Open acoustic system for 6GD-2, 4GD-35

The advantages of open acoustics over closed ones are quite convincingly shown on the site of Siegfried Linkwitz.

His projects “Phoenix” and “Orion” have been repeated by many radio amateurs and have excellent reviews. The site offers to purchase a constructor for assembling these active speakers (with an amplifier), only the cost of the components scares off.

An industrial version of open acoustics, developed with the participation of Linkwitz: “Beethoven-Elite” (Beethoven-Elite) broke all records in terms of sound quality and price.

In short, the essence of the benefits of open acoustics can be summarized as follows:

On the left, there are circular diagrams of radiation of a closed speaker (Monopole, Box speaker) and an open (Dipole) speaker in different frequency ranges (top view).

It is easy to see that an open (dipole) speaker completely lacks radiation in the direction perpendicular to the listener (to the side, up, down), and this is in two of the three directions of three-dimensional space.

Accordingly, in the same directions there is no reason for the occurrence of reflected sound waves, which could lead to distortions due to interference with the direct sound wave created by an open speaker system (as is the case with a conventional closed speaker).

These advantages of open acoustics operating in a closed room can reduce interference (increase noise immunity) of a direct sound wave by 4.8 dB, which is equivalent to a decrease in the power supplied to the speakers required to obtain the same intelligibility as in the case of a closed speaker by 3 times …

However, the loudspeaker heads used by Linkwitz are quite expensive to purchase, as evidenced by the price lists of imported loudspeaker suppliers available to us. In addition, Linkwitz was forced to use three-band amplification due to the complexity or impossibility of building a passive speaker system on the (nevertheless, excellent) emitters available to him.

Nevertheless, the respected Linkwitz applied an erroneous methodology for evaluating the cumulative spectrum of the after-sound of the heads. The assessment of the accumulated energy in the heads should be carried out only when they are connected through filters that correct the frequency response according to the requirements of the project. Therefore, the excitement of fellow developers in terms of finding and acquiring speaker drivers, indicated by Linkwitz as preferred, is a little less than optimal.

It is precisely in this respect that we are in a more advantageous position. Will explain. Our living rooms are small and do not require so much operating power of amplifiers, even for acoustics (AC) with low sensitivity. Most of us already have power amplifiers over 50 watts per channel, which, under certain conditions, makes it possible to use more energy-retention passive filters for loudspeakers.

The available dynamic heads for us are still a clone “Made in the USSR”, which differ in parameters and sensitivity that are not so bad for building open speakers.

In the USSR, there has always been a problem of producing low-mid-frequency heads with a low value of the quality factor Q_ts (full quality factor). For typical heads (25-, 35-GDN, 15-, 20-GDS, 8GD-1, 6GD-2, 4GD-53 (35.8E), 2GD-40, 5GDSH-xx, etc.) the value of Q_ts lies in the range of 0.8-1.8, which complicates their classical use in the form of a closed box or bass reflex, but is ideal for building an open speaker. Moreover, the rise in the frequency response at the resonant frequency of the heads may well be used to compensate for the corresponding drop in the resulting frequency response of the open speaker.

In order not to be unfounded – I give an example of a built and tested, working and bringing real pleasure to its dynamic heads, open type and has the following characteristics:

• Dynamic heads used: LF 6GD-2 (2 pcs), MF 4GD-35, HF HX-125 (BEAG, can be replaced with 2GD-36)
• Operating frequency range: 35-25,000 Hz
• Maximum SPL @ 35 Hz: 105 dB
• Working power in a room of 25 m2: 3 W
• Maximum power: 40W
• Uneven frequency response at the listening point: in the range of 300–25,000 Hz + -2 dB, in the range of 35–300 Hz + -5 dB
• Filter order: LF / MF – 1 order, MF / HF – 2 order
• Frequencies of division of bands: NS / MF – 300Hz, MF / HF – 2000Hz
• Dimensions (height x width x depth): 1100mm x 350mm x 400mm
• Weight: 16KG

The frequency response of the speaker system and each of the bands in terms of sound pressure is shown below (solid lines), the dotted line shows the phase response. Measurements were made with a measuring microphone with a cardioid polar pattern and a spectrum analyzer with 1/6 octave smoothing on white noise at the listening point in a real room of 16 square meters at a distance of 1.5 m from the speaker:

The impulse response of the speaker for sound pressure (under the same conditions) when a pulse signal is applied is presented below:

Why is such a low frequency of the interface between the midrange and the high frequency head chosen?

It’s all about the radiation patterns of the 4GD-35. Characteristic of this speaker at angles 0, 22, 45 and 67 degrees is shown below. Thus, if you choose a frequency of the junction of the bands of more than 2 kHz, then in the 2-4 kHz region there will be a subjective failure – the one with which Alexander Klyachin successfully fights using his swinging microphone technique when creating 2-way systems.

The Agnetta speaker system uses modified first-second-order crossovers, the topology of which performs the following functions:

• shunting of the main mechanical resonance of the heads with a low output impedance of the filters; for the low-frequency section, shunting is carried out by a low output impedance of the amplifier.
• powering the emitters at medium and high frequencies through the high reactance of the filter and eliminating the shunting of the heads at these frequencies by the low output impedance of the amplifier reduces intermodulation distortion at these frequencies.
• suppression of penetration to the output of the back-EMF amplifier of the loudspeaker heads eliminates the disruption of the negative feedback amplifier (if any).
• the minimum deviations of the phase-frequency characteristics of the input impedance of the speaker from the resistive equivalent make it possible to fully use the power characteristics of the amplifier.

The filter topology is shown below:

Source: acustik-master.ucoz.ru