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Subsonic filter needed?
MUFF WIGGLER Forum Index -> Modular Synth General Discussion  
Author Subsonic filter needed?
dbeats
Last weekend I burnt one of my active subwoofers during a small live performance with my Eurorack modular rig. Quite surprising because I had my PA output level well below the internal limiter the whole time, and I´ve been using my active PA´s for years and know how to treat them well.

That brings up my question: Is it possible that subsonic frequencies were the problem? Would anyone recommend using a subsonic filter between my modular rig and my active PA? Any filter you could recommend in particular? I know that my old amps often included a 20hz subsonic filter, dunno if that is still needed today.

I have a medium-size A-100 modular system, my output module is the WMD Performance Mixer, that goes into an Ecler or Allen & Heath analog mixer line input, and the mixer runs directly into my active PA (RCF/Yamaha/Mackie) via symmetrical XLR. I don´t have any 19" devices, so either a Eurorack filter module or a standalone filter device would fit best. If needed.

Any opinions welcome!
Mirolab
Yes.... since a modular synth can very easily generate infrasonic frequencies (yes infrasonic is the correct word).... then yes it's a good idea to filter them before they get to your PA feed. Depending on the PA design, it may or may not have it's own built-in <20Hz filter. Ported subs are more likely to be damaged by <20Hz, since the woofer cone is unrestricted at sub frequencies. I prefer a sealed sub in my studio because the LF is better damped, and the driver is less likely to bottom out. A sealed sub will be less efficient though, and not typically used in PA systems.

You don't mention how robust your PA system was, but Dance, EDM, and Synth music can be especially taxing on the low end. We also don't know how your sub failed... Did the amp blow? Did you burn up the woofer voice coil? Or did the woofer bottom out and damage the voice coil? All these have difference causes.
PrimateSynthesis
Check the fuses/breakers for power and speaker protection. Infrasonic frequencies could have been the problem. But most amps won't pass signals below 20Hz. It was more likely caused by too much audible low end. Which might not be indicated by lights or meters.
hsosdrum
Mirolab wrote:
(yes infrasonic is the correct word


THANK YOU, Mirolab. In the interest of more accurate communication I offer the following to all Wigglers:

Infrasonic: Sound that is lower in frequency than the lowest frequency that can be heard by humans — considered to be between 16Hz and 20Hz. "He damaged his subwoofer by sending it high-amplitude infrasonic program material."

Subsonic: Travelling at a speed slower than the speed of sound — 1187 feet/second at sea level at 0 degrees C. "The Airbus A380's top speed is Mach 0.89, which is in the high subsonic range."

Unfortunately, even manufacturers of audio equipment confuse these terms.
SynthBaron
I didn't try looking to hard, but I don't think there is there is simple module available that does AC coupling or a passive 20hz high-pass filter. It's too bad high-pass voltage-controlled filters are noisy by design...
milkshake
ALL (properly designed) PA systems already have a subsonic filter build in. It's just one capacitor.

The reason yours broke was something else.
ranix
SynthBaron wrote:
I didn't try looking to hard, but I don't think there is there is simple module available that does AC coupling or a passive 20hz high-pass filter. It's too bad high-pass voltage-controlled filters are noisy by design...

the q120 does it I think and it does some other cool stuff too
you could probably make any mult do it by switching out a wire with a capacitor
dubonaire
Mirolab wrote:
Ported subs are more likely to be damaged by <20Hz, since the woofer cone is unrestricted at sub frequencies. I prefer a sealed sub in my studio because the LF is better damped, and the driver is less likely to bottom out. A sealed sub will be less efficient though, and not typically used in PA systems.


I'm not sure that is right, because in a sealed box the lower the frequency the greater the excursion of the subwoofer to play it loudly and accurately. In a ported box you can tune the port to manage this problem.

To the OP, I agree with milkshake, it was probably something else that caused the problem if your amp failed, because unmanaged high energy infrasonics are more likely to cause physical damage.
dbeats
Thx for all your comments so far, including the clarification of infrasonic.

The sub was a Mackie DLM12s, a 12" woofer with a 3" voice coil. I looked up the extended manual now (sorry, RTFM I know) and indeed it says:

"A 36dB/octave high-pass filter at 32 Hz just prior to the low-frequency amplifier prevents very low frequencies from being amplified. Excessive low-frequency energy below 29Hz can damage the woofer by causing it to "bottom out", also known as overexcursion, which is equivalent to a mechanical form of clipping."

Dunno if that should read "...would otherwise damage the woofer..." or "...could still damage the woofer...". But you might be right then, it probably wasn´t because of infrasonic frequencies.

BTW: I´m pretty sure my voice coil got damaged, not the amp and not the front calotte. I just don´t understand why this happened at an output level around 50-75%. Anyway...
SynthBaron
dbeats wrote:

BTW: I´m pretty sure my voice coil got damaged, not the amp and not the front calotte. I just don´t understand why this happened at an output level around 50-75%. Anyway...


I would expect running equipment at those levels with a constant modular synth/frequency generator input is going to break anything over time. While it may be able to handle it for short periods, there's a reason people over spec audio amplification with more speakers and big amps...less heat production. That's what probably killed it.
Dcramer
hsosdrum wrote:
Mirolab wrote:
(yes infrasonic is the correct word


THANK YOU, Mirolab. In the interest of more accurate communication I offer the following to all Wigglers:

Infrasonic: Sound that is lower in frequency than the lowest frequency that can be heard by humans — considered to be between 16Hz and 20Hz. "He damaged his subwoofer by sending it high-amplitude infrasonic program material."

Subsonic: Travelling at a speed slower than the speed of sound — 1187 feet/second at sea level at 0 degrees C. "The Airbus A380's top speed is Mach 0.89, which is in the high subsonic range."

Unfortunately, even manufacturers of audio equipment confuse these terms.


But what do I call it when I take my subwoofer on the Airbus? woah
SynthBaron
Dcramer wrote:
hsosdrum wrote:
Mirolab wrote:
(yes infrasonic is the correct word


THANK YOU, Mirolab. In the interest of more accurate communication I offer the following to all Wigglers:

Infrasonic: Sound that is lower in frequency than the lowest frequency that can be heard by humans — considered to be between 16Hz and 20Hz. "He damaged his subwoofer by sending it high-amplitude infrasonic program material."

Subsonic: Travelling at a speed slower than the speed of sound — 1187 feet/second at sea level at 0 degrees C. "The Airbus A380's top speed is Mach 0.89, which is in the high subsonic range."

Unfortunately, even manufacturers of audio equipment confuse these terms.


But what do I call it when I take my subwoofer on the Airbus? woah


milkshake
Replacement speaker here.

If your voice coil is damaged, you'll hear/feel it scratching when you manually push it inward.


Other reasons why your voice coil can be damaged:
-Shocks. If the sub has gotten some shocks, by for instance driving to hard over bumps in the road, or dropping it on the floor a couple of times, the magnet can move. This can happen over long periods of time, slightly changing the magnets position each time. To prevent this, move the sub with the speaker in up or down orientation.
-Moisture. Simple corrosion can affect the voice coil and the glue simply lets go.

If your voice coil is damaged, your amp can be damaged as well. Before replacing the speaker make sure the amp works.




Some info on bass reflex systems:
At the ports resonant frequency, the speaker itself moves very little. But below it, the speaker "feels" nothing holding it back and excursion is out of control, even a "few" watts can kill it. Hence the highpass filtering.
hsosdrum
dubonaire wrote:
Mirolab wrote:
Ported subs are more likely to be damaged by <20Hz, since the woofer cone is unrestricted at sub frequencies. I prefer a sealed sub in my studio because the LF is better damped, and the driver is less likely to bottom out. A sealed sub will be less efficient though, and not typically used in PA systems.


I'm not sure that is right, because in a sealed box the lower the frequency the greater the excursion of the subwoofer to play it loudly and accurately. In a ported box you can tune the port to manage this problem.


Mirolab is indeed correct. In a sealed subwoofer the enclosed volume of air will restrict the speaker's motion if you feed it a signal lower in frequency than it is designed to receive. This offers a significant degree of physical protection for the speaker.

In a ported subwoofer the port tunes the enclosure for maximum efficiency down to the lowest frequency the speaker is designed for. However, below that frequency the port opening allows the speaker to move freely, where its motion is restricted only by its surround and spider. This situation dramatically increases the chances of damage if the speaker is fed program material lower in frequency than it's designed to handle.

PA woofers are almost always ported because maximum efficiency is of primary importance in being able to do their job. Higher-end PA system electronics usually contain high-pass filters that prevent infrasonic program material from reaching the woofers.
thetwlo
dbeats wrote:
Last weekend I burnt one of my active subwoofers during a small live performance with my Eurorack modular rig.


Perhaps a limiter, like an Aphex Dominator II? they are cheap now, $100+
Paul Perry
Many years ago, i was experimenting with a midi player & transposing midi files way down.. all I could hear was a 'clicking" sound... cranked up the amp (Sansui 1970s hifi) and, pffft that was the speaker coil melted.
So, yes, I can see the point of a cap in line with the output, or some other kind of bottom end stop.
The takeaway: it's unlikely that anyone will damage a speaker using a direct coupled amplifier playing "ordinary music" becuse the overload distortion will stop you exceeding the power level of the speaker. But if you can't actually hear the sound - because it is infrasonic or ultrasonic - then yeah, it's possible, for an idiot like me. Dead Banana
PrimateSynthesis
milkshake wrote:
ALL (properly designed) PA systems already have a subsonic filter build in. It's just one capacitor.


Infrasonic hihi

Yes, they are AC-coupled. However, most amps can produce frequencies most speakers can't reproduce efficiently. Amplifiers that are nearly flat down to 20Hz are common. Speakers that are anywhere near flat down to 20Hz are extremely rare.

So it's not that the OP needs a limiter or a high-pass filter. But was most likely pushing too much bass for the speaker to handle.
milkshake
PrimateSynthesis wrote:
milkshake wrote:
ALL (properly designed) PA systems already have a subsonic filter build in. It's just one capacitor.


Infrasonic hihi

Apparently so.

PrimateSynthesis wrote:

Yes, they are AC-coupled. However, most amps can produce frequencies most speakers can't reproduce efficiently. Amplifiers that are nearly flat down to 20Hz are common. Speakers that are anywhere near flat down to 20Hz are extremely rare.

So it's not that the OP needs a limiter or a high-pass filter. But was most likely pushing too much bass for the speaker to handle.


These subs already have a steep 36dB highpass filter at 32Hz and a compressor/limiter build in.
I think infrasonic frequencies did't damage the subs, but there's alway's a tiny possibility that they did, I think it was something else as I've mentioned here before.
dbeats
milkshake wrote:
These subs already have a steep 36dB highpass filter at 32Hz and a compressor/limiter build in.
I think infrasonic frequencies did't damage the subs, but there's alway's a tiny possibility that they did, I think it was something else as I've mentioned here before.

Again, thx for all your comments. To sum it up: My PA sub did have all the protection you could think of (it has HPF and limiter/compressor and thermal amp protection), and input/output levels were normal, and I still managed to burn the voice coil after a short time. So, either my speaker did already have a general problem (as milkshake pointed out, for example) or all these protection and safety measures aren`t fully reliable when running frequencies from a modular system through it. I still tend to believe the latter, and then really don´t know what to do differently the next time?
milkshake
Sometimes stuff just breaks cry

Get a new woofer, see one of my posts here, check the electronics and keep making sound.
tau_seti
Darn, I was hoping we would have a discussion of how to use infrasound in our work. Just went to a performance tht made extensive use of it. It was great.
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