Reflection on counterperture acoustics


Created on 02.04.2008 18:52.

Updated 04.28.2020 09:52.

Author: Andrey Ramin.

Efforts to improve loudspeakers have been made since their inception. New technologies, engineering solutions, and other concepts have been created for 85 years, but until now fundamental science, as such, has little concern in this area. Perhaps this is due to the lack of interest from the military (since it doesn’t shoot, it’s not necessary) and large corporations (after all, the manufacture of speakers is not the most profitable and voluminous business), sponsoring science when absolutely necessary.

Be that as it may, but the Hi-Fi standards adopted at one time were taken by manufacturers as a basis, since they fully correspond to the ideas of the resolution of human hearing (reproducible frequency range and a measure of nonlinear distortion), and make any attempts to further improve – meaningless.

But, as a practical consequence of the introduction of Hi-Fi standards, the emergence of the Hi-End category was unexpected. Amateurs and professionals are constantly trying to create acoustics that sounds “the way they should”, albeit in violation of the accepted position that “a person will not hear more.” Surely, while reading magazines and the Internet, you have met with constant disputes about sound among audiophiles and simply music lovers.

“These columns are like that, and these are different” – no objectivity.

Agree, this is strange for a long-established science. Imagine how a dispute would look like that in an isosceles triangle, the right thigh is still shorter? Today we invite you to look at acoustics as a science and its practical application, but from a different perspective.

The very word “counterperture” literally means opposing radiating holes (from the Latin apertura – hole). The concept of aperture is often used in optics, but in our case, it is better to understand the term as “opposing sources of sound excitation.” This is the architecture of the speakers built according to the counter-aperture principle. Two identical speakers are located coaxially against each other, and are switched on in phase.

Rice. 1. Duevel Sirius combines elements of horn and counter-aperture design

That is, they work synchronously, without any delays, phase and frequency differences. It would seem that such a design can sound right, because the speakers do not look at the listener and interfere with each other? It’s time to remember the physical foundations.

Sound should not be understood as vibrational air currents spreading through the room. A sound wave is a wave of pressure change. Think of the circles on the water after a drop hit it. The water itself does not flow anywhere, it remains in place, but waves propagate through it, which are the consequences of the displacement of the volume of that very drop. Moreover, this movement is reciprocating. So in the air, the molecules themselves do not move anywhere, but only slightly displace, either towards each other, or vice versa, changing the density of the medium in waves.

With the standard speaker design familiar to us, the discharge / compressed air zone is in front of the speaker. Note that it is not the speaker itself that is the source of the sound, but the change in the concentration of air molecules in front of it. This solution has one significant drawback – the so-called “zone of dominance of the reactive component of radiation”, due to the large length of the low-frequency wave compared to the size of the emitter and the low inertness of the air.

From which it follows that in order to fully listen to bass, you need to be at some distance from the speakers, which is sometimes impossible. Therefore, we usually hear low frequencies reflected from the walls, to some extent even formed there, which naturally does not have a positive effect on subjective sensations. Although objectively the wave is present, however, in a somewhat distorted form.

But what happens if we apply the counterpertural principle? The pressure is created in the air column between the speakers, and the emission point is also between them. A so-called “pressure monopole”, a point source of sound, is created. And it is not distributed frequency at a variable distance from the speakers, but is located between them, evenly radiating in all directions. This is how the problem of the “far zone” was solved.

But another has appeared, which, as our research will show, in general, does not exist.

Counterperture acoustics creates sound pressure, which is always physically a non-directional (scalar) product, which often frightens connoisseurs.

“What about stereo, and the reflections from the walls of the room will multiply at times,” they say.

First, it certainly can’t be a problem. In the case of a stereo set, we have two monopoles, and nature (Huygens’ principle) and your brain do the job of converting the differences between the two channels. When two channels are turned on, no special pressure area is formed in which two points are located.

All the work on the localization of sources is performed by our hearing aid, and it is the task of sound engineers to “trick” the ears by applying phase displacement, delays and changes in the loudness ratio in order to obtain the necessary images. And with reflections, the situation is even simpler. The fact is that directional acoustic systems are in no way free from the problem of reflected sound, they also emit waves in all directions, but unevenly.

And in the so-called “VIP zone”, we hear a normal signal, plus a highly distorted reflected, but attenuated one. However, its weakening is so insignificant, despite all the efforts of the manufacturers, that it does not play a special role. As a result, it turns out that even in a room that is not very good in terms of acoustics, speakers with circular directivity (or non-directional) will be able to sound obviously better.

Another thing is that they will not sound good anyway, since the room itself will not allow them, creating resonances and other unpleasant effects due to their architectural, structural or other features. This problem is nevertheless treated by changing the parameters of the room, and the speakers have nothing to do with it. By the way, counter-aperture acoustics increases the “VIP zone”, because the listener does not need to sit tensely exactly at the top of the triangle created by the stereo pair, being afraid to hear distorted signals coming from the walls.

The uniform field of “pure” sound is wider, and the criticality of the reflected waves is less, because at least to the wall they were the same as they were towards you, in contrast to the speakers of the classical directional design. But what is surprising is that the counter-opening principle allows you to get rid of another misfortune that is inherent in conventional systems, and was previously considered almost insoluble.

The fact is that the human ear is especially sensitive to the effect of Doppler intermodulation. This term, despite its formulation, which is frightening to a person inexperienced in physics, means a rather simple effect: phase and frequency shifts or oscillations of a sound wave due to the movement of a sound source. Musicians and sound engineers know it as flanger.

And for us, we can simply listen to a passing car or train, and note how the perceived sound, for example, of an engine, changes. This effect …