The Sound of Very Low Frequency Notes

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captainmarlow
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The Sound of Very Low Frequency Notes

Post by captainmarlow » Wed Feb 12, 2020 11:52 am

I'm trying to understand why very low frequency notes sound the way they do. I'm talking about notes below C0 like C-2. They sound very clicky or trickling and continue to do so even when I put a high pass filter on the sound. Is this a case of the missing fundamental psychoacoustic effect? But even so why do they sound clicky rather than continuous? And also when I look at the spectrogram I see that the levels of the high frequency harmonics are bouncing up and down which I also don't understand why this would happen.

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BailyDread
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Re: The Sound of Very Low Frequency Notes

Post by BailyDread » Wed Feb 12, 2020 12:36 pm

sounds like you're clipping in the signal path somewhere... a sine wave should be completely clean. unless you're talking about notes that are literally inaudible, at which point you're hearing the peak of each cycle of the waveform without the "blurring" effect that occurs when they repeat fast enough to convey a definite pitch.

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Re: The Sound of Very Low Frequency Notes

Post by captainmarlow » Wed Feb 12, 2020 12:49 pm

Sorry I should have been clearer, this would be while I'm using a saw wave. Also I'm mainly using soft synths in ableton but I don't see any clipping in the meters.

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Re: The Sound of Very Low Frequency Notes

Post by captainmarlow » Wed Feb 12, 2020 12:52 pm

I think C-2 would be around 4Hz so maybe it is the peaks you mentioned? Would this affect higher frequency harmonics too? I quite like the sound so I'm trying to understand it better.

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Re: The Sound of Very Low Frequency Notes

Post by captainmarlow » Wed Feb 12, 2020 12:57 pm

With regard to the peak I'm not don't understand how the peak of the wave would be more audible? Isn't the rate of change of the waveform more important? I'm just imagining how the wave would translate into speaker movement.

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Re: The Sound of Very Low Frequency Notes

Post by captainmarlow » Wed Feb 12, 2020 1:03 pm

I think I see now, if it is a saw wave then there would be a sudden jump to the peak and and inaudible descent

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Re: The Sound of Very Low Frequency Notes

Post by BailyDread » Wed Feb 12, 2020 2:13 pm

i'm not the guy to ask about the specifics of the science of what's going on b/c I'm only referring to my common sense understanding here. basically 4 hz is way below the threshold of human hearing. if it were a sine (100% fundamental and no harmonics), you'd simply hear nothing, even though the speakers would be vibrating at 4 hz per second. this would produce pressure in the air, but it's so low that you have no means of perceiving it, or at least very very limited means of perceiving it (infrasound). what I think you're hearing are the upper harmonics having so much space between them that there are gaps. if you did it with a square wave i would think you'd get a constant clicky/rumbly thing.

i'm at the limit of my capacity here as far as explanation so maybe someone can clarify better but basically 4 hz is only going to give you weird artifacts as a sound. useful as the rate of an LFO tho!

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Re: The Sound of Very Low Frequency Notes

Post by cretaceousear » Wed Feb 12, 2020 2:32 pm

It's the sound of someone tapping the table at 4hz which is equivalent to a click at 240 bpm.
I think when the triangle is transitioning at its peak, it's spitting out more harmonics so you get a higher tish-click at 240 bpm.
Maybe? :hmm:
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Re: The Sound of Very Low Frequency Notes

Post by hsosdrum » Wed Feb 12, 2020 8:01 pm

cretaceousear wrote:
Wed Feb 12, 2020 2:32 pm
It's the sound of someone tapping the table at 4hz which is equivalent to a click at 240 bpm.
I think when the triangle [saw — ed.] is transitioning at its peak, it's spitting out more harmonics so you get a higher tish-click at 240 bpm.
Maybe? :hmm:
This right here.

If it were a pure sine wave you'd "hear" nothing. And remember that almost no loudspeakers on Earth can reproduce a legitimate 4Hz wave at a high enough amplitude that humans could perceive it through their bodies (a phenomenon known as haptics). What you're hearing are the saw wave's harmonics, but not the actual 4Hz.

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Re: The Sound of Very Low Frequency Notes

Post by BailyDread » Thu Feb 13, 2020 1:13 pm

hsosdrum wrote:
Wed Feb 12, 2020 8:01 pm
And remember that almost no loudspeakers on Earth can reproduce a legitimate 4Hz wave at a high enough amplitude that humans could perceive it through their bodies (a phenomenon known as haptics).
just out of curiosity, why is this? is it something to do with the physics of speakers?

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Re: The Sound of Very Low Frequency Notes

Post by cretaceousear » Thu Feb 13, 2020 1:51 pm

dbl post
Last edited by cretaceousear on Thu Feb 13, 2020 6:19 pm, edited 1 time in total.
..the vessel was heavier because “dead sheep do not have the same weight as the live ones”

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Re: The Sound of Very Low Frequency Notes

Post by cretaceousear » Thu Feb 13, 2020 1:53 pm

BailyDread wrote:
Thu Feb 13, 2020 1:13 pm
just out of curiosity, why is this? is it something to do with the physics of speakers?
A metronome at 240 bpm IS playing at 4 hertz. A recording of it is doing the same - you hear the individual ticks as events. Those events are like a PWM square wave with a narrow pulse. So you can hear it!
Woody metronome not a clean tick so each tick has lots of its own thing:
..the vessel was heavier because “dead sheep do not have the same weight as the live ones”

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Re: The Sound of Very Low Frequency Notes

Post by BailyDread » Thu Feb 13, 2020 3:37 pm

cretaceousear wrote:
Thu Feb 13, 2020 1:51 pm
BailyDread wrote:
Thu Feb 13, 2020 1:13 pm
just out of curiosity, why is this? is it something to do with the physics of speakers?
A metronome at 240 bpm IS playing at 4 hertz. A recording of it is doing the same - you hear the individual ticks as events. Those events are like a PWM square wave with a narrow pulse. So you can hear it!
Woody metronome not a clean tick:
oh, i meant why is it that a speaker couldn't produce enough pressure @ 4hz for one to feel it with his or her body? Just wondering if there's some property of speakers that make the SPL required not possible to achieve. does it just take a crazy amount of power amplification?

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Re: The Sound of Very Low Frequency Notes

Post by cretaceousear » Thu Feb 13, 2020 6:32 pm

Yeah I remember seeing a TV program, in my teens I think so a long long time ago .. and it was about Russian experiments using sound as weapons. There was footage of a man subjected to very low, very intense audio and he looked very uncomfortable. I've since Googled it but never found much of anything. As a weapon it can't be directed easily.
I'd have thought you could make a large metal plate say 2 meters square, connected to a piston driven by a motor, to produce 4 hertz, should be fairly straightforward to do mechanically! You'll feel air pressure fluctuations for sure.
..the vessel was heavier because “dead sheep do not have the same weight as the live ones”

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Re: The Sound of Very Low Frequency Notes

Post by beyourdog » Thu Feb 13, 2020 6:55 pm

Low frequency notes have a threshold where you hear the beating instead of the note, you re basically in lfo mode...as the metronome shows...

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Re: The Sound of Very Low Frequency Notes

Post by hsosdrum » Thu Feb 13, 2020 9:13 pm

BailyDread wrote:
Thu Feb 13, 2020 1:13 pm
hsosdrum wrote:
Wed Feb 12, 2020 8:01 pm
And remember that almost no loudspeakers on Earth can reproduce a legitimate 4Hz wave at a high enough amplitude that humans could perceive it through their bodies (a phenomenon known as haptics).
just out of curiosity, why is this? is it something to do with the physics of speakers?
Mostly it has to do with how the human hearing mechanism works. Our hearing is not equally sensitive to sound at all frequencies — in fact, our hearing is tuned to be most sensitive to sounds between 800Hz and 6kHz (one of those 'species-survival' things). The further away a sound is from this window the louder it has to be for us to hear it. That means a sound at 30Hz has to be as much as 60dB louder than a sound at 1kHz for us to be able to hear it at the same volume.

The practical upshot of this is that reproducing low-frequency sounds so we can hear them requires much more acoustic power than reproducing midrange or high-frequency sounds at the same perceived volume, requiring moving much more air at much higher amplitudes (which in turn, requires much more loudspeaker area and amplifier power). That's the main reason why bass loudspeakers need to be very large compared with midrange and high-frequency speakers, and why it requires much more amplifier power to reproduce bass than it does to reproduce midrange and high frequencies.

Scientists Fletcher and Munson performed extensive studies on this phemenon in the late 1920s and published the seminal work about it in 1933. The resulting series of "equal-loudness curves" shown below illustrates how loud sound must be at each frequcency to be perceived at the same volume as we perceive 1kHz (the reference for these studies). The number next to each curve is the dB level at 1kHz. If you follow the curves you can see that the lower in frequency we go (to the left) the louder the sound must be for humans to perceive it at the same volume we perceive 1kHz.

Fletcher-Munson-Curves_S.jpg

Taking the "10" curve as an example, we can see that to perceive 30Hz as being at the same volume as a 10dB 1kHz tone (nearly the threshold of hearing in an anechoic chamber), the 30Hz tone must be reproduced at around 65dB! A 30Hz tone reproduced at that mere 10dB level would be completely inaudible — our hearing simply isn't sensitive enough at those low frequencies. To perceive 30Hz at the same volume as a 40dB 1kHz tone, the 30Hz tone must be reproduced at 75dB.

The studies stopped at around 25Hz since the lower limit of human hearing is around 20Hz. At frequencies lower than that the sensations are felt, rather than heard, and different sensory mechanisms come into play. But the difficulties of creating those sensations using purely acoustic methods (i.e. loudspeakers) only increase the further below 20Hz you try to go.

Although subsequent research has refined the Fletcher/Munson equal-loudness curves shown above, the main principles described by Fletcher and Munson's studies still hold.
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Re: The Sound of Very Low Frequency Notes

Post by hsosdrum » Thu Feb 13, 2020 9:36 pm

cretaceousear wrote:
Thu Feb 13, 2020 1:53 pm
BailyDread wrote:
Thu Feb 13, 2020 1:13 pm
just out of curiosity, why is this? is it something to do with the physics of speakers?
A metronome at 240 bpm IS playing at 4 hertz. A recording of it is doing the same - you hear the individual ticks as events. Those events are like a PWM square wave with a narrow pulse. So you can hear it!
No, the metronome is not reproducing 4Hz, you're sampling the metronome's sound at a 4Hz sampling rate — turning it off and on 4 times a second. To produce an actual 4Hz "tone" you need to create a 256-foot long periodic wave in the air.

Good luck with that...

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Re: The Sound of Very Low Frequency Notes

Post by cretaceousear » Fri Feb 14, 2020 8:12 am

hsosdrum wrote:
Thu Feb 13, 2020 9:36 pm
No, the metronome is not reproducing 4Hz, you're sampling the metronome's sound at a 4Hz sampling rate — turning it off and on 4 times a second. To produce an actual 4Hz "tone" you need to create a 256-foot long periodic wave in the air.
Good luck with that...
Ah caught out!
Would my mechanical plate idea work?
..the vessel was heavier because “dead sheep do not have the same weight as the live ones”

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Re: The Sound of Very Low Frequency Notes

Post by BailyDread » Fri Feb 14, 2020 10:27 am

hsosdrum, great posts! Thank you for this fascinating information.

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Re: The Sound of Very Low Frequency Notes

Post by hsosdrum » Sat Feb 15, 2020 5:21 pm

cretaceousear wrote:
Fri Feb 14, 2020 8:12 am
...Would my mechanical plate idea work?
Yes and no. If you constructed the mechanism in free air it would produce acoustic waves on either side that are out of phase with each other, resulting in lots of cancellation, which would reduce their amplitude. To be effective the plate must be mounted in the center of a baffle that is the length of the longest wave you want to cancel — a 256' x 256' baffle — preventing the waves created on either side from meeting. To get around this impracticality, you could turn the baffle on itself, creating a sealed box (which is known as an "infinite baffle" enclosure). However, it would still be a mighty big box. :eek:

And on top of the concerns about the baffle, you'd also need to consider that flexing a metal plate back-and-forth 4 times a second at excursions long enough to create a perceptible 4Hz wave in the air would very quickly fatigue the metal, causing failure. Such failure would create an air leak, ruining the device's ability to do its job. To get around this you could form the metal plate into a shape that wouldn't flex (like a ribbed cone) and surround it with some sort of flexible material... (See what's happening? You're now beginning to create a cone-type loudspeaker driver with a purely mechanical motor, instead of a typical electro/magnetic motor. The guys on the US TV show Mythbusters attempted this, but the device shook itself apart after only a short period of use.*)

Speaker designers have to wrestle with all of these issues (as well as many others) every time they design a speaker. The only difference is that since they're concerned with audible frequencies they're working with shorter wavelengths (i.e. a 32Hz wave is a "mere" 32 feet in length). And there are other enclosure designs (vented boxes, horns) that involve different concerns and compromises from an infinite baffle. And they have to consider additional practical factors (cost, size, portability, reliability, the ability to manufacture it, etc.).

PLUS, because of the difficulty humans have in perceiving them, such long wavelengths must be created at very high amplitudes, making propagating them dangerous to life, limb and property. (A high-amplitude 4Hz wave could shake apart a structure.) This is one of the reasons why legitimate scientific experiments creating infrasonic waves are few and far-between.

*In the 1980s a very talented speaker designer named Tom Danley designed and marketed a series of professional subwoofers powered by cones that were driven by a motor/belt system, rather than by conventional magnets and electrical coils. The company (Servodrive) is no longer in business, but Danley now leads a groundbreaking professional speaker company named Danley Sound that is having considerable success with more conventional driver designs (mounted in some very un-conventional enclosures). I'm sure a Google search of "Servodrive" will yield some interesting stuff about Danley's mechanically-driven speakers.

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Re: The Sound of Very Low Frequency Notes

Post by cretaceousear » Tue Feb 18, 2020 9:02 am

Most informative post Hsodrum - thanks!

Side note: I had it on good authority that one of the members of KLF, before they got successful, set something infrasonic up in an industrial workshop. Neighbouring units complained of unpleasant vibrations and asked him to stop or they'd get the police round.

This thread needs cross referencing to this one in general discussion:
6Hz at 130dB. We all threw up and cracked a warehouse wall
..the vessel was heavier because “dead sheep do not have the same weight as the live ones”

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Re: The Sound of Very Low Frequency Notes

Post by hsosdrum » Tue Feb 18, 2020 7:35 pm

cretaceousear wrote:
Tue Feb 18, 2020 9:02 am
This thread needs cross referencing to this one in general discussion:
6Hz at 130dB. We all threw up and cracked a warehouse wall
I've been following that thread. Some interesting information about the psychological and physiological effects of infrasound.

Also, it goes to show that when presented with sensory information that doesn't correlate with known experience, humans will desperately grasp at explanations that have nothing whatsoever to do with what is actually happening from a physics standpoint. Thousands of years ago people thought that lightning was sent to Earth by gods; now we know better. The same will be true with phenomena for which we currently have no concrete explanation. We'll eventually understand how it works according to the laws of physics.

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