The TLb -- Radiation Response

A Bipolar Transmission Line Project -- the TL_B
Section 3: System Radiation Response

Fig. 3.0 TL_B Radiation Pattern
The two Vifa P13 drivers are mounted in an push-push arrangement and driven in parallel, in phase. This results in a 4 pi steradian polar pattern that is detailed in Fig. 3.1. In effect this results in a point source radiation pattern from 50 Hz up to about 2-3 kHz.

The dipole pattern results in phase cancellation at 90° and the conventional wisdom is that this is desirable since it reduces the side wall bounce. However the Haas reverberant field data shows that the side wall reflections, if they are in the 20 ms range, enhance the spacial definition.

A comparison with the monopole radiation pattern was done. While a dipole comparison would have been desirable, it will have to await the completion of a seperate study.

Polar Response Data

Fig. 3.2 Vertical TL_B pattern
Horizontal response

A frontal hemisphere is shown in Fig.3.1, but the back hemisphere is the same with the difference that the back tweeter is not time aligned. The polar response from 60 Hz to about 2 kHz is circular and begins to narrow to a 30° bipole pattern @ 10 kHz. The interesting observation is that the HF pattern is wider in the monopole configuration.

The monopole data is green/bipole data is blue.

click images for larger picture

Fig. 3.1 Horizontal TL_B patterns

Vertical response

The vertical response of the TL_B is more complex. The LF response up to about 200 Hz shows the effect of the TL terminus gain and the floor coupling of about +6dB. For the HF the pattern gradually narrows and is shifted to about a +30° axis upward and narrowing to approximately a 40° lobe.

Since these measurements were very difficult to do, only the frontal pattern was characterized.

Impedance Data

To gain an understanding of the characteristics of the TL_B it is necessary to understand the nominal TL's impedance characteristics.

Fig. 3.3 TL36 0.9144 meter line unstuffed line
Impedance for D_tr=1,unstuffed line, note that mrk#1 is at driver F_s and that mrk#2 corresponds to TL line F_r, fundamental resonance displaced by phase shift. Note the presence of line harmonics which are attenuated as the stuffing density is increased from the air's density 0.0745 lbs/cu ft. ie D_tr=1.

Fig. 3.4 TL36 0.9144 meter line under stuffed line
Impedance for D_tr=6.7 Dacron HoloFill fiber. Packing density would fall between the fiberglass and the wool reference curves as given by Bradbury. Note that F_s peak is reduced but more importantly that the F_r peak has shifted in frequency downward. This is the effect of the change in the air velocity in the fiber. Note also the flattening of the phase curve

Fig. 3.5 TL36 0.9144 meter line optimally stuffed line
Impedance for D_tr=9.4. Note that the F_s and F_r peaks are almost equal. This can be used as an approximation of optimum stuffing density for the specific line. The actual value would be dependent on the driver slope attenuation at F_r. A higher value would permit overstuffing the line thus maximizing the attenuation of the harmonics.

For a monopole TL the optimum D_t impedance peak is about 20 ohms.

The TL_B push-push woofer loads a TL line length of 0.944 meters. Note that the impedance is almost flat at 5 ohms and that the phase data is equally good. In contrast to the monopole TL configuration the impedance is reduced from about 20 ohm peak to a minimal energy storage.

The consequences of this can be examined in the pulse response simulations of the closed box vs that of the vented box. The closed box analogy is justified by the TL's near field response which is equal to that of the critically damped closed box.

Fig. 3.6 TL_B Impedance Data

[ Contents | Introduction | Methodology | Radiation Response | Transient Response | Frequency Response | Construction ]

A Bipolar Transmission Line Project -- the TL_B Section 3: System Radiation Response
Fig. 3.0 TL_B Radiation Pattern	The two Vifa P13 drivers are mounted in an push-push arrangement and driven in parallel, in phase. This results in a 4 pi steradian polar pattern that is detailed in Fig. 3.1. In effect this results in a point source radiation pattern from 50 Hz up to about 2-3 kHz. The dipole pattern results in phase cancellation at 90° and the conventional wisdom is that this is desirable since it reduces the side wall bounce. However the Haas reverberant field data shows that the side wall reflections, if they are in the 20 ms range, enhance the spacial definition. A comparison with the monopole radiation pattern was done. While a dipole comparison would have been desirable, it will have to await the completion of a seperate study.
Polar Response Data
Fig. 3.2 Vertical TL_B pattern	Horizontal response A frontal hemisphere is shown in Fig.3.1, but the back hemisphere is the same with the difference that the back tweeter is not time aligned. The polar response from 60 Hz to about 2 kHz is circular and begins to narrow to a 30° bipole pattern @ 10 kHz. The interesting observation is that the HF pattern is wider in the monopole configuration. The monopole data is green/bipole data is blue. click images for larger picture	Fig. 3.1 Horizontal TL_B patterns
	Vertical response The vertical response of the TL_B is more complex. The LF response up to about 200 Hz shows the effect of the TL terminus gain and the floor coupling of about +6dB. For the HF the pattern gradually narrows and is shifted to about a +30° axis upward and narrowing to approximately a 40° lobe. Since these measurements were very difficult to do, only the frontal pattern was characterized.
Impedance Data To gain an understanding of the characteristics of the TL_B it is necessary to understand the nominal TL's impedance characteristics.
Fig. 3.3 TL36 0.9144 meter line unstuffed line	Impedance for D_tr=1,unstuffed line, note that mrk#1 is at driver F_s and that mrk#2 corresponds to TL line F_r, fundamental resonance displaced by phase shift. Note the presence of line harmonics which are attenuated as the stuffing density is increased from the air's density 0.0745 lbs/cu ft. ie D_tr=1.
Fig. 3.4 TL36 0.9144 meter line under stuffed line	Impedance for D_tr=6.7 Dacron HoloFill fiber. Packing density would fall between the fiberglass and the wool reference curves as given by Bradbury. Note that F_s peak is reduced but more importantly that the F_r peak has shifted in frequency downward. This is the effect of the change in the air velocity in the fiber. Note also the flattening of the phase curve
Fig. 3.5 TL36 0.9144 meter line optimally stuffed line	Impedance for D_tr=9.4. Note that the F_s and F_r peaks are almost equal. This can be used as an approximation of optimum stuffing density for the specific line. The actual value would be dependent on the driver slope attenuation at F_r. A higher value would permit overstuffing the line thus maximizing the attenuation of the harmonics. For a monopole TL the optimum D_t impedance peak is about 20 ohms.
The TL_B push-push woofer loads a TL line length of 0.944 meters. Note that the impedance is almost flat at 5 ohms and that the phase data is equally good. In contrast to the monopole TL configuration the impedance is reduced from about 20 ohm peak to a minimal energy storage. The consequences of this can be examined in the pulse response simulations of the closed box vs that of the vented box. The closed box analogy is justified by the TL's near field response which is equal to that of the critically damped closed box.		Fig. 3.6 TL_B Impedance Data
[ Contents \| Introduction \| Methodology \| Radiation Response \| Transient Response \| Frequency Response \| Construction ]