When I entered the venture of building loudspeakers not too long ago I must admit that I did so a little haphazardly. Almost entirely and completely haphazardly honest truth be told. I simply wanted to build speakers with my son, found a design we really liked, and talked to Bjorn Johannesen, the man who invented that design. For the fun of it we built a pair of speakers, and then I thought, wow, these do work and I started a company.
Now with some hindsight I realize that I have gotten rather lucky with our early success in this venture; the thing is, you see, that there are more ways to skin cats than there are actual felines when it comes to designing loudspeakers. There are so many absolutely fantastic approaches as there are gifted passionate people in the industry, and in their homes, advocating one design over another, relentless in their audiophile conquest of the better over the good.
So today I decided to try and put some structure into the madness and talk about the different design approaches that often take completely opposing directions, yet reach surprisingly valid results.
The perfect sound system, it seems, is yet to be invented, and with many basic attributes like size, price, stereo-imaging and soundstage, clarity of the bass, loudness, efficiency, distortion, resolution of detail and last not least aesthetics every loudspeaker ever designed will represent a compromise of some attributes over others, and contingent on where your priorities fall you will come to a completely different conclusion of what´s best for you.
Approaching this in simple terms a loudspeaker enclosure really only needs to fulfill three tasks well. One is to keep the driver from falling to the ground. Most designs are sufficiently good at that one. The other is to keep sound that radiates from the back of the driver to hit the listeners ear at a different time than that coming from the front causing rather nasty cancellations and superpositions of sound. And lastly for many designs it functions to help the driver extend into lower frequencies it can´t naturally express due to size constraints.
So let´s start with a specific design and work our forward. This design is called the “Infinite Baffle”.
Let us, for a minute, suppose size and money are no object and you are unconcerned about your relationship with your neighbors, then a great way to build a fantastic loudspeaker is a simple hole in the wall. A rather big one I might add. This is also called an infinite baffle and serves the purpose to send the backward radiating sound to the neighbors, and leave you only with only the good stuff. It will do nothing in terms of extending the bass, hence a big driver is called for, and since those usually really aren’t that good at expressing higher frequencies you may as well add additional holes for some mids and tweeters to go with it. Now granted this works well, but it does turn your neighbor's condo into a loudspeaker enclosure and some prior negotiation might be required.
So what then? The “Open Baffle”
If we assume your neighbor was hesitant to agree to living inside your loudspeakers what to try next? Well, what if we built our own wall and made it big enough for the backward sound to lose interest to come around and bother us anymore? See, now we just invented the Open Baffle loudspeaker. Those are rather popular in the do it yourself community, quite simple to make and some commercial designs are available. Proponents love the neutrality of the bass, but many struggle with this necessarily always partial solution, since there is some sound coming back at us out of phase, causing cancellations and superpositions, and generally confusing the ear. Some are more sensitive to this and describe such loudspeakers as bouncy and others frankly love them.
So by all means take a listen and see what works for you. This is arguably one of the most polarizing designs out there as people either love the sound of open baffles or it outright appalls them. As a friend in the industry recently put it, now you are in fact living inside your loudspeaker.
The finite baffle becomes the "Sealed Box"
Now if we didn’t want to deal with the shortcomings of a finite wall that are described above. and the neighbors wouldn’t let us use their condo, what if we were to build our own room? And thus we enter into the wondrous world of loudspeaker enclosures. You may begin to wonder how big a room like that would need to be, and the short answer is that depends a lot on the driver you wish to install. As drivers get smaller their parameters tend to shift in a way that the minimum volume of air they need to not feel much that its there anymore is reduced. There are methods to calculate just how big such volume should be. Now as drivers get smaller, cabinets get smaller as well, and so does the loudspeakers ability to extend bass. Now if we still reduce the size further while keeping the driver you have, at some point we will notice a rather happy phenomenon called resonance.
Resonance is the effect of air inside your cabinet compressing and expanding like a spring and it happens at a certain frequency that is a function of the size (volume) of the box, and to some lesser extent its shape. If this resonance frequency is carefully tuned to the type of driver you install you get a welcome benefit that bass notes resonate with the air in the back, leading to stronger bass than you would get had you sent it all back over to the neighbor. You have now officially invented a very common loudspeaker enclosure called a sealed box, and these are indeed everywhere around us. A well designed sealed box has very neutral bass can be rather small for the output, but the bass frequencies are not necessarily that low given the size and capability of any particular driver.
So how can we get better bass keeping the bass? Enter the “Bass Reflex" design.
Now let´s say we want to keep things reasonably small but still get good bass, then there is a well known trick of the trade called Helmholtz resonance that works somewhat like a bottle that is blown over on the neck to produce a particular resonant tone. Take your sealed box as if it were a bottle, drill a small hole, and put a neck on it. Tune things and the right way and you get much stronger resonance to support the driver than we you had before. This design is called a Bass Reflex design, necks are usually pointed inwards for esthetics, and are then called ports.
There are many tools available to choose the right port diameter, length, and box volume to get the bass we want for a given driver. Many less serious manufactures like to overdo things a little to stun the customer into impressive bass, given size and cost, but this comes at a rather dreaded price called “one tone bass” where all the bass notes appear at the boxes resonant frequency sounding as if the bassist really only needed on finger on his left hand. There is also the question that some of the rearward facing sound is now escaping through the port causing phase shifts and the dreaded effect of cancellations.
So what if the finite room becomes a long tube? A "(tapered) Tube Design" is what you get.
If we don´t build a room, but in fact a rather long tube, a tube long enough for the sound sent to the back not escaping much from the tube to bother us? Well that is indeed a choice and not a bad one since such a tube can be folded, bent or curved as sound really doesn´t care much about the direction it travels in. Furthermorew we can expect such tube to not usually inhabited by a neighbour. Such designs were pioneered by people like Paul Voigt in the 1930´s, who also discovered that the longer sound travels down the tube the weaker it gets hence the less wide the tube needs to be without the loudspeaker driver noticing much of its existence. This gave birth to the idea of a tapered pipe.
What if we made this tube a certain length: We get a "(tapered) quarter Wave resonator" or Transmission Line.
Now a very long tapered pipe is arguably the smallest “infinite” baffle design possible, but it still won´t give you more bass than the driver itself can express and hence drivers and in consequence tubes need to be rather large and expensive. Luckily, something rather enjoyable happens if the tube is takes on a rather particular length: That length which corresponds to a quarter wavelength of a particular frequency somewhat below the drivers natural resonant frequency. Now again you enter in resonance but this time with a much stronger and controlled effect. By porting the tube at the end a little this effect can be enhanced strongly by forming a permanent standing wave inside the pipe not entirely unlike an organ pipe or wind instrument, and this time the basses can be extended significantly lower than with bass reflex designs. If well implemented a smooth roll of bass is possible staying well clear from the dreaded one tone base. We have now invented the Transmission line speaker and in its particular form, the quarter wave speaker. These speakers often allow for remarkable bass and clarity with even smaller drivers and the cabinets can be rather small in consequence. Our SoundSommeliers are such design. The downside of such designs is that they are terribly complex to get right, there are few simulation systems available like that of Martin J King, and few people master this art. With a good quarter wave design the sound escaping the port is only in the very low frequency range and in phase with what comes from the front, so superpositions are not much of a problem.
What if we tapered the tube from small to large instead? Come in the "(Back Loaded) Horn" Speaker.
What if the tube got bigger further away from the speaker, rather then larger well, then you would have, what is called, a horn speaker. More specifically a back loaded horn speaker. While there was a time where all loudspeakers were essentially horn speakers, they are now confined to a following of enthusiasts who like the very detailed response that comes with such efficiency and don´t mind the fact that for any significant bass output these designs have to be outright huge. Of course this is the only design that deliberately radiates the backward sound forward and getting coherence and cancellations under control is a dark art that is mastered by few. Other´s say it cannot be mastered.
But what if we put the tapered tube in front of the loudspeaker and not in the back? We have a (Front Loaded) Horn Speaker.
Horns served an ancient purpose from the time of gramophones that relied entirely on mechanical movement of a membrane by the needle in old shellac record´s grooves and of course were not really that loud by nature. A horn helps by providing a sound that is much more directional and hence louder in the area it is projected onto compared to a loudspeaker that radiates over a larger area. A well designed horn can play as loud as 110dB with only 1W of power. The following, rather endearing, video shows how horns can vastly amplify sound.
Directional sound also helps with stereo imaging giving a precise location to each recorded instrument. It is this incredible sensitivity and stereo imaging that make them resurge in times with small tube amplifiers with rather low wattage are making a comeback and there is a loyal enthusiast crowd of builders who come up with remarkable and often beautiful designs. A common misconception about horns is that they do not extend bass much. It may become apparant in the demonstration above that they in fact do. The problem with bass horns, however, is that depending on frequency response they become rather larger making the introduction of a complementary subwoofer using a sealed box or bass reflex design more practical for many.
So which of these is perfect? Well, all of them and none.
So there are many ways to skin a cat and many rather good ones. All have their advantages and disadvantages and none will ever fully satisfy all in all aspects. What is important is that every design requires skill, dedication, experience and passion for the art and the beauty of what can be achievable once all comes together can be outright astonishing. And I haven´t even started to talk about loudspeakers like electrostats that don´t use conventional electromagnetic coils to drive membranes.