Some insight from Dr Heil; [Archive] - Audio & Video Forums


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01-28-2006, 11:47 AM
Doing some research on the Heil Air-Motion tweeter, I came across this explanation of human hearing from Dr Heil. For reference Dr Heil, besides inventing this unusual driver, is also the physicist who invented the FET transistor.

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.

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 speech pattern 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 to 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 extremely 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 characteristic, 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.

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 resonance 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 utilize 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 uniformly like a rigid piston, but will vibrate like a suspended flexible membrane and produce a self resonance with a pitch. (singing saw effect)

02-01-2006, 10:48 AM
You might find this study worth reading as well.


Different patterns of human discrimination learning for two interaural cues to sound-source location (