Ακουστική/ Acoustics 2002 Patras University  

   

 

In the beginning were some historical moments : Roundffusor1's announcement at the University of Patras, September 30, 2002 ,here is a photo report

 

 

 

The Patras paper in English, published in Proceedings of Hellenic Institute of Acoustics (HELINA), University of Patras, © 2003, pages 41-48, ISBN: 960-7620-27-5 . HELINA is a member of the European Acoustics Associations                            

 

 

Wide band sound diffuser with self regulated low frequency absorption and methods of mounting it

 

ΖΑΙΝΕΑ LIVIU NICOLAE

ESR [EDDY’S STUDIO RECORDING]  http://www.zainea.com

E-mail :                                    liviu@zainea.com

 

ABSTRACT

 

           We present here a new kind of a sound diffuser, enough simple in his construction but very active in doing his job. It is composed from a main body and two lateral drivers, which are situated on the inside part of the main body. This two plastic drivers serves to sustain very steady the diffuser upon the surface to be mounted: walls or ceiling. There are two wooden rails ‘’T’’ simply mounted on the wall surface. The main diffuser body with his two drivers is mounted just like a drawer toward the two rails T. The main body is composed from three basically 3D shapes. Each fourth diffusers, displays together a new 3D shape at their nearest point. This diffuser may be used in any kind of room where the critical listening is needed. His main advantage is that it is working simultaneously in two ways: as a clean diffuser for mid and high frequencies and as a self controlled absorber, below 250 Hz, for low and very low frequencies. This is possible, because being mounted in a new way, his compartmented volumes behind the diffuser surfaces works like a complex Helmholtz resonator. Each air space behind each diffuser surface, interact and resonate in a different way. In real time, there is a kind of reconstruction of all music characteristics – parts diffused or absorbed or both. Even for the trained ears, there is no time to percept when and how the music is processed and the listener is just immersed in the sound without any trace of coloration. The ‘’ historical’’ focus or stereo place of listening, becomes obsolete. Diffuser dimensions: 58x58x12 cm, vacuum thermoplastic polystyrene, weight: around 3 Kg (diffuser + 2 woods rails). The Roundffusor1 is Greek Patent Nr. 1004186/22.05.2002.

 

Introduction

 

For many years I’ve thought that, especially in acoustics, what was to be invented was invented. The announced ‘’end of science’’ corroborated with an almost monopolistic worldwide marketing situation, soften the overall ambition for new. Once again, our country, Greece, took the opening flag with an invention which will help all those who intend to build or order a room dedicated to music, whatever big or small. The invention is relative simple as an object but being very easy to mount by anybody with simple tools and being also very cheap will let the people to buy their beloved or professionally electronics and put them in a room where the acoustical environment will become also a professional tool. The live or recorded music will sound beautifully similar from room to room. The acoustic repeatability will be no more an exclusivity for highly specialized acoustic installers...

 

2. What is the Roundffusor1?

 

The shown diffuser, is a device, said apparatus comprising two lateral rigid supports, mounted to the inside of the said diffuser which lateral supports are received , like a drawer , by two wooden rails, section T. The two wooden rails ‘’T’’ are simply mounted on the wall surface with screws or nails. The main diffuser body with his two drivers is mounted just like a drawer toward the two rails T. The main body is composed from three basically 3D shapes. Each fourth diffusers, displays together a new 3D shapes at their nearest point. The angle between each 3D shapes, including the new common shape, is the same. From the total surface of each diffuser there is more than 90 % diffusing surface. Except the wooden rails, section T, the complex diffuser body and his lateral supports are fabricated from vacuum thermoformed hard PVC. The aim of this invention is to radically improve any kind of an existing diffuser, be theoretical or a practical one. Is a big effort, redirecting the whole diffuser philosophy: from absorptive to clean diffusing, simultaneously being only a low frequency self adaptive - variable low frequency resonator.

 

2.1 Something about diffusers…

 

    The diffusers, until now, may be described theoretically and theirs predicted characteristics named diffusion ability are shown as polar plots, for various incidence angles, the additional absorptive curves being supplementary. The diffusion and absorptive ability of predicted acoustical diffusers are common and expected and for the real, practical diffusers. All those polar plots , usually taken each 5 degree of microphone position , shows the acoustic pressure of the reflected waves, and  a semi circle shape is considered the ‘’ideal’’ meaning that the named diffuser controls the interfering reflections in such a way that scatters the incident sound uniformly so that the acoustic glare in all directions is minimized. As much from the wide bandwidth source is scattered uniformly, and the polar plots is taking the semicircular shape, the diffuser have the diffusing ability D tends or equals 1. Fig. 1 shows, for 3 incident angles, such a ‘’ ideal situation ‘’ where the diffusion polar plot’s ark, is like a semi circle. The plots from Fig. 1 are taken from bibliography as an example for prediction for a diffuser of 2,4 x 2,4 meters. The measurement setup for the described situation is similar with our Roundffusor’s measurement and surface (1,8x1,8 meters) and also with the diffuser named as ‘’ from another company’’ which also have dimensions bigger than our Roundffusor. Note, that for the 1,74 x1,74 meters, there are 9 Roundffusors1 mounted in a rectangular formation, a 3x3 matrix.

 

If the diffuser shows one or more irregularities in his polar plots measurements, at that particular angle, the diffusing ability is reduced and D tends to zero. Must be noted that for those frequencies and angles of incidence, the acoustical diffuser is working like as an absorptive surface, the notch being more or less obtrusive toward the supposed source’s linear shape. When the diffusion ability becomes high, a kind of source reinforcement may be perceived. This reinforcement may be associated with the loudness. If it is sound coloration or not, this is a difficult task to respond. Around practical commercial diffusers, there is a confuse approach , because they  appears as having either high diffusing ability and negligible absorption , or the characteristics are shown but the explanations done are misleading even for the specialist. For many years, was kept as advantageous, the absorption ability or more correct the disability of the named diffusers to not have a small percent of absorption. There is a sensible difference as what is perceived and understood as good or bad around the term absorption and his role in music – live or recorded. This difference is wider than expected and differentiated between US and European listeners. Even the highest ‘’ golden ears’’ gifted person are in difficulty of being totally objectives in their appreciations, simply because the educational background is not similar. From the all those notch, the music, as well recorded as may be, will be distorted. There will be certain area in x,y,z directions where the sound distribution / diffusion will be irregular, far from the required symmetry.

We are limited here, but we must note, that the interactive phenomena between purely physical and perceptual aspects of what kind and where the absorption is necessary and if the simple concept of diffusion and nothing else is better practically but less well understood. To conclude, whatever the place for diffusers to be installed, their dimensions, especially the depth, the smaller – the better.

 

2. Study and effectiveness

 

In order to be sure that this diffuser doesn’t have any resemblance with anything else. Being restricted to less than 60x60 cm and less than 15 cm depth (in order to be competitive) was a very difficult task. Practically in study was all existed bibliography. The first advantage of this diffuser is that it works clean and effectively in the 250-6300 Hz, the large area where the voice and musical instruments spectrum are prominent and the human ear is more sensitive. As a result of very small distance between each Roundffusor [R1] – less than 1 mm, and the special mounting procedures, the whole surface of a rectangular team of diffusers [R1] is like a compartmented plate, simply ( but very steady) supported at the two edges, at some distance from the mounting surface. The plate works like a Helmholtz resonator. There is a known experience equation [T1] relating basically the diffuser’s dimensions, distance from the wall and distance between all diffusers. The Helmholtz type of resonance is double. First, the diaphragmatic absorption as if supposed to result from the grouped diffusers, where this total surface is related with the (supposed) bigger wave length for the low frequencies. ‘’ Supposed’’ because until now, especially in larger halls, the panels reflected bigger wave length than that corresponding to their dimensions. Those big panels redirect or reflect the low frequencies. If the panels are mounted at some distance from the mounting surface, they work as an air cavity resonator. Here we have the same surprise, and we encountered absorption down to 5 Hz. For 174x174 cm, the 9 [R1’s] area, this low frequency is much lower than the <critical> 300x300 cm ceiling music hall panels. It is known that those very low frequencies are sub harmonics usually generated in small rooms by the low of modal resonance corroborated with loudspeakers inherent distortion in the very low band. Fact is that a rectangular group of 9 [R1] deal with such a very low frequencies.

 

The parallel architectural acoustics for music hall with the small and medium size rooms is not brought here to confuse, but to show that are still many not clearly explained problems in the two distinct fields of acoustics. Are they so distinct or not, this is another story. The second aspect of the Helmholtz type of resonance encountered in our grouped [R1] is that being mounted compartmented, each line or row of minimum 3 [R1] is a ‘’ sub class’’ or part of the overall diffuser’s surface. All the above analysis is maintained, only that, the resonance frequency bandwidth is higher than the resonance of the whole grouped [R1]. So, the grouped [R1] works simultaneously as an air resonator for different bandwidths. I couldn’t prove ( at least at this stage of our research ) if the total surface of the grouped [R1’s] have some ‘’ classic’’ diaphragmatic absorption , because , being in such a way designed geometrically ,compartmented and steady supported, each [R1] doesn’t resonate as will be expected from a plastic box, meaning a very affordable diffuser. The measurements done with a miniature accelerometer, indicated similar ‘’ box’’ plastic resonance with the heavy wood furniture located in the room. For monaural source / s , all [R1] acts alike, but as soon as the source become stereo, each [R1] above his surface and below, at least at the finger’s edges, feels continuously different. If applied, for 25 mm distance from the wall of 4 grouped [R1], the equation [T1] shows that the smaller resonance frequency will be 58 Hz. This means and imply that the [R1] as hard plastic box don’t have any significant flexures, even at high SPL’s.

 

The equation Τ1 :   fr=√ e/Dd  and  e= r/(r+w ) , where

 

r  =   distance between diffusers

w = diffuser width (for Roundffusor, width = length)

d = depth of diffusers

D = distance from the mount surface

e = percent of open surface between diffusers

fr = fundamental resonance frequency

All this indicate that the grouped [R1] works as a complex Helmholtz air cavity  resonator offering absorption , enough linear , as shown on Fig. 5a, down to some 5 Hz. Such a performance, for only 12 cm depth, is for the first time achieved practically.

 

The other Roundffusor1 [R1] function is his clean diffusing ability. As known, the polar plots are measured for 0, 45 or 60 degrees. Here we have to compare 3 kinds of polar plots : the ‘’ ideal’’ ones from Fig. 1, the Roundffusor1’s from Fig. 2 , 3 and 4 and, the polar plots of a commercial diffuser produced by a known US company. It is also known that except for incident angle at zero degree , where many commercial diffuser present enough near semi circle polar plots , but still unregulated, as we proceed toward 60 degree off axis polar plots, the results are highly irregular for all commercial diffusers. More, the 3D lobes are distinctly irregular. The sole exception until now, the diffuser ‘’ from another company’’ where the overall lobe is somewhat better and may be ‘’ felt’’ at 3-4 meters. What is felt is the diffused field. In our situation, for the Roundffusor1, if we compare the 3 groups of polar plots, the result are more than obvious. May be expressed in words as such: the [R1] is a wide band acoustical diffuser having the ability of low and very low frequency absorption. The Roundffusor1 will be extremely useful in the control rooms, HI-FI, HI-END or music hall and of a big help in music school- for voice or instruments. At the actual technical / cultural level , as much as the electronic device will replace the real organs, the ‘’golden ears’’ of gifted people, able  to hear 1dB or 1Hz  J.N.D.’s and people who loves and remember the timbre of real organs, will become less and less. Thus, the difficult obtained hearing abilities will be slowly lost.

 

2.1.1 Comparisons, perceptual tests

 

To make the story shortly, doing the listening tests in a room filled with only 9 Roundffusors, we obtained similar aural impressions either for near field, far field, and all incidence angles. Until now, the ‘’ historical announcements’’ were outside Greece or Europe’s privileges. Comparing the diagrams, and taking in account the listening tests from specialists or not, doesn’t let many interrogations. Yes , for Greece it is an historical moment, because this invention brings on the market a diffuser, easy and cheap to be built, lightweight and easy to install, doing from just 58x58x11 cm a simultaneous diffusion and absorption, without any porous additional material.

If we compare the Fletcher-Munson curves with the waterfall plot of [R1] and we make their algebraic additions, an almost straight line will result. In a way this means that for our ears, the grouped [R1] is doing an inverse equalization. Because, in any room where the [R1] were installed the aural impression was similar, this means that the inverse equalization observed phenomenon is a self adaptive one; I dare to say, like a digital equalizer. As in all ‘’ historical moments’’ we are accepted to say that the all flat curves and polar plots measured from the Roundffusor1 are very near to the said ‘’ near perfect diffused field’’ situation.

 

2.1.2 ‘’ The perfectly diffuse sound field ‘’

 

     Here is an extract from bibliography, showing which are the conditions for a ‘’ perfect diffused’’ field*. Please note that we not agree with the term perfect, so, near perfect seems more approachable.

‘’ Even though unattainable, it is instructive to consider the characteristics of a diffuse sound field. Randall and Ward’ have given us a list of these: • The frequency and spatial irregularities obtained from steady-state measurements must be negligible.• Beats in the decay characteristic must be negligible.• Decays must be perfectly exponential, i.e., they must be straight lines on a logarithmic scale.• Reverberation time will be the same at all positions in the room.• The character of the decay will be essentially the same for different frequencies.• The character of the decay will be independent of the directional characteristics of the measuring microphone.

These six factors are observation oriented. A professional physicist specializing in acoustics might stress fundamental and basic factors in his definition of a diffuse sound field such as energy density, energy flow, superposition of an infinite number of plane progressive waves, and so on. The six characteristics suggested by Randall and Ward point us to practical ways of obtaining solid evidence for judging the diffuseness of the sound field of a given room.’’  From: '' The Handbook of Acoustics-3rd Editions- F. Alton Everest, TAB Books, page 223’’.

 

3. Listening tests results for the Roundffusor1

The good diffuser has more advantages than induced problems. We will present some of them, as a result from personal listening test and as a kind of statistic based on other’s perception. The degree of openness and spaciousness contained within the recording, such as characteristics of transparency and depth, can also be more easily assessed. This and the followings may be just literature, but when listening test are at such an exceptional level as the measurements, the things must receive the proper attention. Regarding the precedence effect, it is slightly reduced. Axial or left/right head movements don’t arrives to dramatically induced changes. There is an incredible stability of sound image. At the first sound heard from the loudspeakers, the listener, instinctively [meaning very quickly, something between 0.1 to 1 second] focus the ears/ eyes on the location of each musician. During the music, all those spatial references, are easily memorized / visualized by the listener. More, if the musician changes their places, or are moving here and there, their movements are easily sensed. Because the Roundffusor1 is not working on '' phase grating effect '', which involves and produce variable on surface absorption, the subjective phase perception from our diffuser is near zero phase, regardless the center frequencies or music bandwidth. All this phenomena are valid at more than 6-7 meters from the music source and the '' critical focused point of listening '' or ‘’ stereo seat’’ is obsolete. A kind of ''aura'' envelope reconstruction from the room's recording session is obtained. Think that all this arrives from a plastic area [of at least 9 Roundffusors] mounted on the back wall or ceiling, there is nothing to be changed on your electronics. One first conclusion: less absorption on high and mid frequencies: more music. Second conclusion: the automatic bass absorption offered by the Roundffusor1, removes the dynamic masking effects produced by the low and very low frequencies toward the right of the audio band.

Sound impressions in rooms were installed: unveil more and more details in the mix, realistic sound stage and instruments location, outstanding voice & instrument quality & musicality, less fatigue at high levels of listening.  

  

Fig. 1 Roundffusor1 – patent pending, 4  Roundffusor1 mounted together 

 

Fig.2 The ''ideal'' predicted polar diagrams (from bibliography) for situations similar with the Roundffusor's measurement setup. The heavy lines represents 99% diffusion factor, and the incidence angles are: a) 0, b) 30 and c) 60 degree.

 

Fig. 3 Off axis polar diagrams for Roundffusor1. All polar plots were obtained from a 45 deg. incidence angle.

 

Fig. 4 Off axis tile polar diagrams for  Roundffusor1

 

 

 

 

Fig. 5a- Waterfall plots for Roundffusor1, same source and setup, sweeping bandwidth 12-19368 Hz . On right, Fig. 5b- the Fletcher-Munson curves.

 

 

 

Fig. 6 Off axis polar diagrams from another company’s best diffuser

Fig. 7  A big control room in Athens where 14 Roundffusors1 were installed

 

Thanks

Such an invention like this, appear each 40 or 50 years. Without God’s help or an exceptional sudden inspiration, it is no possible to appear an evident result like this. In it’s simplicity but highly complex functioning, even the patent’s text doesn’t explains all, and further research is needed, at least from other researchers if still in disbelieve.

 

Bibliography

1. D‘ANTONIO , P., The Directional Scattering Coefficient: Experimental Determination, J. Audio Eng. Soc. 40, No.12. 997-1017 (December 1992).

2. D’ANTONIO, P., KONNERT, J.H. and KOVITZ, P. The Disc Project. Experimental Measurement of the Directional Scattering Properties of Architectural Acoustic Surfaces, IpAAd2, 14 1144 (June 1994).

3. COX, T.J. and LAM, Y.W. Evaluation of Methods by Predicting the Scattering from Simple Rigid Panels. Applied Acoustics.40 123-1 40 (1993).

4. COX, T.J. and LAM, Y.W., Prediction and Evaluation of the Scattering from  Quadratic Residue Diffusers , J. Acoustic. Soc. Am. 95(l). 297-305. (1994).

5. ISO 66:1997, Acoustics — Preferred frequencies. Geneva, Switzerland: International Organization for Standardization.

6. IEC 61260 (1 995-08) Electroacoustics — Octave—band and fractional—octave band filters. Geneva, Switzerland: International Electrotechnical Commission.

7. DRAFT AES-4id-xxxx , AES information document for room acoustics and sound reinforcement systems-Characterization and measurement of surface scattering uniformity , AES 2000.