Self-Assembly of the Speaker System<. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Self-assembly of the acoustic system. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Speakers

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Here we describe how to create a speaker system with your own hands.

Introduction

Recently, a lot of questions about speakers and subwoofers have been heard. The vast majority of answers can be found within the first three pages of any book written by professionals. The material is addressed primarily to beginners, lazy ;) and rural DIYers, prepared on the basis of books by I.A. Aldoshchina, V.K. Ioffe, partly Ephrussi, magazine publications in Wireless World, AM and (a little) personal experience. Information from the Internet and FIDOnet was NOT used. The material in no way claims to be complete coverage of the problem, but is an attempt to explain the basics of acoustics on the fingers.

Most often the question sounds something like this: “I found a speaker, what should I do with it?”, or “Comrade, do they say there are such subwoofers?” Here we will consider only one option for solving this problem: Using the existing speaker, make a box with optimal low-frequency parameters, as far as possible. This option is very different from the task of the factory designer - to tighten the lower frequency of the system to the value required according to the specifications.

FAQ

Q: On the occasion, I found a large speaker without identification marks. How to find out if you can make a subwoofer out of it?

A: You need to measure its T/S parameters. Based on these data, decide on the type of HF design.

Q: What is T/S parameters?

A: The minimum set of parameters for calculating HF design, proposed by Till and Small:

Fs - resonant frequency of the speaker without decoration

Qts - full quality factor of the speaker

Vas is the equivalent speaker volume.

Q: How to measure T/S parameters?

A: To do this, you need to assemble a circuit from a generator (you can use a computer sound card), a voltmeter, a resistor and a speaker under study. The speaker is connected to the output of the generator with an output voltage of several volts through a resistor with a resistance of about 1 kOhm.

1. We remove V (F) = frequency response of the speaker resistance in the resonance region. The speaker must be in free space (away from reflective surfaces) during this measurement. We find the resistance of the speaker on direct current (it will come in handy), we record the resonance frequency in the air Fs (this is the frequency at which the voltmeter readings are maximum :), the voltmeter readings Uo at the minimum frequency (well, for example 10 Hz) and Um at the resonance frequency Fs.
2. Find the frequencies F1 and F2 at which the V(F) curve intersects with the level V=SQRT(Vo*Vm).
3. Find Qts=SQRT(F1*F2)*SQRT(Uo/Um) / (F2-F1) this is the full quality factor of the speaker, one might say, the most important value.
4. To find Vas, you need to take a small closed box of volume Vc, with an opening slightly smaller than the diameter of the diffuser. Place the speaker firmly against the hole and repeat the measurements. From these measurements you will need the resonant frequency of the speaker in the cabinet Fc.

Find Vas=Vc*((Fc/Fs)^2-1). This technique was written in Audio Store #4 in '99. There are others when the mechanical parameters of the head, mass, flexibility, etc. are measured.

Q: I now have speaker settings, what should I do with them?

A: When designing each speaker, it is sharpened for a certain type of acoustic design. To find out exactly what, let's look at the quality factor.

Qts > 1,2 are heads for open boxes, optimally 2,4

Qts < 0.8-1.0 - heads for closed boxes, optimally 0,7

Qts<0.6 - for phase inverters, optimum - 0,39

Qts<0.4 - for horns
It would be more correct to sort the heads not by the quality factor, but by the value of Fs / Qts. I will cite from memory, reluctance to calculate the formulas.

Fs/Qts >30 (?) screen and open case

Fs/Qts >50 closed case

Fs/Qts >85

Fs/Qts >105 Bandpasses (bandpass resonators)

Elasticity, meatiness, dryness, and other similar characteristics of the sound emitted by a bass speaker are largely determined by the transient response of the system formed by the speaker, low-frequency design and the environment. In order for this system to not have a surge on the impulse response, its quality factor must be less than 0,7 for systems with radiation from one side of the speaker (closed and phase inverters) and 1,93 for two-way systems (screen and open box type design)

Q: Where can I read about open design?

A: Open drawers and screens are the simplest type of decoration. Advantages: ease of calculation, no increase in resonant frequency (only the type of frequency response depends on the size of the screen), almost unchanged quality factor. Disadvantages : large size of the front panel. Sufficiently competent and simple calculations of this type of design can be found in V.K. Ioffe, M.V. Lizunkov. Household acoustic systems, M., Radio and communication. 1984. Yes, and in the old radio, there are probably primitive radio amateur calculations.

Q: How to calculate closed box?

A: There are two types of "closed box" design, endless screen and compression suspension. Getting into one or another category depends on the ratio of the flexibility of the speaker suspension and the air in the box, denoted by alpha (by the way, the first can be measured, and the second can be calculated and changed using filling). For an infinite screen, the ratio of flexibility is less than 3, for a compression suspension it is more than 3-4. As a first approximation, we can assume that heads with a higher quality factor are sharpened for an endless screen, with a smaller one - for a compression suspension. For a forward looking speaker, a closed infinity baffle box has more volume than a compression box. (Generally speaking, when there is a speaker, then the optimal case for it has a uniquely defined volume. Errors that have arisen in the measurement of parameters and calculations can be corrected to a small extent using filling). Speakers for closed cases have powerful magnets and soft suspensions, in contrast to the heads for open boxes. The formula for the resonant frequency of a loudspeaker in a V volume design Fс=Fs*SQRT(1+Vas/V), but an approximate formula that relates the resonant frequencies and quality factors of the head in the case (index "c") and in open space (index "s") Fc/Qtc=Fs/Qts

In other words, it is possible to realize the required quality factor of the acoustic system in the only way, namely by choosing the volume of a closed box. Which quality factor should I choose? People who have not heard the sound of natural musical instruments usually choose speakers with a quality factor of more than 1,0. Speakers with such a quality factor (=1.0) have the least uneven frequency response in the low-frequency region (what does sound have to do with it?), achieved at the cost of a small overshoot in the transient response. The smoothest frequency response is obtained at Q=0.7, and a completely aperiodic impulse response at Q=0.5. Nomograms for calculations can be taken from the above book.

Q: In articles about columns, words like "approximation according to Chebyshev, Butterworth", etc. are often found. What does this have to do with columns?

A: The speaker system is a high pass filter. The filter can be described by a transfer characteristic. The transfer characteristic can always be adjusted to a known function. In filter theory, several types of power functions are used, named after the mathematicians who first sucked on this or that function. The function is defined by the order (maximum exponent, i.e. H(s)=a*S^2/(b2*S^2+b1*S+b0) has a second order) and a set of coefficients a and b (from these coefficients you can then move on to the values of real elements of the electric filter, or electromechanical parameters.) Further, when it comes to approximating the transfer characteristic with a Butterworth or Chebyshev polynomial or something else, this must be understood in such a way that the combination of the properties of the speaker and the housing (or capacitances and inductances in an electrical filter) is such that the frequency and phase characteristics can be adjusted to one or another polynomial with the greatest accuracy. The smoothest frequency response is obtained if it can be approximated by a Butterworth polynomial. The Chebyshev approximation is characterized by a wave-like frequency response and a greater extent of the working section (according to GOST up to -14 dB) in the region of lower frequencies.

Q: What type of approximation to choose for a phase inverter?

A: So, before building a simple phase inverter, you need to know the volume of the box and the tuning frequency of the phase inverter (pipes, holes, passive radiator). If we choose the smoothest frequency response as a criterion (and this is not the only possible criterion), then we get the following table A) Qts < 0,3 - the quasi-third order curve will be the smoothest B) Qts = 0,4 - better described by Butterworth curves C) Qts > 0,5 - you will have to allow waves on the frequency response, according to Chebyshev. In case A), the phase inverter is tuned 40-80% above the resonance frequency; in case B), at the resonance frequency; in case C) below the resonance frequency. In addition, in these cases there will be a different volume of the case. In order to find the exact tuning frequencies, one must take the original formulas, which are cumbersome enough to be given here. Therefore, I send those interested to the AudioMagazin for 1999, after this educational program it will already be possible to figure it out there, or to Aldoshina's books. And even Ephrussi's articles in Radio for 69 will fit.

Conclusion

If, after reading all this, you still have a desire to rivet something yourself, then you can take some program on the Internet (such as WinspeakerZ) and calculate it all yourself, bearing in mind that you can’t make candy out of G.. You should not get carried away with reducing the cutoff frequency; in no case should you try to compensate for the decline in the frequency response with an amplifier. The frequency response may even out a little, but the sound will be enriched with a mass of harmonics and subharmonics. On the contrary, the best results, in terms of pleasantness to the ear, can be achieved by forcibly losing the lowest frequencies at the PA input, i.e. frequencies below the cutoff frequency of the woofer. Another note regarding bass reflexes: an error in setting the bass reflex resonance frequency of 20% leads to a surge or drop in the frequency response of 3 dB.

Yes, I almost forgot to mention subwoofers, which are actually bandpass resonators. The quality factor of the speakers for them should be even lower. The simplest bandpass is also calculable, but that's where my courtesy ends.

Publication: cxem.net

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