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MODEL VERIFICATION
The acoustic response measured in nearfield is superimposed over the model prediction below. The measured response has been normalized to the maximum output level measured below 100Hz. Note that the measured frequency response drop-off, as frequency increases, is artifact of the measurement method[2]. For this application, the response below about 50Hz is of interest. Ql =10.
The acoustic response measured in nearfield is superimposed over the model prediction below. The measured response has been normalized to the maximum output level measured below 100Hz. Note that the measured frequency response drop-off, as frequency increases, is artifact of the measurement method[2]. For this application, the response below about 50Hz is of interest. Ql =10.
ENCLOSURE
Photographs of the enclosure used in this study are shown below. The cabinet panels are cut from 1.5" thick MDF sheet. Bracing is extensive. Cabinet is veneered in American Walnut, oil finish.
Photographs of the enclosure used in this study are shown below. The cabinet panels are cut from 1.5" thick MDF sheet. Bracing is extensive. Cabinet is veneered in American Walnut, oil finish.
ACOUSTIC RESPONSE
The acoustic response, using the near-field method described by Keele [2], is shown below. The response was measured using CLIO FW (v10.31). Green diaphragm near-field, purple ducted port near-field, red phasor summed response.
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The impedance modulus, Z, of the system is shown below. The measured Fb of 19.2Hz is somewhat lower than design intent and is associated with a longer duct length than is required for an Fb of 20Hz. Measured responses for area of the ducted port, Sv, equal to 45.5in^2 with and end-corrected length, Lv, equal to 13.75in. For preliminary testing, a duct length approximately 5% longer than prediction recommended. The duct length can be trimmed to adjust.
The acoustic response, using the near-field method described by Keele [2], is shown below. The response was measured using CLIO FW (v10.31). Green diaphragm near-field, purple ducted port near-field, red phasor summed response.
The impedance modulus, Z, of the system is shown below. The measured Fb of 19.2Hz is somewhat lower than design intent and is associated with a longer duct length than is required for an Fb of 20Hz. Measured responses for area of the ducted port, Sv, equal to 45.5in^2 with and end-corrected length, Lv, equal to 13.75in. For preliminary testing, a duct length approximately 5% longer than prediction recommended. The duct length can be trimmed to adjust.
ENCLOSURE MODEL FROM
SPECIFICATIONS
Modeling begins by first considering the driver small-signal parameters [1] as specified by the manufacturer. In our situation, a large extensively braced enclosure with Vb equal to 0.48m^3 (16.95 ft^3) was available for experimentation. The large size allows for adjustments to Vb (additional bracing or adding solid volumes such as cellophane wrapped bricks).
A ducted port enclosure, with tuning ratio equal to unity (Fb=20Hz) was modeled across a range of enclosure Quality factors (5-20) using the Thiele-Small vented-box Excel spreadsheet. The predicted responses, Eq. 58 in [1], are shown below.
The plot shows that F-3 will range from about 22 to 24Hz over a reasonable range of enclosure Quality factors for the conditions specified.
The effect of variations in Qes, for fixed Vas (=25.9ft^3), Qms (=2.90) and enclosure Q (=10), are considered and the results shown below.
The plot shows that magnetic damping lower than about 0.45 will produce a reasonably "flat" magnitude response for the conditions specified.
Our analysis of the small-signal parameters measured at 3.5Vrms demonstrate that specification Qes and Qms are somewhat larger than reported by the manufacturer. Here, the magnitude response for the enclosure conditions specified is shown at the Qes, Qms and Fs values measured.
Modeling begins by first considering the driver small-signal parameters [1] as specified by the manufacturer. In our situation, a large extensively braced enclosure with Vb equal to 0.48m^3 (16.95 ft^3) was available for experimentation. The large size allows for adjustments to Vb (additional bracing or adding solid volumes such as cellophane wrapped bricks).
A ducted port enclosure, with tuning ratio equal to unity (Fb=20Hz) was modeled across a range of enclosure Quality factors (5-20) using the Thiele-Small vented-box Excel spreadsheet. The predicted responses, Eq. 58 in [1], are shown below.
The plot shows that F-3 will range from about 22 to 24Hz over a reasonable range of enclosure Quality factors for the conditions specified.
The effect of variations in Qes, for fixed Vas (=25.9ft^3), Qms (=2.90) and enclosure Q (=10), are considered and the results shown below.
The plot shows that magnetic damping lower than about 0.45 will produce a reasonably "flat" magnitude response for the conditions specified.
Our analysis of the small-signal parameters measured at 3.5Vrms demonstrate that specification Qes and Qms are somewhat larger than reported by the manufacturer. Here, the magnitude response for the enclosure conditions specified is shown at the Qes, Qms and Fs values measured.
REFERENCES
1. R. Small, J. Audio Eng. Soc., v.21 no. 7, pp. 549-554, 1973.
2. D. B. Keele, J. Audio Eng. Soc., v.22, no. 3, pp. 154-162, 1974.
1. R. Small, J. Audio Eng. Soc., v.21 no. 7, pp. 549-554, 1973.
2. D. B. Keele, J. Audio Eng. Soc., v.22, no. 3, pp. 154-162, 1974.
BASKET AND VOICE COIL ASSEMBLY
The photos below (click on each image) are the 6174-8 with the magnetic structure removed to reveal voice coil, former and dust cap. The former appears to be composite and is identified as Nomex in the literature. The voice coil is in-wound thus will expand into the inside diameter of the former when it gets hot. The top-plate of the magnetic structure is located by the counter-bore machined in the basket. Note the three tapped holes, these accept the cap screws that sock the magnetic assembly to the basket.
The photos below (click on each image) are the 6174-8 with the magnetic structure removed to reveal voice coil, former and dust cap. The former appears to be composite and is identified as Nomex in the literature. The voice coil is in-wound thus will expand into the inside diameter of the former when it gets hot. The top-plate of the magnetic structure is located by the counter-bore machined in the basket. Note the three tapped holes, these accept the cap screws that sock the magnetic assembly to the basket.
The magnitudes of Qes, Qms and Qts are determined by using a constant voltage approach. The parameters required to determine the Quality factors are provided in the EXCEL spreadsheet developed specifically for this test. The parameters were measured at two drive voltages, 3.0 and 3.5V. The test gear used to make the measurements:
FLUKE 8012A digital multi-meter > voltage drop across source resistor
LEADER LMV-182A AC analog meter > voltage applied to driver
FLUKE 1910A digital multi-counter > frequency counter
KRON-HITE 6200A phase meter > phase angle
B&K 4010A function generator > sine wave generator
McIntosh MC2105 amplifier > voltage source
Source resistor > 10W non-inductive (7.45 Ohm)
The tables below (click on each) are the results of the 3.5Vrms (left) and 3.0Vrms (right) measurements.
As evidence in the measurements, the magnitude of Qes measured is essentially the same at each drive level (0.46). The value reported here is somewhat higher than the magnitude specified by the supplier for this driver (0.38).
MAGNET ASSEMBLY
The magnetic assembly was removed from the basket and photographed. The loudspeaker was purchased new from a McCauley distributor. Note factory application of adhesive (white material).
McCAULEY SOUND 18" LOW FREQUENCY TRANSDUCER MODEL 6174-8
DESCRIPTION (FROM McCAULEY SOUND WEBSITE)
The 6174 18" transducer is an ultra-high performance woofer designed to reproduce low and extended low frequency material at very high SPL's. The 6174's twin coupled magnet structures increase the power handling to 800w RMS / 1600w PROGRAM, while the system's convective cooling chambers actively dissipate heat. The 6174 offers over 2" of controlled, peak to peak cone excursion.
The unit examined here (S/N129911, Motor S/N14086) was purchased new in 2004. The unit has seen light use in a residential environment consisting of music and home theater sound (no touring, no portable sound, no sound reinforcement, no musical instrument applications). Supplier specifications for the 6174 are here.
DESCRIPTION (FROM McCAULEY SOUND WEBSITE)
The 6174 18" transducer is an ultra-high performance woofer designed to reproduce low and extended low frequency material at very high SPL's. The 6174's twin coupled magnet structures increase the power handling to 800w RMS / 1600w PROGRAM, while the system's convective cooling chambers actively dissipate heat. The 6174 offers over 2" of controlled, peak to peak cone excursion.
The unit examined here (S/N129911, Motor S/N14086) was purchased new in 2004. The unit has seen light use in a residential environment consisting of music and home theater sound (no touring, no portable sound, no sound reinforcement, no musical instrument applications). Supplier specifications for the 6174 are here.