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Calculating High Pass Filter & Input Power for Subwoofer Below Recommended HPF S

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Ben Sparkes:
I previously posted this on Reddit over at r/livesound and r/soundsystem, however someone suggested posting it here. Sorry if a thread on this topic already exists somewhere on the forum; I wasn't able to find one from my searching. The original Reddit thread is here.

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Hi all,

I have a stack of 4x FLH subwoofers known as "Super Bass Horns". They are loaded with RCF LF18N401, a 1200w 8ohm driver, which I power from a Lab Gruppen FP14000 (2 on each channel in parallel). On the amplifier, I set the peak voltage limiter to 138vac (1200w @ 8o gives an RMS rating of 97.9vac, and then multiplying by 1.41 (3db crest factor) to find the peak rating -- this is as per the amp manual's recommendation). The gain structure of the crossover/dsp is then configured according to this voltage limiter using various sine waves (ie. 30, 40, 50hz). I'm unsure if this method of gain structuring is the correct way to do things (since this does not account for the 3-6+ dB crest factor of normal music); so would appreciate some input here.
Following the recommended settings for this enclosure and subwoofer, I have the HPF set to 32hz 24db/oct and the LPF at 90hz 24db/oct.
This setup gives me great output under normal operating conditions.

I was recently reading on Meta Acoustics' website for the Terrahorn subwoofer here how you can adjust the HPF to 20hz, when used with an input power of just 300w (1/4 of their normal rated RMS), for "special effects usage".
I want to experiment with this myself, and I'm wondering how I go about calculating the input power I should use when decreasing the HPF frequency (say to 20hz). Am I right in thinking that decreasing the HPF too low can cause damage to the driver (aside from the obvious increase in excursion of the subwoofer which could cause the surround to tear); and if so, why does this happen? Does it cause the voicecoil to overheat more rapidly? Surely the power level is the same no matter the configuration of the pass filters, assuming the limiter(s) and gain structure is set up accordingly? Any help is much appreciated!

I also have modeled the sub in Hornresp, so I can share specific data about the enclosure if that is helpful.

Thanks!

Steve-White:
Moving the HPF below what the enclosure loading on the driver with subsequent power decrease won't stress the voice coils.  Mechanical damage from over-excursion of the drivers will occur at some point in the equation.

Exactly what will fail in the driver first?  That's going to be on a case by case basis.  It may or may not cause the surround to tear.  Spider damage may occur, or just slamming the voice coil into the structure, or clear up out of the gap.

They already did the testing for you stating 1/4 power at 20HZ HPF center frequency.  That recommendation came most likely from testing and not any math formula.

More output at lower frequency = add subs.

Ben Sparkes:

--- Quote ---Exactly what will fail in the driver first?  That's going to be on a case by case basis.  It may or may not cause the surround to tear.  Spider damage may occur, or just slamming the voice coil into the structure, or clear up out of the gap.
--- End quote ---
This I should be able to calculate using Hornresp. RCF advertise an Xmax of 8.8mm in their T/S parameters, but also state a maximum peak to peak excursion "before damage" of 52mm (or 26mm each way). I would hazard a guess that value is because of the 25mm voice coil depth, but that's besides the point.

Using Hornresp, modelling the box & driver at 97.98vac, I get a peak excursion of 9.8mm at 30hz, using a 24db/oct L/R filter at 32Hz. This would make sense, assuming the designer of the cabinet was aiming for a figure of Xmax + 10-15%.
Dropping the HPF to 30hz gives a peak excursion of 19.2mm, which I assume is far too much for this driver (woud I be right in saying that once the Xmax is exceeded the driver no longer produces "sound", even if it does not cause any physical damage?).

Interestingly, looking at the acoustic power in a stack of 4, changing the HPF to 20hz also increases our peak output at 33hz to 141.0dB, up from 136.7dB with a filter of 32hz. Is this correct to what you would measure in the real world? Would I be right in thinking here that if we had a driver with an Xmax of say, 20mm, we would be able to drop the HPF to this lower frequency and see higher SPL numbers or is it more complex than that?

Playing around with the numbers somewhat, I can calculate that with an input voltage of 54.6v and a 12db/oct HPF filter set at 17hz, we can reach as low as 115dB @ 20hz, while not exceeding a displacement of 9.7mm (at 28hz). Interestingly, this value (372w) is not far off the 300w figure that Meta Acoustics recommends for their subwoofers. In fact, if I am to model for a 24db/oct 20hz HPF, I get very close (48v / 288w).
Do these line numbers also line up with what we would expect to see in the real world, and could I go ahead and try this on my setup without anything exploding?


--- Quote ---They already did the testing for you stating 1/4 power at 20HZ HPF center frequency.  That recommendation came most likely from testing and not any math formula.
--- End quote ---

Just to be clear, I'm not using the same subwoofers as the website talks about. They are the same theory (front loaded folded horns) but a different design (terrahorn vs superbasshorn).

Chris Grimshaw:
With this type of horn, you can drive them to a very low frequency. They don't benefit much from the horn at those frequencies, though - you end up somewhere near the output of the drivers in sealed boxes.

Generally speaking, once you start pushing past Xmax, things get unpredictable. The driver is no longer acting linearly, so while Hornresp may predict 20mm of excursion (and the output that goes with that), you're more likely to get 14mm of excursion and a load of harmonic distortion. I often test drivers in free air to figure out where they get nasty. Some drivers go past Xmax fairly gracefully, while others barely get there.

At low frequencies, you'll hit excursion limits before thermal limits. Go with a CEA burst at 20Hz, start quiet and push the fader slowly. You'll notice the drivers struggling. Set the limiters a bit below that, and you'll be fine for SFX.

Chris

Art Welter:

--- Quote from: Ben Sparkes on January 04, 2022, 09:20:45 PM ---1)This (Exactly what will fail in the driver first?) I should be able to calculate using Hornresp.
2)RCF advertise an Xmax of 8.8mm in their T/S parameters, but also state a maximum peak to peak excursion "before damage" of 52mm (or 26mm each way). I would hazard a guess that value is because of the 25mm voice coil depth, but that's besides the point.
3)Dropping the HPF to 30hz gives a peak excursion of 19.2mm, which I assume is far too much for this driver (woud I be right in saying that once the Xmax is exceeded the driver no longer produces "sound", even if it does not cause any physical damage?).
4)Interestingly, looking at the acoustic power in a stack of 4, changing the HPF to 20hz also increases our peak output at 33hz to 141.0dB, up from 136.7dB with a filter of 32hz. Is this correct to what you would measure in the real world? Would I be right in thinking here that if we had a driver with an Xmax of say, 20mm, we would be able to drop the HPF to this lower frequency and see higher SPL numbers or is it more complex than that?
Do these line numbers also line up with what we would expect to see in the real world, and could I go ahead and try this on my setup without anything exploding?

--- End quote ---
Ben,

Good questions.
1) No, Hornresp can not calculate the failure mode of a driver, it will only approximate whether the driver would exceed the excursion or power you set as a limit.
2) The RCF LF18N401 top plate (magnetic gap) thickness is 15mm, the voice coil depth 25mm, 25-15=10mm.
Traditionally, half that excursion would have been considered Xmax, (linear excursion) but the Bl (magnetic strength) still is strong enough to be considered linear past that point, RCF still considers the excursion to be "linear" up to 9mm excursion, at which point 4mm of the coil has left the gap.
At 27.5mm excursion (25/2+15), the entire voice coil would have left the gap, the motor would have very little driving force regardless of input voltage. Hornresp can not predict non-linear functions, it considers the suspension and Bl to be constants.
3) Exceeding Xmax generally increases distortion, which may "sound" louder. The Bl and suspension determine how "bad" the driver sounds before damage.
4)Not likely. Doubling Xmax (and power) results in 6dB more output- a driver with 20mm Xmax should have more than 6dB more output than one with 9mm, but unfortunately, Xmax is measured in different ways by different manufacturers, so direct comparisons are impossible, and as the suspension progressively "puts on the brakes", impossible to determine what's going to "explode" first.

I'd recommend pushing the driver by hand (or sine wave at Fs in free air progressively advanced in voltage, leaving time for cooling between measurements)  to determine if the coil former hits the top plate at 26mm ("clack"), or the suspension reaches it's limits first, and what the limits sound/look like.

Art

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