Air exiting bell

[Bach5G]

There has been discussion about how much air exits the bell of a brass horn. Apparently not much. I’ve been trying to find the posts without success. Can anyone send me a link?

[Posaunus]

There has been discussion about how much air exits the bell of a brass horn. Apparently not much. I’ve been trying to find the posts without success. Can anyone send me a link?

Please post anything relevant. Thanks.

[Doug Elliott]

I'm not worried about the little bit of air coming out of the bell.

What I think is concerning is that every player will still be breathing without a mask when they're NOT playing. Distance helps, but a group indoors without masks is still risky.

[PaulT]

The amount of air that exits a horn is equal to whatever amount of air you put into the horn.

If this question is related to virus spread, my expectation is that bands would present a similar degree of virus spread risk as a vocal choirs, both subject to number of and spacing of participants. The reports I have heard of virus transmission with vocal choirs has not been encouraging.

If the question has another end, disregard the previous paragraph.

But air out will equal air in and the velocity of exit, while variable, will generally exceed speaking by fair margin.

https://www.startribune.com/coronavirus ... 571704252/

[Posaunus]

The amount of air that exits a horn is equal to whatever amount of air you put into the horn.

... related to virus spread, my expectation is that bands would present a similar degree of virus spread risk as a vocal choirs, both subject to number of and spacing of participants. The reports I have heard of virus transmission with vocal choirs has not been encouraging.

But air out will equal air in and the velocity of exit, while variable, will generally exceed speaking by fair margin.

Larger brass instruments require higher flow rates, but have larger bells, so the exit velocity from a (large-bell) trombone will probably be similar to that from a lower-flow (smaller-bell) trumpet. But still much higher velocities than from speaking. And perhaps also higher than from a singer's mouth, since a singer does not typically have to create such a large airflow. The only redeeming benefit for us brass players is that most of the moisture (droplets) blown into our instruments condenses on the way through the horn (to be removed via a "water valve"). But I'd guess there is still plenty of fine "aerosolized" moisture (not big droplets) – which could be filled with tiny virus particles – in the exhaust flow from such instruments.

This makes me reluctant to sit "downstream" of any brass players.

Please take care when playing with others, especially indoors!

[Bach5G]

I also asked this on Facebook, Trombone Pedagogy. A couple of posts suggest that the air pressure of air exiting the bell is quite low. There’s a video in German of various orchestral wind instruments, as well as a video of a trumpet player.

I got out my trombone and directed my bell towards a candle, about 6 to 8” from the bell. The flame barely flickered. YMMV. But it appears that there is not a high volume stream of air spreading virus exiting the horn. And, of course, the air from your lungs has already moved 10’ by the time it exits the bell. A lot of the aerosols and droplets will have condensed I would think.

I rather think the risks of playing together (apparently we’re welcome to return to our rehearsal space this Sept) can be minimized with social distancing, masks, hand hygiene and are not significantly increased by playing instruments as long as the rest of the precautions are in place.

[shider]

According to the laws of fluid dynamics the flow rate (speed of the flow X cross section of the pipe) stays constant in such a system. So yes, you put a rather high flow of air into the instrument, but for each crosssection of the instrument the amount of air per second (litres/sec) remains constant. That means that the speed of the flow decreases dramatically towards the bell because the circular area does not increase linear.

To summarize:
An Input of 5l of air equals an output of 5l of air.
An Input of 5l of air at e.g. 5m/s does not equal an output of 5l of air at 5m/s.

quick sidenote: some music stores in Germany started selling "masks" for instrument bells, which is a piece of fabric with a rubber band sewn in. But not one of those stores actually says what those are supposed to do

[Bach5G]

Does the flow rate vary passing through the venturi of the throat of the mouthpiece (.276”), into the slide section (.547”), and out through the bell section?

[Posaunus]

The air flow rate is of course constant through the entire instrument. The exit air velocity is not really the issue. We know it's low. The concern is what the air entrains (including virus particles).

It would be easy to put an air flow meter in the flow path of a trombone to get some real data. (Not very musical, but you could readily measure the volume flow rate.) The velocity is the flow rate divided by the cross-sectional area of the flow path. Let's say the diameter of the leadpipe "venturi" is 13mm, so its cross-sectional area is ~133mm2. If the "effective" exit diameter of the bell is, say, 70mm, the exit area is ~3850mm2. So the exit velocity will be reduced to 133/3850 = ~3.5% of the venturi velocity. No wonder the candle barely flickers even if you blow pretty hard into your trombone.

But the outflow from the bell still (by definition) equals the inflow to the mouthpiece. Think of how much air you blow through your trombone (and why you may "run out" of breath). Compare that to the airflow when you are singing. Or speaking. I bet it's quite a bit more. And if you have Covid-19, every breath you exhale entrains millions of virus particles. Most of the virus will be attached to relatively large "droplets" which don't travel very far. (And which will mostly condense out inside a trombone.) But some will be very small "aerosolized" (micron-size) particles, which persist in the air like an invisible fog. If the virus load in this "fog" is substantial, it could become a vector for virus transmission for one person to another. This is what worries me.

I actually like the idea of a "bell mask." I occasionally use a Crown Royal cloth bag (1.75L size) as a sort of poor-man's Softone mute. It fits nicely over a tenor trombone bell (up to 8" diameter), and creates a pleasant, lightly-muted sound. But like a cloth face mask, it would also filter out some of the the larger droplets in the exit air. (Of course the micron-size particles will still freely flow through a facemask or a Crown Royal bag!)

What to do? Take precautions. Don't play indoors. If playing outdoors, keep at least 6-8 feet from your nearest neighbor. Don't share your air.

[Burgerbob]

And perhaps also higher than from a singer's mouth, since a singer does not typically have to create such a large airflow.

Please take care when playing with others, especially indoors!

Singing is possibly the most dangerous, actually.

[BGuttman]

In a private communication, Baileyman reported seeing a Tyndall effect from the bell of his horn while playing. For those of you who don't know what the Tyndall effect is, it's the reflection from suspended particles in a fluid when a strong beam of light is shone through it. Ever use a flashlight in a fog?

The microdroplets causing the Tyndall effect have very low settling velocities. This is what characterized an aerosol. Originally we thought the aerosol droplets were not contagious, but later research shows that extended contact with COVID contaminated aerosols can transfer the disease; just not as quickly.

Incidentally, Burgerbob is right about singing. The vibrating vocal cords create a lot of aerosol.

[timothy42b]

And perhaps also higher than from a singer's mouth, since a singer does not typically have to create such a large airflow.

Please take care when playing with others, especially indoors!

Singing is possibly the most dangerous, actually.

I did a short recording in an empty church. (I'll share if you beg me. hee, hee) Nobody would be in there for at least another week, and I was playing a Bb tenor, so 9 plus feet of tubing. The floor was masonry, no carpet to trap moisture. So I thought I was okay.

But as I left a flute player showed up to do the same thing. None of his aerosol production gets trapped in the instrument, and I bet it sprays pretty far. So I had second thoughts about what I'd done - making a lot of fine distinctions between instruments that spray and those that don't probably doesn't work.

[Cotboneman]

Could there be one instance when brass players aerosol particles at a more voluminous rate? I'm referring to blowing air through a brass instrument while holding open the water key (s). Granted the condensation largely goes on the floor (or a towel if you are kind), but droplets expelled are in aerosol form. We may well have to be more cognizant of that when emptying the water from our horns.

[BGuttman]

The aerosol droplets from you water key don't disperse that rapidly unless you empty into a gale. They should remain pretty close to the ground. The big problem with aerosols comes from the talking and discussion that invariably happens in a rehearsal.

[jthomas105]

This is probably the most comprehensive study being done.

https://smtd.colostate.edu/reducing-bio ... ming-arts/

I know here in Texas band directors are waiting for this to be released. Release was/is supposed to be today (7-13-2020) but it may be delayed.

[Posaunus]

This is probably the most comprehensive study being done.

https://smtd.colostate.edu/reducing-bio ... ming-arts/

I know here in Texas band directors are waiting for this to be released. Release was/is supposed to be today (7-13-2020) but it may be delayed.

Please let us know when the results are published. (Hope the delay is not due to political interference!) And post a link to the completed study. This could be a matter of life and death!

[Bach5G]

This is probably the most comprehensive study being done.

https://smtd.colostate.edu/reducing-bio ... ming-arts/

I know here in Texas band directors are waiting for this to be released. Release was/is supposed to be today (7-13-2020) but it may be delayed.

Please let us know when the results are published. (Hope the delay is not due to political interference!) And post a link to the completed study. This could be a matter of life and death!

Political interference? What kind of crazy, deranged individual would play politics with the health and safety of school children? Oh…

[Bach5G]

I can’t interpret this, but here is a preliminary report.

https://www.nfhs.org/media/4029952/prel ... Hwkqrcsd0I

[Cotboneman]

I couldn't get the link to work, but a former colleague emailed me the Pdf file. The trombone video in the attachment is surprising. I will try to figure out what it all means at some point. I hope this opens.

preliminary-testing-report-7-13-20.pdf

[shider]

But as I left a flute player showed up to do the same thing. None of his aerosol production gets trapped in the instrument, and I bet it sprays pretty far. So I had second thoughts about what I'd done - making a lot of fine distinctions between instruments that spray and those that don't probably doesn't work.

In Germany there was an experiment with the Bamberger Symphoniker where they let musicians play their instruments while inhaling a kind of mist to visualize the exhaled air movement.

The Wiener Philharmoniker did something similar:
https://wien.orf.at/stories/3049099/
The article is in german, but there are some interesting pictures you could look at.
And yes.. The biggest culprit was the flute...
The experiments though just showed the moving air, not the amount of aerosol being expelled after travelling through a brass instrument (or any other)

Because of those studies playing music (on wind instruments) is starting to become allowed again, even in the local community bands. We still have to go by some restrictions regarding distance, but our hobby music scene is starting to come back slowly.

[jthomas105]

I can’t interpret this, but here is a preliminary report.

https://www.nfhs.org/media/4029952/prel ... Hwkqrcsd0I

This report is from University of Colorado-Boulder. It is concurrent with the one from Colorado State

[BGuttman]

I put this in another similar topic but it really belongs here:

https://www.nfhs.org/media/4029952/prel ... zVU3G-cnC4

[ronnies]

This is an interesting experiment:



Ronnie

[timothy42b]

I put this in another similar topic but it really belongs here:

https://www.nfhs.org/media/4029952/prel ... zVU3G-cnC4

Off thread, but interesting - did you notice the changes in air direction in the alphabet test? I would not have predicted it, and that may have an effect on our playing.

[elmsandr]

This is an interesting experiment:



Ronnie

This does very little for me... Conservation of mass, we know the amount coming out is dispersed over the bell diameter, sure, it may not go straight into the flame, but it goes into the room. Additionally, if we are sitting next to somebody, usually almost shoulder to shoulder, there is a lot of breathing that happens outside of putting air through the horn.

We need real data on the dispersion of material and virus, and the candle test, while fascinating, is entirely irrelevant in my opinion. We know SOMETHING is coming out. We do not know if that matters from this, and as a test it isn't designed to find that answer.

Cheers,
Andy

[Posaunus]

This is an interesting experiment:



Ronnie

This does very little for me... Conservation of mass, we know the amount coming out is dispersed over the bell diameter, sure, it may not go straight into the flame, but it goes into the room.

We need real data on the dispersion of material and virus, and the candle test, while fascinating, is entirely irrelevant in my opinion. We know SOMETHING is coming out. We do not know if that matters from this, and as a test it isn't designed to find that answer.

Cheers,
Andy

Of course this little experiment clearly demonstrated the significant reduction in the velocity of the airflow exiting the mask or the trombone bell. This will reduce the immediate dispersal range of the exhaust air, so is indeed somewhat relevant. But as you point out, it shows nothing about how, or how much, or how effectively virus is transmitted. Those answers are yet to be revealed.

[tim]

correct me if i'm wrong but when air condenses, doesn't the water become distilled?

[BGuttman]

correct me if i'm wrong but when air condenses, doesn't the water become distilled?

Well, yes. BUT.

Your breath contains droplets of saliva and body fluids. In some cases the droplets are really tiny (in which case they are called aerosols). The droplets land on the walls of the instrument or if they are aerosols they don't land at all; they just travel through the instrument. Condensing moisture from the breath now washes these droplets down to the water key. So the stuff you drain out of the water key is a mixture of pure water plus whatever it washed out of your trombone.

[timothy42b]

correct me if i'm wrong but when air condenses, doesn't the water become distilled?

Bruce explained well but I'll add what convinced me.

We know we get lime deposits, enough to jam a valve if we don't keep it clean. Distilled water doesn't contain lime, so it follows that a significant amount of saliva gets into the horn. Saliva does contain lime though in very small amounts.

[baileyman]

I have heard that the aerosols in question may show up in black light, which I keep forgetting to try in a dark room.

[BGuttman]

I have heard that the aerosols in question may show up in black light, which I keep forgetting to try in a dark room.

No, the aerosols will show up in a strong beam of light like you described to me in a private communication. It's called the Tyndall effect. Just think of car headlights in the fog.

[Oslide]

There is a new article (July 13) in JAMA, "Airborne Transmission of SARS-CoV-2".
https://jamanetwork.com/journals/jama/f ... le/2768396

The authors suggest "that long-range aerosol-based transmission is not the dominant mode of SARS-Cov-2 transmission".

[BGuttman]

Originally aerosol was not suspected as a major form of transmission. Then it was discovered that it could be a vector.

My guess is that there is a minimal "dose" of virus to create an infection. Brief encounter with contaminated aerosol is not enough, but probably extended contact with infected aerosol may be enough. Shorter contact with large droplets is a faster way to get enough contamination.

More will come out as we learn more.

[Posaunus]

There is a new article (July 13) in JAMA, "Airborne Transmission of SARS-CoV-2".
https://jamanetwork.com/journals/jama/f ... le/2768396

The authors suggest "that long-range aerosol-based transmission is not the dominant mode of SARS-Cov-2 transmission".

This is a very important, fact-based observation. All who have commented on this thread should read it.

The bottom line:
__________ __________ __________ __________ __________ __________ __________ __________ __________

"What this means in practice is that keeping 6 feet apart from other people and wearing medical masks, high-quality cloth masks, or face shields when it is not possible to be 6 feet apart (for both source control and respiratory protection) should be adequate to minimize the spread of SARS-CoV-2 (in addition to frequent hand hygiene, environmental cleaning, and optimizing indoor ventilation).

" ...the balance of currently available evidence suggests that long-range aerosol-based transmission is not the dominant mode of SARS-CoV-2 transmission."
__________ __________ __________ __________ __________ __________ __________ __________ __________

Droplets appear to be far more deadly than aerosols. Read the entire article for details.

[Basbasun]

Does the flow rate vary passing through the venturi of the throat of the mouthpiece (.276”), into the slide section (.547”), and out through the bell section?

The air flow rate decreases while the pipe volume increases, as it does in a conically trombone.
Well I think you know that allready?

[baileyman]

..."(in addition to frequent hand hygiene, environmental cleaning, and optimizing indoor ventilation)."...

Handwashing surely is good, but I wonder why gloves have not become popular. When I have my gloves on, I know for sure I have not gotten something on my hands, and I cannot inadvertently pick my nose. Seems like a winner.

[brtnats]

..."(in addition to frequent hand hygiene, environmental cleaning, and optimizing indoor ventilation)."...

Handwashing surely is good, but I wonder why gloves have not become popular. When I have my gloves on, I know for sure I have not gotten something on my hands, and I cannot inadvertently pick my nose. Seems like a winner.

Just a guess from a former life in food service: Gloves are germ and dirt magnets without proper training and significant diligence. Regularly washed hands are almost always preferable to gloves.

Regarding the NFHS study: This is the one all the teachers seem to be watching. The preliminary results have a lot to say about air circulation in enclosed spaces and give some best-practice advice about staggering time in the space. That’s something I’m going to use extensively when school restarts. As noted above, the big unknown is how far droplets can recirculate indoors before becoming inert. The risk is not 0 with wind instruments, but it looks like the major risk is everyone breathing the recirculated air at the same time.

[JohnL]

Does the flow rate vary passing through the venturi of the throat of the mouthpiece (.276”), into the slide section (.547”), and out through the bell section?

The air flow rate decreases while the pipe volume increases, as it does in a conically trombone.
Well I think you know that allready?

The air flow rate (the total amount of air over a given time, generally measured in cubic meters per minute or cubic feet per minute) does not change through a change in tubing diameter; It's the air velocity that changes (increasing if the tubing gets smaller or decreasing if the tubing gets bigger).

[Basbasun]

The air flow rate decreases while the pipe volume increases, as it does in a conically trombone.
Well I think you know that allready?

The air flow rate (the total amount of air over a given time, generally measured in cubic meters per minute or cubic feet per minute) does not change through a change in tubing diameter; It's the air velocity that changes (increasing if the tubing gets smaller or decreasing if the tubing gets bigger).

Hm. I just phoned a former professor I used to do work with. The pressure falls when the pipe gets more conical. A gas that is flowing through the tube slows down in the conical part. Air that is blown in the mpc with some speed comes out of the bell with much slower speed. If you messure the air speed coming out of just the slide with no bell, the speed would be faster. But maybe that is the velocity? Ok, the air comming out the bell moves much slower than air comming out of the mouth when speeking.

[BGuttman]

Air is not inelastic (otherwise we would not be able to play). Conservation of mass indicates that whatever air we put in the horn an equivalent amount of matter will come out. But air is compressible to some extent. So when we blow in we increase the pressure a little so the volume goes down At the other end of the trombone the pressure is atmospheric so the volume goes back to what it should. In general, the smaller diameter tubing will have faster air flow and larger diameter tubing (including, and especially the bell flare) will have slower air flow.

Note that air flow velocity is WAY less than the speed of sound.

[JohnL]

But maybe that is the velocity?

Yes. For the purposes of this discussion, "speed" and "velocity" can probably be considered interchangeable (sorry to any science teachers who are reading this); it's how fast the air is moving.

[timothy42b]

Hm. I just phoned a former professor I used to do work with. The pressure falls when the pipe gets more conical. A gas that is flowing through the tube slows down in the conical part. Air that is blown in the mpc with some speed comes out of the bell with much slower speed.

This is correct for this case. When the duct is wider, and the same amount of mass flowing, obviously it must flow faster - for this case.

Two comments.

As Bruce said, the air flows very slowly relative to the sound wave. What makes air move is more pressure on one side. The pressure past the mouthpiece is higher than that outside the bell. But it's not a lot larger, so it doesn't move very fast. I've measured it at 1 inch of H2O above ambient in a tee-fitting that I had the mouthpiece inserted into and that fit on the receiver. To the sound wave, the air is largely stationary.

Second comment. I mentioned "in this case." It isn't always true, if the velocity of the gas approaches that of the speed of sound in that gas. See choked flow:
https://en.wikipedia.org/wiki/Choked_flow

and supersonic flow:
https://history.nasa.gov/SP-440/ch5-2.htm

[VJOFan]

I'm not worried about the little bit of air coming out of the bell.

What I think is concerning is that every player will still be breathing without a mask when they're NOT playing. Distance helps, but a group indoors without masks is still risky.

There are these types of things being invented... masks that you can play with and bell coverings.

http://www.unitedsound.org/mask?fbclid= ... BkQfzvnPPk

[Bach5G]

This is as good a place as any to put this.

From the WSJ:

How Exactly Do You Catch Covid-19?

There Is a Growing Consensus that Surface contamination and fleeting encounters are less of a worry than close-up, person-to-person interactions for extended periods

By Daniela Hernandez, Sarah Toy and Betsy McKay
Updated June 16, 2020 10:39 am ET
Six months into the coronavirus crisis, there’s a growing consensus about a central question: How do people become infected?

It’s not common to contract Covid-19 from a contaminated surface, scientists say. And fleeting encounters with people outdoors are unlikely to spread the coronavirus.

Instead, the major culprit is close-up, person-to-person interactions for extended periods. Crowded events, poorly ventilated areas and places where people are talking loudly—or singing, in one famous case—maximize the risk.

These emerging findings are helping businesses and governments devise reopening strategies to protect public health while getting economies going again. That includes tactics like installing plexiglass barriers, requiring people to wear masks in stores and other venues, using good ventilation systems and keeping windows open when possible.

[baileyman]

Air is not inelastic (otherwise we would not be able to play). ...

Seems pressure waves would still travel in inelastic situations, for instance singing pipes in apartment houses, Brooklyn perhaps.

And regardless of the speed of air coming out of the horn the volume is the same as breathing. I still need to test that blacklight thing just to see for myself. It was shocking, warming up in the garage with glancing morning light in the windows highlighting the fog of my buzz.

No group buzzing, okay?

[BGuttman]

Get off the blacklight fetish. All you need is a strong flashlight beam in a darkened room. You will see all the aerosol there is. Tyndall reigns supreme.

[baileyman]

Okay Bruce, experiment concluded. Bach 8 standard slide, NY, 11C, dark garage, blacklight mounted adjacent to bell in Workmate vise top, forte played on middle Bb, F, low Bb. Observation of aerosol partials in blacklight, zero.

Faith-based science prevails.

To be fair will re-try in the glancing morning sunlight.

[Basbasun]

Air is not inelastic (otherwise we would not be able to play). ...

Seems pressure waves would still travel in inelastic situations, for instance singing pipes in apartment houses, Brooklyn perhaps.

And regardless of the speed of air coming out of the horn the volume is the same as breathing. I still need to test that blacklight thing just to see for myself. It was shocking, warming up in the garage with glancing morning light in the windows highlighting the fog of my buzz.

No group buzzing, okay?

The same as breathing yes, but thinking on how long time it takes to play a long tone, and breath out the same amount of air with the same speed it shows that the breathing out is very slow, right? Much slower the our usual exhalation.

[VJOFan]

Seems pressure waves would still travel in inelastic situations, for instance singing pipes in apartment houses, Brooklyn perhaps.

And regardless of the speed of air coming out of the horn the volume is the same as breathing. I still need to test that blacklight thing just to see for myself. It was shocking, warming up in the garage with glancing morning light in the windows highlighting the fog of my buzz.

No group buzzing, okay?

The same as breathing yes, but thinking on how long time it takes to play a long tone, and breath out the same amount of air with the same speed it shows that the breathing out is very slow, right? Much slower the our usual exhalation.

But how long can you speak on one breath? Our respiration when at rest is different from our respiration when doing anything else. A colleague of mine made the point that over time we have to average the same amount of gas exchange because breathing more or less than what your body requires causes detrimental physiological effects. I can’t see anyone putting a smaller volume of air out while they play than when they don’t.

[Basbasun]

This is a good demonstation of the subject. The air speed from the bell is very very slow.