Cardioid bass

Why cardioid bass?
Cardioid constructions
Resistance enclosure
Variable volume resistance enclosure
Mixed cardioid
Mixed cardioid of Alien2004

Why cardioid bass?

Response variations

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Juha Backman, "Low-frequency polar pattern control for improved in-room response", presented at the AES 115th Convention, New York, 2003 October 10-13, preprint 5867:

"As a summary of the results presented above for the room-speaker interaction it can be stated that cardioid source has more immunity against changes in source placement or room absorption in sparsely modal range. Below the lowest mode the cardioid speaker does not have any advantage over the monopole source, but both exhibit higher output and less source position dependence than the dipole speaker. These results indicate that creating a loudspeaker that has unidirectional polar pattern in the sparsely modal region and omnidirectional below the lowest mode represents a good compromise between low-frequency output capability and avoiding room coloration effects."
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The following polar pattern collection should explain why different cardioids are quite immune against changes in source position. Typically dipole has the weakest side wall reflection, but all other early horizontal reflections are weaker with cardioids. Supercardioid is probably more immune and flexible than basic cardioid.
DipCardSuper.png

Excessive immunity for placement may also be a disadvantage. Speaker placement closer/farther to front or side wall may not change bass level enough - like with dipoles and monopoles. Electrical tone control or different listening position may be needed if overall tonal balance is not satisfactory.

Cardioid cannot fight against reflections from back wall - of course. Single back wall reflection may cause deep dip because other reflections are too weak to fill it. See blue, magenta & orange curves in the following response set.
On-axis room modes will be exited if front and back walls are solid, and speakers are close to front wall. See the red curve. Rotated dipole may be better choice in that case if EQ or other room correction is not possible.
Cardioid_placement_immunity.png

Dipole woofer is not so sensitive for a single back wall reflection, but otherwise response variations are bigger:
Dipole_placement_immunity.png

Placement map for previous two measurements. Mic is at 220 cm towards arrow. Time window 500 ms, smoothing 1/24 oct.
Placement_immunity_map.PNG

Timing

Total and excess (in-room) group delay of cardioid woofer (2x18") at 240 cm, on-axis:
KS-2125_GD_240cm.png

Total and excess (in-room) group delay of dipole woofer (2x12") at 240 cm, on-axis. Dipole woofer is located 115 cm from front wall:
Revo_GD_240cm.png

Response variations cause also variations in minimum group delay. GD peaks of (bigger) cardioid are shorter compared to (smaller) dipole. However, these examples don't have measurable excess group delay (<250 Hz) caused by reflections from front wall and corner. Excess group delay 50-200 Hz is based on low-pass filtering of the woofer.
Lower attenuation of front wall/corner reflections or front-back unsymmetry (rear transmission line) or excessive rotation of dipole woofer could cause excess group delay when reflected sound pressure overrides direct. For example dipole woofer rotated 60° toe-in:
Revo_GD_240cm_rotated.png
Excess group delay jumps to the sky where direct sound loses the game. This could happen with any type of source, but the risk is reasonable small with unidirectional source which is rotated towards the listener.


Cardioid constructions

Traditional resistance enclosure

Resistance_Normal_Back.PNG Resistance_Normal_Mix.PNG Resistance_Normal_Side.PNG

Rear-ported is the most sensitive in this group. Trade-off's are loss of directivity at frequency band below membrane's directivity, and distance dependence - both caused by the distance/delay between front and rear radiators. It's applicable as a subwoofer, and polar pattern may spread to omni already at upper bass.
Side-ported is full-range application but it's not so sensitive as rear-ported.

Variable volume resistance enclosure

Resistance_Variable_Side.PNG

This is close to traditional side-ported resistance enclosure, but may have better polar pattern when properly tuned.

Horn loaded resistance enclosure

Resistance_Horn_Mix.PNG Resistance_Horn_Side.PNG Resistance_Horn_Mix2.PNG

Horn loaded resistance enclosure is more sensitive and kicking than other resistance boxes. Side-ported is more directive at high frequencies but not so sensitive as versions where resistance ports are located more back.

Mixed dipole and monopole

Hybrid-1.PNG Hybrid-1b.PNG Hybrid-2.PNG Hybrid-2b.PNG
Hybrid-3.PNG Hybrid-4.PNG Hybrid-5.PNG Hybrid-6.PNG

There are several ways to construct cardioid with mixed sources. Basic rule is that acoustic centers of dipole and monopole should be as close as possible. Any transmission lines on the back should be damped to avoid higher acoustical impedance -> higher sound pressure and curves in responses -> difficulties in adjustment. Vertical (and horizontal) symmetry is good but not must (see 4th example). Monopole should radiate back if it's not on both sides, to eliminate back wave as high frequencies as possible.

Main advantage of this design is adjustable polar pattern. Everything from dipole thru hyper- and super-cardioid to cardioid is possible by adjusting level of monopole. Even monopole is possible if dipole is muted and monopole is also on the front (5th example).
Mixed cardioid is not full-range application because of gap between dipole and monopole. However, bass range could be covered without severe problems. Volume requirement of monopole enclosure is one disadvantage. Also polar pattern is distance dependent - just like others (e.g. dipole) having delay between front and back radiators.

Dual monopole with a delay

Dual-mono-1.PNG Dual-mono-2.PNG Dual-mono-3.PNG

Two monopoles require more volume than other cardioid constructions. Delay requirement adds complexity and may require DSP. This is mainly sub-woofer application if delayed monopole radiator is located on the back. Placement closer to front baffle allows wider band but decreases sensitivity.

Dual-mono-4.PNG Dual-mono-5.PNG

Separate volumes are not must with sub-woofer application when phase difference between front and back radiator(s) is continuously close to reversed. This also decreases volume requirement radically. Filling would decrease inter-modulation at mid-range.

U-frame

U-frame.PNG

Advertised as cardioid at lower and mid-bass if properly damped. This would be simple but it's not even full bass range application if the woofer is at least 10".
Leaking (resistance ported) transmission line could improve polar pattern.

Horn

The most effective cardioid, but also largest construction.


Resistance enclosure

Dimensions are 360 x 680 x 420 mm and net volume is 2x 35 liters. Drivers are SLS-12" in parallel. Leaking gaps are 20 x 600 mm, 40 mm steps, 9 gaps per side. Leaking area is totally 200 % Sd. There's one layer of SPO Yhtymä Oy's Lido Ural synthetic felt carpet in the resistance ports. Thickness of felt carpet is about 2.5 mm. Felt is clued onto outer surface with PVAc and stapled with 11x6 mm. Inner surfaces of the box are covered with 70 mm E25 plastic foam. 2/3 of volume is filled with polyester.

Img_0865.jpg

Img_0858.jpg Img_0860.jpg

Enclosure drawing: KS-702.pdf

Frequency response

0-180º. Measuring distance 250 cm.
KS-702_FR_0-180deg_250cm_1p3oct_lg.png
Polar pattern is close to supercardioid which is flexible for positioning.
This is out-doors measurement. Wind was not totally calm and MLS length was only 64 kt. The result is not accurate and reliable at the low end.


Variable volume resistance enclosure

Dimensions are 330 x 580 x 430 mm and net volume is 2x 25 liters. Drivers are SLS-10" in parallel. Leaking gaps are 16 x 235 mm, 35 mm steps, 10 gaps per side. Leaking area is totally 112 % Sd. First gap is 110 mm from front edge. There's one layer of SPO Yhtymä Oy's Lido Ural synthetic felt carpet in the resistance ports. Thickness of felt carpet is about 2.5 mm. Felt is clued onto outer surface with PVAc and stapled with 11x6 mm. Box is filled with polyester.

IMG_0701.JPG

Secondary chamber is just an additional volume for lower frequencies which are low-passed into back chamber thru 50 mm (diameter) port. Back chamber material is 19 mm MDF. It's 100% filled with polyster to damp Helmholtz-resonance. There's also one layer of air filter above the port, on drivers' side.

Enclosure drawings for 2x12" and 2x10" drivers: MT-Resistanssi-3.pdf.

IMG_1220.JPG

Horizontal directivity (response compared to on-axis)

Measuring distance 200 cm.
MT-Resistanssi-3_200cm_DR.gif
Polar pattern is excellent and close to cardioid below upper midrange.
This is out-doors measurement. Wind wasn't totally calm, but MLS length was 512 kt. Anyway, the result is not very accurate and reliable at the low end.

Average directivity

Measuring distance 200 cm, 0-180º
MT-Resistanssi-3_200cm_DI.gif

Harmonic distortion

On-axis, signal 2.83 V.
MT-Resistanssi-3_THD_2830mV_drv.gif

Harmonic distortion at resistance port

Signal 2.83 V.
Non-linearity of port signal is higher at bass range. Midrange is more linear.
MT-Resistanssi-3_THD_2830mV_port.gif

Impedance response

Drivers in parallel.
MT-Resistanssi-3_ZR.gif

Note! This design is not recommended because of limited SPL capacity at low frequencies.


Mixed cardioid

This is quick and dirty mix of dipole and monopole. Construction is composed from miscellaneous boxes and baffles lying on the garage floor. Dimensions of dipole baffle are 440 x 560 x 38 mm. Dipole drivers are SLS-10". Closed monopole box is 277 x 560 x 325 mm (33.5 liters) with XLS-10". Gap between dipole and monopole is 110 mm. Target is to locate acoustical centers of dipole and monopole as close as possible to stretch controlled directivity up to midrange. Narrow gap increases acoustic impedance and causes response curves, which are better to damp with polyester.

IMG_0679.JPG IMG_0680.JPG

Near fied response of monopole is slightly rising. Required EQ is 2nd order shelving low-pass +10 dB @ 30 Hz.
Near field response of dipole is slightly falling. This dipole needs the same shelving low-pass as monopole. In addition, there is band stop filter -6 dB @ 300 Hz, Q=2.0, which compensates dipole peak and transmission line effects between the baffle and the box. Monopole and dipole (far field) levels are set equal at lower midrange. Near field responses of one dipole and monopole driver intersect at mid-bass:
Hybridi_kartiot_FR.gif

Frequency response of dipole

At 5 cm on-axis and 180 degrees, without polyester damping. Tight gap between baffle and box increases pressure from 140 to 500 Hz. Damping is needed for sure.
Hybridi_valin_resonanssi.gif

Frequency response of mixed system

0°, 90° and 180°. Measuring distance 70 cm:
Hybridi_vanua_70cm_FR.gif
The result at bass range is quite good. Also listening tells that it's decent cardioid bass.
All measurements are done in-doors at short distance; reliability is not very good.

This kind of mixed cardioid is easy to configure up to 150 Hz. Lower midrange is mission impossible; front baffle of monopole box is too large and dipole baffle is not fully open on the top. One problem was XLS-10" with 400 Hz maximum.


Mixed cardioid of Alien2004

Monopole is 60 liters closed with Peerless CSX-10" or XLS-10". Dimensions of enclosure are 520 x 320 x 520 mm (W x H x D). Box is filled with polyester.
Dipole driver is Peerless SLS-12". Dimensions of this "open horn" are 520 x 375 x 230 mm (W x H x D). Width of back baffle is 360 mm. Material is 24 mm plywood.

CARDSUB_BOX_FRONT.JPG CARDSUB_BOX_SIDE.JPG CARDSUB_BOX_BACK.JPG

Crossover and EQs

See Alien2004.

Frequency response

- Measuring distance 250 cm, time window 170 ms, smoothing 1/6 oct.
- drivers SLS-12" + CSX-10"
CARDSUB_MEAS_CSX.GIF
Polar pattern is slightly omnipolar at 250 cm. Level of monopole is too high for dimensioning at 250 cm. Result would be better closer to speaker where relative level of dipole part will increase at lower and mid-bass. This is working system but proto was not perfect with common power amplifier for both radiators.


Page updated 14.05.2012

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