Acoustic design with JBL Speakershop software. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Speakers

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The acoustic design, or box, that you choose for your subwoofer will be the determining factor in achieving quality sound in the low-frequency bass region." From the JBL description "Automotive Subwoofers".

Wasting time arguing about what acoustic design is and why a subwoofer needs acoustic design, and, accordingly, software for its calculation (see "12 Volt Master" N 1/97, p. 52, N 5/97, p. 18, N 1/98, p. 62), probably not necessary. Let's proceed immediately and directly to the subject of consideration - the computer software SPEAKERSHOP, prepared by JBL specialists for the development and calculation of the parameters of the acoustic design of subwoofers. Let's make a reservation right away that the program will work well for home acoustics, but this is not our case, and that it allows you to make calculations not only for JBL speakers, but actually for a variety of products - the values ​​\uXNUMXb\uXNUMXbof the necessary characteristics would be known.

JBL SPEAKERSHOP is software known to some extent to Russian installers. He got to them in various ways, including through the Internet. This year, JBL has an exclusive distributor in Russia in the car audio section - MMS. Now SPEAKERSHOP is available to everyone, and MMS customers receive its original version along with a detailed description in Russian.

SPEAKERSHOP consists of two independent and complementary parts: the Enclosure Module for calculating the acoustic design and the Crossover Module for calculating the parameters of crossover filters. So let's start in order.

Enclosure Module

This software helps to determine the volume and dimensions of the cabinet and evaluate the sound quality. The structure is analyzed in two stages. First of all, it is determined how it will work at normal listening levels. This procedure is called small-signal analysis and includes calculation of amplitude (frequency) response, voice coil impedance response, phase response, and group delay. Secondly, the maximum volume mode is modeled for the structure. This stage is called large signal analysis and includes thermal acoustic power norms in the medium frequency range and the maximum power characteristic at various deviations.

Two ways to use the program

There are two ways to construct enclosures using the SPEAKERSHOP Enclosure Module. One of them involves designing a cabinet for certain selected speakers. In this case, the characteristics of the body vary. Another way is to find the right speakers for your existing cabinet: you match the speaker models. The construction method can be selected using the Variable command on the Options menu.

When the SPEAKERSHOP Enclosure Module program is started for the first time, the default setting is the mode in which the adjustable values ​​are the characteristics of the acoustic design.

The spreadsheet contains columns for the construction of six cases. The first three are designed to calculate enclosures with a phase inverter - for optimal, custom (i.e., designed by the master himself) designs and for enclosures designed for a specific frequency band. The following column is for a custom enclosure design with a passive radiator. The last two columns are for optimal and custom design for enclosed enclosures. Because the spreadsheet shows different types of constructs at the same time, you can easily compare them. The speaker options are shown in the lower left area of ​​the spreadsheet. The graph below is the same for both methods.

The mode when the variable is the loudspeaker itself is set using the Variable-Loudspeaker command in the Options menu. This is in case of choosing suitable speakers for an existing case. The mode is very convenient for calculating the sound reproducing systems of cars, when it is necessary to select a speaker for a strictly specified volume, as it allows you to quickly check the operation of several different acoustic systems in a specific case or in a certain limited space.

Variable-Loudspeaker mode uses a different kind of spreadsheet menu. Instead of showing six different cabinet designs like the Variable-Box mode does, six different drivers are shown simultaneously. This makes it possible to quickly compare up to six different models.

Speaker Options

If you're new to speaker cabinet design, or if you're in a hurry and want to enter only the minimum parameters needed to build your cabinet, select the Parameters-minimum option from the Loudspeaker menu. A window will appear in which you can enter the minimum parameters, including the name of the manufacturer (Manufacturer), the name of the model (Model), Fs, Vas and Qts. Nominal efficiency or sensitivity only needs to be entered when designing enclosures with a phase inverter.

To enter complete parameters (mechanical, electrical, combined), select the appropriate command. Next, we give a brief explanation of the parameter designations.

Mechanical parameters

Fs - Natural resonant frequency of the speaker (Hz).

qms - The quality factor of the speaker at the frequency Fs, when its mechanical (non-electromagnetic) losses or attenuation are taken into account.

You - A volume of air having a resilience equivalent to that of the speaker suspension (cubic feet or inches, as well as liters).

Cms - Coefficient of mechanical compliance of the suspension (inches per pound or millimeters per newton).

MMS - The mechanical mass of the diffuser, taking into account the aerodynamic load (ounces or grams).

rms - Mechanical resistance in the speaker suspension (pounds per second or kilograms per second).

Xmas - The maximum or peak linear amplitude of the speaker's voice coil oscillations (inches, centimeters or millimeters). Usually defined as the distance that a coil can travel in one direction while maintaining the ability to maintain a constant number of oscillations across the magnet gap. This parameter determines the maximum oscillation amplitude at which distortion does not appear.

Sd - "Piston/cone area" of the speaker (square inches or square centimeters). Represents the area of ​​the moving part of the speaker.

Day - "Piston diameter" (inches or centimeters).

Combined Options

Qty - The quality factor of the speaker for the frequency value Fs, taking into account all electromagnetic and mechanical losses.

ho - Nominal speaker efficiency at half volume acoustic load (reflector positioned at infinity). The efficiency is entered as a percentage.

SPL - Rated sensitivity of the speaker at an acoustic load of half the volume (the reflector is located at a distance to infinity). Entered in decibels. Sensitivity is taken as measured along the axis at a distance of 1 meter when an electric power of 1 W is applied to the speaker. Since many manufacturers test their speakers at a fixed voltage of 2,83V instead of 1W, there is a 2,83V option in the Full Loudspeaker Parameters window.

Electrical parameters

what - Q dynamics for the frequency value Fs. Allows only electromagnetic (not mechanical) losses or vibration damping.

Re - DC resistance of the voice coil (Ohm).

Le - Voice coil inductance (millihenry).

Z - Nominal electromagnetic impedance of the speaker (usually 8 or 4 ohms).

BL - Speaker drive power (Newton/Amp, Meter/Tesla, Pound/Amp or Feet/Tesla).

Pe - Thermally limited maximum electrical power (W) that the speaker can handle. Usually represents the maximum electrical power that has yet to burn out the voice coil.

Speaker database

The database stores the values ​​of all the necessary characteristics of a large number of speakers from various manufacturers. The "shooting sector" is very wide, it is enough to list as an illustration several companies from the beginning of the list: A&S Speakers, Acoustic Research, AcousticPro - and from its end: Xtasy Audio, Yamaha, Zachry. Of course, if you have not found the model you are looking for, then it can be added to the database along with the characteristics, increasing the information contained in it. Moreover, if you have the opportunity to measure the amplitude-frequency characteristics of the speaker in a special test case-screen or receive these data from the manufacturer, then a variant of point-by-point introduction of experimental values ​​is provided. Of course, the addition of experimental data will increase the accuracy of the calculation result.

The program also allows you to automatically select speaker models that meet predetermined conditions. It is enough to determine the range of Fs and Qts values ​​- and the program will instantly offer a number of models that are suitable for the selected acoustic design.

Acoustic enclosures and their parameters

1. Phase inverter

The goal of optimizing the design of a bass-reflex enclosure is to select a volume that provides the most even and smooth amplitude response in the tuning frequency region of the bass-reflex port.

1) A system with a large bass response and a system with a more "smooth" bass response; 2) Underdamped system (box volume is small) and overdamped system (box volume is large)

The advantages of this design are wider mid and low frequency response, less distortion due to smaller cone amplitude, higher efficiency and lower overall cost.

The bass-reflex cabinet design is relatively sensitive to changes in speaker parameters. Speakers with sufficiently low Qts (from 0,2 to 0,5) work better in such a case. Bass reflex cabinet designs allow for significantly higher resonant frequencies (Fs), as well as shorter pitch voice coils (low Xmax) and stiffer suspension (small Vas) than closed cabinet designs. Downsizing the bass reflex cabinet will require lower Qts and lower Vas.

2. Band-Pass Design

Band-Pass - a box design that allows you to control the amplitude response in both the low and high frequencies through the use of a double chamber housing. Moreover, the speakers are inside the case. (If there is more than one speaker, three-chamber cases, etc. may be used.)

The Band-Pass design means you can use drivers with a higher Q value (smaller magnets) than drivers used with other bass reflex cabinet designs. It provides lower distortion (high-order distortion is filtered out), increased efficiency in the operating frequency band, and virtually no need for a low-pass crossover filter.

The disadvantages of Band-Pass include high-order resonance of the "organ pipe" for the port, which determines the cutoff of the upper frequencies, as well as the complexity of the design.

The Band-Pass design is very sensitive to the quality factor of the speaker. A 4th order design works best with speakers having a Qts close to 0,4, and a 6th order design works best with speakers that have a Q factor close to 0,5. In general, the higher the Qts, the narrower the bandwidth. The lower Qts, the wider it is, but at the same time, the unevenness of the characteristic in the operating frequency band also increases. The coefficients Vas and Cms do not have much effect on the design.

3. Acoustic design with a passive radiator (emitter)

A passive radiator (similar to a regular speaker, but without the magnet system and voice coil) acts as a port on the cabinet. For this reason, a passive radiator enclosure behaves like a bass reflex enclosure in many cases.

The advantages of the passive radiator case design are the same as those of the bass-reflex case, plus the possibility of using a smaller case, which, however, does not always fit a port of the required size. This ensures minimization of re-radiation of the internal noise of the case and a decrease in the amplitude of the speaker cone in the region below the resonance of the system. The latter benefit is a result of the passive radiator's ability to drive the driver at very low frequencies.

The disadvantages of the design of the case with a passive radiator include, as one would expect, the disadvantages of the case with a phase inverter plus a poor transient response at the resonant frequency of the passive radiator (Fp). A passive radiator usually requires the possibility of large linear displacements of the cone compared to a woofer. The complexity of the design is, of course, also a disadvantage.

4. Closed box

The advantages of the closed case design are its simplicity and usually small size. Deviations in speaker characteristics often have less effect on sound quality. A flatter amplitude response and the possibility of using it with high-power amplifiers (because the speakers do not unload at low frequencies, as happens when working with bass-reflex cases) is also a plus.

The disadvantages of the design of a closed case - less efficiency than when using a case with a phase inverter. Usually in a closed design, speakers with a quality factor of more than 0,3, low Fs and high Xmax and Vas perform well. Reducing the duct volume will require lower Qts and Vas.

Underdamped system (box volume is small) and overdamped system (box volume is large)

Below are the parameters of acoustic boxes used in the calculations.

Vb - The internal volume of the box.

F3 - Rated frequency (Hz) at half power -3 dB. Represents the point 3 dB below the break in the amplitude response at which the low frequency rolloff begins.

Fb - Resonant frequency for bass reflex cabinet (Hz).

QL - The value of the quality factor for the case is the sum of all losses. Cases under 11 cubic feet (311 liters) typically have a QL value close to 7. Larger cases have a QL of approximately 5.

steam - A volume of air having a resilience equivalent to that of a passive radiator suspension (cubic feet or inches, as well as liters).

Fp - Natural resonant frequency of the passive radiator (Hz).

Qtc - The value of the quality factor for the case of a closed type.

Dv - Diameter or cross-sectional area of ​​a port or duct in a bass-reflex enclosure.

Lv - Length of a port or duct in a bass-reflex enclosure.

Received "output" graphics

In this program, you can access six graphs of various characteristics. These graphs are: normalized frequency response (often called frequency or amplitude response), amplitude response when a 2,83 V signal is applied to the input, maximum sound power, voice coil impedance characteristics, phase and group delays.

Special note

Characteristic "hump" due to the transfer function of the cabin

This remark concerns the transfer function of the car interior. The peculiarity lies in the fact that the calculated amplitude-frequency characteristics of the system, displayed by the resulting graphs, most seriously depend on the specific car (size, design, etc.) in which the entire bass speaker system will be placed. The above graph shows that the interior of the car leads to significant changes in the frequency response with a "hump" emission at frequencies in the range of 30-50 Hz. The issue of the transfer function of the cabin was considered in "Master 12 Volt" N 1/98, and the experimental results of measurements are given in the next article in the same issue of the journal.

In most calculation programs, the transfer function is taken as a universally averaged one, and SPEAKERSHOP is no exception in this respect. Although a point-by-point input of the transfer function measured experimentally is provided. The option of using experimental data can significantly increase the accuracy of calculations. Well, if there are no such data, then in the question of what will happen to the amplitude-frequency characteristics of the bass in various car models, Their Majesties Experience and Inspiration of the installer come first.

Crossover Module

This software allows you to calculate two- and three-way passive crossover systems from the first (6 dB / oct) to the fourth (24 dB / oct) order and a number of filter types: Bessel, Butterworth, Chebychev, Gaussian, Legendre, Linear-Phase and Linkwitz Riley.

As a result of the calculations, an electrical diagram of the crossover system selected by the user will appear on the monitor screen, indicating the exact characteristics of its elements.

As a result of the calculations, an electrical diagram of the crossover system selected by the user will appear on the monitor screen, indicating the exact characteristics of its elements.

Publication: cxem.net

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