Coaxial Technology Advantages

KEY CONCEPTS NECESSARY TO UNDERSTAND THE ISSUE:

• Sound source – For the purpose of this article, these can be both the individual components of a speaker system such as woofers, mid-range drivers, or tweeters, as well as the entire, ready-made system. If the sound source is a single speaker, it will be referred to as having a limited frequency response range, as opposed to a complete system that can be called broadband.
• Frequency response (of a system or individual source) – The range of frequencies reproduced by a speaker appropriate for its type of driver. For example, a woofer may have a range of 50-500Hz, a mid-range driver 300-4000Hz, and a tweeter 1000-20000Hz. Of course, the operating ranges of different speakers may vary.
• Coaxial Speaker System – The subject of this article.
• Traditional speaker system – A system of two or more speakers where the drivers are positioned one above the other: the woofer at the bottom, the mid-range driver (if present) in the middle, and the tweeter at the top of the column. There are other spatial configurations used by designers (e.g., D'Appolito), but for simplicity, they will be omitted in this article.
• Audio crossover – An electrical or electronic circuit consisting of passive components such as capacitors, coils, and resistors (or digital DSP), designed to limit the frequency range reproduced by speakers operating in different frequency ranges. A properly constructed crossover is matched to a specific (not any) set of speakers, horn, and enclosure, with particular attention to the crossover point appropriate for these elements, the linearity of magnitude (SPL versus frequency), and the phase linearity of the entire system. The effect of the filters described above is to attenuate portions of the signal that travels to the speaker. Filters used in crossovers have their slope characteristics (the degree of signal attenuation) expressed in decibels per octave. This means, in simple terms, that there is always a specific, narrower or wider range of the spectrum in a set where more than one speaker is playing. More on the theory and classification of such circuits can be found here: [Audio crossover](https://en.wikipedia.org/wiki/Audio_crossover).
• Spatial crossover – Functions in the medium through which sound travels, which, in the case of this article, is air. It is related to both the frequency response and the space in which the speaker system operates. It is the area where two or more sound sources reproducing the same signal interact simultaneously. This is the area where the most problems, such as unwanted amplification or even complete signal cancellation (frequency gaps), can be expected. These issues are minimized or eliminated in Favo Audio's coaxial systems.
• Phase – Sound waves rely on a cycle of regular repetitions. Phase describes the point in the cycle at which a given wave is located. It is expressed in degrees, where one cycle is 360 degrees, half a cycle is 180 degrees, and so on. The main aspect of phase relevant to this article is the influence that two identical waves (signals, sounds) reproduced by two different sources (different speakers) have on each other. Such waves can add up, causing sound reinforcement, or cancel each other out, resulting in complete silence. One of the elements influencing this behavior of both waves is their relative phase. The phenomena of sound cancellation or reinforcement related to phase are associated with differences in the time it takes for sound to reach the listener from different sources. These, in turn, are related to the placement of these sources in space relative to each other and the listener.
• Comb filtering – The effect of overlapping two or more identical audio signals, where one is delayed relative to the other, causing constructive and destructive interference. The frequency response of such a signal consists of a series of regular cancellations, taking the form of a comb.

In the following video, we can observe the overlay of two signals where one is gradually delayed. Each increase in this delay (and thus also the distance from the listener) causes signal cancellations at increasingly lower frequencies. Each "dip" area represents a lack of information reaching the ear, resulting in a gradual loss of clarity and speech intelligibility to a complete degradation of sound reminiscent of a booming noise.

Here's a short video explaining the problem of comb filtering (polish language, with english subtitles option):

1. Differences in listener distance from individual sound sources with changes in listening position:

   Changing the position of the measuring device or the listener's ear causes a difference in distance from the sound sources, leading to phase changes and all the negative consequences associated with this (comb filtering in the area of the spatial crossover). In a coaxial speaker, the listener's distance from both sources is always the same, regardless of changes in listening angle and axis.

2. Frequency gaps in the spatial crossover area heard from different angles:

   The tonal balance of a speaker system is usually set and provided by manufacturers for a single point in space along the system's axis. The narrow frequency response of tweeters and the size of woofers make it impossible to maintain proper tonal balance at angles other than 0 degrees in the horizontal plane. Changing the listening angle of most traditional systems, especially where the woofer is larger than 10" and the driver is small, clearly highlights gaps in the mid-frequency range. The solution in the traditional approach is to use large and expensive, low-playing driver and horn systems (the larger the horn mouth, the lower the frequencies it can reproduce), which are less commonly used due to their cost and poorer high-frequency reproduction.

   This problem is illustrated in the following video. We have a traditional system from a top-tier manufacturer in the USA, Meyer Sound. As the listening point (black and white point moving from bottom to top) shifts, there are significant changes in horizontal dispersion width (observed in the blue "section view" window), resulting in changes in the sound's tonal balance in the crossover area (XO square) when listened to from different angles

Video illustrating the problem:

(film provided courtesy of Merlijn van Veen)

Due to the lack of difference in distances from the sources in a coaxial speaker and the use of the highest quality drivers with a large 3" coil and the widest possible frequency range, this problem has been resolved. The crossover point of both sources is set low enough to maintain a constant coverage angle (for VSC models, it is 60x40 degrees) and consistent frequency response at different angles.

3. Sound reflected from hard reflective surfaces:

In any enclosed space, sound waves reflected from walls, the ceiling, and the floor always reach the listener later than the sound directly emitted from the source. Any time difference between these two signals will cause problems in the form of comb filtering, which is practically perceived as a gradual cancellation of tones from high to lower frequencies as the distance from the sound source increases. In the case of a traditional set, there is an additional difference resulting from the distance between the sources.

 

Fig. 1. Classic construction

 

Fig. 2. Coaxial construction