JBL Technical Note - Vol.1, No.3 电路原理图.pdf

Technical Notes Volume 1, Number 3 Choosing JBL Low-Frequency Transducers A. INTRODUCTION When JBL converted its low-frequency transducers from Alnico-V magnets to SFG ferrite structures in 1978, we took the first steps in simplifying and rationalizing the line by reducing the number of models. There had been needless duplication and functional overlap in the line, and this often served more to confuse the user than help him. The present line-up of products includes models of the important performance classes in each size, and it is the intent of this Technical Note to explain to the professional user how LF transducers are chosen for specific jobs. We will also explain in detail the elements of construction and design of the products which optimize them for their particular jobs. B. THE BASIC MATRIX The primary JBL transducers are grouped into a three-way matrix, shown as follows: TABLE I. SENSITIVITY: MEDIUM HIGH MAXIMUM EFFICIENCY: 0.5% - 3% 2% - 6% 4% - 10% LINEARITY: HIGH MEDIUM LIMITED 460 mm (18) 2245H 2240H (E155-8) 380 mm (15) 2235H 2225H,J 2220H,J (E140-8) (E130-8) 300 mm (12) (128H) 2202H (E120-8) Those models enclosed in parentheses are not in the normal 2200-series line-up, but they fit structurally into the matrix in the slots indicated. The model 128H is used as the LF driver in the 4411 monitor system. C. SENSITIVITY VS. EFFICIENCY It is important to understand the difference between sensitivity and efficiency, and the various methods of rating sensitivity. The reference efficiency is a function of the electro-mechanical parameters of the loudspeaker. It represents the low frequency output within the piston band limit (where sound propagates omnidirectionally), and will usually correlate with the measured output in the 300 Hz to 400 Hz range for 460 mm (18 inch) to 300 mm (12 inch) transducers. Sensitivity relates to the measured output on-axis for a given input, which may or may not relate to the low-frequency efficiency level. JBL Professional Series low frequency loudspeakers are rated with a 1 watt input averaged from 100 Hz to 500 Hz, which correlates directly with their efficiency ratings. JBL E Series loudspeakers are rated with the same input but averaged from 500 Hz to 2.5 kHz, since they are primarily designed for full range use. If they were rated as the Professional Series are, for low frequency use, their sensitivities would be lower. The reference efficiency percentages give the most consistent method of comparison for E series, Professional Series, and competitive loudspeakers in low-frequency applications. These can be compared by taking 10 log of the efficiency ratio. D. DETAILED DESCRIPTION OF THE MATRIX In this section, we will ba describing in some detail the internal parts of a loudspeaker. For those who are not familiar with the construction details of a typical JBL LF transducer, we show in Figure 1 a labeled cutaway drawing of a typical JBL Profesional series transducer. 1. MEDIUM-SENSITIVITY TRANSDUCERS: a. Uses: Primary use is as LF transducers in monitor systems as well as sub-woofer designs for recording studios and motion picture theaters. For these purposes, long travel at low frequencies is essential, as is low distortion. Smooth HF response* is also a requirement for proper transition to mid- and high-frequency transducers in the 400-to-1 kHz range. b. Sensitivity and Power Handling: TABLE IL MODEL RATED SENSITIVITY (1 W, 1m) CONTINUOUS PROGRAM POWER REFERENCE EFFICIENCY 2245H 95 dB 600 WATTS 2.1% 2235H 93 dB 300 WATTS 1.3% 128H 91 dB 200 WATTS 0.86% c. Internal Design and Construction: VOICE COILS: Flat copper wire (See Table VI), extended over-hanging winding for maximum linearity (See Figure 2A). 100 mm (4) voice coil diameter used in larger models; 75 mm (3) voice coil diameter used in 128H. VOICE COIL FORMER: Made of aluminum, Kapton polyimide plastic, or a composite for effective heat sinking and mechanical integrity (See Table VII). SPIDER (Inner Suspension): All models use a special design which reduces dynamic offset and instability at high drive levels, resulting in unusually low distortion and a tight transient character. 3 CONE: Straight-sided and ribbed for stability. Fairly large mass for optimum balance of efficiency and bass output, and low distortion. Aquaplas (TM) coating used on 2245 and 128H to optimize stiffness and mass, mass ring on 2235 for desirable efficiency and bass balance. SURROUND (Outer Suspension): Half-roll of polyurethane foam for high compliance and long travel (See Figure 3A). DUST DOME: Made of similar felted material as cone for smoothest HF response. 2. HIGH-SENSITIVITY TRANSDUCERS: a. Uses: Primary use is in sound and music reinforcement, mounted in horn and direct radiator reflex-type enclosures. For these purposes, LF cone excursion has been limited and sensitivity increased, relative to the transduers of the medium sensitivity class, in order to get greater output in the 50 or 60 Hz range up to 800 Hz. b. Sensitivity and Power Handling: TABLE ill MODEL RATED SENSITIVITY (1 W, 1 m) CONTINUOUS PROGRAM POWER REFERENCE EFFICIENCY 2240H 98 dB 600 WATTS 5% 2225H.J 97 dB 400 WATTS 3.5% E140-8 100 dB 400 WATTS 4.9% 2202H 99 dB 300 WATTS 6.0% c. Internal Design and Construction: VOICE COILS: Flat copper wire, 100 mm (4) in diameter. Slightly over-hung in the magnetic gap (See Figure 2A) for proper balance of sensitivity and linearity. VOICE COIL FORMERS: Made of aluminum, Kapton polyimide plastic, or a combination, for effective heat resistance and sinking, and rigidity. SPIDER (Inner Suspension): All models use a special design which reduces dynamic offset and instability at high drive levels, resulting in unusually low distortion and a tight transient character. CONE: Straight-sided and ribbed for stability; however, total moving mass is less than in the case of medium efficiency transducers, without the added dampening compounds or mass rings. SURROUND (Outer Suspensioh): Multiple half-roll (See Figure 3B) provides controlled travel. Stiffness is optimized by depth of rolls, weave of cloth, and damping treatment. DUST DOME: Made of similar felted material as cone for smoothest HF response. The E140 is similar to the 2225, but with an aluminum dome and less voice coil overhang for slightly more midrange efficiency, peaked high-frequency output, and a punchy sound character. The E145 and 2234 are specialized transducers that combine high efficiency with high linearity. The E145 has a short copper coil moving in a very deep magnetic gap (Figure 2B) for maximum voice-coil control and excursion linearity. This design also provides maximum low-frequency output from a given enclosure size. The E145 can move as much air as a high efficiency 460 mm (18 inch) transducer. The 2234 is identical to the 2235 with the deletion of the mass ring. This raises the efficiency (mid-band sensitivity) for multiple woofer applications. The 2234 is used in the 4435 studio monitor. 3. MAXIMUM SENSITIVITY TRANSDUCERS: a. Uses: Primarily in music reinforcement for driving LF horn systems. For these applications, the range of linear travel has been restricted in a trade-off for greater sensitivity in the 50 or 60 Hz range up to 1200 Hz. 5 b. Sensitivity and Power Handling: TABLE IV MODEL RATED SENSITIVITY (1 W, 1 m) CONTINUOUS PROGRAM POWER REFERENCE EFFICIENCY E155-8 100 dB 600 WATTS 4.9% 2220H.J 101 dB 200 WATTS 8.7% E130-8 105 dB 300 WATTS 8.6% E120-8 103 dB 300 WATTS 8.6% c. Internal Design and Construction: VOICE COILS: Flat copper or aluminum wire, depending on transducer sensitivities. Voice coils either slightly underhang (2220) or overhang (E155) the top plate, or are of equal length (E120, E130) (See Figure 2C). Sensitivity is at a premium and is more important than linearity. VOICE COIL FORMERS: Aluminum and Kapton polyimide plastic composite, for effective heat sinking and rigidity. SPIDERS: Stiff, to keep resonance high. CONE: Curvilinear on E120, E130, and 2220 and shown in Figure 4C. Curvilinear cones are thin and exhibit controlled high-frequency break-up for extended output. One piece cone/ compliance construction provides increased top-end on E155, while straight sides and deep cone angle give rigidity. SURROUND (Outer Suspension): Paper, integral with cone on E155. Multiple half-roll on E120, E130, and 2220. DUST DOMES: Thin paper on 2220, aluminum on E120, E130, and E155 for extended HF response and minimum mass. The E130 is similar to the 2220, but with an aluminum dome, and an aluminum voice coil equal in length to the magnetic gap, for maximum high-frequency output. 6 TABLE VI. VOICE COIL WIRE MATERIAL CHARACTERISTICS USED ON ALUMINUM FRAGILE DIFFICULT TO SOLDER LIGHT HIGHEST EFFICIENCY HIGH RESISTANCE - LOWER INDUCTANCE INCREASES HIGH- FREQUENCY OUTPUT CONE TRANSDUCERS FOR MAXIMUM EFFICIENCY AND HIGH-FREQUENCY OUTPUT (SENSITIVITY) HORN COMPRESSION DRIVERS COPPER RUGGED - HIGHEST .POWER HANDLING HEAVY - ADDITIONAL MASS REDUCES EFFICIENCY LOW RESISTANCE - MORE TURNS INCREASES Bl FACTOR (MOTOR STRENGTH) LOW FREQUENCY TRANSDUCERS FOR RUGGEDNESS, RESISTANCE TO THERMAL COMPRESSION EFFECTS. RIBBON WIRE VOICE COILS USE FLAT MILLED WIRE FOR GREATER WIRE DENSITY WITHIN THE AVAILABLE MAGNETIC GAP SPACE, HENCE GREATER EFFICIENCY. TABLE VII. VOICE COIL FORM MATERIALS SUPPORT TUBE, BOBBIN, SUPPORTS VOICE COIL WITHIN MAGNETIC GAP AND CONNECTS COIL TO CONE. MUST MAINTAIN CONCENTRICITY (ROUNDNESS) AND STRENGTH AND AT HIGH TEMPERATURES AND FORCES. NAME TYPE CHARACTERISTICS KRAFT PAPER BOND PAPER WOOD PULP RAG, CLOTH LOW POWER DESIGNS LIGHT, BUT FRAGILE NOMEX (DuPONT TM) HIGH TEMPERATURE NYLON PAPER (POLYAMIDE) TEMPERATURE RESISTANCE; GOOD ADHESION PROPERTIES FOR BONDING (GLUING) KAPTON (DuPONT TM) PLASTIC FILM (POLYIMIDE) INSULATOR. HIGHER TEMPERATURE THAN NOMEX, HIGH STIFFNESS, DIFFICULT TO GLUE. ALUMINUM THIN FOIL FILM. VARIOUS THICKNESSES STIFF, RIGID. CONDUCTS HEAT FOR ADDITIONAL VOICE COIL HEAT SINKING. CAN ACT AS SHORTED TURN ON MOVING ASSEMBLY, CREATING EDDY CURRENTS WHICH ADD DAMPING TO REDUCE BASS, HIGH END. 7 FIGURE 1. CUTAWAY DRAWING OF TYPICAL JBL LOW FREQUENCY TRANSDUCER /SURROUND, COMPLIANCE MOUNTING GASKET. CENTERING SPIDER CENTER POLE PIECE FOAM VENT PLUG OR SCREEN FOILCAL LABEL 8 CENTER CAP, DUST DOME CONE FRAME INPUT TERMINALS INPUT TINSEL LEADS PROTECTIVE MAGNET TIRE BACK PLATE TOP PLATE REAR VENT FRAME MOUNTING SCREWS VOICE COIL FORMER, TUBE MAGNET VOICE COIL SHORTING RING FIGURE 2. THREE VOICE COIL/MAGNETIC GAP TYPES CROSS-SECTIONAL VIEWS OF LOUDSPEAKER MOTOR VOICE COIL AND MAGNET STRUCTURE (2A) LONG COIL/SHORT GAP Coil moves outside magnetic gap, overhang provides linear travel. Requires precise choice of suspension elements to guarantee linearity. Both magnetic gap and voice coil depths are scaled up in 460 mm (18 inch) transducers. Used on: Slight overhang: 2202, E140 Moderage overhang: 2225, 2240, E155 Large overhang: 2235, 2245, 128H, 2234 (2B) SHORT COIL/DEEP GAP Coil moves totally within gap for controlled response. Large heat sink, but expensive due to thick metal, large magnet required. Used on: 2220, slight gap overhang; E145, very large overhang. (2C) EQUAL HEIGHT COIL AND GAP Maximum efficiency; short, light coil completely within maximum flux magnetic gap. Minimal low-frequency linearity, only what is provided by fringe flux. Used on: E110, E120, E130. 9 FIGURE 3. DIFFERENT TYPES OF SURROUNDS/COMPLIANCES 3A. HALF-ROLL OF POLYURETHANE FOAM Low stiffness (high compliance) for long travel, but requires precise choice of centering spider for controlled linearity. Used on: 2235, 2245, 128H, 2234 3B. DOUBLE HALF-ROLL CLOTH Shape of rolls can precisely tune stiffness characteristic. Used on: 2202, 2220, 2225, 2240, E120, E130, E140, E145 3C. MULTIPLE-ROLL ACCORDIAN PLEAT Long travel, but prone to rim-resonance response dip problems. Used on: 2213 (4312 woofer) 3D. ONE PIECE CONE/COMPLIANCE WITH TREATED EDGE Stiff, non-linear suspension, provides HF resonance peak Used on: E155 10 FIGURE 4. DIFFERENT CONE SHAPES 4A. STRAIGHT-SIDED CONE WITH REINFORCING RIBS Attempts to simulate theoretical piston action Used on: 2225, 2235, 2240, 2245, 128H, 2234, E140 4B. STRAIGHT-SIDED SMOOTH CONE WITH DEEP CONE ANGLE Uses deep cone angle to maintain rigidity. Used on: E145, E155 4C. CURVILINEAR CONE Cone flexing at mid-points provides greater high-frequency output. Used on: 2202, 2220, E110, E120, E130 li E COMPARATIVE FREQUENCY RESPONSE: The three families of LF devices are normally mounted in different enclosure types; however, it is instructive to look at the frequency response of the three types mounted under similar conditions. Figure 5 shows the 2235, 2225, and 2220 mounted in a 280 liter (10 cubic foot) sealed enclosure. The curves were run in half space; that is, the enclosure front was flush with a large baffle surface. At low frequencies, below about 60 Hz, all three drivers exhibit similar response. Above about 80 Hz, the three drivers diverge, and the 2235, the lowest sensitivity model of the three, levels off at a mid-band plateau of 93 dB. The 2225 continues to climb and does not level off until about 400 Hz at 97 dB. The 2220 levels off hardly at all, but we can see something of a plateau at about 101 dB in the 400 Hz range. The mid-band levels of the three drivers are a direct result of their respective efficiencies, as is the reduction of the LF bandwidth of the curves. Note the HF response of the three drivers. The 2235 begins to roll off at 1 kHz, and the slight peaks and dips in its roll-off characteristics are due to resonances in the polyurethane surround. Because of its lower mass, the 2225 reaches about 1600 Hz before it begins to roll off, and it exhibits the smoothest roll-off characteristic of the group, due to its straight, ribbed cone and stiff, multiple half-roll surround. The 2220 exhibits the most extended response of the group, going out to 4 kHz before it rolls off sharply. This is due to break-up modes in its curvilinear cone. Remember that each of the three transducer types is designed for a particular kind of enclosure and application, and that its sensitivity and bandwidth have been optimized for those applications. FIGURE 5. RELATIVE RESPONSES OF JBL MODELS 2220 (TOP CURVE), 2225 (MIDDLE CURVE), AND 2235 (BOTTOM CURVE) FREQUENCY, Hz dB SPL 12 F. THIELE-SMALL PARAMETERS While the high-frequency performance of a LF transducer is often unpredictable due to break-up modes and surround resonances, the precise nature of LF response can usually be accurately plotted beforehand through the use of the Thiele-Small parameters. All JBL ported enclosures have been designed using these parameters and, where possible, it is recommended that sound contractors use them. For the convenience of those who have a need for desi