It describes how to create speaker system with your own hands.
Introduction
Recently I heard a lot of questions about speakers & subwoofers. The vast majority of the responses are available on the first three pages of any book written by professionals. The material is intended primarily for beginners, lazy ;) and rural DIY experts knows, based on the books by I. A. Aldoshina, V. K., Joffe, partly Ephrussi, magazine publications in Wireless Worrld , AM, and (some) personal experience . You did NOT use information from the Internet and FidoNet. The material is in no way intended to be complete lighting problem, but is an attempt to explain on fingers basics of acoustics.
Often the question sounds like this: "I found the speaker is, what to do?" or "Comrade, as they say these subs are there?". Here we consider only one solution to this problem: On the momentum to make a box , with optimal parameters on bass as possible. This option is very different from the tasks of the factory constructor to pull the lower frequency of the system necessary to THAT value.
Question-answer
Q: Found a large speaker on the occasion unmarked. How to find out whether you can make it a subwoofer?
A: you Need to measure T/S parameters. Based on these data to make a decision about the type of bass design.
Q: What is the T/S parameters?
A: the Minimum set of parameters to calculate the LF design proposed by Thill and Small:
Q: How to measure T/S parameters?
A: you need to assemble the circuit from the generator (you can use the sound card of a computer, voltmeter, resistor and studied the dynamics. The speaker connects to the output of the generator with an output voltage of a few volts through a resistor of about 1 kω.
1. Remove V(F)=frequency response of resistance dynamics in the field of resonance. The speaker needs during this measurement to be in free space(away from reflective surfaces) . Find the resistance of the dynamics at constant current (useful), recordable frequency of resonance in the air Fs (this is the frequency at which the maximum voltmeter reading :) , the voltmeter Uo at minimum frequency (well, for example 10 Hz) and Um at the resonance frequency Fs.
2. Find the frequencies F1 and F2, in which the curve V(F) intersects the level V=SQRT(Vo*Vm).
3. Find Qts=SQRT(F1*F2)*SQRT(Uo/Um) / (F2-F1) is a complete q-factor of the dynamics, we can say that the most important value.
4. To find Vas need to take a small closed box volume Vc, with a hole slightly smaller than the diameter of the diffuser. Tightly place the speaker into the hole and repeat the measurement. From these measurements will need the resonant frequency dynamics in the hull Fc.
Find Vas=Vc*((Fc/Fs)^2-1). This technique written in the Audio Shop No. 4 99 a year. There are other, when measured mechanical parameters of head, weight, flexibility, etc.
Q: I now have the parameters of the dynamics, what to do with them?
A: Each speaker when designing sharpened to a certain type of acoustic design. To find out what it is, look at the quality.
It would be better to sort your heads not in q, and the largest Fs/Qts. Lead by memory, don't want the formulas to calculate.
Elasticity, Masatoshi, dryness and other similar characteristics of the sound of bass speaker, is largely determined by the transition characteristic of the system composed of the speaker, woofer design and the environment. So in this system has not been release to the pulse characteristics, the quality factor must be less than 0.7 for systems with radiation of one party dynamics (sealed and vented enclosures) of 1.93 and for two-way systems (registration type screen and open the drawer)
Q: Where can I read about open source design?
A: Open the boxes and screens, the simplest type of design. Advantages: ease of calculation, no increase in the resonant frequency (of the size of the screen depends on the frequency characteristics), almost the same figure of merit. Disadvantages : large size front panel. Quite literate and simple calculations of this type of design can be found in Joffe V. K., M. V. Lizunkov. Home audio system, Moscow, Radio I Svyaz . 1984. And in the old Radio there are probably primitive Amateur radio calculations.
Q: How to calculate a sealed box?
A: Making "closed box" is of two types, an endless screen and suspension compression. Getting into a particular category depends on the ratio of the flexibilities of the speaker's suspension and air in the box, denoted by alpha (incidentally, the first can be measure up, and the second to calculate and change with padding ). For infinite screen ratio of flexibilities less than 3, for compression of the suspension 3-4 more. In a first approximation we assume that the head is more sharpened by a factor endless screen, with a smaller under suspension compression. For advance taken, the dynamics of the closed body type endless screen has a greater volume than the compression box. (Generally speaking, when there is a speaker, the optimal body under him is definitely a certain amount . Any errors resulting from the measurement and the calculations can slightly improve by filling in). Speakers for sealed enclosures have powerful magnets and soft suspensions unlike heads for open boxes. The formula for the resonant frequency dynamics in the design volume V FC=Fs*SQRT(1+Vas/V),and an approximate formula relating the resonant frequency and quality factor of the head in the body (index "C") and open space ("s") Fc/Qtc=Fs/Qts
In other words, it is possible to implement the required q-factor of the acoustic system, the only way, namely by choosing the volume of the sealed box. What factor should I choose? People who didn't hear the natural sound of musical instruments, usually choose speakers with good quality bolee1,0. Columns with this factor (=1.0) of the smallest frequency response in the lower frequencies (and what does that sound?), achieved at the cost of a small release on the transient response. Very smooth frequency response is obtained by Q=0.7, fully Periodica impulse response at Q=0.5. Nomograms for calculation you can take in the above book.
Q: In articles about speakers often words like "approximation by Chebyshev, Butterworth," etc., What does this have to the speakers?
A: speaker system is a high-pass filter. The filter can be described by the transfer function. The transfer characteristic is always possible to fit a known function. In theory filters use several types of power functions, named for mathematicians, the first obsessie a particular function. Function is determined by the order(maximum exponent, i.e. H(s)=a*S^2/(b2*S^2+b1*S+b0) has the second order) and a set of coefficients a and b (these coefficients can then move on to the real values of the elements of the electrical filter, or Electromechanical parameters.) Further, when it will go on the approximation of the transfer characteristic polynomial Butterworth or Chebyshev or something else, it must be understood so that the combination of dynamics and properties of the body (or capacitance and inductance in an electrical filter) turned out to be such that most accurately the frequency and phase response can be tailored to a particular polynomial. Most smooth frequency response is obtained if it can be approximated by a polynomial of Butterworth. Chebyshev approximation is characterized by wolnoobraznae frequency response, and a greater length of the work area (according to GOST to -14 dB) in the region of the lower frequencies.
Q: What kind of approximation to opt for the phase inverter?
A: so before building a simple bass reflex need to know the volume of the box and the tuning frequency of the bass reflex(pipes, vents, passive radiator). If as a criterion to choose the most smooth frequency response( and this is not the only possible criterion), we get the following table (A) Qts < 0.3 to most smooth curve will quanitities order B) Qts = 0.4 to better describes Butterworths curves In) Qts> 0,5 - have to prevent the waves of the amplitude-frequency characteristic, according to Chebyshev. In the case of (A) the phase inverter is configured by 40-80% higher than the resonance frequency In the case B) is based on the resonance frequency, In the case In) below the resonance frequency. In addition to these cases, and will be different case.. in order to find the exact frequency settings, it is necessary to take the original formula, rather cumbersome to bring them here. So I refer those interested in Audiomagazine 1999, after this learning there may be already understand, or book Aldoshina. And even article Ephrussi in Radio for a 69 year amiss.
Conclusion
If after reading this You still have a desire riveted yourself, then you can take on the Internet some files (like WinspeakerZ) and calculate it yourself, bearing in mind that from G. do not make candy. Do not be carried away by a decrease of the cutoff frequency, in any case do not try to compensate for the decline in the frequency response of the amplifier. AFC maybe a little bit more and equalized, but the sound is enriched with a lot of harmonics and subharmonics. On the contrary, better results in terms of amenity to the ear, can be achieved forcibly ruined inlet the lower frequencies, i.e. frequencies below the cutoff frequency WOOFER speakers. One more note regarding vented enclosures, the error in setting the resonance frequency of the bass reflex 20% leads to a surge or decline curves 3 dB.
Oh, I almost forgot to say about the subs that actually bandpass resonators. The quality of the speakers for them to be even lower. The simplest bandpass is also discernible, but courtesy of my ends.
Publication: www.cxem.net