Created on 3/14/2008 07:03 PM.
Updated on 16.02.2020 17:24.
Author: G.S., Gendin.
Now on sale (at least in large cities) you can buy the most varied in power, design, overall dimensions and cost of acoustic systems for almost any taste, ranging from small volume of 2-3 dm3 up to floor volumes over a cubic meter. However, the overwhelming majority of these systems have one common feature: they are all type of a closed box.
This means that the acoustic design is tightly closed and the loudspeaker diffuser works like a piston, the cylinder of which has a constant volume of enclosed air inside.
The acoustic design of a closed box has a number of indisputable advantages, among which the most important are the following:
- An acoustic short circuit between the front and rear sides of the loudspeaker diffuser is completely eliminated, which increases the relative (but not absolute!) Response at extreme low frequencies and, therefore, reduces the overall unevenness of the frequency response due to this part of the spectrum
- Due to the fact that the diffuser works like a piston in a closed cylinder, the resistance of the internal volume of air in the case increases sharply, which leads to a rapid attenuation of the free vibrations of the diffuser, and this is equivalent to an increase in the damping factor.
- By increasing the emission of low frequencies (see item 1), it is possible to significantly reduce the overall dimensions of the case while maintaining the sound quality in the bass register.
However, as they say, only cheese in a mousetrap is free. Everything else comes at a price. In the case of compression loudspeakers, the payoff is their efficiency and, therefore, the electrical power that must be supplied to the system in order to obtain sufficient sound volume.
Readers probably paid attention to the fact that most modern portable and compact receivers, radio tape recorders, as well as their automobile twins, have a passport output power of 50, 60, 100 and even 300 W regulated! Meanwhile, the overwhelming majority of old tube radios and radios of even the highest class had an output power 10–20 times less.
For example, the top-class “Symphony” console stereo radio had an output power of each channel not exceeding 6 W, the first-class table receivers “Latvia”, “Mir” and “T-689” had an output power of 5 W, although the volume of their sound was by no means less, but rather more than today’s car radio with a rated power of 2×30 watts.
What’s the matter?
And the fact is that before the widespread use of transistor radio equipment, not compression, but exclusively open emitters were used as acoustic systems, i.e. those in which the rear side of the loudspeaker diffusers communicated with the air volume of the room through the perforated rear wall of the case.
Although such open-ended loudspeakers did not have the advantages of compression systems, they nevertheless provided excellent sound quality with significantly less electrical power input.
A comparison of the two types of speaker systems is provided so that the radio amateur can make the right choice. The fact is that the current range of powerful terminal transistors makes it possible to obtain undistorted output power of 50 and 100 W with extremely high efficiency, since special circuit solutions allow these transistors to operate in class B with practically no noticeable nonlinear distortions. In this case, the use of compression acoustic systems is not only possible, but also fully justified.
The situation is different with tube amplifiers. Modern tube power cascades can only operate in pure class A.
This is necessary to ensure an acceptable level of THD. But this, as you know, is the most uneconomical regime. In addition, powerful terminal lamps consume a large current through the filament circuit, so it turns out that even with an output power of 10-15 W, the amplifier consumes more than 100 W from the network.
It is clear that it is simply pointless to create a tube amplifier with an output power of 100 W or more for normal swinging of a sufficiently powerful compression system: it will consume at least 1 kW from the network and, accordingly, generate heat on a par with an iron or an electric stove.
It follows that an open-type speaker system is preferable for a tube amplifier. But it is precisely such systems that today are not produced by practically any company, either in Russia or abroad. What is left for the reader to do? It remains for him to build such a system himself.
For those who have never done this, we inform you that it is not at all as easy as it might seem at first, and that building a high-quality speaker system is no easier than a high-quality amplifier. Therefore, we will give not only a detailed description of one of the systems (far from the most complex), but also accompany it with explanations and comments that will help to competently approach the choice of types of loudspeakers, determine the shape and size of the case and construction materials for its manufacture.
Begin the design of the speaker system by setting the basic parameters. The main indicators of any speaker system are:
- Really reproducible frequency range in terms of sound pressure.
- Frequency response unevenness in this range.
- The real value of the sound pressure.
- Nonlinear distortion factor.
- Power consumption of the audio signal.
Directly related to these parameters is the choice of the types and number of loudspeakers capable of solving this problem. Here again, a small digression into the field of theory is required, without which much of the further reasoning may turn out to be incomprehensible. Let’s start by looking at how a loudspeaker works.
For effective radiation of the lowest frequencies, the loudspeaker diffuser must have the largest possible radiating surface (cone area), an extremely soft suspension (elastic corrugation and a small suspension elasticity), which entails a sufficiently large inertia of the entire system. However, at the lower frequencies of the range, this practically does not negatively affect the sound quality of bass instruments.
To effectively reproduce the higher frequencies of the range (from 8-10 kHz), the requirements for the loudspeaker are reversed. The diffuser can be small, but always rigid: very often to achieve this, a paper diffuser is impregnated with bakelite varnish, and for the most expensive models (mainly Western firms) they are made of plastic or lightweight duralumin. The coil suspension is made rigid and inertia-free as much as possible.
Even what has been said is enough to understand that one loudspeaker is indispensable for efficient emission of a wide spectrum of frequencies. Indeed, the vast majority of broadband speaker systems consist of three or more different emitters.
Why three and not two? Because good …