Acoustic design for broadband heads


Created on 20.06.2007 16:44.

Updated on 31.01.2020 23:53.

Author: V.A. Vasiliev ..

The acoustic design of the dynamic drivers consists of a box or a screen designed to equalize the amplitude-frequency characteristics of the drivers in the low-frequency range of the audio range. Dynamics heads that do not have an acoustic design do not work well at low frequencies. The main reason for this is that the front and rear surfaces of the head diffuser excite sound vibrations that are equal in amplitude but opposite in phase. At low frequencies, where the radiation is nondirectional, the antiphase oscillations add up and cancel each other out, as a result of which the acoustic output of the head drops sharply.

To eliminate the mutual influence of vibrations generated by the front rear surfaces of the diffuser, the dynamic head can be placed in the simplest case in the center of a square screen made of thick plywood or chipboard with dimensions LxL in meters, where L = 0.06 / Fmin… Here Fmin – the minimum frequency of the reproduced signal, kHz.

For example, with a minimum signal frequency of 50 Hz, this formula determines the side of the square equal to 1.2 m. It is obvious that it is inconvenient to use a broadband head in such a bulky acoustic design. Therefore, acoustic screens are rarely found in amateur constructions. Instead of them, various multifaceted boxes are widely used – boxes with and without a back wall. It should be noted that the design of the acoustic design of dynamic drivers requires complex calculations and accurate information about the characteristics of the driver. It is beyond the scope of this book to address these issues. Below we will consider examples of how foreign radio amateurs carry out the acoustic design of broadband heads.

Ultra-flat bass reflex

Bass reflex boxes are boxes made of thick plywood or other similar wood material (pine boards, chipboards), in which there are two holes. One is for the installation of a dynamic head, the other is for the implementation of the acoustic connection of the internal volume of the box with the external environment.

When selecting certain ratios between the dimensions of the diffuser of the head, the second hole and the box itself, taking into account the frequency properties of the head, it is possible to achieve that at low frequencies the oscillations excited by the rear surface of the diffuser of the head, at the output of the second hole of the phase inverter will be in phase with the oscillations excited by the front surface diffuser.

This remarkable property of the phase inverter has long been widely used in amateur and professional installations.

The phase inverter has one drawback – the bulkiness of the box, which is close in shape to a cube. In this respect, the design of the ultra-flat phase inverter deserves attention. A design drawing is shown in Fig. 1.

Rice. 1. The design of the ultra-flat phase inverter.

As follows from the description of the original, the body of the bass reflex box is assembled from dry pine boards or pieces of chipboard with a thickness of about 20 mm. Overall dimensions of the case are 150X1580X630 mm. The case is designed for the installation of a broadband dynamic head, similar to the domestic head of the 4GD-35 or 4GD-36 type. The bass reflex hole measures 50×125 mm and is located under the head hole.

The diffuser holder of the head is fixed externally, flush, which reduces losses at all frequencies. To eliminate unwanted vibrations of the front and rear surfaces of the body, there are five spacers, indicated in the drawing in Fig. 1 by dashed showers, which connect the front and rear walls. Spacers measuring 20X109X200 mm are made of the same material as the box body. Connect the body parts with screws and nitro glue.

The outer finishing of the case is reduced to draping the front panel with radio fabric, preferably loose, and pasting the walls with a decorative film or veneering them. The phase inverter fits well into the interior of a modern room and can be placed directly against the wall.

For a stereo setup, two speakers are required. When repeating the design, it is necessary to clarify the dimensions of the bore holes for a specific type of dynamic head. In addition to those mentioned above, here it is possible to use heads of the type 4GD-4, 4GD-7, 4GD-28.

Loudspeaker – organ pipe.

High requirements for the quality of the acoustic installation, on the one hand, and the limited area of ​​the living space where loudspeakers can be placed, on the other, force designers to look for solutions that meet these requirements, or find an acceptable compromise.

In this respect, the design of a single-way loudspeaker with two broadband dynamic heads connected in phase in series is of particular interest. The loudspeaker is described in American amateur radio literature. Sketches of its front panel and cross-section are shown in Fig. 2.

Rice. 2. Organ pipe.

The described acoustic design has a casing in the form of a column with a cross section of 200×200 mm, a height of 800 mm with two slots at the base formed between the floor and side walls shortened by 26 mm, as shown in Fig. 2.

In the upper part of the cabinet there are two holes for installing the speaker drivers on the front and rear walls of the cabinet. The dimensions of the cutout and the characteristics of the heads allow the use of domestic dynamic heads of the ZGD-38 type in this design. The walls of the cabinet are made of plywood or chipboard with a thickness of about 15 mm.

The elongated shape of the loudspeaker housing, the small floor area it occupies, make it possible to use such loudspeakers in stereo and quadraphonic installations. In addition, the loudspeaker of the described design features improved low frequency response and an extended horizontal radiation pattern.

The first is due to the phenomenon of standing waves observed inside the loudspeaker housing, which behaves like an organ pipe tuned to a frequency of about 100 Hz. It is at this frequency that one quarter of a wave is laid along the body of the loudspeaker, which contributes to the rise of the low frequencies.

At a frequency twice as high, equal to 200 Hz, on the contrary, exactly half the wavelength fits along the length of the body. At the same time, suppression of radiation from the lower part of the case is observed, which eliminates the unpleasant booming sound characteristic of acoustic installations with large linear dimensions.

The second advantage is associated with the use of two heads emitting in opposite directions. By changing the direction of the radiation of the speakers by rotating them about the longitudinal axis, you can achieve the best sound reproduction in a particular room. It should not be regretted that half of the radiated power is directed in the opposite direction relative to the listener.

Low and middle frequencies, reflected from walls and furniture, give multiple …