To PR or not to PR in a compact portable three-way.

I am setting off on a new build of a compact three-way mono battery powered dsp controlled speaker.
I have 2x SB Acoustics SB15NRXC30-4 for bass duty. They will be placed one on the front and one on the rear. Push-Push to cancel vibrations.
2x Accuton P173 passive radiators.
Scanspeak 10F/4424G for midrange in a separate chamber.
Scanspeak D2004/60200 for highs.
It will be powered by a 6S Li-ion battery and a Zoudio AIO438 4x 38W dsp Bluetooth amp.
The internal volume without drivers will be circa 10 litres.

Before I begin I want to check the design principle, and I have come across a thread questioning the need for the passives. One person suggested that they would only boost around the tuning level 1db but would help with excursion and power handling.

I have all drivers in hand and a pair of the passives from a previous project.

As proposed, the passives are going to be placed on the sides of the cabinet (a square tower).



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Instead, suspension resonance.

Dave, this is a bit of a false equivalency IMO.

The port self-resonance adds higher harmonics and, at higher velocities, chuffing noises. These are external noises that combine with the driver output and can be audible (not in a good way).

A PR has a mechanical resonance much like a driver has. AFAIK this does not generate any harmonics or other objectionable noises. It only causes a dip in the response below tuning, steepening the rolloff a bit.

Am I missing something about PRs?
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Ok thanks!
It depends on your lower extension and max spl requirements.
PRs reduce driver excursion at low frequencies (and eventual resulting distorsion), potentially allowing more output.
Simulation programs will show max spl vs frequency for both options.
A port or PR extends the bass response using its resonance with the box air. The cone excursion is also minimized at the tuning frequency. With limited input power, these systems permit more bass compared to a sealed box. With only 38W you cannot boost the bass much before all your power is used up... Have you done (much) speaker design in the past?
Hmm, the driver 'wants' a ~56.5 Hz max flat tuning, but the PR's Fs = driver Fs, so ideally it needs some EQ in a bigger box otherwise it's a sealed box with a bit of over excursion protection as previously noted, ergo in this app a waste of $$ nowadays with cheap DSP if tuned correctly.
Thanks gents.

I haven't done any speaker designing previously, but I have built several, and have been reading forum posts here for the last 15+ years. I know enough to know that I don't know enough.

Max spl will be whatever the amplifiers will allow. The device will be used indoors and outside also. The priority will be normal listening levels rather than party use, but party use will inevitably occur.

So what is the consensus given my parameters? Ditch the PRs and just use EQ to get the best out of the driver/box limitations?
@SpeakerBob He already has the PRs in hand, as explained in the first post.

@dublin78 Box modeling. Have you done that before when building speakers? You can use the free program UniBox if you have Excel. Model the drivers in the desired box volume with and without the PR and adjust PR mass to change tuning, etc. What are the two best scenarios that you find? If you need help with modeling, post an ask here.


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just as a rough example I did a maximum SPL simulation in hornresp with the drivers in closed box and with PRs.
each simulation uses two drivers in parallel, which for this simulation is the same as using parallel amps+drivers.
box volumes are totals for both drivers.
the PR tuning to ~ 50 Hz is just a first shot, will need further optimization.

the blue lines indicate maximum SPL is amplifier output voltage limited (using 38 W @4 Ohm = 12,3 V max)
the red lines indicate maximum SPL is excursion limited (obviously independent from eventual equalization).

the graph with more output between 40 and 200 Hz is of course the PR version.

max SPL in the excursion limited area for closed box will not change with box volume change, thus I just took arbitrary Qtc = 0,6

tuning the PR box lower will increase lower bass output while decreasing upper bass output. this may be sensible, using some bass eqalizing if needed.

second graph is the group delay. apologies for the strange graphics, i had to hugely adjust the y-axis to match!
as to be expected the PR resonator creates big group delays at the resonance.
again - it might be sensible to tune the PR lower to get the group delay peak out of useful frequency range.

(peaks and dips in the upper frequency range of both graphs are enclosure resonances).

happy simulating!


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If a reflex or passive is tune to 50 Hz
No electrical reproduction all port or passive.

50 Hz is typical boost frequency
So the port or passive is in the worst place it could be.

Tune lower or .8 Qts sealed for boost.

Everyone freaks out with extended bass shelfs.
When superior for EQ boosting.
Thanks again for all of your replies and help.
I will have a go at modelling tomorrow. There's no time today.
It seems (to me) that there are advantages to using the PRs if they are tuned lower. Shall I start at 40hz?
Which free modelling software is simplest to learn? I have Excel.
Rob: I had considered two extra bass drivers instead of PRs, but I already have 4ohm units and also limited amp channels. Two 8ohm drivers in parallel give me 4ohms and +3dB? But the 8ohm versions are 3dB down, I think.
I can/am happy to sell my drivers and PRs if a better solution arises.