Rooms & Speakers

 

There is much fretting and hand wringing in the world of high end audio. The analog vs digital debate rages on. Thermionic based units vs solid state. Component synergy and cable differences. Real and imagined.

Audio reviewers can be counted on to miss the elephant in the room. The pachyderm lurking in the parlour is...speaker dispersion patterns!

There are 5 basic speaker design categories. Monopole, Dipole, Bi-pole, Omnidirectional and Point Source Omnidirectional.

Monopole

99% of all loudspeakers fall into this camp.They can be small bookshelf units the size of a tea cup or large floor standing ones with the size and heft of a garden shed.The drive units, whether a single full range or multiple woofers, midranges and tweeters are all parked on the front of the cabinet and fire directly at the listener.As can be seen on hundreds of measurements of this design, the frequency response curves taken on axis will always be very impressive when the microphone is placed directly in front.All is lovely until we gaze at the response above 4 or 5 KhZ taken at 30 degrees off axis.They donʼt dare show you what happens at 90 degrees horizontal or 60 degrees vertical. The curves are completely different when compared to the curves taken on axis.Why does this matter when we only sit in front?The sound reflected off of walls, ceiling and furniture will be different than the sound on axis since they effectively have dissimilar sources. The net result is like listening to two different speakers at the same time or, more to the point two different sounds arriving at a slight time delay caused by the reflections.It becomes very fatiguing.This is why monopoles cannot sound lifelike even when using distortion free drive units and perfectly damped enclosures. Audiophiles will invest thousands of dollars and months of screwing around with all manner of sound damping material to eliminate these annoying reflections. To the point where the room becomes claustrophobic. Spousal units are not amused by any of this. Because of the simplicity of monopole designs they can be churned out at a respectably low price. Walmart has them at $30 a pair or less. There are some boutique versions of monopoles that can cost North of a half million dollars (cough). You can spend a gazillion dollars on this design. The problem is that they always sound like speakers.

Dipole

This includes open baffle as well as electrostatic and planar types. The sound leaving the front of the speaker is out of phase with the rear. There is a cancellation of the signal to the sides which certainly cuts down on side wall reflections, neatly dispatching the typical problems of a monopole. Thereʼs also a steep roll off of the low frequencies which must be carefully corrected with equalization. The optimum listening window can be narrow. The main advantage is that there are none of the usual cabinet resonances to tame.

Bipole

This is basically two monopole speakers placed back to back in the same cabinet. The drive units work in phase without the side cancellation of signal found with dipole speakers.

Omnidirectional

There can be much mischief in the marketing departments with this design.

There are far too many versions with drive units firing every which way claiming to be omnidirectional. With a dogʼs breakfast of a dispersion pattern itʼs anyones guess as to where they are supposed to be firing.One such example is a large Bi-pole speaker with many drive units plastered on front and rear. Itʼs touted as being an omni. It is no more an omni directional speaker than the family sedan is a rocket ship.

Then thereʼs a hybrid effort with cone woofers and electrostatic mid/tweeters. A noble effort but thereʼs a “crease in the fabric” sort of discontinuity between driver types. Itʼs sort of an omni in the horizontal plane but suffers in the vertical.In other words donʼt stand up. They cost a stinking fortune.The marketing mafia will try to fob off some versions with a woofer mounted on the front and a tweeter on the top. Others have the woofer on top and a tweeter on the front.

Some examples will have woofers/tweeters parked on all four sides, sort of a North, South, East, West arrangement. The drawbacks of a monopole ie: lobing effects , diffraction and time smears are simply multiplied by four. They can play fairly loud though. Just donʼt sit in the South-East corner of this design.Others may have an array of drive units firing six ways to Sunday to achieve a more “spacious” sound. Sadly, there will always be an incoherent wave launch due to multiple drivers with inherent time delays and diffraction. Lobes of frequency accentuation and nulls are also compounded.

There is an omnidirectional design which has the woofer, mid and tweeter spaced quite a distance apart. In order to make the stereo image “gel” in this approach the listener must be sitting fifteen to twenty feet away. This is why these speakers are always displayed in huge rooms. Given the price youʼd have to live in a castle anyway.

Point Source Omnidirectional

Note: The marketing whiz kids have snared the term “point source” to market just about anything. There are some full range single speakers with the drive unit plunked on the front face of a box. This is touted as a point source as opposed to physically displaced drivers and no crossover. What does this get you? Bugger all really. Itʼs still a monopole with all the usual ills of limited dispersion.

The theoretical perfect speaker is a “pulsating sphere”. Think of a vibrating basketball somehow suspended in midair. The point source omni is as close as we can get to the illusive sphere. Itʼs also an engineering nightmare to build correctly.The goal of this design is to create the illusion that the speakers disappear. It can be a wonderful experience to hear the individual instruments in a string quartet right before your eyes without bobbing and weaving in front of an electro-mechanical device with a restricted dispersion pattern.

The drive units of a point source omni speaker must be snuggled as physically close together as possible while maintaining a hemispherical signal launch pattern. All frequencies must be reproduced into a solid angle with no diffraction, beaming or frequency deviations. A correctly executed point source omni speaker has three unique features that no other design type can accomplish:

First - the listener can sit as close as two feet from the speaker at any angle and not be able to discern that he is listening to an electro-mechanical device.
Second - the listener can be anywhere in the listening area and not hear peaks and valleys in the frequency response. There are no diffraction effects or lobes of drive unit overlapping problems to cause distraction.
Third - the perspective presented is natural. This is of paramount importance for lifelike reproduction.This is immediately recognized by folks who attend live un-amplified concerts.

There is no incessant yammering about imaging, depth, or “sound staging.” It simply sounds natural. Owners of conventional speakers are sometimes confused by the natural and lifelike presentation of a point source omni : “it doesnʼt sound like a speaker.”

Room Interactions

One would think that the worst type of speaker to place in a room would be a point source omni and the best would be a monopole. Actually just the opposite is true. A monopole speaker - drive units on the front, will exhibit a totally different frequency response pattern when measured at 0 degrees on axis and 90 degrees to either side. It will also have a different response pattern when the listener stands up. The listener will always need to carefully measure tweeter height from the floor for the optimum listening position. No one takes a tape measure to the concert hall.When we look at the frequency response of the point source omni on axis at 0 degrees compared to 90 degrees to either side or 180 degrees away from the listener the pattern is identical. You can also stand up and not have the image collapse in a heap. Thatʼs the advantage of the design.The reflected sound off any walls or ceiling has the same source as on axis.The ear/brain quickly adjusts as it does in real life. You can accept the sound of someoneʼs voice in a reverberant room and easily understand him. If another person was talking to you at the same time from the side only a wee bit delayed it would be rather difficult to follow the conversation - thatʼs the net effect of listening to a monopole design.It becomes fatiguing since it is incapable of a natural and lifelike sound. Audiophiles with monopole speakers spend thousands of dollars and months of screwing around to tame the reflections. The premise is flawed in the first place when a life like presentation is the goal. No one plasters the walls of a concert hall with yards of sound deadening material.

In 1933 Paul Voigt introduced his corner horn speaker with the objective of reproducing a live music event in the living room. Voigt continued development of his products and worked for a time with Alan Blumlein who is considered the inventor of stereo. Blumlein was tragically killed in a plane crash in 1942 at the age of 39. Voigt also worked with Donald Chave, Peter Lowther and later Victor Brociner. All of these gentlemen later associated with Stewart Hegeman. Hegeman is considered one of the grandfathers of accurate stereo reproduction in the home. His work with Harmon Kardon in the late 50ʻs and early 60ʻs resulted in the Citation series of preamps and power amps. These items are sought after by collectors all over the world. Hegeman also introduced a point source omni speaker at a price far too low to be taken seriously by the high end audio press. In other words there was no money to advertise the product. Hegeman handed the reins over to me in 1977. I am deeply indebted to these individuals for paving the way for me to evolve the speakers to their current state. The goal for these 83 + years has been singular. That is, to park the illusion of musicians performing in your living room. The laws of physics and acoustics and a love of music being the constant guide. The Morrison Model 29 is the result.