Created on 28.07.2006 16:05.
Author: Andrey Elyutin.
In one working day, if you take it, you can find from fifty sources, from smart books to indistinct mumbling on the Internet, where it will be said in one way or another: the simplest acoustic design of a dynamic loudspeaker is a closed box. Probably, it is already clear: I cannot agree with this.
We have not met for a long time (in this section), for a long time I did not surprise you with anything. I’ll start right now. Consider: A closed box as a way of acoustic design of a loudspeaker is the subject of an invention. Which has an author, patent number, and other American pies.
The application for this patent was filed in March 1954, and granted in December 1956, so now, we can say, the half-century anniversary is in full swing. The patent, issued in 1956, was challenged in court and annulled six years later.
Although by this time the author of the invention had sold licenses to two companies to use the fruits of his intellectual kurtosis and to rely on payments he lived happily ever after. Despite the legal conflict, a sample of the first serial loudspeaker in acoustic design of the “closed box” type became an exhibit of the largest polytechnic museum in the world.
Intellectual excesses aimed at improving (!) A closed (H) box (!!!), not only did not stop during the aforementioned half century, they continue to this day. If for you the listed above is associated with the “simplest” – we put different meanings into this word. Now – in order.
Life has become better
The name of the inventor of the closed box is Edgar Vilchur. Citizens with this surname also live here, mostly lawyers and doctors. Before the war, the American Wilchur graduated from college in the field of fine arts, fought in the Pacific Ocean, was demobilized with the rank of captain, settled in New York and opened a radio workshop there.
To match, the certified artist Vilchur found it difficult to obtain another higher education, this time – a technical one, which eventually made him famous. The war ended, life in America became better, life in America became more fun, the demand for audio equipment (in the then idea of it) and its service grew by leaps and bounds. It was then that the reserve captain Vilchur made one of his two great inventions.
The second in 1958 was the dome squeaker. Today this is not the topic, but only one who does not have a heart can not take off his hat in front of this person. The absence of a hat is not accepted as an excuse.
It is logical to ask: what, until the mid-50s, not a single speaker was inserted into a box with a closed volume? It turned out, but somehow differently. See how the acoustic design of loudspeakers evolved before Vilchur. The speaker (the head itself) cannot reproduce low frequencies, because an acoustic short circuit occurs: instead of creating a pressure wave in front of the diffuser, the speaker pumps air from the front of the diffuser to the rear, moving in the opposite direction.
To prevent this from happening, the speakers were placed in an acoustic screen. The speed of propagation of the pressure wave (it is the speed of sound) in air is finite and known. Until the pressure wave runs around the acoustic screen and allows itself to be extinguished by the antiphase wave at the rear of the diffuser, the speaker will not emit sound as expected.
When the distance from the front of the diffuser to the back along the shortest path (over the edge of the screen) equals the half-wavelength at this frequency, the radiation intensity will begin to fall. Now estimate the price of the question: with a meter per meter screen, the distance from the front of the diffuser to the rear (with a turn over the edge of the screen) will be the same meter, and this is half a wave at a frequency of 67 Hz.
The loudspeaker AR-3, which incorporates both Wilchur’s inventions, an acoustic suspension and a dome sounder, is part of the Smithsonian Institution’s US exhibit.
They quickly realized that the screen could be non-flat, and wrapped it in a kind of semi-box. So, in fact, the acoustic systems of radio receivers and radio transmitters of the middle of the last century were arranged for many years. Elementary everyday logic suggested: what if “to attach the bottom to the bins of the Motherland”?
Close the drawer at the back, closing and the issue of acoustic closure. Tempting? Yes, but another question immediately opened up. If, with an open box, the diffuser drove the air back and forth almost unhindered, then the closed volume began to resist, possessing elasticity.
The resonant frequency of the speaker, which was in no way affected by its installation in a screen or in a “semi-box”, immediately jumped: the elasticity of the air volume was equivalent to an increase in the rigidity of the suspension. The boxes had to be made so large that the elasticity of the air inside was much lower than the elasticity of the head suspension itself. Bottom line: The movement for high fidelity of sound reproduction, which originated in America in the early 50s, and is still known as Hi-Fi, gave rise to monstrous speakers.
Actually, what Vilchur proposed was called and is still sometimes called acoustic suspension – acoustic suspension. Many of those who were interested in the question sincerely believe that this was invented in order to reduce the size of the loudspeakers. This actually happened, but this was not the main, but a secondary goal of Vilchur.
About a year ago, in the context of the ongoing anniversary, Vilchur was interviewed by Stereophile magazine, where he explained clearly what led him to the idea of a closed box. Its goal was to reduce the harmonic distortion introduced by loudspeakers. In the 50s, it was easy to buy an amplifier (tube, people lived!) With nonlinear distortions of less than 0.5% at a power of almost 50 watts.
However, a loudspeaker connected to such an amplifier immediately increased the total harmonic distortion at least twenty times. Wilchur asked himself the question: why? And he answered: because of the nonlinearity of the suspension.
In those years, the suspension of the diffuser of the dynamics of the classical design, already at that time, consisted of a rigid centering washer, often made of getinax in the form of a kind of spider (hence the word spider preserved in the English literature in relation to this element of the speaker structure) and a corrugation, which was then really corrugation – wavy part of the diffuser, which provided it with the ability to move along the axis of the magnetic system. Together, this forms a spring, which, together with the mass of the cone and voice coil, creates an oscillatory system.
A popular explanation of the acoustic suspension principle in one of Wilchur’s articles
Any man-made spring is not perfect. Its response to compression or tension is not completely linear, it can be compressed to stop (saturation effect), its parameters depend on time and many other factors. But there is an ideal spring, this is the elasticity of the air volume.
The air does not get tired, does not wear out, the air spring is always linear. Or much more linear than any other. And then Vilchur decided to make a speaker …