Alpha Audio Academy – Series 2 – Episode 2 – Types of Speakers

When we think of a speaker, we almost immediately think of a cabinet with round units in it that move back and forth… A so-called dynamic speaker. But that’s just one type… In this episode we walk past the various types and try to point out the pros and cons. Note: this will be a longer episode, so grab a cup of coffee already!

Dynamic speaker

Let’s start with the most widely used technology: the dynamic speaker. Now that may sound a little crazy: dynamic. Because what is so dynamic about this technology that other types of speakers do not possess? Well… there’s an explanation for that.

The name dynamic speaker is derived from the dynamic microphone (also called moving coil microphone) which, simply put, works in reverse: the diaphragm in the microphone vibrates and thus generates voltage via induction (the coil passes through a magnetic field). These diaphragms are also called dynamic diaphragms (after all, they move). And partly because of this, we call speakers – and microphones – based on this technology dynamic speakers and microphones.

Now one unit can never properly reproduce the complete frequency spectrum. There are wideband units, but they either compromise in the low range, or in the high range. Partly for this reason, most speakers possess two or more units. However, these units must work together. And this is done in most cases by using a filter. This filter separates the incoming signal into two or more frequency bands.

For example, the tweeter gets higher frequencies and the mid/woofer gets the rest of the spectrum. Where the separation is made varies from speaker to speaker, as it depends on numerous things: unit bandwidth, resonant frequencies, et cetera.

The advantage of a dynamic speaker is not so directly explained. It is also a matter of taste. As is often the case in our hobby. An advantage could be that you really feel the sound pressure. Especially in bass there is real impact. That is not the case with an electrostat and magnetostat. There you hear the bass, but you do not feel it. Partly for this reason, subwoofers are often used to help in that area.

Another advantage is the dispersion. A dynamic speaker – especially with a coaxial unit where the tweeter is placed in the middle of the midrange – is a point source. That means that the “heart” comes from one point. We humans find that a point souce sounds natural.

One disadvantage of a dynamic speaker is that the majority use a filter to make multiple units work together. A filter introduces problems in terms of phase and coloration. Especially if it is not implemented properly. Phase in particular is a problem, since most filters use a capacitor. And while that can be compensated for, it remains tricky we notice. To ensure a smooth transition with minimal coloration, we see quite a few^1st order filters. These require fewer components, which in turn causes less coloration.

However, a unit must be able to do its work over a wider range without problems. Read: outside the resonance area. After all: the attenuation is only 6dB per octave…

Watch this episode (Dutch)

Electrostatic speakers

A totally different approach than the dynamic speaker, is the electrostatic speaker. A very well known brand in this market is Quad with the ESL line and of course Martin Logan.

Now Quad is often seen as the mother (or father) of the Electrostatic speaker, but that is not quite correct. Arthur Janszen – an American – created the first electrostatic. Not so much to make a nice speaker, but as an HF source to find torpedoes. Only after the war did he make a hybrid for music reproduction (JansZen electrostatic speakers (70’s)). This company was not very successful, but what is nice to know is that Wilson Audio used JansZen tweeters in its WAMM and Modulator Monitor from that era.

However, Quad did make the first full-range electrostatic speaker: the Quad ESL (later renamed the ESL57 because of the year of its introduction) And it also had by far the most impact in the industry.

The mastermind behind this full range electrostat is Peter Walker, founder of Quad. Rumor has it that many ESLs have gone up in smoke because it is no easy task to keep a conductive foil – stretched between two panels (called stators or grids) – through which high voltages pass, in one piece. In the end, however, he succeeded, as we all know.

The beauty of a full range electrostat, is that there is one foil for the entire bandwidth. The greater the area and the greater the movement the foil can make, the deeper the bass can be.

However, here we come to the first problem: full range models are often quite large. And they are usually incredibly difficult to drive. Partly for this reason, they are not actually made anymore. What you will see very often, are hybrid models: an electrostatic for the middle and high and a dynamic unit for the bass. Where the crossover starts to do its work varies from model to model. Often a manufacturer will try to stay as far as possible from the voice area; our ears are very sensitive there so it is not wise to let a filter do its work.

What is often very beautiful in an electrostatic – and what is almost impossible to match with a dynamic speaker – is the openness, air and purity of the middle and high. There is simply no crossover in the mid and high range and that makes for a very pleasant playback without phase problems and coloration at the transition from midrange to tweeter: it really is one whole. A magnetostat has that too. It is really very addictive to listen to that.

What can be a problem is the connection of low to mid and high. It’s just a different type of unit. And to get that coherent is very difficult. With modern technology – think of electronic filters and room correction – it’s much better.

Another problem is that an electrostatic speaker projects forward as well as backward: it is a dipole speaker as it is called in jargon. This has consequences for the placement. Especially if we look at reflections and because of that, arrival differences in some frequency bands, which in turn can cause smearing (you get quite directly the reflection of the rear wall in the reproduction).

Now every design has something – a dynamic speaker suffers more from room-modes – but our experience is that a deep and tight stereo image is a bit harder to achieve with a dipole. Sidewalls – so first reflections – and the ceiling, however, have less impact on the reproduction.

Magnetostatic speakers

Like an electrostatic speaker, a magnetostatic speaker uses a thin foil with a larger surface area to realize air pressure differences. However, the principle is different.

Whereas an electrostatic speaker uses a thin conductive film between two panels (stators), a magnetostatic speaker uses a thin film with conductive strips placed in front of a permanent magnet. On those conductive strips – usually copper wires for the bass and thin aluminum foil for the mid and high – comes the music signal. And in this way the entire (mylar) foil is attracted or repelled by the permanent magnets behind the foil. Physics at its best shall we say.

Also a magnetostatic speaker is a dipole, which means that it sends energy forward as well as backward. And again, this means that you have to take placement into account. It simply needs to be placed differently than a conventional, dynamic speaker. However, certain rules play a role there as well; especially in terms of room-modes, about which more in another episode.

The strength of a magnetostatic speaker is that – unlike many an electrostatic speaker – it works fine full range. Modern Magenpans sound tight and detail-rich. And can reproduce surprisingly good low end. Also, the mids and highs are very clean and detail-rich. Yet not everyone will fall for a full range magnetostat: it lacks a certain slam or impact in the bass that a dynamic speaker can offer.

In addition, the radiation behavior of a panel speaker – which includes magnetostats and electrostats – is such that the listener really has to sit in the middle. Something to take into account.

One of the best known manufacturers of magnetostatic speakers, is Magnepan. Apogee is also well known in our industry, but they have not been making them for some time. In the field of tweeters – better said AMTs – more on that later – there are more manufacturers to be found.

Omni’s, dipole, open..

We have had most types of speakers: from dynamic through electrostatic to magentetostatic. Now there are so-called ‘resonators’ that you can attach to a flat surface and then use the surface as a resonator – think shop windows, cabinet doors or a panel of a TV – but these are very little used techniques. And they’re not really ‘hi-fi’ shall we say (though I’m sure there are good implementations).

Now what about the various flavors of speakers? Dipole, omni’s… open back… That kind of work? Basically, the name says it all. An omnidirectional speaker is one that radiates in all directions.

Now that sounds ideal. After all, energy is distributed nicely and there is a sweet spot everywhere, right? Yes, it certainly has advantages. There is energy everywhere in the room. And that sounds wonderful. But it also has disadvantages: all room modes are continuously energized, which has the consequence that the acoustics must be in perfect order.

With a dipole this is already a little less. At least with a panel speaker. With a dynamic dipole this is not always the case. Now there are not very many – we know for example Dutch & Dutch and the KII Three – and the models that we know, use a DSP to control the rear units.

This creates a cardioid characteristic, which is to say that the speaker only plays forward and uses the rear wall purely as a boundery.

Then open models. Now of course electrostats and magnetostats are open. That’s inherent in the design. But in dynamic speakers we really don’t see it that much. We still know the Jamo R909. A brutal design. But you also needed a brutal amplifier to get anything out of it.

Special tweeters

AMTs, ribbons, JETs or tweeters made from a special material like beryllium or diamond. Lighter, faster, stronger, less resonance and ultra-low distortion. That is the goal of every manufacturer when it comes to speaker units.

Of course, we can’t cover all the materials with their pros and cons here. Entire books have been written about that, because it is genuinely complex. Big brands spend years researching combinations of materials that do dampen resonances, but are still stiff enough not to obscure the finest details in the signal. And then it also has to be light and durable. And preferably easy to manufacture. You understand the complexity.

But just to point out some interesting things. Especially in tweeters we see a lot of diversity. And that in itself is not surprising: the tweeter plays in a sensitive area, has a huge impact on the reproduction and must cover a wide spectrum. Much more than the woofer and midrange. And it is not surprising that manufacturers put a lot of time and money in this unit.

Tweeters come – with the exception of the large magnetostats and electrostats where no separate tweeter is used – in the form of a hard dome, soft dome and ‘ribbon-like’ shapes. Unfortunately, the plasma tweeter is off the market. How cool is it to see a flame on the speaker!

Dome tweeters usually have a diamater of about 1-inch. Sometimes a little larger, sometimes a little smaller. Commonly used materials are fabric-like materials (silk, for example) for the soft dome, aluminum, titanium and beryllium and diamond for high-end speakers. Sometimes ceramic materials are also used.

Besides domes, we also see magnetostatic solutions. And almost without exception it is a variation on the Ribbon. By that we mean that the AMT, Ribbon and JET are basically the same. It’s based on magnetostatic technology. They are variations on that theme. The AMT is like an accordion, just like the JET and ribbon.

Of course, each manufacturer will apply their own unique sauce, materials and underlying controls. And yes: that will create performance differences. Just as with dynamic units that is the case.

Concluding

We have come to the end of this long episode around types of speakers. We hope you found it interesting. In the next episode: from broadband to 4-way…

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