T60 Limit

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Signalsdrone
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Re: T60 Limit

#76 Post by Signalsdrone » Thu Aug 30, 2018 5:43 am

Your brain fills in missing info all the time for you. No one on this board sees their ocular blind spot, yet it is there. Your brain takes info from your other eye and what it already knows of what you are looking at and creates a picture in your head. That is what you "see" though not what your eyes really pick up.

The brain is a mysterious thing, Mine has had 46 years of experience duping me every second of the day. I'm pretty used to it.

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Re: T60 Limit

#77 Post by SethRocksYou » Thu Aug 30, 2018 1:38 pm

Bill Fitzmaurice wrote:
Wed Aug 29, 2018 8:46 pm
Harmonics and harmonic distortion aren't the same thing, but they're closely related. Harmonic distortion is when the harmonic content present in what's heard is different than that in the original signal. Usually that means there's more harmonic content, but in rare instances there can be less.
So, which type of distortion is it that's naturally suppressed in a folded horn?

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Bill Fitzmaurice
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Re: T60 Limit

#78 Post by Bill Fitzmaurice » Thu Aug 30, 2018 1:56 pm

Harmonic distortion. IM distortion is reduced in all horns.

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Re: T60 Limit

#79 Post by SethRocksYou » Thu Aug 30, 2018 1:59 pm

I had a bright idea as I woke up this morning. I don't know why, but I just seem to wake up with good ideas. Anyway, I thought of another way to prove, disprove, or just add some clarity to the question of whether we hear 25hz or not.

Record what the earbuds output with a 25hz sine wave, then play that recording back, but through a 40hz high pass filter. If what Bill's saying, that it's harmonics and not the fundamental 25hz sine that we hear, it "should" sound very similar... shouldn't it? Am I thinking that through correctly?

It could well be that I'm not awake enough first thing in the morning to realize that what seems like a great thought isn't really as brilliant as it seems. LOL

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Re: T60 Limit

#80 Post by CoronaOperator » Thu Aug 30, 2018 2:03 pm

SethRocksYou wrote:
Thu Aug 30, 2018 1:59 pm
I had a bright idea as I woke up this morning. I don't know why, but I just seem to wake up with good ideas. Anyway, I thought of another way to prove, disprove, or just add some clarity to the question of whether we hear 25hz or not.

Record what the earbuds output with a 25hz sine wave, then play that recording back, but through a 40hz high pass filter. If what Bill's saying, that it's harmonics and not the fundamental 25hz sine that we hear, it "should" sound very similar... shouldn't it? Am I thinking that through correctly?

It could well be that I'm not awake enough first thing in the morning to realize that what seems like a great thought isn't really as brilliant as it seems. LOL
Why not just record it and then look at it in Room EQ Wizard and see if there is any content above 25hz? You will see a plot of the frequency response. Of course how do you know if your recording device isn't creating its own harmonics? Anyways a 40hz highpass filter isn't a cliff, it is a slope and will still allow some content through.
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Re: T60 Limit

#81 Post by SethRocksYou » Thu Aug 30, 2018 2:23 pm

CoronaOperator wrote:
Thu Aug 30, 2018 2:03 pm

Why not just record it and then look at it in Room EQ Wizard and see if there is any content above 25hz? You will see a plot of the frequency response. Of course how do you know if your recording device isn't creating its own harmonics? Anyways a 40hz highpass filter isn't a cliff, it is a slope and will still allow some content through.
Yeah, it's my intention to run a spectrum analyzer on the earbuds. That should indicate the actual content of what I currently believe is a fairly pure 25hz sine wave with a very low level of harmonics.

The "record it" method is just another, if not additional, test method. Crazy thought worth sharing... I thought

A 40dB/octave 40hz high pass should sufficiently remove a 25hz fundamental, yet allow harmonics through.... I would think. No? What does that equate to?.. like a hundredth of the original signal or something like that? (didn't bother mathing it out, just shooting from the hip on that figure)

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Re: T60 Limit

#82 Post by SethRocksYou » Thu Aug 30, 2018 2:37 pm

Bill Fitzmaurice wrote:
Thu Aug 30, 2018 1:56 pm
Harmonic distortion. IM distortion is reduced in all horns.
If I understand correctly, harmonic distortion is suppressed in folded horns and intermodulation distortion is reduced in all horns.

The harmonic distortion that's suppressed in a folded horn... I'm guessing they're mostly harmonics that occur outside the pass band of the horn and that the first harmonic or two of lower frequencies will not get suppressed. Is that essentially the case? That's just a guess on my part.





Wikipedia's intermodulation Distortion https://en.wikipedia.org/wiki/Intermodulation

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Re: T60 Limit

#83 Post by SethRocksYou » Thu Aug 30, 2018 2:45 pm

SethRocksYou wrote:
Thu Aug 30, 2018 2:23 pm
A 40dB/octave 40hz high pass should sufficiently remove a 25hz fundamental, yet allow harmonics through.... I would think. No? What does that equate to?.. like a hundredth of the original signal or something like that? (didn't bother mathing it out, just shooting from the hip on that figure)
(edited to adjust figures)

Just decided to think this through...

40dB per octave at 40hz would be down to 1/10,000th at 20hz, right?
Down 20dB or 1/100th at 30hz
Should put it down 30dB or roughly 1/1000th at 25hz... 0.001?

Am I doing that right? Wouldn't surprise me if I'm not.

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Re: T60 Limit

#84 Post by SethRocksYou » Thu Aug 30, 2018 5:38 pm

I was able to get to it earlier than I thought!

25hz YouTube "sine wave" https://www.youtube.com/watch?v=mRQ9qfxStg0&t=988s

Earbud in one end of tubing, mic in the other and plugged into my Samsung S6, running the free Spectrum Analyze app
Image

The red trace is max volume, green trace is lower volume and still clearly audible

Image

Thoughts? Insights? Input?
Last edited by SethRocksYou on Thu Aug 30, 2018 5:40 pm, edited 1 time in total.

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Re: T60 Limit

#85 Post by Bruce Weldy » Thu Aug 30, 2018 5:40 pm

Pretty sure the question here isn't whether your phone can hear it, but whether you can hear it.

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Re: T60 Limit

#86 Post by SethRocksYou » Thu Aug 30, 2018 5:53 pm

Hahaha

Well... it was suggested that it's the harmonics and not the 25hz that I'm hearing. In the red trace you can easily see the 3rd, 5th, and 7th harmonic. But they're pretty absent in the green trace.

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Re: T60 Limit

#87 Post by CoronaOperator » Thu Aug 30, 2018 9:25 pm

There is still a fair bit of energy at 30hz in that green trace
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Re: T60 Limit

#88 Post by Bill Fitzmaurice » Thu Aug 30, 2018 10:30 pm

CoronaOperator wrote:
Thu Aug 30, 2018 9:25 pm
There is still a fair bit of energy at 30hz in that green trace
There's a pretty fair amount of energy a lot higher than that when you level compensate the chart using an equal loudness curve. Then it's pretty obvious to me that what's being heard is the harmonics even at the level of the green trace, let alone the red.

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Re: T60 Limit

#89 Post by SethRocksYou » Thu Aug 30, 2018 11:59 pm

Good point Bill. I can see that.

Do you think it would sound very similar if I created a "missing fundamental" situation by recording the output and playing it back through a high pass filter? It seems to me, if it is indeed the harmonics I'm hearing, that it "should" sound much the same.

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Re: T60 Limit

#90 Post by CoronaOperator » Fri Aug 31, 2018 12:42 am

SethRocksYou wrote:
Thu Aug 30, 2018 11:59 pm
Good point Bill. I can see that.

Do you think it would sound very similar if I created a "missing fundamental" situation by recording the output and playing it back through a high pass filter? It seems to me, if it is indeed the harmonics I'm hearing, that it "should" sound much the same.
I'm pretty sure that is how the Waves MaxxBass plugin works. That is used on most modern recordings to give the impression of low bass without stressing the speakers.
Built:
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In progress:
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For best results, point the loud end of the array towards the audience

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