What Makes a Good Loudspeaker Driver?

A beautifully made loudspeaker driver can be completely wrong for a particular loudspeaker. So can an expensive one. The specifications may look impressive, or its materials may sound exotic. The manufacturer may have an excellent reputation.

None of these things answers the most important question: Is this driver suitable for the job we are asking it to do? That distinction is fundamental to loudspeaker design.

There Is No Best Driver

A woofer designed for very deep bass may be inefficient. A highly efficient woofer may require a large cabinet. A small woofer may reproduce the midrange beautifully but lack the ability to produce adequate bass at realistic listening levels. None is necessarily better than the others. They were designed to do different jobs.

Choosing a driver, therefore, begins by defining its job. How low must it reproduce? How loudly? How high in frequency must it operate before another driver takes over? How much amplifier power is available? How large can the cabinet be? Only then do specifications become meaningful.

Firstly, Look for Bad Behavior

One of the first things we examine is breakup. Ideally, a loudspeaker cone moves forward and backward as one rigid surface. Real cones cannot do this indefinitely. At sufficiently high frequencies, different portions of the diaphragm begin moving somewhat independently. The cone has entered breakup. This can create resonances, stored energy, distortion, and abrupt changes in the way sound radiates into the room. A frequency response graph may show the result as a prominent peak. Sometimes the problem is less obvious. The important question is not simply, “How high can this woofer play?” A better question is, “How high can this woofer play well?” Those are very different questions.

Look Sideways Too

Most published frequency response graphs show what a driver does directly in front of it. That is useful, but incomplete.

As frequency rises, a large driver generally begins concentrating its sound into a progressively narrower area. This is called increasing directivity.

Suppose a woofer becomes quite directional before reaching the tweeter. The tweeter may suddenly spread sound over a much wider angle. Directly in front of the loudspeaker, the frequency response might look excellent. Move sideways and it may not.

The room also receives this uneven sound and reflects some of it back toward the listener. Good driver selection therefore requires examining not merely frequency response, but dispersion. The drivers must work together in space as well as in frequency.

Resonances Have Consequences

Every physical object has resonances. A loudspeaker driver contains a cone or dome, suspension, voice coil, former, surround, and other structures. Each can store and release energy. A strong resonance is particularly troublesome because the driver continues emphasizing that frequency regardless of whether the music asks it to.

This is one reason two drivers with similarly flat frequency response graphs can sound surprisingly different. One may stop cleanly while the other may ring.

Distortion Changes With Level

A driver that behaves beautifully at low volume may behave quite differently when asked to play loudly. Producing sound requires moving air. Producing bass requires moving a great deal of it. As a diaphragm travels farther, its suspension and magnetic system may become less linear. Distortion increases. Eventually, the driver reaches its safe mechanical limits. This is why bass extension, loudness, diaphragm size, and excursion cannot be considered independently. There is no magic material that eliminates physics.

Specifications Need Context

Driver specifications are extremely useful when properly understood. They can help predict bass performance, enclosure requirements, electrical loading, sensitivity, and power requirements. Yet individual numbers should not become trophies. A huge magnet does not guarantee a good driver. A lightweight cone does not guarantee good transient response. A high advertised power rating does not guarantee that the driver will play loudly without objectionable distortion. An exotic diaphragm material does not guarantee freedom from breakup. Engineering requires looking at how the characteristics interact.

The Crossover Cannot Fix Everything

It is tempting to select an attractive driver and assume that any undesirable behavior can be corrected later with the crossover. Sometimes it can. Often it cannot.

A crossover can reduce the electrical signal sent to a driver outside its useful range. It can compensate for some frequency response irregularities. It cannot make a badly behaving diaphragm stop behaving badly. It cannot restore broad dispersion after a driver has become highly directional. It cannot remove energy that has already been stored mechanically. The better approach is to choose a driver that behaves naturally well over the range where it will be used. Then the crossover has a much easier job.

Measurements, Then Ears

Measurements tell us where to look. Listening tells us whether we have understood what we found. A promising driver should therefore survive both. Does its measured behavior make sense? Are resonances controlled? Is its dispersion appropriate? Does distortion remain reasonable at the required level?

Then listen. Does a voice retain its natural character? Do cymbals acquire an artificial metallic signature? Does bass remain articulate as level increases? Does the driver become tiring after extended listening? When measurements and listening agree, confidence grows. When they disagree, investigate.

Suitability Is What Matters

A good driver is not necessarily the newest, most expensive, or most technically exotic one. Nor is it necessarily the driver with the flattest published frequency response. A good driver is one whose strengths match the requirements of the design and whose weaknesses can be kept safely outside the range in which it will operate. That leads to one of the simplest rules in loudspeaker engineering: Do not ask how impressive a driver is. Ask how well it does the job.