Audiofilkina diploma: educational program on acoustic design of speakers

7 min


The previous article on the closed box showed that some readers have questions about the differences between different types of acoustics. People wondered what a closed box is in general, what is its difference from a bass reflex design and from other types. Most of the survey participants answered that when buying a speaker, in principle, they will not be interested in acoustic design.

I think it will not be superfluous to focus the readers’ attention on the main strengths and weaknesses of various types of acoustic design and conduct a small educational program on this topic. This time I will not touch upon too rare and exotic types, but I will try to describe in comparative detail the advantages and disadvantages of the most common ones. The part of the post devoted to ZJ is an answer to the question posed by lair, who did not understand why “the middle class is not ready to sacrifice the volume of small apartments”, in other words, for what reason a closed box usually needs more volume than the vast majority of modern speakers with bass reflex design.

Briefly about the dependence of sound on the speaker cabinet

The acoustic design of the enclosure mainly affects the frequency response, as well as some other parameters. Depending on the calculations and the chosen design, this effect can improve or worsen the fidelity of the reproduction. Any solution in acoustics is a kind of compromise between practicality (and often aesthetics of form) and engineering solutions that seek to improve the fidelity of reproduction. The problem of fidelity of reproduction rests on the laws of physics that limit it, acoustic design is an attempt by engineers to reduce the influence of factors that worsen the fidelity of reproduction, while obtaining operational properties acceptable to the end user.

I suppose most readers know that without design the speakers will not sound right – there will be a so-called. acoustic short circuit. The pressure wave reproduced by the speaker with a length commensurate with the dimensions of the diffuser is compensated by the rarefaction of air from the rear of the diffuser.

The ideal speaker system is an endless wall. If the region of the ideal is not affected, then the path from the center of the outer side of the diffuser to its center on the back of the diffuser should be more than half the maximum length of the emitted sound wave. Especially many problems arise here with low frequencies. So, at 20 Hz (the lower threshold of perception), the wavelength is just over 17 meters. Naturally, the speaker in the form of a wall of this size is a little too big for a commercial series. For this reason, walls are not built, but boxes are preferred, which do not completely solve the problem, but are able to largely compensate for them.

Problems exist not only with acoustic short circuit, but also with other properties of the speaker. For example, any speaker has a resonant frequency, below which there is a steep drop in the frequency response, approx. 12 dB per octave. When operating at the resonant frequency, a lot of harmonic distortion occurs. It is impossible to solve the problem of the frequency response drop and nonlinear distortion by too sharp a decrease in the resonant frequency, since the huge amplitude of the resonant oscillations will break the diffuser.

It is known that the vibration amplitude of a diffuser is inversely proportional to the square of the frequency, i.e. at an equal sound pressure at 50 Hz, the amplitude will be 4 mm, and at 25 Hz -16 mm. Thus, the larger the diffuser, the lower the resonant frequency can be with relatively safe oscillations for the speaker. In other words, the lower the resonant frequency of the speaker, the better.

Body resonances and shape

All loudspeaker cases are cavity resonators (be it an open box, FI, ZY or labyrinth), which have a huge number of natural resonances. This is clearly seen from the formula for calculating resonances for a closed box:

where a, b and l are the sides of the resonator body, and m, n and g are integers

Resonances are determined by standing waves that occur inside the case, which significantly affects the frequency response, as a rule, not in the best way. To remove them, use the same dampers that reduce the Q-factor of resonances, but they are not completely removed.

We can say that resonances directly depend on the shape of the case, otherwise, on the aspect ratio. The popular rectangular pillar shape today is extremely unfortunate when it comes to cabinet resonances. And the shape of the cube, on the contrary, allows you to smear resonances throughout the frequency response and make them less noticeable. A spherical shape of the body is also sometimes used for ZP and FI, which prevents the formation of standing waves, but is also not able to completely eliminate them.

A few words about the open box

Despite the fact that today this design is difficult to find in mass-produced devices, it has one unique advantage. An open box does not affect the resonant frequency of the speaker. It was for this feature that he was loved in the past. The big problem with the open box is its impressive dimensions. Without them, it is not able to reproduce low frequencies with sufficient sound pressure. For this reason, these speakers today are mainly the lot of amateurs and custom shops who produce them as genre models for music, the lower threshold of the frequency range of which ends in the region of 200 – 300 Hz. An acoustic impedance panel in the form of a thin perforated back wall was used as an acoustic damper in open boxes.

Benefits:

  • Does not affect the resonant frequency of the speaker
  • has low intrinsic resonances

Disadvantages:

  • Big sizes
  • Limitations on the lower threshold of the frequency range approx. 300 Hz.
  • Almost impossible to find

Closed box

The closed box is a housing that completely isolates the speaker in the internal volume. The design of a closed box leads to an increase in the resonant frequency of the dynamic head, since in addition to the rigidity of the suspension of the diffuser, the elasticity of the air in the inner volume of the box begins to influence. The smaller this volume, the higher the resonance frequency.
The first option for a closed box is to make the volume of the box so large that it cannot appreciably affect the resonant frequency of the speaker.

The second version of the closed box was proposed by Edgar Vilchur. He drew attention to the fact that the linearity of the gas spring, which was actually air in a closed volume, is higher than the linearity of the suspension of the diffuser. Wilchur was the first to propose to reduce the stiffness of the diffuser suspension as much as possible in order to actually replace the mechanical suspension with a pneumatic suspension, as much as possible. And thus increase linearity.


Fragment of Edgar Vilchur’s patent application for a closed box

Both options, like everything else in acoustics, have their own advantages and disadvantages. Vilchur’s variant did not allow reducing the harmonic distortion, since the diffuser cannot be kept only in air, and the mechanical parts are preserved in the structure, let the internal volume take over part of the suspension function. Moreover, it turned out that at low volumes and in piston operation, the air is also non-linear. To avoid such non-linearity, the volume of the box must be equal to the volume of the room in which it is located. That practically negates all the advantages of the Wilchur variant.

The version with a large volume does not require special conditions for the construction of the speaker and works comparatively well, having dimensions slightly smaller than an open box with an equal SPL (sound pressure) at low frequencies. In this case, the limit of the frequency range at the low frequency, with smaller dimensions, can be significantly lower than in an open box. In order to smooth out the humped frequency response, damping sound absorbers are used.

Advantages:

  • The lower limit of the frequency range is 65 Hz and below with appropriate dimensions)
  • Smaller than open drawer
  • Frequency response, as a rule, is smoother than bass reflex

Disadvantages:

  • With competitive characteristics, a really large body (2-3 times larger than that of a PHI with equal SPL at the low frequency and the lower limit of the frequency range)
  • The presence of significantly larger body resonances than an open box (especially with insufficient damping and non-optimal shape)
  • Significant influence of the shape on the Q-factor of resonances (the most preferable are cube and ball with a low Q-factor)

Capricious bass reflex

The principle of FI acoustics is familiar to many. The oscillation phases inside and outside in the same closed box are opposite. Installation in the body of a pipe of a certain length allows you to turn the phase by 180 degrees. Thus, at the exit from the phase inverter pipe, the sound at its resonant frequency becomes in-phase EEE sound from the outside of the diffuser, they add up and the sound pressure increases.

The presence of additional resonance increases the frequency response rate by 6 dB per octave. In insufficiently wide pipes, vortices arise due to the high speed of air circulation, which is reflected in the sound in the form of pronounced extraneous sounds and additional nonlinear distortions. Also in FI often there are so-called. organ midrange resonances, turbulent and other overtones. Getting rid of all these “charms” is worth a significant effort on the part of the column builders (cross out) engineers. For this reason, we can say that PHI acoustics, for all its popularity, is the most problematic.

It is absolutely impossible to buy FI acoustics without first visiting the showroom and listening, as you can run into very beautiful, but buzzing rattling speakers.

In view of the above, there is no need to talk about PHI acoustics as some kind of universal solution. The main advantage is the enhanced reproduction of low frequencies at the resonant frequency of the FI, for which the user pays with the linearity of the frequency response, a high probability of resonance problems and extraneous overtones.

Advantages:

  • Loud bass
  • Small size
  • Very common

Disadvantages:

  • Risk of increasing kg
  • Failure frequency response between res. frequency FI and res. speaker frequency
  • Parasitic resonances and overtones
  • Solving inborn design problems is expensive and sometimes high-tech

Transmission line

This is one of the options for labyrinth acoustics, which I wrote about in detail here.

Eventually

Thus, the simplest and least problematic types of acoustics from an acoustic point of view require more volume, and any tricks, in particular phase inversion, are fraught with distortion and overtones. From the above, we can conclude that the market makes a choice in favor of loud, or, more precisely, bass speakers of smaller dimensions, and practically ignores logical decisions implying a higher fidelity of reproduction.

90% of home speakers for the hi-fi market that are produced today in the world are bass reflex acoustics of mainly two types: floor columns and small bookshelf speakers. For some people, the problem with the FI is solved by the plugs that close the FI, which turns the AC into a PS.

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