Dr. Oskar Heil, noted physisist and inventor of the Field Effect Transistor, began his research into loudspeaker design, not with abstract theory of how a loudspeaker should work, but with a study of the peculiarities of the human listening aparatus. The result of this intensive year-long research program led to his discovery of the principle on which the Oskar Air Velocity Transformer is based. By applying this principle to the design of a loudspeaker diaphragm, he was able to achieve a revolutionary breakthrough in solving the fundamental problems of diaphragm mass, inertia and self resonance. In the following, we describe the results of Dr. Heils research and how it led to the development of the Heil Air Motion Transformer speaker.
The research behind the Oskar A.V.T. ( AMT)
As a physicist, Dr. Heil concentrated his study on how nature designed and constructed the human ears. Then his studies concentrated on animals of a small proportion, which can produce a loud sound, especially compared to their size. These studies led to Dr. Heil’s formulation of his basic diaphragm design theory and the subsequent development of the Oskar A.V.T. ( AMT) Air Velocity Transformer.
Spurious Diaphragm resonances:The design of the Oskar A.V.T. ( AMT) diaphragms makes the use of dampening material with all of its detrimental effects, totally unnecessary
Non Uniformity of Driving force: In the Oskar A.V.T. ( AMT), the driving force is applied uniformly over the entire surface of a structurally rigid diaphragm by means of the conductive aluminium foil strips.
Ability to move air efficiently: The A.V.T. ( AMT) diaphragm’s pleats propel air at a speed of 5.3 times their own velocity.Oskar Heil AVT/AMT
Ability to differentiate sounds:
A principal function of the ear is to identify voices and for this it has developed an extraordinary ability to differentiate sounds. Single sound sources, such as a distant voice can be separated from other sounds by concentrating our hearing apparatus upon the voice and ignore noise or other voices which we do not want to hear
——-airconditioner noise – – – – desired information noise as discriminated by the ear is always below the level of the desired information
Volume ( Intensity) variations. The ear has little sensitivity to sound level “jumps” or to the relative loudness of different sound’s which are audible at the same time. For a loudspeaker, sound output levels (amplitude) over a range of frequencies are valid criteria, but are of less importance for our ears. Our ears are protected from damage by a construction which makes them relatively insensitive to amplitude changes. The difference in amplitude between a whisper and normal volume speech is not just 1:2 or 1:4, but 1:100’000.The relative loudness of different sounds, within certain limits, is therefore not too important to us, since the ear has the ability to adjust to different levels. This explains why street noises do not necessarily disturb conversation level. It also explains why we can hear an opera singer even though the sound level of the orchestra is many times that of the voice itself. Frequency variations: In contrast to its relative insensitivity to amplitude variations, the ear is extremely sensitive to minute fluctuations in the frequency of sounds, especially in the mid. frequency range. a half-tone in the musical scale represents a frequency change of 6% while the frequency shift in the vibrato of a violin is approximately 0.5%. In the critical midrange of 250 – 6000 Hz, we can differentiate between two tones even when the frequency difference is as little as 0.06%.
It is this sensitivity to frequency variations that enable us to identify different voices. When we speak, we do not produce constant tones, but tones which are constantly varying. We can usually recognize a familiar voice immediately even over the telephone and can often tell the mood of the other party by the differences in speechpattern produced by the changing of the tension of his vocal cords.
Frequency variations verses amplitude variations: It is commonly accepted that the smallest change in amplitude that the ear can detect is 1 dB, which is a power difference of 26%. Compared to the ear’s sensitivity to frequency variations of 0.06%. Contrasting this relative insensitivity to amplitude changes with the ear’s extreme sensitivity to frequency variations, it is difficult to understand the loudspeaker industry’s obsession with the minor loudness variations of 1 or 2 dB in the frequency response of a loudspeaker, while completely ignoring the audible shifting or fluttering or high frequencies which can result from changes in membrane stiffness as a sound wave spreads transversely across a diaphragm.
Phase Differences: The Ability to Localize Sounds A listener’s ability to localize sounds is made possible by phase differences ( time delays) resulting from the difference in path lengths from a sound source to each ear. This ability is frequency dependent and is more pronounced in the critical range of 500 – 3000 Hz. than at lower and higher frequencies. This is why the speed of response of a loudspeaker diaphragm is extremly important to the faithful and realistic reproduction of music. If the loudspeakers diaphragm cannot respond fast enough to enable it to reproduce these transients, or if it distorts them, the listener’s ability to recognize and localize the sound source is greatly diminished and the realism of music reproduction and the pleasure of listening is seriously reduced.Problems of loudspeaker design
Spurious diaphragm resonances: Any solid material which is made to vibrate by striking it or otherwise setting it in motion will produce a unique pattern of resonances characteristic of that particular material. If made to vibrate at a specific frequency by an external driving force it will, in addition to this frequency introduce its own resonances. In music, the pattern of these resonances or harmonics is peculiar to each instrument and enables us to distinguish between the sound of a saxophone (metal), for example, and an oboe (wood) even though both instruments are playing the same fundamental note.This charactristic, useful in recognizing musical instruments, constitute a major problem for the loudspeaker designer, since spurious resonances generated by a diaphragm will distort and mask the musical signal.
In order to move a large amount of air with minimum loss and provide fast response to the transients, the diaphragm must be extremely lightweight. However, if the diaphragm material is too thin and light, it will not be sufficiently rigid to prevent it from flexing and producing its own resonances. If the deformation occurs between the center area and the edges, that portion will vibrate independently of the music signal and produce standing waves or bell shaped vibrations which are clearly audible as distortion. In addition, the diaphragm will store the resonant energy and, when the music signal stops, it will continue to move in order to dissipate this energy. The continued vibration of the diaphragm will dampen (absorb) the sharp rising transients of the following music and seriously affect the quality of the music reproduction
single point of energy: ( flaps)
dual point of energy ( reduces flapping)
energy applied over the surface, (no flapping)
Efforts to Eliminate Unwanted resonances: Attempts by designers to minimize diaphragm resonances usually consists of coating the diaphragm with silicon rubber or other substances (this is called dampening) to increase its rigidity and prevent it from flexing. There is a trade-off, however, while the damping material may help to reduce resonances, it adds to the weight of the diaphragm increasing its inertia and resulting in a slower speed of response to the transients of complex musical wave forms. The ability of the diaphragm to move air efficiently is also reduced on many loudspeakers to a mere 0.25%
Large diaphragms and differentiated driving force. Efforts have been made to minimize unwanted diaphragm resonances by applying the driving force more evenly over a large area of the diaphragm.
Electrostatic speakers distribute the driving force over a large, flexible plastic panel suspended on a framework.
EMIT and magnetostatic speakers utillize a differentiated driving force applied to different areas of the diaphragm to compensate for the varying flexibility of its surface. However, when a flat or conical diaphragm supported at its edges is caused to vibrate only part of the diaphragm oscillates in a direction perpendicular to its surface. At the outer edges, where it is suspended, it cannot oscillate in the same manner since the surface of one side will stretch with each + sinus oscillation, while the reverse side will be compressed or “crunched” and vice versa. Thus the entire diaphragm will not move uniformely like a rigid piston, but will vibrate like a suspended flexible membrane and produce a self resonance with a pitch. (singing saw effect)
Large diaphragms and differentiated driving force. Efforts have been made to minimize unwanted diaphragm resonances by applying the driving force more evenly over a large area of the diaphragm. Electrostatic speakers distribute the driving force over a large, flexible plastic panel suspended on a framework. EMIT and magnetostatic speakers utillize a differentiated driving force applied to different areas of the diaphragm
Spurious Diaphragm resonances:The design of the Oskar A.V.T. ( AMT) diaphragms makes the use of dampening material with all of its detrimental effects, totally unnecessary.
Non Uniformity of Driving force: In the Oskar A.V.T. ( AMT), the driving force is applied uniformly over the entire surface of a structurally rigid diaphragm by means of the conductive aluminium foil strips.
Ability to move air efficiently: The A.V.T. ( AMT) diaphragm’s pleats propel air at a speed of 5.3 times their own velocity.Oskar Heil AVT/AMT
How the OSKAR A.V.T. operates.
The unique design feature of the OSKAR A.V.T. which distinguishes it from all other speakers is an extremely lightweight diaphragm, folded into a number of accordeon-like pleats to which aluminium foil strips are bonded. The Diaphragm is mounted in an intense magnetic field and a music signal is applied to the aluminum strips.
This causes the pleats to alternately expand and contract in a bellows-like manner in conformance with the music signal forcing air under pressure out of the pleats and sucking the air in on the other side, the airmovement is 5 times bigger than the movement of the membrane, therefore also the velocity must be 5 time bigger.The total moving mass is approx. 1 gram, we have therefore an almost perfect transducer system. This principle can be demonstrated very simply by taking a sheet of DIN A 4 paper with a surface of 616 cm2, folding it in the center lengthwise and bending the long edges together to form an opening of 5 cm on the one side. We imagine, that the upper and lower part of the structure is closed and move each side 2.5 cm together. With a frontal surface of 140 cm2, we have now moved 770 ccm of air, compared with the 350 ccm of air moved by a flat diaphragm. Our transformation is now 1:2.2, by making the triangle (top view) a square form, we doubled the transformation to 1:4.4 The selected transformation ratio with the Oskar A.V.T. is 1:5.3.
Unlike conventional speakers, whos diaphragms move air only in a direct proportion to their own movement with the inherent inertia. The A.V.T. multiplies (transforms) the Air Velocity by a factor of 5.3 (with a total mass of less than 1 gramm) and is, therefore, appropriately called an “AIR VELOCITY TRANSFORMER.”
To prevent destruction of the hearing by sudden loud noises, our ears are by design quite insensitive to amplitude changes. e.g. the difference in power between whispered speech and speech spoken at normal volume is not one in two or three, but one in 100,000!
The ear has the ability to adapt to different levels, both upwards and downwards. The relative loudness of different sounds is therefore, within certain limits, meaningless to us. This phenomenon explains why, during a conversation, we perceive the surrounding street noise as not so disturbing, even though it is only 10 dB below the conversation level.
A singer is heard even if the volume level of the orchestra is many times louder than the voice.
Conclusion: We show little sensitivity to level jumps and also to the relative loudness of two or more different sounds that are audible at the same time.
The frequency response of a loudspeaker is therefore a criterion that is not solely decisive for our ears.
Pitch: We are very sensitive to changes in pitch (frequencies). Especially in the frequency range of 250-3000 Hz, where we perceive differences between two tones of only 0.06%. In the upper and lower frequency range this ability is not so well developed and is around 0.4%.
For comparison: a semitone step is a frequency change of 6%, a vibrato of a violin is about 0.5%. Our pitch sensitivity is primarily for recognising voices. Speech does not produce constant tones, but constant variations. These speech patterns are very precise. Every voice has its own “face”. We recognise a voice we know even on the telephone.
We can even recognise the mood of the speaker. (That is, the resonance patterns and changes in pitch due to the different tensions in the vocal cords).
Phase: We are very sensitive to phase shifts because they help us locate the source of the sound. We can detect a sound delay of about three hundred thousandths of a second. This ability is frequency
dependent. It is at its best in the 500-3000 Hz range, which has historically been the most important for humans.
In his studies on sound localisation, Dr.Heil found that the beginning of a tone, (rising edge) is most important for directional information. The transient response of a loudspeaker is therefore
extremely important.
In a loudspeaker these problems manifest themselves as “inaccurate, no depth gradation, sound definition and no resolving power”
Cone loudspeaker. Eliminating random resonances that mask the music signal is the main problem in designing a loudspeaker. To move the air without much loss, the diaphragm should be light and stiff. If the diaphragm is stiff, it is not light; if it is light, it is flexible and can produce resonances. When a diaphragm is in a state of resonance, its shape undergoes various changes: E.g. at a certain frequency it will curve from the centre to the edge, this curvature vibrating perpendicular to the membrane. Not only the gong has a sound character, ( resonance) but also the material in the loudspeaker membrane.
As long as the resonance frequency continues to be fed to the diaphragm, it will store the energy. When the exciting signal is no longer supplied, the diaphragm will continue to vibrate to dissipate the stored energy.
The sharp, defined rising edge of a fast following impulse is impossible because the stored resonance energy keeps the diaphragm oscillating.
Electrostats have a uniform drive and magnetostats only a conditionally uniform drive. When flat diaphragms radiate a sound wave perpendicular to the surface of the body, the diaphragm body is set into vibration perpendicular to the surface. The diaphragm surface is thus dented at the suspensions. If it is bent downwards, the surface is inevitably stretched, the underside compressed and vice versa. (see singing saw). If a three-dimensional structure is thin in one dimension and thick in the other two, it will always behave in the way described. Pitch fluctuations are unavoidable
Magnetostat Loudspeaker, relatively heavy diaphragm, almost uniformly driven, not very true to impulse and a pronounced self-resonance
