic types of tooling: Proprietary tooling and open tooling. Often proprietary tooling is copied, or the logo is simply ground off.

Proprietary tooling is tooling that that particular manufacturer designs and pays for themselves, and then they own the tooling and retain the rights to it forever. There are many distinctive tweeter faceplates, woofer frames, or plastic satellite cubes which we associate with a particular manufacturer’s look and style, and that’s because they’ve done their own tooling. The manufacturer will often protect their tooling appearance with a design patent, in order to make their design immune to counterfeiting and unauthorized copying.

Good examples of proprietary, patented tooling are Boston Acoustics’ automotive Pro Series cast aluminum woofer baskets and Bose’s famous satellite cubes.

Boston Acoustics Pro Series Woofer (left pic) ; Bose mid woofer (right pic)

The other kind of tooling is what’s known as “open tooling.” Open tooling is tooling that a vendor of raw parts undertakes themselves in anticipation that many of its customers will order their parts, thus making their (the vendor’s) speculative investment in the tooling worthwhile in the long run. Examples of open tooling are the cast woofer baskets that many driver vendors make for their line of general distribution woofers, for example, or the round plastic frames for ceiling speakers offered by many Far East vendors to “brand name” speaker companies. You can look closely at the metal woofer baskets or round plastic ceiling mounting frames from many speaker companies and find that they are identical, absolutely identical.

There is nothing wrong with that, of course. The frame, top plate, back plate, and magnet are known as the speaker’s “hard parts.” It’s expensive for small speaker companies to tool and manufacture those, so they buy them pretty much “off the shelf.” The “soft parts” of a speaker—the cone, surround, voice coil, spider—are easier to customize and change from run to run, so that’s where a lot of differentiation comes in for the speaker companies. Brand A and Brand B may use the same basket, top plate and back plate, all tooled by Vendor C, but they specify and customize different soft parts, and the resulting woofers are completely different, each according to that particular company’s engineering design goals and marketing objectives. Again even unique custom tooled motor designs do not remain proprietary. They are copied. That's progress.

Cast baskets are more rigid than stamped alternatives

Rigidity of Cast vs Stamped Baskets

Regarding resistance to shipping damage, this is a real concern. In the early 1990’s when magnetic shielding was important for speakers in a home theater so as not to interfere with CRT televisions, many speaker manufacturers added “bucking magnets” onto their speakers’ existing magnet structures. This was another magnet, reversed, so its external magnetic field would cancel out, and then the entire structure was typically enclosed in a shielding “can” to further reduce stray leakage.

The problem was that now these speakers’ frames were being asked to carry significant added weight for which they weren’t designed. One major company was using a plastic frame and the additional weight of the bucking magnet and shielding were causing these frames to crack and break off during shipping. Another company was using stamped steel frames on their midranges, but the struts were not reinforced with crimped sections—they were flat and thus very prone to bending because of the added weight. The entire first shipment of these new speakers went out to the dealers and the mids were all inoperative, because the weight of the magnetically-shielded structures had torqued the frames so badly in shipping that the voice coils were all frozen out of alignment. Both of these companies redesigned their baskets with additional reinforcing ribs and added internal cabinet supports around the drivers to keep them from moving and being damaged in shipping. So there is no question that the added rigidity and precise alignment afforded by cast frames is their biggest advantage. Cast baskets tend to hold the parts in alignment more accurately and with greater longevity. It has been proven countless times that such baskets survive shipping drops better than stamped baskets.

Bottom Line on Driver Baskets

Stamped baskets are less costly to produce than cast baskets and are at a disadvantage in some aspects (strength, cooling, etc), but respectably good performance can still be achieved with stamped frames. You will typically find the very best loudspeaker designs on the market will take advantage of the properties of cast baskets and thus employ them in ALL of their drivers. Plus—cast-basket drivers look so darned cool when they’re photographed by themselves! Any speaker marketing department worth its salt knows that!