speaker-tone-center

How understanding why speakers distort can make all the difference in crafting your tone.

Let’s jump right in and continue with Part II of our discussion that began with “Your Signature Distortion” (September 2009). Part I covered the DNA of a speaker and how the materials used factor into your tone. This month we’re diving in deeper to breakdown how a speaker distorts. We’ll also answer the question, “Is the speaker producing what the amp can deliver?”

There are a number of ways to achieve the distorted tone you’re looking for. If you have a tube amplifier, you can get distortion from clipping your preamp and power amp tubes, not to mention the fact that you can clip your input stage, thus adding a bit of gain and compression right from the input of your amplifier. Of course, you also have a plethora of distortion pedals on the market to give you an infinite variety of distortion. But the funny thing is that all of these distortion makers fall to the mercy of the speakers. Why? Because it’s the way the speakers interpret this signal that will determine your ultimate tone.

How does a speaker distort?
When a speaker distorts, it produces two types of frequencies. The first type is harmonic distortion: this is heard as additional tones which are multiples of the original note played. For example, if the original sound produces 100Hz, you would also get 200Hz, 400Hz and so on, even though these tones are not part of the original sound. The second type is non-harmonic distortion, also known as odd harmonic distortion, and often referred to as a buzz or a rattle in the sound. For example, if the original sound produces 100Hz, the odd harmonic distortion would produce frequencies of 300Hz, 500 Hz, and 700Hz, etc.

When setting your tone, there are a series of specs you need to consider, starting with your guitar and amplifier. The signal from a guitar pickup is mostly all midrange and is not rich in harmonics, with practically nothing coming through above 4000Hz. The sixth string “E” tuned to pitch comes through at about 80Hz, two full octaves above the 20Hz low frequency our ears can pick-up. The standard 4-string bass has a range one octave below the guitar, with the low E at about 40Hz.

The way typical guitar amplifier circuits, such as Marshalls and Fenders, are designed also affects how the speakers will respond. The most obvious difference is that the Marshall circuits let more signal pass through, and the tone controls offer less frequency range. The higher signal means that the preamp tube stage can overdrive the output tube stage more. Additionally, the Marshall circuits have a slight dip in the midrange section, almost an octave higher than Fender amps, metering in around 700Hz. A Fender’s midrange dip is around 400Hz, while the bass response on both amplifiers meter in around 10Hz. Fender’s tone controls allow for a higher midrange frequency to pass with the treble response, meaning more dynamic range for that sparkling, tight sound they’re famous for.

To save time, I’ll spec out the three most popular speakers associated with Fenders and Marshalls, starting with a 25-watt, 12" speaker with a sensitivity rating of 98dB, 1.75" copper voice coil, ceramic magnet and a resonance frequency of 75Hz. Another popular choice is a 30-watt, 12" speaker with a sensitivity rating of 100dB, 1.75" copper voice coil, ceramic magnet and a resonance frequency of 85Hz. The speakers in Fenders are designed to stay clean, so they spec out at 100 watts with a sensitivity rating of 99dB, a voice coil inductance of 1 kHz, ceramic magnet and a resonance frequency of 104Hz.

How does this translate to your guitar tone?
It’s these specs that are directly responsible for your tone, and it’s here where the “disconnect” usually happens for most musicians. We’re not trained to translate these features into the sounds we hear, but it’s this knowledge that can serve as a guideline for you to build your own “tone formula.” Here is an example of a classic one: 50-watt tube Marshall running through two 25-watt speakers (100 watts with four 25-watt speakers will produce similar results). The Marshall circuit allows more signal to pass through, meaning that the guitar input section, the preamp and power section are going to distort. The lower wattage speakers with the smaller magnet and voice coil are going to break up faster at a lower volume. The voice coil will clip and compress, giving you that “sizzle” in the upper frequency range.

This is the classic Marshall tone, but here’s the rub: the lower-wattage speaker distorts so fast that the speaker will not be able to produce the lower frequencies (bass response) that the amp can put out, thus giving you the illusion that the amp has no bottom end. By adding a 30-watt speaker, you can increase the bass response but cut the level of distortion. Now, if you went to the other end of the spectrum and had a Fender amp that was too clean and brittle sounding, you could add a lower wattage speaker with a smaller magnet and voice coil and achieve some nice, mild distorted tones. Changing the magnet to alnico would also add a bump to the midrange section.

Next up in Part III: the “secret weapon” in your tone formula. Stay tuned!

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The Role of Loudspeaker Diameter in the Relentless Pursuit of Tone

These days, guitar speakers are available in a range of sizes from two or three inches, right up to 15". Smaller speakers are great for bedroom blasters and practice amps, where reduced output at low frequencies can minimize sound spillage between rooms and keep the neighbors sweet. If you’re like most, though, chances are you’ll be using 12" speakers for much of your recording and live work. But with an ever-increasing range of quality 10" speakers, and even some interesting 15" speakers available, one has to ask the question, “What role does speaker size play in the ‘relentless pursuit of tone?”

Inside the Mind of the Speaker Designer
Practically all of a guitar speaker’s constituent parts contribute in some way to its sonic signature. Chief among them are voice coil, magnet assembly and cone [but also influential are the suspension, surround, dustcap, cone treatments, etc.]. Each of these factors interacts with the others, together contributing to overall tone. These interactions, though in some cases very complex, are governed by certain principles of physics, in particular:

Output level [a.k.a. sensitivity] is determined by how efficiently the speaker converts electrical energy into movement of air.

Sound dispersion is controlled by the directional nature of high frequency sound and the tendency of certain cone shapes to focus the output signal in different ways.


This laser doppler image indicates vibration modes within the body of the cone.

For guitar speakers in particular, vibration “modes” within the body of the cone add much of the harmonic complexity and coloration that significantly contributes to great tone.

The speaker designer uses their expertise to find the right mix of all of these factors to hit a given “tone target.” Now, imagine we want to use a small speaker with a thin and light cone. There would be more intense vibration modes within this type of cone [compared to a cone of greater thickness, which would be more resistant to these vibrations], resulting in a richer, more harmonically complex tonality. However, use the same cone thickness with a larger diameter speaker and that cone might lack sufficient stiffness to withstand the proposed power handling, and could buckle under the force of the moving voice coil.

In this situation there would need to be some “trade-off” between tonality and power handling, requiring the designer to make both musical and technical choices to reach a desirable and workable solution. An experienced speaker designer will have the capability to identify the “right” choices to make in these situations, and use the opportunity to create a completely new sounding speaker.

What This Means For Tone
So, we see that attributes like size, harmonic complexity, power handling and high-note dispersion are clearly linked in the design process. Over time, the 12" speaker has come to be regarded as having the best balance of these attributes. However, 10" and 15" speakers can offer some alternative, interesting and even exotic flavors!

Good sounding 10" speakers can deliver a fast, punchy sound at wider listening angles with reduced “boom” on small stages. They can offer increased portability, reduced cost and the ability to push your amp into overdrive at reasonable levels without having drumsticks aimed at the back of your head. A well-designed 15" speaker can move more air so you can gig those wonderful little valve amps. The vocal range can be creamier, with extended low end and lots of detail and harmonic complexity, giving surprising richness to some otherwise scratchy-sounding guitar and amp combinations.

Which Size is For You?
It’s becoming more widely understood that changing speakers has a greater impact on tone than swapping guitar, pickup, or even amplifier. So ask yourself, why just one size of speaker? As players, all we need to do is select the right one according to situation, application … and desire.

Try this:
For the recording or practice session, why not try a small amp through a sweet, wellbalanced 10"?

At your big break support gig on the city hall stage, how about a wall of 4x12s?

Need to add some beef to your retro “plasticaster”? Break out a 1x15 cabinet.

What’s more, just as boutique amp makers have mixed different models to increase harmonic detail, you might even try and take this a step further by mixing 10", 12" and even 15" speakers to create that unique signature sound. But that’s another column.

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The history of the Aluminum-Nickel-Cobalt magical combination we know so well

Alnico Magnets Today
Alnico permanent magnets are made from an alloy containing varying percentages of iron, aluminum, nickel, cobalt, copper and sometimes titanium. It is produced in different grades, which offer increased magnetic strength (maximum energy product) and resistance to demagnetizing forces (coercivity). Alnico offers the best temperature stability of any standard magnet material produced, but it is also the most susceptible to demagnetizing forces. It is often selected for modern products that must operate at extremely high temperatures. In guitar speakers, alnico is usually selected to help recreate the sound of ‘50s and ‘60s blues, jazz and rock and roll music.

Electrodynamic Loudspeaker History
One of the first modern-style (electrodynamic or “moving coil”) loudspeakers, the Magnavox, was demonstrated in 1915 by Edwin S. Pridham and Peter L. Jensen. Their 1920 patent application (US Patent 1448279) describes “...an annular coil rigidly connected to a diaphragm. This coil is disposed, so as to be freely movable, in a strong concentric magnetic field produced either by a permanent or an electromagnet.” Today, the vast majority of speakers use permanent magnets; however, in the 1920s the permanent magnets available were relatively weak, and those capable of producing a strong magnetic force were costly and difficult to make. Most early electrodynamic loudspeakers used an electromagnet (also known as the field coil).

Electromagnet vs. Permanent Magnet
An electromagnet is composed of a coil wrapped around an iron core. When DC current passes through the coil it generates a steady magnetic field. Electromagnets may be thought of as temporary magnets that lose their magnetism when power is shut off, while permanent magnets do not require an electrical power source to remain magnetized.

Alnico Magnet Development
In the early 1930s, most of the world was in the depths of the economic downturn known in the US as the Great Depression. Early patents relating to the development of alnico express their objective to provide a permanent magnet composed of relatively low-cost materials, which could be fabricated inexpensively and have superior magnetic characteristics.

In US Patent 2027994 (Applied For: 1/20/1932; In Japan 3/09/1931), inventor Tokushichi Mishima of Japan explains how a strong permanent magnet comprised of nickel, aluminum and iron could be produced economically and with superior magnetic characteristics to the “magnet steels” available at the time, including tungsten, chrome, and chrome manganese. Months later, he added to this the discovery that the addition of cobalt could further improve the magnetic characteristics of the alloy, as well as improving its tenacity and ductility (US Patent 2027996).


An advertisement from QST amateur radio magazine (April 1945, p. 75)
In the Nov. 4, 1935 issue of Time magazine, an article titled “Science: Industrial Insides” describes researcher William E. Ruder of GE Schenectady demonstrating the power of the new alnico permanent magnet by swinging a 55-lb. radio cabinet from an alnico disk of less than a pound. “Alnico is being groomed to displace small electromagnets in motors, transformers, and loudspeakers, lowering the cost and simplifying the construction.”

In US Patent 2295082 (Applied For: 6/29/1939; In Germany 12/06/1938), inventor Gottfried Bruno Jonas of the Netherlands explains that an alnico alloy in the anisotropic form can yield a permanent magnet with a 50% to 200% higher maximum energy product than the isotropic version. This is the discovery behind what would become known as Alnico 5. This application was patented on Sept. 8, 1942 in the United States.

Alnico had been developed into a robust permanent magnet by 1939. In that same year, Charlie Christian joined Benny Goodman’s band and is credited with popularizing the electric guitar as a lead solo instrument on par with the trumpet and saxophone, but his Gibson EH-150 amplifier used a field-coil speaker. It was not until about 1947 when Gibson and Fender began stocking their guitar amps with alnico speakers. What could account for such a delay?

Don’t you know there’s a war on?
The Nazis invaded Poland in 1939 and WWII began in Europe. In January of 1942, a month after entering the war, the United States government established the War Production Board to regulate the production and allocation of raw materials. Basic metals including steel, copper, and aluminum were heavily regulated to supply military demand, and only essential civilian products would be allocated a percentage of these metals. In order to stay profitable, many American companies had to shift their production to supply parts and equipment the military needed. For example, Gibson made radar assemblies, glider skids for airplanes and precision rods for use in submachine guns. It would not be until after the war ended in 1945 that many peacetime products could make full use of alnico, the miracle metal.

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