What Makes A Good Speaker?

A loudspeaker has a deceptively simple job: convert an electrical signal into sound. Doing that is easy. Doing it accurately is extraordinarily difficult.

Every loudspeaker changes the signal to some degree. Its diaphragms bend. Its cabinet vibrates. Its drivers interact. Sound reflects from its surfaces and from the room. The challenge is not eliminating every imperfection. That is impossible. The challenge is identifying which imperfections matter most and solving them in the right order.

Start With the Driver

A loudspeaker driver should behave as much like a simple piston as possible within the range of frequencies assigned to it.

Real diaphragms are not perfectly rigid. At some frequency, portions of a cone or dome begin moving differently from other portions. This is called breakup. Breakup can create resonances, distortion, stored energy, and changes in the way sound spreads into the room. A crossover network can reduce some of the symptoms, but it cannot make an unsuitable driver suitable.

This is why good loudspeaker design begins with selecting drivers that naturally behave well over the frequencies they are expected to reproduce.

Control Resonance

Strike a bell and it continues ringing after the strike. A loudspeaker should not.

Energy stored in a diaphragm, cabinet, or other structure continues producing sound after the original musical event has changed or stopped. That sound was not present in the electrical signal. The loudspeaker added it.

This can make voices sound colored, obscure fine detail, and make different recordings acquire a suspiciously similar character. A good loudspeaker releases energy when the music tells it to, then stops.

Make the Drivers Work Together

Most loudspeakers use more than one driver because no practical driver reproduces the entire audible range equally well. A woofer handles lower frequencies. A tweeter handles higher frequencies. Some systems add a midrange driver.

The difficult part is making several sound sources behave as though they were one. At the crossover between drivers, their outputs overlap. Their relative position, phase, dispersion, and frequency response all affect what reaches the listener.

A poorly integrated loudspeaker may sound like bass coming from one place and treble from another. Voices may subtly change character as their frequencies move between drivers. A well-integrated loudspeaker should become difficult to hear as a collection of individual parts. It should simply produce a coherent acoustic event.

Sound Goes Everywhere

Frequency-response graphs are usually measured directly in front of a loudspeaker. Listeners, however, hear much more than that. Sound also travels toward the floor, ceiling, and walls before being reflected back toward the listener. Consequently, the sound a loudspeaker produces away from its central axis is extremely important.

A speaker with excellent response directly in front of it but badly irregular response elsewhere may behave quite differently in a normal room. Good loudspeaker design therefore considers not merely how much sound is produced, but where that sound goes.

Music Happens in Time

Frequency response matters, but music is not a collection of steady test tones. A drum is struck. A string is plucked. A singer begins a syllable. These events happen in time.

The loudspeaker must start, stop, and change rapidly enough to preserve the relationships within the original performance. Stored energy, poorly integrated drivers, and other time-related errors can blur those relationships even when a conventional frequency-response graph looks impressive. This is why a flat graph alone does not define a good loudspeaker.

Distortion Matters

Every loudspeaker produces some distortion. The objective is to keep it sufficiently low that it does not obscure the music. This becomes especially difficult at low frequencies because producing bass requires moving considerable air. As a diaphragm moves farther, its suspension and magnetic system become less linear. Distortion rises. There is no substitute for adequate radiating area, appropriate excursion capability, and sensible engineering.

Measurements and Listening

Measurements are indispensable. They reveal problems, test predictions, and allow one design change to be compared objectively with another. But a measurement is evidence, not a verdict. A loudspeaker ultimately exists to reproduce music. For that reason, a serious design should survive both objective measurement and careful listening against known references. When the two disagree, neither should simply be discarded. The disagreement should be investigated. That is often where something important is learned.

Solve the Right Problems First

Good loudspeaker design is largely a matter of hierarchy. Correct a serious diaphragm resonance before worrying about a tiny frequency-response irregularity. Choose compatible drivers before designing an elaborate crossover to force incompatible ones together. Control stored energy before attempting to disguise its effects with equalization.

Every decision affects the decisions that follow. When the fundamental problems are solved first, the rest of the design becomes easier. When they are not, complexity tends to accumulate as one compromise is used to conceal another.

A good loudspeaker is therefore not defined by exotic materials, an impressive specification, an elaborate crossover, or a fashionable technology. It is defined by something much simpler: It reproduces the musical signal while adding as little of itself as practical.