Passive Radiator Excursion

[HTRookie]

Hi,
I recently completed a dual passive radiator sub, closely based on Adrian Mack's design, using a 12" Peerless XLS driver with two 12" Peerless passive radiators. The PR's are the 425 gram versions, with an additional 275 gms. of weight added. In use, the active driver seems to move noticeably more than the passive cones. Is this normal, or have I done something wrong?

Thanks for your help!

HTR

[Daveis]

I also built a dual 12" sub using TC sounds drivers, but using the same Peerless XLS passive radiators.

I'd say that could be normal depending on what material you are playing.

If possible put a sine wave sweep tone through it, but remember your ears get less sensitive as the frequencies get very low. So it could sound like it isn't flat but it really is. Also, your room can make 10-20db peaks/dips in frequency response.

Soundeasy or Praxis are some software that might help but they aren't cheap.

The problem I experienced was a flapping sound coming from one of the passive radiators.

If your bass is ultra weak then possibly your box volume needs to be reduced.

[Cal Weldon]

Hello Bruce, nice to hear of your latest spawn. Modesty is definitely not in order, we'd like to see a pic or two please.

Assuming you've already done a pressure check and different room placements, I think you know PR's are a favourite of mine and here's some things I've thought about translated to "after beer speak".

The drones are reactive so the movement is likely to be 50% if you have matched mass and cone areas, compliance and perfectly sealed cabinets.

A heavier drone has a hard time moving so you lose the higher frequencies.

700 grams means you are reinforcing the really low end.

There's a lot of high excursion info in the 70 - 125 hz area so the big heavy passives can't just can't keep up while the woofer appears to be merrily flapping away. If you are lacking warmth, remove the extra weight and listen again. What shows up on a graph doesn't always make your ears happy.

How are things? long time no speak.

[Ron E]

If you post the parameters of both woofers and passives, and the box size you use, someone could model it and see how simulations match up with your reality.

At very low frequencies, it will be as Cal says, but the passive does more than just match the woofer excursion in the area around tuning. Above tuning (more than say 2-3x tuning) the passive won't move much at all.

[HTRookie]

Hello again,

Thanks for the information. I have attached a photo of my sub. It is ~50 litres internal volume, constructed from dual layers of 3/4" MDF, with a vertical "I beam" centre brace laminated froom hardwood and Baltic Birch plywood. West System epoxy is used along with polyurethane adhesive. The amplifier is an Adire ADA1200 (gain turned down!). The steel circles you see on the cabinet are used to secure the grille frames, which have rare-earth magnets epoxied in them.

I know it will not match the bigger 15" subs out there, but I hope it will do a pretty good job.

I appreciate your assistance!

HTR

[HTRookie]

Here are the photos, sorry :<(.....

[HTRookie]

Here are the photos, sorry :<(.....


Peerless 830500 12" XLS Driver


Electrical Data

Nominal impedance
Zn
8
ohm

Minimum impedance
Zmin
4.7
ohm

Maximum impedance
Zo
64.2
ohm

DC resistance
Re
3.5
ohm

Voice coil inductance
Le
4.2
mH

Capacitor in series with x ohm
Cc
38
uF

T-S Parameters

Resonance Frequency
fs
18.1
Hz

Mechanical Q factor
Qms
3.7


Electrical Q factor
Qes
0.21


Total Q factor
Qts
0.2


Force factor
Bl
17.6
Tm

Mechanical resistance
Rms
5.12
Kg/s

Moving mass
Mms
166.3
g

Suspension compliance
Cms
0.46
mm/N

Effective cone diameter
D
24.4
cm

Effective piston area
Sd
466
cm2

Equivalent volume
Vas
139.2
ltrs

Sensitivity (2.83V/1m)

90.6
dB

Ratio BL/�ã(Re)

90


Ratio fs/Qts
F
--



Power handling

100h RMS noise test (IEC)
--
W

Long-term Max Power (IEC 18.3)
--
W

Max linear SPL (rms) @ power
--
dB/W

Short Term Max power (IEC 18.2)
--
W

Voice Coil and Magnet Parameters

Voice coil diameter
51
mm

Voice coil height
33
mm

Voice coil layers
4


Height of the gap
8
mm

Linear excursion +/-
--
mm

Max mech. excursion +/-
--
mm

Flux density of gap
1.04
mWb

Total useful flux
2.5
mWb

Diameter of magnet
147
mm

Height of magnet
35
mm

Weight of magnet
2.42
Kg
______________________________________

Peerless 830548 12" Passive Radiator (2 used)

TS Parameters
Resonance Frequency fs 10.4 Hz
Mechanical Q factor QMS 15.2
Mechanical Resistance Rms 1.8 Kg/s
Moving Mass Mms 425 g
Suspension Compliance Cms 0.55 mm/N
Effective Piston Area sd 466 cm^2
Equivalent Volume Vas 169.6 ltrs

[Ron E]

model attached. I didn't enter Qp correctly but the effect should be small. This is with the full input of a 200W/8 ohm amp or a 400W/ 4ohm amp. You are just reaching driver excursion capability above tuning, but below tuning you are in the danger zone. You might want to put a 20Hz highpass if your amp doesn't have one. It depends on how much power you have on tap. - to reduce excursion by half at any frequency, you must reduce power by 4 times.

Note that except around tuning, the PR excursion is less than the woofer - a lot less.

edit - nice looking sub, BTW.

[HTRookie]

Thanks Ron for your efforts. The box is sealed, 54 litres gross, approx. 50 litres net internal volume. I am using the Adire ADA1200- 1200 watt (4 Ohm) plate amp. I am reviewing the manual as the amp does have a number of filter features on it. As Cal said, perhaps I have too much weight on the passive radiators- did your modelling indicate that?

I glued the weights on using a special elastomeric adhesive, but I could get them off if need be with some effort. I just followed Adrian Mack's design, as it seemed a sound one and was well thought of in the DIY ranks.

Thanks to all for your feedback



HTR

[inksplat]

Over half a cycle you have one active cone moving in therefore the two remaining passive cones will move out half as much. It's simple piston area maths imho