Jet-Bass in more Detail
Achieving the Maximum by Doing a Minimum

Bass - the persisting Problem in Home Entertainment

The persisting problem in sound reproduction is accurate bass, until today and that is what I'm talking about, nothing else than bass - sub bass - down to the audible limit. For those laymen and maybe also for more skilled readers who want to know more about speakers, I recommend my jetbass-blog where I explain speaker- and particularly bass-related matter in a way that I think to be easy understandable also to those who are not  acoustic sound specialists.

For excellent reproduction of deep bass at home, I have developed a solution that works for me and can work for you, as well. Within years of my retirement I built more than a dozen horns to test different aspects, digged deep to identify the problems and probed for ways to overcome them.
   
 

How to use a big horn while maintaining a small enclosure

I found a surprising answer because commonly the first idea is, how to make it small which until now never lead to the desired sound quality. My approach was a horn without compromise at first which led to a horn of maximum size. After that I probed for the best way to integrate it optimally within a room and you guess right - throughout a corner but not by building it within as is always done, because I envisioned the corner like a mirror cabinet where the waves are reflected as in optics by a rectangular prisma and the horn appears as coming into the room virtually from far behind the corner. That imagination gave the correct result with a graphical method, but I still had no way to calculate and predict it. The solution was combining known with new pieces and I had to go back in history to the beginnings of sound reproduction.
 
 

Back to the Roots

In the beginnings of telephone, sound recording and reproduction only very little power was available, therefor horns were frequently used, the bigger the better. When amplifiers already became more powerful, a German Hifi-Norm was defined in 1966 that required a minimum of 40Hz for good reproduction, which nowadays is still referred to by some sellers, aiming to suggest good quality to consumers, but technically it was already far surpassed before 1930. And since then the demand for bass in music and video went much further, down to the audible limit.

We know, that at constant amplitude wave energy increases quadratically with the frequency and because bass frequencies are the lowest audible ones, they also have the lowest energy and therefor shouldn't be so difficult to generate.






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32Hz - Horn 1929
Western Electric







Link tested: 5.5.2018,
                        http://www.royaldevice.com/indexITA.html
Experimental
Sub-Bass-Horn
in Basement,
Italy, 2008


Why is Bass still such a Big Problem
after Hundred Years of Sound Reproduction?

At bass, a diaphragm is always small in relation to waves of up to 22m length at 16Hz. Therefor an acoustic short between both sides of the diaphragm must be avoided which was accomplished with cones and horns intrinsically. If one diaphragm side radiates into a 1/8th section and the other side into seven 1/8th sections, the displaced air volumes are equal on both sides, but the air speed within the wide side is only 1/7th the small section which is inversly to the section area size.
 
 

A Simple Calculation Reveals the Dilemma

Because the resistance of air increases quadratically with the speed, the emitted energy at the restricted side is 7²=49 times higher and 98% of the energy is drawn at the restricted side and converted to pressure waves.

It's opposed at the wide open side of the diaphragm. There air can escape easy in seven directions with low resistance and only as little as 2% of the energy is drawn and radiated to the wide area.

Bass horns are huge and consumers prefer smaller speakers. Using a closed box to prevent an acoustical short circuit works for mid and high frequencies, but at bass arise a number of serious problems because physically you are trying to radiate bass to the ambient at the unrestricted side, which can radiate only very poor, as said above - so for bass it's the bad side.
 
 

Trying Hard an Inadequate Concept

Closed box bass speakers therefor struggle with very low wanted radiation and much unwanted side effects like cushion forces, vibrations, reflections and standing waves within the box that all affect the diaphragm and generate distortion of the original sound, but must not be heard outside. And the best audio-engineers still are busy in designing more powerful speakers, only to fix some of the symptoms, but the physical fact of an inadequate enclosure-type for bass is incurable.
 
 

Horns have Problems, too

Meanwhile bass horn designers have to deal with very large waves that need a horn mouth of about 7m in diameter at 16Hz to be well radiated. This results in a long horn which causes a time-delay with respect to other speakers for mid and high frequencies. Sensible listeners can already perceive 8ms delay, which relates to a length of 2.75m. But for horns of less than a quarter wavelength (5.5m at 16Hz) other problems arise and affect the sound quality.
 
 

A Proper Horn doesn't fit Within a Corner

It is known that restricting the expansion by seizing a corner reduces the required horn mouth area by eigth and it's diameter by sqrt(8) to 2.5 Meter at 17Hz. Nevertheless only few sub horns of that size exist world wide and are of experimental character only. Because this is still much too large to be applied in a living room, it is impossible to fulfill the requirements at home and audiophile listeners use horns for mid and high bass only, because until now nobody has found a solution for that dilemma.
 
 

Seize the Opportunity for a Horn Without a Mouth

Now, a corner itself resembles a horn of conical type. Using it to improve bass was patented first by Edward Sandeman in 1929 and improved by Amar Bose in 1959. It occupies less space volume than other horns and has many advantages, the most remarkable of them are lack of a mouth, lack of a cut-off frequency and any reflections from adjacent walls, no time delay and an equally dispersed sound in all directions. For 51Hz Amar Bose dispersed 22 pieces of 4"-speakers on a 1/8th sphere, which quadruples to 88 pieces for 26Hz and would be 230 pieces for 16Hz. Unfortunately too much to be normally accepted by consumers and engineers don't have another ace up their sleeve, but I considered it a sin not to take advantage of the unique opportunity of having such a big horn for free, despite of apparent obstacles that seemingly prevented its use.
 
 

Digging Deeper

I dug still deeper and found
  1. Trick: Combining Horns

    Combine two horns, this trick was first teached by Harry Olson in 1940. He said that a short preceding horn of higher flare doesn't change the characteristics of the following one. Substituting the huge radiating area with that bunch of speakers by a preceding horn therefor becomes an option. The connection has to be at equal area and same time equal expansion, then no reflections occur but there is not much advantage in size with an exponential horn, because equal expansion occurs at a very large area only. Also there isn't enough space volume to build a proper preceding horn within the corner.
     
  2. Trick: Modify Hyperbolic Expansion

    for better hornshape. Horns are calculated for plane waves but really neither radiate plane wave fronts nor really spherically ones at the mouth. The calculated horns provide up to about 60% smaller areas than needed for spherical shape at the mouth. Hyperbolic-, Traktrix-, Kugelwellen- and Le Cléac’h horns aim to improve that by somewhat increased expansion towards the mouth and really achieve better shaped wavefronts, but the correction should still be more.
     
  3. Trick: Fix the Expansion Gap

    Bridge the gap between radial and spherical expansion.
    Exponential horns have a constant expansion rate. Conical horns have a gliding one. While both need the same radiating area for the same low frequency limit, a corner cone has double the expansion at that area, whichever frequency is chosen. This requires a horn of double flare and additionally a corrective action for the forementioned 60% of missing area to be provided by an increase in flare rate, in all about three times the flare rate at the connection to the corner cone. To retain good hornloading at the throat, I use a modified expansion that is low at the throat and adjustable to the required flare towards the end at the connection. Thereby the whole structure can preserve the 'infinite' characteristic because nowhere happens a sudden change in the expansion, but at the expense of  some higher attenuation at lowest frequencies, which is still small compared to the losses of a direct radiating speaker. And without a constant expansion rate of the preceding horn, no cut-off frequency is introduced. Unfortunately there is not enough space volume within a corner to build any proper horn within, so you always need some extra space in a garden, subsolo, a bay or maybe a room next door.
     
  4. Trick: Seize the Corner Reflections

    A corner cannot only be used for a closed back chamber or to build a speaker within to economise some wood. Actually it can be of much higher value when the corner cube area becomes employed as a 3-D retroreflective means and fold the last portion of the preceding horn within. It is then sufficient to house the small tail of the preceding horn and radiate from an outlet at a certain distance towards the vertex. Some areas within the corner cube then are used two or three times in different directions by the waves and thus the missing space within the corner is virtually provided. Thereby all trajectories have equal length until the 1/8th spherical joint to the corner cone throat, so the waves arrive there in perfect spherical shape and without distortion. You not even need any extra baffle nor deflecting members and no sealing to the walls.
     
  5. Trick: Seize Asymmetry for Separation

    Radiating asymmetrically towards one of the walls is a method known from optics, to separate the two paths for the in- and out-going waves which avoids any reflections back into the small outlet of the preceding horn. It works with bass because the emerging waves from the outlet are nearly plane type. So the outlet can be positioned near either wall before the corner, ideally radiating in paralell to the room diagonal, see figure 'Top View'.
 
Having created an infinite bass horn resolves the problems that direct radiating bass speakers are suffering from, same time the problems of mouth reflections with exisiting horns. Finally the door opened for an opportunity to reduce the enclosure to a tiny fragment containing only the 'horn-tail' and speaker alone, without affecting the working of the whole.
 
This can be housed e.g. with a 10"-speaker in a gross 60 liter tower or a functional shaped design that is appealing to the eye, see Photo, accentuating but not dominating your room design and not displaying an annoying big black hole to the listener. Nobody sees a big horn but it works as an infinite horn of room size, everybody can hear it's marvelous sound and for sure will ask what sub bass speaker you are using.
 

Q.E.D.






Dipole in 1/8th-Space







E. Sandeman 1929







Amar Bose in 1959








Harry Olson in 1940








Speakers Removed







Horn Loaded Corner







Flares






Mirrored Horn







Reflective Corner-Cube







Functional Enclosure







Top View







Jet-Bass 7", 40 Watt


In my ebook "Design of Small Sub-Bass Corner Horns" you find what you need to design and calculate a horn and an enclosure with tips for proper folding and proposals of realized prototypes. Media & Contact

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