3 ms·

> However nothing existed to produce significant enough output to change this belief. I thought it was more accurate to say that the lowest frequency a human c

by duped 3y ago

> However nothing existed to produce significant enough output to change this belief. I thought it was more accurate to say that the lowest frequency a human can perceive as sound is also the threshold between determining if a sequence of pulses is a sequence of distinct events and one continuous sound, whose period is around 5ms (or 20Hz) and this is pretty easy to verify experimentally, even with woofers that can't produce a pure sine below 20Hz. That said there is another factor which is that we don't just experience pressure waves in air with our ears, but with our full body of senses. And if you can definitely be impacted by infrasound even if it isn't just vibrating your ears, since it will absolutely vibrate your chest cavity and the rest of your body and you can feel that in your bones (or so to speak). There's also a long history of scientific/pseudo scientific and almost apocryphal stories about infrasound. Off the top of my head I've heard of fans in a building causing subsonic vibrations that disoriented people, subsequent research into weaponizing it, whistles that were designed to vibrate below the threshold of hearing, the legendary "brown note" (a sound low enough and powerful enough to cause evacuation, of a building or a person) and even Havana syndrome was allegedly attributed to infrasound. It's very interesting to use a fan to create such waves when an electromagnetic piston might fail. But I wonder how useful it is since making a piston vibrate at low frequencies isn't that hard, it's enclosing it that is the problem. I suspect a large enough enclosure with a big enough woofer and enough power in an amplifier (eg, a commercial subwoofer) can do just as well without being so exotic as to require custom electronics.

madengr 3y ago
[dead]
CognitiveLens 3y ago
Vibrating a piston at low frequencies isn't hard, but getting it to move enough air is very hard. You're right that "big enough" with "enough power" could solve that problem, but the mechanics of a conventional driver result in unavoidable output roll-off as you go lower, whereas the fan/rotary solution won't roll off because it has essentially infinite excursion - it just keeps blowing in the required direction at the required pressure level for any low-enough frequency, even fractions of Hz. Re: custom electronics - my understanding is that the electronics just need to handle very low frequencies (in part by being able to sustain enough current without fading), which would also be true for a conventional subwoofer system trying to operate at those frequencies. I remember reading about this thing a long time ago and wondering how close I could get with a DIY solution. It would probably sound terrible but it would be fascinating as an transducer learning platform.
- johnvanommen 3y ago
  The Thigpen Roatary sub used an entire hotel room as it's enclosure. I'm willing to bet you could outperform it using about $1000 worth of 18" subwoofers and 10,000 watts or so.
  - explaininjs 3y ago
    And where you you put those subwoofers? Probably in a room of some sort. Thus considering the room as part of the BOM for one but not the other is a bit disingenuous.
johnvanommen 3y ago
> It's very interesting to use a fan to create such waves when an electromagnetic piston might fail. But I wonder how useful it is since making a piston vibrate at low frequencies isn't that hard, it's enclosing it that is the problem. I suspect a large enough enclosure with a big enough woofer and enough power in an amplifier (eg, a commercial subwoofer) can do just as well without being so exotic as to require custom electronics. See my comment above. Tom Danley came to the same conclusion that you did, and he's literally a rocket scientist.
- duped 3y ago
  I feel like anyone with a basic understanding of loudspeaker design would come to the same conclusion. Ultimately, it's well understood that the "infinite baffle" design will give you the best low frequency performance for a given woofer but it's impractical given the amount of volume behind it to actually act as a practically infinite baffle. The goal of loudspeaker design is how to choose a port geometry and enclosure volume that makes the best tradeoff of low end rolloff and resonance for a given woofer at a price point for consumers. The cleverness of a waveguide is that it can manipulate the combined acoustics of small woofer and enclosure volume to provide good sounding low end that is extremely surprising compared to the infinite baffle design for the same woofer. That's why big speakers sound good but are not surprising. There's an inherent tradeoff between low end response and acoustic compliance which is proportional to the volume of the enclosure. A large enough enclosure gives you effectively infinite volume behind it, hence the "infinite baffle" design. That volume tends to be prohibitively large for commercial products. If you choose a smaller volume you get a noticeable rolloff in the bass, but you can trick the listener by turning the enclosure into a Helmhotz resonator by adding a port that creates a resonator with critical frequency below the lower rolloff of the enclosure that extends the bass response. Waveguides further allow better extension by creating a natural acoustic amplifier for a range of frequencies that behave much better than the Helmhotz resonator. It's not rocket science, although it is control systems and signal processing. In short I don't think an exotic loudspeaker is better, because under ideal conditions it still has to deal with the physical properties of whatever encloses it. It's interesting to consider how they interact in a different way that affects the model but I don't think there's anything fundamental that makes it better than a traditional piston model. It's still just moving air and working against the compliance of the air behind it in the enclosure. Essentially the problem of low frequency reproduction is not moving air in front of the piston that is the problem, it's isolating the pressure wave behind it and the effects of the compression of that volume that prohibit its movement at low frequencies (and how they diffuse around the front) that contribute to the issue of reproduction. I don't see how this design fundamentally alters such problems.
explaininjs 3y ago
I’d be very interested to hear a recording of large waves crashing at the ocean with and without this woofer. That comes to mind as the ultimate low-frequency soundscape I know.

by duped 3y ago

madengr 3y ago

[dead]

CognitiveLens 3y ago

Vibrating a piston at low frequencies isn't hard, but getting it to move enough air is very hard. You're right that "big enough" with "enough power" could solve that problem, but the mechanics of a conventional driver result in unavoidable output roll-off as you go lower, whereas the fan/rotary solution won't roll off because it has essentially infinite excursion - it just keeps blowing in the required direction at the required pressure level for any low-enough frequency, even fractions of Hz. Re: custom electronics - my understanding is that the electronics just need to handle very low frequencies (in part by being able to sustain enough current without fading), which would also be true for a conventional subwoofer system trying to operate at those frequencies. I remember reading about this thing a long time ago and wondering how close I could get with a DIY solution. It would probably sound terrible but it would be fascinating as an transducer learning platform.

johnvanommen 3y ago
The Thigpen Roatary sub used an entire hotel room as it's enclosure. I'm willing to bet you could outperform it using about $1000 worth of 18" subwoofers and 10,000 watts or so.
- explaininjs 3y ago
  And where you you put those subwoofers? Probably in a room of some sort. Thus considering the room as part of the BOM for one but not the other is a bit disingenuous.

johnvanommen 3y ago

> It's very interesting to use a fan to create such waves when an electromagnetic piston might fail. But I wonder how useful it is since making a piston vibrate at low frequencies isn't that hard, it's enclosing it that is the problem. I suspect a large enough enclosure with a big enough woofer and enough power in an amplifier (eg, a commercial subwoofer) can do just as well without being so exotic as to require custom electronics. See my comment above. Tom Danley came to the same conclusion that you did, and he's literally a rocket scientist.

duped 3y ago
I feel like anyone with a basic understanding of loudspeaker design would come to the same conclusion. Ultimately, it's well understood that the "infinite baffle" design will give you the best low frequency performance for a given woofer but it's impractical given the amount of volume behind it to actually act as a practically infinite baffle. The goal of loudspeaker design is how to choose a port geometry and enclosure volume that makes the best tradeoff of low end rolloff and resonance for a given woofer at a price point for consumers. The cleverness of a waveguide is that it can manipulate the combined acoustics of small woofer and enclosure volume to provide good sounding low end that is extremely surprising compared to the infinite baffle design for the same woofer. That's why big speakers sound good but are not surprising. There's an inherent tradeoff between low end response and acoustic compliance which is proportional to the volume of the enclosure. A large enough enclosure gives you effectively infinite volume behind it, hence the "infinite baffle" design. That volume tends to be prohibitively large for commercial products. If you choose a smaller volume you get a noticeable rolloff in the bass, but you can trick the listener by turning the enclosure into a Helmhotz resonator by adding a port that creates a resonator with critical frequency below the lower rolloff of the enclosure that extends the bass response. Waveguides further allow better extension by creating a natural acoustic amplifier for a range of frequencies that behave much better than the Helmhotz resonator. It's not rocket science, although it is control systems and signal processing. In short I don't think an exotic loudspeaker is better, because under ideal conditions it still has to deal with the physical properties of whatever encloses it. It's interesting to consider how they interact in a different way that affects the model but I don't think there's anything fundamental that makes it better than a traditional piston model. It's still just moving air and working against the compliance of the air behind it in the enclosure. Essentially the problem of low frequency reproduction is not moving air in front of the piston that is the problem, it's isolating the pressure wave behind it and the effects of the compression of that volume that prohibit its movement at low frequencies (and how they diffuse around the front) that contribute to the issue of reproduction. I don't see how this design fundamentally alters such problems.

explaininjs 3y ago

I’d be very interested to hear a recording of large waves crashing at the ocean with and without this woofer. That comes to mind as the ultimate low-frequency soundscape I know.