acme bass

Andy Lewis on Horns


Is it possible to play bass using a so-called "folded horn" loudspeaker? Would you want to? Is there any advantage to this type of loudpeaker which would make it worth the trouble of hauling one around? Why don't any credible manufacturers still produce them, as they used to? Why have they all but disappeared from the scene, and remained exclusively in the realm of the hobbyist?

Friends and customers have asked me to comment on the operation and the viability of horn loaded loudspeakers, and variants thereof. There seems to be an interest in home-built variants of horn loaded speakers for bass guitar.

What is A Horn Loaded Loudspeaker?

Horn loading is a technique whereby a speaker diaphragm is coupled to a column of air, which increases in cross sectional area as a function of distance from the diaphragm.

Effective horn loading occurs when two conditions are met: (1) the distance from the diaphragm to the mouth of the horn is equal to the wavelength of the tone being produced, or greater, and (2) the cross sectional area of the mouth of the horn is equal to, or greater than, that of a circle of a diameter equal to the wavelength divided by PI (3.14159). More on this later.

(These figures are actually conservative by a factor of 1.25, according to my ancient text.1)

Horns can be designed to have different "flare rates." In other words, some horns increase in cross sectional area faster than others. Common flare rates include conical, exponential, hyperbolic, and "tractrix," which is a combination of exponential and hyperbolic. Each of these flare rates, or shapes, has its advantages and disadvantages. Flare rate is chosen to control "radiation resistance" in the "throat" of the horn. The exponential horn is the most widely used, by far, because it's seen as a good compromise between the low-end extension and low distortion in the throat.

Advantages and Disadvantages of Horn Loaded Loudspeakers

Advantages of horn-loaded loudspeakers include greatly increased efficiency and decreased distortion when operating within their range of true horn loading. Another practical advantage is that a horn can be shaped to achieve a predictable dispersion pattern in the room, with resulting tight control of coverage. This is great for feedback avoidance, as in a public address system, and for "long-throw" applications, as in a deep rectangular room.

Disadvantages of horn loading include, ironically, their tight pattern control, which can be bad in a situation where wide dispersion is desired, and their large size. We will see that the size issue is what makes horn loading impossible to achieve in practical bass guitar systems.

Another disadvantage of the folded horn, in particular, is the coloration, the response irregularities, and the phase effects caused by the convoluted path through which the sound must travel before entering the listening area. An acoustic guitar or a human voice reproduced through such a loudspeaker will be greatly colored and distorted.

This is not to say folded variants of horn loaded systems can't "sound good," only that if they do, it's not because of their horn shape per se. And they're undoubtedly larger than they need to be to sound as good as they do.

My Opinions of Horns

In my opinion, horns are like any other family of loudspeakers. They have a place. They can be used correctly, or they can be used incorrectly. They can be well designed, or they can be thrown together or ill conceived. When they are used and designed intelligently, they can be fabulous. When they are designed or used inappropriately, they can be silly, obnoxious, and too big.

Some of my favorite loudspeakers in the world are horn loaded. Among the classics, of course, is the venerable Klipsch Corner Horn, which albeit huge, overcomes the size problem of the true bass horn by converting the entire corner of the room in which it is used into a horn mouth. Electrovoice actually produced a larger version, under license from Klipsch, called the Patrician, a four-way all horn loaded system. Didn't sound that great, but they had a mojo of Biblical proportions. I owned some of them at one time.

The classic Acoustic 360 and 370 systems are probably the most celebrated horn-variant systems in the bass guitar world. Although not true bass horns, they are very cool. I had an opportunity to hear one locally not long ago, and that old pig still sounded great! It had an 18" Vega in it, before they called it Cerwin Vega.

The Electrovoice ST350A is a personal favorite. A radial tweeter with a phenolic diaphragm, very sweet. JBL and Electrovoice have produced, and do produce zillions of great horns. I love the sound of the modern JBL's at the movie theater. I have a fondness for early fiberglass horns from Community Light and Sound. They made a straight bass horn called the Leviathan (in the Whole Earth Catalog- Google that, kids!), which was huge, and came in two pieces, which when fastened together, would put a horn with a huge mouth area in front of a pair of 15" woofers.

I would be remiss to not mention the Altec 311 Series. The 311-90 is a highly sought after, antique, cast metal exponential horn, said to have a low frequency cutoff of 300 Hz. It's approximately, I estimate from memory, 30 inches wide, and 16 inches high. Completely bodacious, and a thing of beauty. It has an Altec-size throat of 1.4 inches, as I recall, and is driven by the Altec 280 and 290 series drivers, with their 3-inch aluminum diaphragms. Killer! Collectible, valuable, and mostly in Japan by now...

Google some of my faves. You won't be disappointed.

Bottom line: I love horns, when designed and used correctly. I've built and used lots of them. I still own some.

Car Horns, and Variants of Horns, Be Useful to Bass Players?

Well, yes. Sort of. They can get you through the gig in an emergency, when you can't find anything smaller or with better low-end extension, or with less midrange distortion, and if you can find a goon to carry it in for you. Other than that, it's hard to recommend them.

To explain, allow me to divide what are referred to as "horns" into two categories: True horns, which can achieve horn loading at bass frequencies, and "variants," which cannot.

We will see that the former is nonexistent, and that the latter is impractical.

The True Horn

The lowest note on a 5-string bass, as some of you might know, is 30.87 Hz. Let us calculate the size of a 30.87 Hz truly horn loaded system from the above rules:

Wavelength at 30.87 Hz = 1120 feet per second divided by 30.87 Hz = 36.28 feet.

A Low B note has a wavelength of 36.28 feet. This has alarming implications for anyone who wants to achieve horn loading at this frequency.

To wit: From the above conservative guidelines, a bass horn would have to have a length of 36.28 feet, and, assuming a circular shape, a mouth diameter of 11.55 feet (36.28/3.14159).

The true Low B bass horn is about 36 by 12 feet. Anything with a mouth or length smaller than that cannot achieve horn loading at this frequency. I know it's alarming, but those are the conservative facts.

This first type of horn is nonexistent, in my experience. I think I'd remember a bass rig of that size it if I saw it. So would the singer, who would fire the bass player, and get someone a little less OCD for the next gig.

The Horn Variant: A Midrange Horn

So what about the second type, the horn variant, which fails to achieve horn loading at bass frequencies? Let's take a look at what it really is.

The key to understanding the horn variant is to recognize that it transitions from true horn to non-horn behavior at a much higher frequency than the true bass horn. Thus, there are two modes of operation, above and below this frequency, and a limited range in between, where it behaves progressively less horn-like as it transitions.

This horn variant, in fact, is a true horn. It is a midrange horn, but not a bass horn. It's too small to be a bass horn. There is a frequency below which it cannot operate as a horn, based on its size.

How Low Can it Go?

It is easy to calculate the frequency at which the horn loading breaks down. How big is the mouth? How long is the horn?

What would be the lowest horn loaded frequency of a horn variant? Let us look at an example. Let us assume we built a so-called folded horn, in which the length of the horn was folded back on itself, or around, to manage a convenient shape. 

Let's say we have a horn with a folded length of 6 feet, and a mouth of 2 by 3 feet, or 6 square feet. This would be similar in scale to the typical bass horn variant, and would not be considered a small load by anybody but a linebacker. How low would this horn "work?"

Based on the mouth size, there is a frequency below which the unit can't operate as a true horn:

This horn's 6 square foot mouth would be equivalent to a circular horn with a diameter of 2.76 feet.

This would, from the above guidelines, predict a horn cutoff frequency wavelength of PI times this 2.76, or 8.68 feet.

A tone with a wavelength of 8.68 feet has a frequency of 130 Hz.

Below 130 Hz, this is not a horn. Its mouth isn't large enough to permit horn loading.

But it gets worse. The ability of this midrange horn to behave as a true horn is also limited by its length of 6 feet, and in fact, is longer than what is to be found in most bass "horns" and kit plans for them. This horn can't behave as a true horn below the frequency whose wavelength is 6 feet. What is this frequency? It is 1120 feet per second divided by 6 feet, or 186.7 Hz.

This very large loudspeaker cannot operate as a proper horn below 186.7 Hz, which is more than two octaves above Low B. In the low bass, this horn variant isn't a horn at all, it's just a good way to fill up a van and break your back moving it around.

Behavior Below Horn Cutoff

Below the frequency at which the horn loading begins breaking down, the horn structure becomes "invisible" to the wave, as the frequency becomes lower, and as the horn becomes smaller in comparison to the wavelength. The cone movement is no longer controlled by the horn, and the dispersion pattern in the room widens to virtual omnidirectionality at low bass frequencies, exactly like a conventional vented or sealed enclosure- because that's what it is at low frequencies.

When the horn is inconsequential, at these low frequencies, the actual air in the vicinity of the throat has a significant mass, which is "dragged" by the cone. This mass loads the cone of the woofer just as a wad of silicone or modeling clay stuck to the cone would. The unit literally becomes a mass-loaded woofer in a small sealed enclosure- the horn is gone. At low frequencies, you carried that behemoth into the gig for no reason, other than to meet girls.

(Please see my piece on "Should You Download Your Woofer," in the Science section of this website. You will find, if you look, a detailed analysis of the addition of an air mass in a confined space to a speaker cone, and how it changes the effective parameters of the woofer. All of the math applies to a horn well below cutoff frequency. It's important to understand these principles when choosing a driver for use in one of these horn-variant systems. Qualified suppliers of kit plans should be able to explain these principles to budding home-builders.)

As the frequency falls below the horn-cutoff frequency, the distortion-reducing qualities of your horn disappear. Cone excursion increases as the unit transforms to simple mass-loaded sealed enclosure. As cone-escusrion increases at low frequencies, intermodulation and "doppler" distortion of the high frequencies increases as a result. In fact, the conventional vented system is able to control cone excursion much more effectively than the horn variant in the low frequencies, where it counts, and is able to reproduce the lowest notes with lower distortion as a result.

Where Did All the Folded Horns Go

There was a time, primarily in the 1970's, when folded horn bass rigs were everywhere. Partly because of the great sound, and the success of the Acoustic 360 and 370, and the visibility of Jaco Pastorius, nearly all manufacturers took a stab at the folded horn for mass production. I remember, as a drummer, helping bassists lug around designs from Ampeg, Fender, Traynor, Peavey, Carvin, and others. They were everywhere. I built a few of them myself. Like everyone else though, I abandoned the idea over 20 years ago as goofy, gave my last folded horns away, and haven't touched them since. As a musician, while I was still playing gigs, I crossed a line after which I declined to help my fellows carry their ridiculous equipment, such as Yamaha Electric Grand Pianos, Hammond B-3's, Leslie speakers, and folded horns. "You're on your own, pal."

The market has spoken. Folded horns for use in sound reinforcement applications are still used, but for bass players, they have gone the way of the Stanley Steamer for obvious, stubborn, and irrefutable reasons.

Can you play bass through a folded horn? Of course you can. It's just been proven to be impractical. They're just too stinkin' big and heavy, and average at best in terms of sound reproduction. For decades now they've been pretty much gone from the scene. They won't be coming back.

Horns have always had a great fascination for loudspeaker hobbyists, because they're so "cool," and they probably always will. Horns are sexy, no doubt about it.

But it's basically a huge box that doesn't have the low end that a vented or sealed box of similar size would have. It has a high low-end cutoff as it loses horn "loadedness."

This is to say nothing of the issue of the distortion in the midrange caused by the bending of the folded horn. Musicians seem to be much more concerned about coloration now than they used to be. I believe this is because of the proliferation of home studios in the last 15 years, and the experience musicians have had in hearing their instruments reproduced with relative accuracy.

I will never build or recommend a folded horn for a bass player. It's a dumb idea, which has failed to pass the test of time. Folded horns have been relegated to the well-intentioned world of the loudspeaker hobbyist and to the landfill. So they shall remain. Take it to the bank.

1. Hi-Fi Loudspeakers and Enclosures. Abraham B. Cohen, Hayden Books, 1956.

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