Horn acoustic systems

One way to increase the response is to use horn acoustics. Remember the old gramophones: thanks to the horn, small vibrations of the needle connected to the membrane created a very loud sound. In gramophones, a part of the horn is folded and “hidden” inside the case. The open cover serves as a sound reflector.

Horn loudspeakers are used for sounding open areas.

A professional horn loudspeaker can be up to 25–35% efficient. A good horn loudspeaker is hardly available to radio amateurs, but something can be done from ordinary drivers.

Of the author’s available, good results were shown by a small-sized industrial-made speaker in a narrow and deep plastic case with one 1GD-40 head. Its output, quite large and without additional measures, was noticeably increased by a folding horn (Fig. 1) made of cardboard folders for papers. Radiation from the rear of the driver cone, exiting the open rear of the loudspeaker, is reflected from the middle of the horn loudspeaker and is directed by its expanding side walls towards the listener. The top and bottom of the system is also covered with cardboard lids.

Another foldable horn loudspeaker can be easily made from an old “diplomat” with rigid fiber walls (soft absorb sound). The design (side view) is clear from Fig. 2. Two elliptical heads 1 (for example, 1GD-40) are fixed in one line on a narrow reflective board 2 located in the front of the suitcase. Horn acoustics is formed by a bottom and a slightly curved plastic panel 3.

The cracks formed at the side walls must be closed. Coming out of the wide rectangular opening, the sound is reflected by the open lid of the suitcase 4, heading towards the listener, and adding up with the sound emitted from the front of the diffusers. For better reflection, “acoustic mirrors” 5 made of plastic are installed near the loops. There is enough space in the “diplomat” to accommodate the output transformer, receiver and other useful things.

The wide-throated bass reflex is sometimes made in the form of a “horseshoe”, which was described above, or a “labyrinth”. Both designs are shown in fig. 3, and their front view (along A) is practically the same – the hole for the head is on top, and the outlet of the horn is on the bottom. The side walls can be made vertical, but it is better – widening towards the horn opening. The sizes are not given as they are not critical.

With the path length of the sound wave inside the “horseshoe” or “labyrinth” of the order of a meter, one should expect reproduction of lower frequencies up to 70-100 Hz. Of course, these horn speakers can also be positioned horizontally by aligning them, for example, with a bookshelf.

Matching transformers have already been mentioned in the previous section.

Where to install it, in the receiver or in the speaker?

The answer is unambiguous – only in the acoustic system, otherwise the losses in the connecting wires can significantly reduce the efficiency. For example, if the resistance of the wires from the receiver to a four-ohm speaker is only 1 ohm, the efficiency will be 80%. By increasing the AC input impedance to 4–10 kOhm with a transformer, we obtain 100% efficiency and the ability to use thin wires.

But the blocking capacitor must be installed in the receiver, since it closes the high-frequency currents of the detector. If the dimensions of the acoustic horns are still large, it is better to use “solid” transformers – they have less loss. TVK and TVZ from old TVs are well suited.

Their approximate data: cross-section of the core 2.5-3 cm², primary winding – 3000 turns of PEL 0.15, secondary – 100 turns of PEL 0.7. You can also use “silovichki” – network transformers of power supplies with a transformation ratio of 220 / 6-12.

Source: amfan.ru