> 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.
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.
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.
> 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.
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.
What sticks in my mind is that I think its a rate problem. Meaning audible bass volume is relative to volume of air displaced.
It's like when we design pumps. There are positive displacement pumps, flow equals stroke volume times stroke rate. Stroke rate is frequency, stroke volume is stroke times surface area. So bigger subwoofer diameter equals bigger surface area. You can also increase the stroke. Anyway, that all integrates to flow.
We also have rotary pumps like centrifugal and axial flow pumps. With those flow is a function of rpm and pressure gradient across the pump. (Pressure gradient across the pump is not a factor in positive displacement pumps). To me this is why they needed such a large chamber. Below a certain size they were sucking down the Pressure too much. Also it's much flow that the pressure gradients ar such that it limits its frequency response to 10 hz or so.
All this to say, I think the mistake was simple. They said existing subwoofers are 12" so let's make a 12" rotary sub.
I think they should have said a 12" sub has a volumetric displacement rate (diameter times stroke times frequency) of X mm^3/s and if we design a rotary sub to have that same volumetric displacement what are it's dimensions.
It would be far less than 12", likely 2" or so, and very compact yet powerful. That form would also dramatically change its frequency response and performance.
This is what I've been wanting to build for years, a 2" rotary sub with a very powerful fan.
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.
Also very expensive.. which makes sense, it's mechanically complex product (variable-pitch propeller) with strict requirements (don't want things to rattle or while) which is marketed to rich people (hi-fi fans).
I wonder if modern power electronics and brushless motors can achieve the same effect with a fixed-pitch propellers. This could be much cheaper and more accessible to hobbyist.
Skimming through the site, the magic behind this is that the fan spins at a constant speed, but the pitch of the blades can be modulated at very low frequencies - positively and negatively. I had to look carefully at the .GIF on this page to have the ah-ha moment when it made sense. http://www.rotarywoofer.com/howitworks.htm
That page also answers the obvious question of why the fan thingie is needed and why these frequencies can't be achieved with a traditional loudspeaker membrane.
I love the Leslie speaker and that's what I thought of at first (by the name) as well but upon closer inspection it really isn't very similar after all. Check the videos linked here in other comments https://www.youtube.com/watch?v=NZKCxIuJ-5M to get an intimate understanding and then you'll see how it really is a linear actuator.
* I heard the Thigpen Rotary Subwoofer when Thigpen premiered it at the Rocky Mountain Audio Fest in 2005. Something that's not obvious from most of the published pics is that the rotary subwoofer was using an entire hotel room to serve as an enclosure. IE, you're not going to easily get loud bass at 10Hz without an enclosure, and the Thigpen sub used an entire room.
* Tom Danley is a national treasure and designed something similar at Sound Physics Labs, about 22 years ago. Tom also licensed the technology to Phoenix Gold for use in their "Cyclone" car subwoofer. I've only met Tom once, but as I understand it, he abandoned the technology because subwoofers from companies like Faital, 18 Sound, MTX and Eminence offered comparable performance with improved reliability. Basically the subs he made for Sound Physics Labs were crazy, but reliability was an issue, especially since they used a lot of custom parts.
If anyone is curious about these designs, let me know, I love this stuff.
He had a hand the design of the BassTech 7 servo drive subs as well. Their not being made anymore as I think they went out of business 20 years ago or so. All the reports I've read about those indicated that those subs were a pinnacle of rock concert audio technology and were devastatingly loud, lol.
Once BassTech folded, he worked with an online forum known as the Live Audio Board (LAB) to open source the design and replace the servo based driver with a conventional voice coil and cone driver.
The resultant design became known as the LAB Sub. A friend of mine built 12 of these beasts ~ 45x45x22 inches and let me tell you, they were amazing performers, that you could build yourself for around $500 (at the time).
Mr. Danley truly is a treasure to audio engineering and loudspeaker design.
Seems like they do the same in their subs what Backes & Müller and Silbersand do for every speaker. Cinema for the ears, if you ask me. Even at volumes below speech, you can hear EVERYTHING! Even low bass, from a closed enclosure!
> Something that's not obvious from most of the published pics is that the rotary subwoofer was using an entire hotel room to serve as an enclosure. IE, you're not going to easily get loud bass at 10Hz without an enclosure, and the Thigpen sub used an entire room.
This is called "infinite baffle." Basically just a wall that provides isolation from the other side of it. It is merely a type of speaker enclosure.
The film "War of the Worlds(2005)" has effects going down to 20 Hz; to play it you will need an ATMOS AV receiver that has an LFE port to feed the signal to an amp that has LFE input.
Not all amps which have an LFE input can cleanly output 20 Hz, let alone anything lower, so you would need to check the specs.
I wouldn't be surprised to discover that the original "Iron Man" movie reproduced effects at that level, but I know the "War of the Worlds" film does.
Why atmos? Wouldn't any receiver with a subwoofer out do? Many subwoofers have a crossover and you can could also wire your audio out through the subwoofer and then to your speakers if you really want.
A 64ft organ stop goes down to 8Hz, but your DAC might not. Attempting to play extremely low bass too loud is an easy way to damage speakers/headphones, and most people don't have rotary woofers, so it's common for playback equipment to filter everything below 20Hz.
Audacity can generate sine waves of arbitrary frequency, but take care not to set the volume too high and break things if you want to experiment.
ELO's album "Time" had an opening ("Prologue") that had a pipe organ kick in around 00:26s that would occasionally cause the needle to hop on my turntable. Probably pushing the RIAA equalization limits for vinyl.
The rotary subwoofer would have paired nicely with theatrical sensurround https://en.m.wikipedia.org/wiki/Sensurround whos heyday was in the 1970s and employed large drivers and enclosures the size of a few refridgerators, large enough to require the removal of seats to make room for them, at a loss of revenue...
I feel like this is a lot more approachable with the advent of cheap mass-produced RC helicopter parts (modulating the collective pitch with the audio waveform).
(Which has me wondering if you could do some fun directional things with the sound? Of course low bass is not exactly known for its directionality)
Sound with a wavelength of 6 inches is 2.25 kHz, a high pitched tone far above typical voices, around 100-250 hz. Pretty sure the ears can measure the delay between peaks in the waveform with wavelengths much longer than the width of a head.
That is true, in fact directionality is actually harder to achieve in higher tones (everyone remember those mosquito ring tones that went around a little over a decade ago?) When the wavelength gets very small your ears have less and less information to distinguish which ear is receiving the signal first, other than the brute force which ear actually heard it first calculation. Distinguishing waveforms becomes near impossible at high enough frequencies. It's interesting though that a low enough frequency might also be difficult to distinguish due to there being not enough information rather than too much. If the waveform is as long as a room how much difference is there really across the size of a head.
> The air density is much too low to be a good impedance match with a cone loudspeaker. Take your hand and try and grab some air or flatten your hand and try and push some air as fast as you can. This illustrates the futility of trying to move air to create a small amount of pressure at low frequencies with a cone speaker. You can barely feel the resistance of the air to the motion of your hand, the air molecules simply slip around.
> Now take your hand while driving down the road and stick it outside the window of a car. The faster you go the thicker the air feels.
So could you have a normal fan blowing air into a normal subwoofer to increase the pressure, and get more bass?
That's a Leslie speaker in the video, it doesn't operate on the same principle. Leslie speakers creates sound using a conventional vibrating membrane, whereas the rotary woofer does so by using a variable pitch fan. The rotating part effectively changes the geometry of the baffle, creating a tremolo effect. For the rotary woofer, the whole room acts as a baffle.
I mean, if we could find something that knocks off a zero at least this might make an interesting project but at this price point, it's for die-hards only.
53 comments
[ 2.7 ms ] story [ 44.9 ms ] threadI 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.
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.
I'm willing to bet you could outperform it using about $1000 worth of 18" subwoofers and 10,000 watts or so.
See my comment above. Tom Danley came to the same conclusion that you did, and he's literally a rocket scientist.
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.
What sticks in my mind is that I think its a rate problem. Meaning audible bass volume is relative to volume of air displaced.
It's like when we design pumps. There are positive displacement pumps, flow equals stroke volume times stroke rate. Stroke rate is frequency, stroke volume is stroke times surface area. So bigger subwoofer diameter equals bigger surface area. You can also increase the stroke. Anyway, that all integrates to flow.
We also have rotary pumps like centrifugal and axial flow pumps. With those flow is a function of rpm and pressure gradient across the pump. (Pressure gradient across the pump is not a factor in positive displacement pumps). To me this is why they needed such a large chamber. Below a certain size they were sucking down the Pressure too much. Also it's much flow that the pressure gradients ar such that it limits its frequency response to 10 hz or so.
All this to say, I think the mistake was simple. They said existing subwoofers are 12" so let's make a 12" rotary sub.
I think they should have said a 12" sub has a volumetric displacement rate (diameter times stroke times frequency) of X mm^3/s and if we design a rotary sub to have that same volumetric displacement what are it's dimensions.
It would be far less than 12", likely 2" or so, and very compact yet powerful. That form would also dramatically change its frequency response and performance.
This is what I've been wanting to build for years, a 2" rotary sub with a very powerful fan.
Also very expensive.. which makes sense, it's mechanically complex product (variable-pitch propeller) with strict requirements (don't want things to rattle or while) which is marketed to rich people (hi-fi fans).
I wonder if modern power electronics and brushless motors can achieve the same effect with a fixed-pitch propellers. This could be much cheaper and more accessible to hobbyist.
https://en.wikipedia.org/wiki/Leslie_speaker
Although its rotation was for tremolo, not bass.
https://youtube.com/watch?v=G5fI3X9BdrQ
* I heard the Thigpen Rotary Subwoofer when Thigpen premiered it at the Rocky Mountain Audio Fest in 2005. Something that's not obvious from most of the published pics is that the rotary subwoofer was using an entire hotel room to serve as an enclosure. IE, you're not going to easily get loud bass at 10Hz without an enclosure, and the Thigpen sub used an entire room.
* Tom Danley is a national treasure and designed something similar at Sound Physics Labs, about 22 years ago. Tom also licensed the technology to Phoenix Gold for use in their "Cyclone" car subwoofer. I've only met Tom once, but as I understand it, he abandoned the technology because subwoofers from companies like Faital, 18 Sound, MTX and Eminence offered comparable performance with improved reliability. Basically the subs he made for Sound Physics Labs were crazy, but reliability was an issue, especially since they used a lot of custom parts.
If anyone is curious about these designs, let me know, I love this stuff.
He had a hand the design of the BassTech 7 servo drive subs as well. Their not being made anymore as I think they went out of business 20 years ago or so. All the reports I've read about those indicated that those subs were a pinnacle of rock concert audio technology and were devastatingly loud, lol.
Once BassTech folded, he worked with an online forum known as the Live Audio Board (LAB) to open source the design and replace the servo based driver with a conventional voice coil and cone driver.
The resultant design became known as the LAB Sub. A friend of mine built 12 of these beasts ~ 45x45x22 inches and let me tell you, they were amazing performers, that you could build yourself for around $500 (at the time).
Mr. Danley truly is a treasure to audio engineering and loudspeaker design.
Never thought I'd see these words together.
Danley is awesome and I would kill to have synergy hornes for my apartment (if they'd fit)
Seems like they do the same in their subs what Backes & Müller and Silbersand do for every speaker. Cinema for the ears, if you ask me. Even at volumes below speech, you can hear EVERYTHING! Even low bass, from a closed enclosure!
This is called "infinite baffle." Basically just a wall that provides isolation from the other side of it. It is merely a type of speaker enclosure.
https://en.m.wikipedia.org/wiki/Loudspeaker_enclosure
Normal subwoofers can also be used with infinite baffle. You can use an attic, basement, or crawlspace to create it.
Looks like they have a demo DVD. Was hoping for more.
I wouldn't be surprised to discover that the original "Iron Man" movie reproduced effects at that level, but I know the "War of the Worlds" film does.
Audacity can generate sine waves of arbitrary frequency, but take care not to set the volume too high and break things if you want to experiment.
https://youtu.be/NZKCxIuJ-5M?si=C545ivmg8TvhKhYg
(Which has me wondering if you could do some fun directional things with the sound? Of course low bass is not exactly known for its directionality)
> Now take your hand while driving down the road and stick it outside the window of a car. The faster you go the thicker the air feels.
So could you have a normal fan blowing air into a normal subwoofer to increase the pressure, and get more bass?
https://www.youtube.com/watch?v=NZKCxIuJ-5M
It's the tall cabinet behind Money Mark in the Beastie Boys video for "Gratitude" at around 2:19.
See https://en.wikipedia.org/wiki/Leslie_speaker
I mean, if we could find something that knocks off a zero at least this might make an interesting project but at this price point, it's for die-hards only.