Speakers have attained so much ‘cult’ status among guitar players of late. We speak with the utmost reverence when names like Jensen or Celestion are passed through our lips. What strikes me as strange is the one ‘camp’ that I will refer to as the ‘Fender Boys’ swear by Alnico magnet Jensen speakers while the ‘Marshall Deputys’ over at the other ‘camp’ swear by ‘Greenback‘ 25 watt Celestion speakers, with their ceramic magnets! Would a Marshall amplifier sound like a Bassman if you put Jensen speakers in the cabinet? Would a Twin Reverb sound like a Marshall with Celestion ‘Greenbacks’? Many British amplifiers used EL34’s or Celestion speakers, yet sounded nothing alike, just as all the American amplifiers with 6L6’s and Jensen speakers didn’t sound similar. While the speaker has a tremendous influence on the tone of your amplifier, don’t give all of the credit to the magnet. And please don’t call a speaker ‘British‘ sounding, or ‘American‘ sounding. Speaker brands can be associated with ‘American’ or ‘British’ built amplifiers, but most Celestion speakers we covet were manufactured after the AMERICAN company ‘Rola’ purchased Celestion (the catch-phrase ‘pre-Rola’ is another story). This is now the time to teach you about speaker basics. There is far more to speaker theory than I can ever learn myself, let alone teach you. This is simply a place to start the learning process.
The simplest construction idea is (poorly) represented below. It is essentially a permanent magnet with a voice coil centered on the ‘pole’. On the voice coil end is glued a flexible cone material (usually paper) which in turn is glued to a ‘surround’. The whole enchilada is mounted to a frame. At the risk of sounding repetitive, the actual sound you are hearing is the air displacement of the cone itself. You don’t ‘hear’ the magnet. The air displacement is the result of the piston-like action of the cone when an AC signal is applied to the voice coil.
If an AC signal is applied to the voice coil, as illustrated below, the polarity of the magnetic field set up in the voice coil is at first is similar to the polarity of the permanent magnet. Thinking back to Grade 9 science class, since ‘like’ charges repel each other, the cone is pushed out.
As the AC signal cycles through, it will reverse polarity, and since opposite charges attract one another; the cone will pull in, as shown below.
Seems pretty straightforward, doesn’t it? Only after this little basic theory, everything else is not so straightforward. Let’s agree that the audio (sound) doesn’t actually travel through the magnet material itself. Then what accounts for such a difference in tone between a speaker with an Alnico magnet and a speaker with a ceramic magnet? When the speaker manufacturers changed the magnet material in their speakers, they changed a heck of a lot more while they were at it. This was also the time when the quest was on for more powerful amplifiers. Speakers needed to be able to handle this increased power output. Enter larger voice coils made of kapton, heavier cone paper, and much higher power handling capabilities. Whereas the average speaker power handling many years ago was 20 watts, today it is very hard to find a speaker that cannot handle at least three times that amount. Thicker cone paper will have less distortion of the cone itself. Larger voice coils usually means larger plate gaps, yielding a less efficient speaker. Certainly this will affect the tone, and much more so than what type of magnet you have. The complete answer will always remain a mystery because I doubt you will ever see two speakers, one with an Alnico magnet and one with a ceramic magnet, that had exactly the same ‘ingredients’ (cone paper/voice coil/plate gap/spider/surround/etc.). These ‘ingredients’ have a tremendous role in the final tone of the speaker.
Here, in the proverbial nutshell, are a few points to consider about altering these seemingly ‘unimportant’ ingredients in your speaker. These observations also account for the disappointing results in the reissue Alnico speakers; the manufacturers had hoped just having an Alnico magnet would be enough to lure folks over. Keep in mind that at the volumes you probably play at, and with a restricted frequency range, many of these differences may not be ‘heard’ as obviously as if we were designing Hi-Fi speakers. This is just some food for thought. Let us now start off by trying to imagine a small weight attached to a spring. We attach this spring to the ceiling and ‘drop’ the weight; it will bounce or ‘oscillate’ at a certain frequency, much the same as if we chose to swing the weight like a pendulum. This frequency can be thought of as a complete cycle of how long it takes for the weight to extend the spring downward and retract back to its original starting point; or we can think of the frequency as the length of time it takes the weight to swing forward and an equal distance backward from the neutral starting point.
If we start with a small mass on a stiff spring (as seen above), and set the mass in motion, it will oscillate at a certain ‘frequency’. This frequency is called the resonance, or natural resonance, of the mass. Suppose we wish to alter the frequency of this resonance. We have two options;
Firstly, we can increase the size or weight of the mass. This will create more pull on the spring, and decrease the frequency of the oscillations as we see a longer, slower swing when we set the mass in motion. With respect to speakers, this is similar to using thicker cone paper. Increasing the thickness of the cone material (to increase power handling capabilities and reduce that awful distortion when driven hard) will lower the resonant frequency of the speaker. Just for reference, the typical Celestion speaker has a resonance of about 90Hz. We could go about lowering that resonant frequency another way, as well.
Here we go back to our small mass, but this time we decide to use a relatively weaker spring. The weaker spring cannot support even the smaller mass, and as a result the spring is stretched out and the mass will vibrate at a lower frequency. With respect to speakers, this is similar to using a softer surround or suspension. The typical Jensen P10R speaker actually has a tight or stiff surround, with a (small) light-weight cone material, together yielding a resonant frequency of about 100Hz. The 7-ounce magnet does not offer much in the way of damping. Each speaker has its fundamental frequency of resonance that is determined by the mass of the cone and the compliance of its suspension. Larger cones, having greater mass, will have a lower frequency of resonance than smaller cones. At this resonant frequency, the speaker is most efficient at converting electrical energy into sound; by its greater voice coil movement, however, the speaker produces more back EMF at this frequency than at any other. The magnet controls the cone movement at resonance by damping it. In my opinion, if the magnet had much of an effect on the ‘tone’ of any speaker, it could only happen at the resonant frequency. We can have different magnet materials with the same effective gauss density, and therefore the same damping qualities. For the Hi-Fi buffs out there, bigger magnets offer more damping, although too much magnet can ‘over damp’ a speaker, reducing the bass response. If you recall the ‘Classic Article’ about softening the surround of your speaker, the goal was to lower the resonant frequency by literally stretching the surround until it was like a looser spring. Not a good idea, but something to study and learn that people thought about these things a long time ago.
Cone material thickness isn’t something any ‘guru’ discusses in books or magazine articles, although it certainly does have an influence on the tone of the speaker, and more so than any magnet can. Thicker paper will have less deformation of the cone itself under high level input signals. This means that rather than the cone moving in and out ‘smoothly’ like a piston, the paper cone will move out further at some points along it surface than at other points. Not a random deformation, the cone will distort in a ‘harmonic’ pattern, adding overtones to the original signal. The picture below is from the book Audio Design Handbook (H.A. Hartley). It shows the ‘nodes’ that develop along the surface of the cone paper. I have added the red coloring to highlight the basic speaker shape, so you can distinguish the ‘normal’ versus ‘distorted’ cone shapes.
Now think back to the analogy using a weight and spring. Heavier cone paper will have less distortion of the cone itself, and this in turn will lower the resonant frequency. Can you see compromises developing? Personally, I like the cone distortion and overtones it adds; and I do not need (or even want) a speaker that can reproduce frequencies down to 20Hz. This is why I try to use a speaker rated for ‘just a bit more’ more power than the amplifier is supplying. Guitar players like 25-watt Celestions in a 4X12″ cabinet with a 100-watt Marshall. This isn’t always a safe thing to do, but it isn’t all bad either. Since the amplifier is never putting out 100-watts continuously, we can find 40-year old Marshall cabinets with the original speakers in fine order. Also, the closed box design restricts the cone excursion slightly, adding a small margin to the handling capabilities.
In 1983 Donald Brosnac published ‘The Amp Book‘. While today it seems like light-weight reading material, for its’ time this was the only book that looked at tube guitar amplifiers. Filled with history and trivia, the chapter on speakers is especially worth mentioning here. A few excerpts that I found enlightening include the following:
- “Sometimes as much as three times the amount of ceramic magnet mass is needed to equal an Alnico magnet. This situation resulted in a redesign of speakers. Alnico speakers generally had the magnet inside the voice coil, but ceramic magnets of equal power are too big to be inside a voice coil. Nearly all present day guitar speakers have ceramic magnets held by iron or steel plates; this assembly surrounds the voice coil.”
- “As for the differences in the tonal response of Alnico speakers compared to ceramic, few people hear any difference.”
The book also contains a treasure chest of information not regurgitated in other ‘feel good’ books, including a chapter entitled “(The) Effect of Surround, Cone, and Speaker”. This is required reading for even the most educated ‘guru’ who claims he can hear magnets. I put this person on a level with Uri Gellar, the (in)famous Russian psychic who seems to have the power to bend spoons using only the power of his mind. Just as we can’t ‘prove’ his feats are elaborate hoaxes, few actually take him seriously.
Sound cannot and does not physically travel through any magnet.
The reissued ‘classics’ really missed the proverbial boat with their Alnico magnet speakers. As an example, in the August 1999 issue of Guitar Player Magazine reviewing 15 Alnico magnet speakers, the reissued Jensen P12N did not do very good! To justify their disappointment, it is rationalized that these new Jensen speakers are manufactured over in Italy. ‘Jensen’ bet the outhouse that just having any Alnico magnet would be the ‘ticket’. Never mind the higher power rating (80 watts!) with the necessary heavier cone paper/voice coil winding! This should (hopefully) prove to you that the magnet material is not the ‘magic’ ingredient. The ‘winner’ in the review? The Celestion ‘Blue’, weighing in at 15 watts! The review also goes on to mention that the Celestion speaker was the ‘loudest’ of the 15 speakers tested. Celestion and Jensen each have their own ‘recipe’ that yields their own unique sound. If you prefer Celestion speakers, you like the sound of a big spider with a tight surround. The smaller voice coil usually paired up with a tighter plate gap, giving you a more ‘efficient’ speaker. This makes sense when you ponder why the Celestion ‘Blue’ was the ‘loudest’ of the speakers tested; it had the lowest power rating. Also too, keep in mind that the magnet will ‘soften up’ with age. So, do you want an Alnico II magnet or an Alnico V magnet? At what gauss density? This only matters because a softer/weaker magnet yields a less efficient speaker.
The baffle board plays an unusual role in the tone of your amplifier. Traditionally, the baffle board is only there to prevent phase cancellation as the sound waves from the rear of the speaker travel and meet up with the sound waves from the front of the speaker. This would usually happen at the lower end of the audio spectrum, as these frequencies have a longer wavelength. How does this work? Assuming that a raw (unmounted or unbaffled) speaker is the sound source, as its cone moves forward it produces a compression wave in front of it. Simultaneously, the rear surface of the cone is creating an equivalent rarefaction. Compressed air moves around the rim in an attempt to equalize the difference in air pressure. The net result is a partial or almost complete cancellation of the sound you hear. At higher frequencies, the cone moves fast enough that the air from the rear of the speaker simply doesn’t have enough ‘time’ to travel around the speaker.
Without an adequate size baffle board, low end frequencies would phase cancel themselves right out of a job!
The baffle board is primarily used to prevent a loss of a speaker’s bass frequency response through phase cancellation.
The baffle board needs to be large enough (in size area) to ‘block’ these bass frequencies from the rear of the speaker, and it needs to be thick enough to support the speaker. When the baffle board is too thin, or not held down very tightly, the board itself can resonate with the speaker movement. This results in a very inefficient baffle board, canceling some of the bottom end. The resonance also adds overtones to the final sound, and this sound is what makes the whole Bassman/wood issue last as long as it has. Uneducated people let the tail wag the dog, as it were. If you like the sound of the resonating baffle board, fine. The baffle could be made from plywood, MDF, or cryogenically frozen camel turds; just make it thin, OK? For the record, every Fender has used a 1/2″ baffle board, save the Bassman (5/16″) and the Twin (3/4″). Of course, open-back cabinet designs have their own quirks, as opposed to a closed-back speaker box. If you are interested in the differences, check out any speaker ‘cookbook’. In a nutshell; closed back speaker ‘boxes’ have a few detrimental effects, which may not be detrimental to you. Since some of the sound energy produced is being dissipated inside the box (where no one can actually hear it), the amplifier power applied to such a system must be somewhat higher than it would be if all the sound energy available were directed toward the listener. The ‘plus’ of closed back systems is that such a system can offer smooth low-frequency response with a minimum of sound coloration or enclosure vibration and resonance. Whether professional or homemade, speaker boxes must be solidly built, and large enough that the air trapped inside does not restrict the movement of the speaker cone. However, some folks like the inefficiencies of a poorly built cabinet that resonates. C’est la vie. For a glimpse into the world of speaker advertising, check out…..
Your assignment for this lesson includes locating one vintage book on speaker theory. New Wireless Pioneers/Brampton Books (1-716-681-3186) has a few. Since their stock will change regularly, it’s hard to recommend any specific titles. Previous titles have included Loudspeakers (G.A.Briggs) and Audio Design Handbook (H.A. Hartley). Next, you hook up your little tweed Deluxe to various speakers and hear the differences for yourself. Which one do you like best? Does your favorite have cone paper thicker or thinner than the others? How does its’ power rating compare to the others? Does your favorite have a larger or smaller voice coil? Try your favorite speaker on a 1/2″ baffle board first, and then on a 3/4″ baffle board. Can you hear a difference? Which do you prefer? You now have done more research than most tube amplifier ‘guru’ types, who just assume the Fender way was the best way.