Sound question

[Schneibster]

Be very, very careful saying the ear is not sensitive to phase, studiot. In fact, the ear is very sensitive to phase and uses it to provide direction information. That's the reason for all the intricate whorls and ridges in your external ear.

 

Furthermore, the cochlea has muscles that allow it to filter phases, and to filter frequencies. It is this that allows you to hear a voice even over background noise.

 

As far as the original OP, I agree with the majority of posters that this is destructive interference caused by the shape of the bell/chalice and present at certain spatial nodes and absent at others. I have experienced numerous similar effects while playing music, engineering musical recordings, arranging studios and live performance stage setups, and watching my home theater.

[studiot]

 

Be very, very careful saying the ear is not sensitive to phase, studiot. In fact, the ear is very sensitive to phase and uses it to provide direction information. That's the reason for all the intricate whorls and ridges in your external ear.

 

Furthermore, the cochlea has muscles that allow it to filter phases, and to filter frequencies. It is this that allows you to hear a voice even over background noise.

 

As far as the original OP, I agree with the majority of posters that this is destructive interference caused by the shape of the bell/chalice and present at certain spatial nodes and absent at others. I have experienced numerous similar effects while playing music, engineering musical recordings, arranging studios and live performance stage setups, and watching my home theater.

 

 

Thank you for your comments.

 

1) Phase.

 

Try the following experiment.

 

Take a loudspeaker and listen to it.

Now reverse the connections.

Can you hear any difference, the phase between the two connections is as far apart as possible?

 

You can also do this with a pair of loadspeakers, with the same result.

 

2) I agree, the ear is a complicated sound receiver with many signal processing and conditions features.

 

3) I have yet to see any of the multitude of those claiming the measurable difference to offer anything other than hand waving statements in support of their claim. Nor have I seen anyone else prepared to make experiments or measurements.

I have offered both measurements and solid mathematics that results in the same values as measured.

 

When I read the OP I had not come across this effect so I experimentally confirmed it for myself. Since it is interesting I have not yet finished and yesterday received a real time audio spectrum analyser with a much finer pickup than my own meter. Preliminary results have confirmed the overpressure levels, but produced one suprise so far. I have not had a chance to complete the measurments yet so will hold off reporting more detail until I have proper data. This is taking some time as I must refresh my technique on this kit as I last used it a couple of years ago when doing some consultancy audio work on the noise of rotary aero engines. Unfortunately I have not yet gained any access to accelerometers or vibration monitors to detect the actual frequency my "bell" is vibrating at - I can only test the sound in the air.

 

A really good challenge for alternative theories is "Can they predict what this suprise is in the new measurements"

Edited April 10, 2014 by studiot

[Schneibster]

 

Thank you for your comments.

 

1) Phase.

 

Try the following experiment.

 

Take a loudspeaker and listen to it.

Now reverse the connections.

Can you hear any difference, the phase between the two connections is as far apart as possible?

 

You can also do this with a pair of loadspeakers, with the same result.

 

In fact this is incorrect. If you reverse the phase of one speaker of a stereo pair using a stereo source program, you will be able to hear the difference.

 

Furthermore, you can also hear the difference if you use headphones, in reversing left and right, or reversing the phase of either one alone.

 

This is standard audio engineering. I can present links to a couple good books on it, but personally I learned it by experimentation in the real world.

 

You can avoid this by reversing the phases of both speakers simultaneously.

 

All of this also applies to 5.1 and 7.1 systems. In fact, phase is even more important with such complex setups.

Edited April 10, 2014 by Schneibster

[studiot]

 

In fact this is incorrect. If you reverse the phase of one speaker of a stereo pair using a stereo source program, you will be able to hear the difference.

 

What you say is true, but not relevent since the experiment I described did not present that condition.

 

 

Furthermore, you can also hear the difference if you use headphones, in reversing left and right, or reversing the phase of either one alone.

 

Yes, of course you can, the signal is then different. What does that have to do with the case in point?

 

 

This is standard audio engineering. I can present links to a couple good books on it, but personally I learned it by experimentation in the real world.

 

 

So I look forward to your prediction about the results I am now measuring.

 

[Schneibster]

Phase is a meaningless word unless two waves are compared.

 

Never argue with a sound engineer about sound.

[studiot]

 

Phase is a meaningless word unless two waves are compared.

 

Never argue with a sound engineer about sound.

 

 

I have no idea what you mean by either of those statements.

Edited April 10, 2014 by studiot

[Schneibster]

Define phase.

[studiot]

Consider the wave defined by

 

[math]\zeta[/math] = [math]A\sin (\omega t - \varphi )[/math]

 

The phase is defined to be the value of [math]\varphi [/math]

[Schneibster]

No, I didn't say describe it, I said define it.

 

Have you ever used an oscilloscope? Do you know what the "trigger" control does? And do you understand that by manipulating the sweep speed, you are defining the oscilloscope's frequency, and comparing the signal's phase with it, using the trigger to control the relative phase?

 

Now, that oscilloscope (and I have two in my garage, one in need of calibration) is calibrated so that those sweep speeds are extremely precise; this allows extremely precise measurement of frequencies and time intervals, which is one of the main functions of an oscilloscope. In addition it is calibrated so that its voltage scale is also extremely precise. These are adjusted by a professional calibration service, that maintains the special batteries that are certified to provide exact voltages, and the special oscillators that are certified to provide exact frequencies, so that I don't have to have such incredibly expensive and touchy devices.

 

So "phase" is defined in terms of an exact time reference, compared to a reference frequency: the oscilloscope.

 

There is no absolute phase. Phase is always a comparison between two signals: a reference and a datum.

 

The oscilloscope provides the reference with its sweep speed, triggering adjustment, and variable holdoff reference. And even then the variable holdoff is used by defining the beginning and ending measurements. "0" is basically meaningless, though they put it close enough that you're guaranteed to be able to measure a full screen width. It can't be kept that tight; you have to measure it relative to itself every time, two numbers not one.

 

You know the little knob that looks like a safe combination dial? That's the variable holdoff; the one you use to get four digits out of the 'scope when you're measuring. Against the reference.

 

Cute math. Now let's talk about reality.

 

Never argue with a sound engineer about sound. And never argue about waves with an EE.

Edited April 11, 2014 by Schneibster

[studiot]

 

Never argue with a sound engineer about sound. And never argue about waves with an EE.

 

 

This looks to me like the classic fallacy "Appeal to Authority"

 

Are you claiming to be that authority?

 

 

Cute math. Now let's talk about reality.

 

Are you stating that the definition, which you will find in many mid level textbooks of physics and applied maths, is incorrect?

 

Unlike some here I don't want "to be right", I want "the right answer". If a better theory is offered that mathematically analyses the observations and offers further measurable predictions, different from mine, then I will happily embrace it.

 

So far alternative derivable mathematics has been sadly lacking.

Edited April 11, 2014 by studiot

[Schneibster]

When you can measure what you're yammering about with an oscilloscope you can come talk to me. Until then you're a n00b.

[studiot]

 

There is no absolute phase. Phase is always a comparison between two signals: a reference and a datum.

 

 

Defining phase in terms of oscilloscope knobs is nonsense.

 

If you have trouble with mathematics, I will happily explain.

 

Your difficulty appears to be failure to distinguish between phase and phase difference.

 

You have been describing phase difference.

 

This is a common error, and applies to many quantities as well as phase, such as potential and pressure.

 

You cannot describe a wave without first setting up a coordinate system. Such a system is inherent in the equation I quoted (which was the simplest I could think of).

 

As soon as you have set up a coordinate system you have a reference or datum. That is its purpose.

 

That is why you do not need a second wave.

 

If you do compare two waves the comparison is meaningless unless you reference them both to the same coordinate system.

 

When this is done the result is the phase difference.

 

In each case the two quantities are measured in the same units which I think exacerbates the confusion.

 

What do you think the units of phase are?

Edited April 11, 2014 by studiot

[Schneibster]

Sonny I measured a lot of phase and it's always been two signals' relations to one another.

 

Maybe you can go sell your stuff to a freshman physics student.

[studiot]

 

Sonny I measured a lot of phase and it's always been two signals' relations to one another.

 

Maybe you can go sell your stuff to a freshman physics student.

 

 

I feel that this particular digression has gone far enough.

 

So I will say no more until someone posts some predictions as to what my new measurements will show or I have finished them and post them here.

Edited April 11, 2014 by studiot

[Schneibster]

What "new measurements?"

 

You think somehow there's gonna be "new measurements" that define phase instead of describing it, and that don't require a reference for phase to be measured against?

 

Yay Don Quixote. Kill that windmill. Good luck buddy.

[studiot]

 

You think somehow there's gonna be "new measurements" that define phase instead of describing it, and that don't require a reference for phase to be measured against?

 

 

Actually no, I refer to the last two paragraphs of post52.

[Schneibster]

Sorry, sonny, I stick with oscilloscopes and generally figure people who deny what they say are idiots.

[swansont]

!

Moderator Note

Enough with the sniping already. If you can't converse in a civil fashion, don't post.

[Lizzie L]

 

3) I have yet to see any of the multitude of those claiming the measurable difference to offer anything other than hand waving statements in support of their claim. Nor have I seen anyone else prepared to make experiments or measurements.

I have offered both measurements and solid mathematics that results in the same values as measured.

 

 

I'm not sure exactly what you are referring, to, but Schneibster is correct that detection of phase-difference in the sound reaching the two ears is helps us locate the sound source in space, although loudness difference is another cue.

 

 

Here is a google-books textbook page:

 

http://books.google.co.uk/books?id=evbdjAQYA1cC&pg=PA201&lpg=PA201&dq=phase+sound+ears+difference+location&source=bl&ots=zNWELveXgs&sig=xx1rRZsBuxhgswTA4lerQU2joCU&hl=en&sa=X&ei=ekNIU7noCauI7Abgn4DABQ&ved=0CEkQ6AEwAw#v=onepage&q=phase%20sound%20ears%20difference%20location&f=false

[studiot]

Hello, Lizzie, welcome to the thread.

 

 

I'm not sure exactly what you are referring, to, but Schneibster is correct that detection of phase-difference in the sound reaching the two ears is helps us locate the sound source in space, although loudness difference is another cue.

 

Firstly you are correctly referring to a phase-difference phenomenon, which S was not. I don't want to dwell on the departed, but I got my ear chewed out for making the correction to phase difference.

 

I actually said originally, "the ear is not sensitive to phase", which I stand by and offered an simple experiment to demonstrate.

 

You can actually take a single sound (which is what we are dealing with in the thread), run it through a phase shifter to induce whatever phase shift you like and then into a loud speaker and your ear will not be able to detect any difference in the sound.

 

Just so long as you are not comparing it with the same signal received via another source.

 

But we are not discussing binaural spatial location here.

 

In any case for very low frequency sounds we posess no sense of aural location. That has already been discussed earlier as well.

 

Really though this was a digression from the thread which is why the sound is measurably of lower volume inside a vibrating vessel than outside.

[Lizzie L]

Yes, I have no answer to that!

[Acme]

...

When I read the OP I had not come across this effect so I experimentally confirmed it for myself. Since it is interesting I have not yet finished and yesterday received a real time audio spectrum analyser with a much finer pickup than my own meter. Preliminary results have confirmed the overpressure levels, but produced one suprise so far. I have not had a chance to complete the measurments yet so will hold off reporting more detail until I have proper data. This is taking some time as I must refresh my technique on this kit as I last used it a couple of years ago when doing some consultancy audio work on the noise of rotary aero engines. Unfortunately I have not yet gained any access to accelerometers or vibration monitors to detect the actual frequency my "bell" is vibrating at - I can only test the sound in the air.

 

A really good challenge for alternative theories is "Can they predict what this suprise is in the new measurements"

I haven't found a suitable 'bell' to experiment with yet, so I'm just venturing a guess at your findings based on all that has been said.

 

Guess: The surprise is that when you put the pickup deeper into the bell than your ear can go, the pressure level is higher than when the pickup is located at ear level. ??

[studiot]

 

Guess: The surprise is that when you put the pickup deeper into the bell than your ear can go, the pressure level is higher than when the pickup is located at ear level. ??

 

 

No the actual pressure levels obviously depend upon how hard I thump the bell, but the values quoted in post 4 are indicative.

I did try to find nodes and antinodes but could really find any discernable variation either inside or ouside. This was noted in later posts.

 

I am feeling like Agatha Christie here. All the clues are in the previous posts in fact.

Edited April 11, 2014 by studiot

[Acme]

No the actual pressure levels obviously depend upon how hard I thump the bell, but the values quoted in post 4 are indicative.

I did try to find nodes and antinodes but could really find any discernable variation either inside or ouside. This was noted in later posts.

 

I am feeling like Agatha Christie here. All the clues are in the previous posts in fact.

Well, it's all got my head ringing like a bell. I have been off reading on bell acoustics and that has pretty much broke my clapper (tongue in bell parlance apparently.) I reread #4 & #52 and I'm no closer to the murderer than I was before. Mr. Plum in the conservatory with the lead pipe? IDK

 

I did run across some node mappings but confusion soon set in trying to differentiate between modes & nodes in the explanations. Egat Brain!!

 

This is a specific bell, but is it how bells vibrate in general? Will it be of any help to me in understanding what's going on inside? Help me Obi Wan.

 

source: >> http://www.acoustics.org/press/133rd/4pmu4.html

 

Edit: So I found a stainless pot lid about 8" across to experiment with. Definitely louder outside near the rim than inside at the plane of the rim. But sticking my ear further inside the loudness did increase as I was suggesting earlier. So is 'loudness' not the right term for what we're investigating?

I also rotated the lid as it was ringing and with my ear on the outside near the rim the loudness seemed to go up & down. Is that because of the nodes & antinodes?

Edited April 12, 2014 by Acme

[studiot]

Your graphics department definitely produces prettier work than mine.

Do they work for free?

 

 

Your ears are not reliable equipment for detailed investigation of the phenomenon.

Even function, who is much younger than I am, and deserves all the credit for noticing this, does not have good enough ears.

 

But remember, it is the sound in the air we hear, not the vibrations in the bell.

 

Here is the key phrase, extracted from your reference, which exactly mirrors my suprise finding.

 

 

 

acoustics link in post #74

The fundamental frequencies of the internal acoustic field are not well matched with the natural frequencies of the bell, which is different with what most of the researchers expected.

 

The diagrams from your link are similar to those from my post#14 and the comment noted above is exactly what I said about my diagram c in post#14.

 

 

studiot post#14

So there is significant acoustic impedence mismatch between the systems.

 

I did not fully pursue the implications of that at that time but it is the underlying mechanics of the effect.

Edited April 12, 2014 by studiot