Technology is no secret, but the implementation of physical knowledge and the experiment to test the physical model in reality
Intro:
Here I will give an overview of the design criteria of loudspeakers in general, to summarize and simplify the acoustic and electrical connections that create and influence a sound image. Practical projects and further information can be found on our DIY pages.
Passive speakers with drivers for different frequency
ranges of audible sound essentially consist of three modules:
1. the cabinet
2. the drivers
3. the crossover
network
It is important to harmonize these modules and to create a harmony - or in other words - a homogeneity in the sound image.
the cabinet needs to be optimized in five disciplines
1. the driver arrangement
2. the sound diffraction at the edges of the cabinet, also known as baffle
design
3. the sound insulation of the cabinet shell
4. the resonance poverty in the interior of the box
5. the bass playing ability
The arrangement of the drivers in the context of the frequency filter of the crossover and the diffraction behavior of the cabinet surface is designed so, that the best possible omnidirectional characteristic is generated, which is one for human hearing - ie the auditory signal processing of the brain - appears as a natural perceived sound. The temporal behavior of the electric filter is to be considered, which can also lead to a depth offset of drivers. In this context, the area and filter properties (cut-off frequencies and slope) of the driver is of crucial importance, as this has a direct effect on the diffuse sound field in the listening room and thus determines the tonal embedding of the presented, as well as the acoustic quality at unfavorable listening positions.
Directly connected with this point is the topic of the sound diffraction at edges of the cabinet. For the installation of speakers in walls or ceilings, it is basically possible to neglect this issue, but as soon as a loudspeaker is free in space, there are structurally related geometrically limited surfaces, at the edges of which the sound is diffracted and thus deforms the uniform omnidirectional behavior frequency-dependent. The diffraction behavior in relation to the tweeter has a special effect, since slight changes in the geometry have a significant influence on the high-tone sound image, for example the reproduction of the human voice.
The soundproofing of
the speaker cabinet is crucial for clear playback. At thin-walled boxes, the
wall resonates like a particularly poorly crafted resonance box of an instrument
and thus provides additional noise and overtones (distortion) that the driver
does not play. This is also known as the drumming noise of a box when the ratio
of thin-walledness to high sound levels is extreme.
Therefore a case have to be very soundproof, so that the speaker also sounds
very good. For this purpose, heavy tropical wood, which has perfect rigidity and
high mass / density, is a very good base.
The subject of the
resonances inside a loudspeaker cabinet is very demanding, since the cabinet can
not simply damp without this having a serious effect on the driver's playing
style.
The idea of dampening a volume behind the driver - so that the driver plays
sound into it and no sound comes back - is just an illusion. If you want to make
a model of the behavior inside the box, you find that the driver is playing
against a damped air spring. The association with a car suspension helps with
the problem analysis, that both an over-damping, as well as a too low damping,
result in a problematic behavior. The same applies to the way the driver plays.
If it plays in the undisturbed volume, then it hops like a car with broken shock
absorbers and plays a very wavy frequency response with echo-like decay
behavior. If the driver plays in a totally damped box, he sounds pressed and
without fine dynamic details, just like the hard shock absorbers of a car, lead
to a stubborn driving behavior without fine response. Damping and optimizing the
housing shape is made more difficult by the fact, that the drivers themselves
bring a substantial damping. It exist the thesis, that the loss factor Rms of
the driver provides representative information about the damping behavior and
thus the ability to image acoustic details, but this measure is useful only as a
vague orientation.
On the one hand, there is the possibility to design the port to be particularly
large in order to obtain a particularly high precision in the bass, with the
disadvantage, that at the same time, disturbances in the upper bass and midrange
from the system are also emitted very clearly. On the other hand it is possible
to make the system vibrate very much with a particularly small and short port,
so that the bass still plays in dead spaces or with muted drivers, but with the
deficit that the bass loses precision, however, disturbances in higher frequency
ranges can be almost neglected. Quite different is the variant because of
passive membranes to install a "high" moving mass in the port. Benefits of this
design only in the compact sub-bass. Here, however, there is the basic problem
of non-linear behavior, so that such systems rather quiet signals and not loud
with a lot of "droning" play.
Other types of cabinets are, the transmission line, a waveguide that is
stimulated like an organ pipe to play the deepest notes, but it is not easy to
design because of its overtones. A further variant are bass horns, which are
arranged according to a trumpet to increase the level in front of or behind the
driver and provide with gigantic geometrical dimensions for impression the
viewer. Even designs with internal woofer, as well as designs with multiple
chambers are variants to optimize the efficiency and the sound performance, as
well as the level stability of speakers in the bass.
The drivers are often made very individually and there are offered great amounts of different typs. I take this diversity rather as helplessness, because everyone is somehow trying to make something special, without there are a large selection of high quality playing driver really exist. Basically, there are selection criteria, that very quickly sort out a lot of drivers.
1. linear frequency response,
2. distortion poorness
3. minimization of nonlinear behavior
4. large usable frequency range
The frequency response linearity means, that the driver has few resonances, steps or other artifacts. If a driver plays quite close to the physical behavior of Tiele and Smal (ie the frequency response determined by the Tiele and Smal parameters agrees fairly well with the real frequency response), then it is also likely that the driver has an ideal temporal behavior that resonaces in the speaker design has been avoided and that the driver does not play against internal frequency-dependent springs - such as an air cushion. Whether the driver was designed with a rising or falling frequency response at the end has no relevant influence on this topic.
Low-distortion is a topic for high-end applications.
For the level of distortions there are standardized limits, which distortions
can still be heard. With regard to the question of timbre and authentic
perception in comparison to reality, it is easily possible to perceive even 20
dB lower distortions below the normative hearing limit. There are undoubtedly
frequency-dependent sensitivities of the human ear corresponding to the hearing
curves. Thus, even in the case of very high quality demands, distortions in the
bass range remain tolerable to a greater extent than in the mid-high frequency
range.
The low distortion is also important in assessing the
level stability, as high level peaks / pulses, as generated by percussion, must
be reproduced precisely by the speaker.
Nonlinear behavior is a topic that regularly disqualifies seemingly good drivers. Typical of nonlinear behavior are the mechanical properties of rubber surround. The elastic deformation of rubber - as well as its attenuation based on the amplitude - is not linear. Practically, one can carry out measurement series of the Tiele and Smal parameters for the used volume (ie the desired dynamic range) and determine that they nevertheless change significantly. As a result, the dynamics of the electrical signal do not match to the dynamics of the acoustic signal. This has barely noticeable effects on the rough dynamics, but this is a relevant factor for the detail reproduction of music. Unfavorable but unavoidable is the fact, that, in addition to the playability of the drivers out of the box, the damping behavior in the cabinet contributes to additional non-linear behavior. Understandably, the constructive aim is, that the behavior of the cabinet should intentionally behave inversely to the behavior of the driver in order to obtain approximately a high linearity with respect to the impulse response.
The usable frequency range is a rather simple topic, there it is basically the question, in which frequency range the criteria of the frequency response linearity are met. This requires a high degree of perfection of the membrane, as these significantly limit the upper limit frequencies - ie the useful range towards the high tone. At low frequencies, essentially distortion is the limiting quantity which, in other words, limits the level stability of the driver.
There are now the subjectively justified selection criteria for drivers, that a paper membrane sounds better than a metal or ceramic membrane. Practically, this is a question of distortion with simultaneous "natural" so very fine-dynamic play ability. On the one hand, there are drivers that play very "lively" and in which the overtones (distortions) characterize the sound pattern. To the other, the trend to more damped playing drivers is recognizable, in this case, a sound of the membrane material is not audible due to missing harmonics. The second, however, are less suitable when it comes to seeking authentic music reproduction. It is therefore essential to listen to the "sound" of the drivers and select suitable types for the speakers. For this purpose, no selection criterion / measurands or the like has proven useful for me.
The crossover is surrounded by myths like nothing else in the speakers. There are a variety of objectives for the design of crossovers, on the one hand very steep filters, on the other hand very flat filters, very few components, as well as the correction of all possible - such as the perfect linear frequency response on the axis - all with the goal to create the ultimate sound.
There is no question
that components of the crossovers do not behave ideally. These deviations from
the physically ideal behavior are used for all possible effects. As an example
the Copper Foil Inductor - a single-layer wound coil of thin copper sheet. From
the electrical behavior of this coil results the reduction of fine details,
which has the consequence that a rich in detail, but also distortion rich
playing driver muddy plays and so the distortion - also described as hardness -
is no longer perceived as such clearly. This can specifically counteract errors
of both players and drivers. In other words, to avoid being annoyed by the
precision of the high-end devices, you need components that round off the sound
image, comparable to the anti-aliasing function of a screen.
The same is true for capacitors, even with these components, the non-linear
behavior is measurable and acoustically audible. Particularly low-loss
capacitors also contribute to the dynamics of the enemy, but allow only for very
mature speakers the desired goal of natural reproduction. A little too much
detail and the sound is only perceived as "hard" and "exhausting".
As optimizing
variables for natural, detailed and low-distortion music reproduction, a
relatively linear frequency response is sought, in order to counteract masking
effects that overlap details, to aim for perfection of the acoustic phase in the
treble for the natural acoustic stage image, and the geometric arrangement of
the coils in the cabinet to minimize unwanted inductions.
This catalog of requirements leads to complex and rather expensive crossovers,
whose components have to be distributed at several locations in the loudspeaker
cabinets in order to come close to the goal of authentic reproduction.
A disadvantage, for example, core coils have been found in the point optimization, since with high acoustic image quality, the effects of hysteresis were always heard. Also interactions with amplifiers, as they are normal in third-order turnouts, result in increased distortion, so that for this purpose the impedance of the loudspeaker is to be optimized relative to the amplifier. These factors lead to the realization, that few components can not be a relevant objective, as well as the dense arrangement of coil on a board to correct the frequency response.
The aspects listed
here are indispensable design features of a loudspeaker, but not sufficient to
maintain a balanced presence of the presented in the entire frequency spectrum.
The presence of frequency bands of a loudspeaker is a decidedly subjective
attempt to describe the fact, that loudspeakers of seemingly the same quality
reflect different details of the same music. The presence is concerned with the
description of the authenticity of what is offered in the different pitches.
Here too, the energy frequency response of the loudspeaker, which forms the
diffuse sound field in the listening room and thus the background noise level,
is an essential criterion. However, a uniform energy frequency response (which
in conjunction with a linear frequency response on the axis leads to a uniformly
increasing bundling) is no guarantee for detailed music reproduction, since the
origin of the music material has not emerged even under perfect conditions.
One of the sources of error is the mixing monitors, where the material presented
was mixed. Studio monitors are not perfect speakers, but, like the mixing rooms,
have shortcomings that can be found again in the analysis of music playback.
Therefore, every loudspeaker on the new is just an attempt to provide a good
playback, but never a universally applicable ideal of speakers that can afford a
perfect reproduction for all recordings.
In addition to these topics, there are other topics, such as the bass reproduction of speakers. The question of the interaction between space and loudspeaker is also a topic of its own. Some topics are also listed in the book and lecture by Floyd Toole - Sound reproduction – art and science/opinions and facts which I always like to link to.
