Ribbon loudspeakers

The ribbon loudspeaker design was patented in 1928 by Gerlah engineer from European Acoustic Laboratories. However, it became possible to start their production only in the 30s, when permanent magnets appeared.

The general principle of ribbon loudspeaker construction is shown in Fig. 1. A long thin corrugated aluminum foil ribbon is placed between the poles of the magnet.

When an alternating current is applied to an aluminum conductor in a magnetic field, the mechanical Lorentz force begins to act on it, directed perpendicular to the surface of the ribbon and equal to

F = B x L x I

, where B is the induction in the gap of the magnet, L is the length of the conductor, I is the current. Since the ribbon is very light, it receives a fairly large displacement with a small amount of applied current. Since the ribbon turns out to be quite wide (relative to the diameter of the voice coil conductor), its active resistance is quite small (about 0.047 Ohm), therefore, emitters are usually used with input transformers (which makes it possible to increase their input impedance).

The ribbon is usually crimped to reduce unwanted resonance vibrations as well as to increase its flexibility. A closed volume is often made behind the ribbons, the flexibility of the air of which creates additional elasticity. To reduce the resonances of this air cavity, it is usually plugged with a porous absorbent material.

The absence of diaphragm resonances allows obtaining a homogeneous frequency response up to very high frequencies. To reduce distortion caused by asymmetry and non-uniformity of the magnetic field in the gap (which is a big problem for conventional electrodynamic loudspeakers), the design of the magnetic circuit poles is optimized in such a way as to obtain the most uniform field in the gap.

The induction in the gap is usually small, ~ 2500 G, but due to the small mass of the ribbon, this is sufficient to obtain a high sensitivity (~ 95 dB / W / m). The relationship between the required sensitivity and the ribbon area can be obtained using the following approximate formula:

S = 1.52 x 102 x 10 x L / 20 / F_n x A

, where S is the area of ​​the ribbon, L, dB is the level of the characteristic sensitivity of the loudspeaker, A is the amplitude of the ribbon displacement in mm, F_n Is the lower cutoff frequency.

Let’s say a ribbon tweeter is being designed with a sensitivity of 110 dB / W / m and a lower cutoff frequency of 5000 Hz. The amplitude of the ribbon displacement should not exceed 0.2 mm to ensure a low level of nonlinear distortion, then from the formula it turns out:

S = 1.52 x 102 x 10 x 110/20/5000 x 0.2 = 9.6 cm²

If we take the width of the ribbon 0.8 cm, then the length with an area of ​​9.6 cm² will be equal to 12 cm, which is usually used in tweeters. Since the ribbon has a large area, it cools quickly, and therefore quite a lot of power can be supplied to the loudspeaker.

Due to its long length and narrow width, the loudspeaker has a wide horizontal response (up to 70 degrees at 20 kHz), which is useful for wide stereo imaging. True, in the vertical plane the loudspeaker has a narrow directivity characteristic. This is sometimes useful as it helps to avoid unwanted reflections from the ceiling and floor.

The undoubted advantages of tape loudspeakers over conventional electrodynamic ones are the short attack and decay time of the sound due to the small weight of the tape (weight ~ 0.01 g, thickness 9 microns), low level of nonlinear and intermodulation distortion, flat frequency response, etc., which makes it possible to obtain transparent and clear sound at high frequencies.

Although the fundamentals of the design of industrial tape loudspeakers were described in the 70s, they were not especially widespread due to the need to use a rather large and expensive magnetic circuit, low impedance, low reliability and high cost.

In 1997, AL Butler at the ASA (Acoustical Society of America) congress made a report “The re-birth of the ribbon loudspeaker”, which triggered a new surge of interest in it. During this period, new magnetic materials appeared, which made it possible to significantly reduce the dimensions of the magnetic circuit. The most widely used magnetic materials are neodymium, which have a magnetic energy almost three times higher than conventional barium-ferrite magnets.

The technology for making ribbons has also changed (in some cases, a durable polyimide laminated with a layer of aluminum is used, for example, in the SA8535 by Stage Accompany), etc. As a result, modern designs of ribbon emitters are sufficiently reliable and affordable.

Currently, dozens of companies are engaged in the production of such loudspeakers and acoustic systems: Stage accompany (Holland), Expolinear (Germany), Philips (Holland), Bohlender-Graebener (USA), Apogee Sound (USA), Image Acoustics (USA), Dali (Denmark), Newform Research (Canada), Wisdom Audio (USA) and others.

Typically ribbon loudspeakers are used as the tweeter in conjunction with a woofer dynamic loudspeaker unit, however, broadband ribbon loudspeaker models are available. Ribbon loudspeakers are widely used in consumer electronics.

In recent years, they have begun to be used in automotive acoustics, sound equipment, etc.

Source: audioakustika.ru