Welcome to the Piano World Piano Forums
Over 2 million posts about pianos, digital pianos, and all types of keyboard instruments
Join the World's Largest Community of Piano Lovers (it's free)
It's Fun to Play the Piano ... Please Pass It On!

Gifts and supplies for the musician
SEARCH
the Forums & Piano World

This custom search works much better than the built in one and allows searching older posts.
(ad 125) Sweetwater - Digital Keyboards & Other Gear
Digital Pianos at Sweetwater
(ad) Pearl River
Pearl River Pianos
(ad) Pianoteq
(ad) P B Guide
Acoustic & Digital Piano Guide
Ad (Piano Sing)
How to Make Your Piano Sing
Who's Online
122 registered (Anne'sson, anamnesis, AndyP, accordeur, A Guy, Alegretto, 37 invisible), 1552 Guests and 17 Spiders online.
Key: Admin, Global Mod, Mod
Quick Links to Useful Piano & Music Resources
Our Classified Ads
Find Piano Professionals-

*Piano Dealers - Piano Stores
*Piano Tuners
*Piano Teachers
*Piano Movers
*Piano Restorations
*Piano Manufacturers
*Organs

Quick Links:
*Advertise On Piano World
*Free Piano Newsletter
*Online Piano Recitals
*Piano Recitals Index
*Piano & Music Accessories
*Music School Listings
* Buying a Piano
*Buying A Acoustic Piano
*Buying a Digital Piano
*Pianos for Sale
*Sell Your Piano
*How Old is My Piano?
*Piano Books
*Piano Art, Pictures, & Posters
*Directory/Site Map
*Contest
*Links
*Virtual Piano
*Music Word Search
*Piano Screen Saver
*Piano Videos
*Virtual Piano Chords
(ad) Estonia Piano
Estonia Pianos
Page 4 of 4 < 1 2 3 4
Topic Options
#2269165 - 04/30/14 04:38 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: RMT]
Olek Offline
7000 Post Club Member

Registered: 03/14/08
Posts: 7904
Loc: France
Hello . About 4 hours for the plate to expand ans slightly raise the lowered pitch . It does not get back to original however.

If you have an ETD just laying the thumb for 20 seconds on a string will show some lowering..

Interesting.. i have read those tests made in the 90 . It was in PTG journal.

Regards
_________________________
It is critical that you call your Senators and Representatives and ask them to cosponsor S. 2587 and H.R. 5052. Getting your legislators to cosponsor these bills


Top
(ad PTG 757) The Value of PTG Membership
The Value of a PTG Membership
#2269171 - 04/30/14 04:51 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: A454.7]
prout Offline
500 Post Club Member

Registered: 11/14/13
Posts: 903
Originally Posted By: A443
Originally Posted By: prout
What interests me about this discussion is that there is a huge amount of data available on moisture content in wood, how to dry it, protecting it, what the amount of EMC in the wood is for a given RH and temperature - everything a house, boat, or furniture builder could ever want to know, including precise data, measured in 1/000ths of an inch of the seasonal variations expected in every region of the world of just about every species of wood, but NOTHING about short time span (2 hours) changes.


YES, prout, that is what has fascinated me for a long time now: there is a lot of well-reseachered data out there on spruce and EMC, just not exactly what we want to know regarding its relationship to the piano!

We could probably test this--and produce some data--if someone has an idea of how to stabilise RH/temp in an controlled environment (e.g., a small box or something). Any ideas?


FINALLY, some data that helps.

Moisture Content eventually comes to an Equilibrium with air moisture, approximately, in the case of sitka spruce to

relative humidity % 30 35 40 45 50 55 60 65 70 75 80
moisture content % 6.2 6.9 7.7 8.5 9.2 10.1 11.0 12.0 13.1 14.4 16.0

(Notice that temperature is not a factor. The variation in MC, for a given RH, due to a temperature change of 10 C is &#8776;0.1)

At MC lower than the fiber saturation point (usually 35%), moisture change takes place by diffusion within the wood. The standard diffusion equation may be written as

t = L / D

where

D = 1x10-6 cm/s transverse and radial.
L is the length along the direction of diffusion.
t is the time to 1/e of the moisture change, that is to 63% of the equilibrium change.

So, if you have a soundboard approx. 1 cm thick that is at 8.5% MC (RH 40%) and you want to estimate how fast it will come to equilibrium in your home at 70% RH (13.1% MC) if exposed to air both sides, L = 0.5 cm and the diffusion equation gives t = 2.5x105 s, or about 3 days. The equilibrium MC change required is 4.6%, so in 3 days you can expect 63% of 4.6% = 2.9% higher MC, that is 11.4% total MC. That leaves 1.7% to go, and you can expect 63% of that 1.7% to take place over the next 3 days, to 12.4% MC. So, a little over 1 week should be enough time for 1 cm thick wood to come into equilibrium. According to the literature, most seasoned temperate woods change moisture at a rate within 20% of this.

The MC change over two hours for a soundboard in the environment above, would be negligible, on the order of 0.08%.

Source: http://web.ncf.ca/bf250/wetwood.html

Edit: "Approximate" symbol not shown


Edited by prout (04/30/14 04:53 PM)

Top
#2269174 - 04/30/14 05:01 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: Grotriman]
DoelKees Offline
1000 Post Club Member

Registered: 05/01/10
Posts: 1766
Loc: Vancouver, Canada
String length does not change when temperature changes, only the tension changes. You need the tensile modulus to figure this out.

Kees

Top
#2269184 - 04/30/14 05:39 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: prout]
A454.7 Offline
1000 Post Club Member

Registered: 04/28/12
Posts: 1577
Loc: Manywheres
Originally Posted By: prout
Originally Posted By: DoelKees
String length does not change when temperature changes, only the tension changes. You need the tensile modulus to figure this out.

Kees


You bring up a good point. Using the nominal speaking lengths of wires on the piano, does the tension have a noticeable effect on the wire length? I am thinking of high voltage wires whose length is much greater than the distance between towers, and whose resultant resonant fundamental freq. would be affected by some interaction between length and tension.



Ohhhhh...that is something that I have never thought about considering...in other words: is the piano's string length actually shorter than what it appears to be because there is tension involved?!?

See what happens when we all take the time to have a thoughtful discussion; it's nice, right?!? People from different fields actually have a lot to contribute. :-D


Edited by A443 (04/30/14 05:54 PM)
_________________________
Masters degree in piano technology, +factory(s) training, etc., blah, blah, yada, yada, yada...[uncensored break-out in song]..."it don't mean a thing, if you aint got that swing."
--Klavierbaukuenstler des Erwachens--
Email: klavierbaukuenstler@gmail.com

Top
#2269192 - 04/30/14 05:51 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: RMT]
A454.7 Offline
1000 Post Club Member

Registered: 04/28/12
Posts: 1577
Loc: Manywheres
prout, THANK YOU SO MUCH for finding info on soundboard EMC equilibrium! I am still mentally processing the info--I'm sure I'll have some questions for you later so I can do the math on my own and adjust for variables.

We'd also need to factor in the bridge height, but it looks like this can easily be calculated...

Any info/idea on how a layer of lacquer/polyester/shellac would slow down this diffusion (i.e., how to calculate for this)?
_________________________
Masters degree in piano technology, +factory(s) training, etc., blah, blah, yada, yada, yada...[uncensored break-out in song]..."it don't mean a thing, if you aint got that swing."
--Klavierbaukuenstler des Erwachens--
Email: klavierbaukuenstler@gmail.com

Top
#2269198 - 04/30/14 05:56 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: A454.7]
prout Offline
500 Post Club Member

Registered: 11/14/13
Posts: 903
Originally Posted By: A443
prout, THANK YOU SO MUCH for finding info on soundboard EMC equilibrium! I am still mentally processing the info--I'm sure I'll have some questions for you later so I can do the math on my own and adjust for variables.

We'd also need to factor in the bridge height, but it looks like this can easily be calculated...

Any info/idea on how a layer of lacquer/polyester/shellac would slow down this diffusion (i.e., how to calculate for this)?


Only that it does slow down the diffusion. Too many variables.

Top
#2269205 - 04/30/14 06:08 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: RMT]
A454.7 Offline
1000 Post Club Member

Registered: 04/28/12
Posts: 1577
Loc: Manywheres
Grotriman, prout, et.al, if anyone could figure out the tension issue to predict the change in pitch via temperature, that would be amazingly helpful...to many technicians!

I guess, though, it would need to be the entire length of the string, not just the speaking length. Right? Since the whole thing is expanding...

The plate also needs to be factored in. It takes more time to change, but I know from experience [a very unfortunate one] that it too matters and has a big impact on the pitch level.
_________________________
Masters degree in piano technology, +factory(s) training, etc., blah, blah, yada, yada, yada...[uncensored break-out in song]..."it don't mean a thing, if you aint got that swing."
--Klavierbaukuenstler des Erwachens--
Email: klavierbaukuenstler@gmail.com

Top
#2269215 - 04/30/14 06:32 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: A454.7]
Eric Gloo Offline
1000 Post Club Member

Registered: 05/28/01
Posts: 1266
Loc: Richfield Springs, New York
Are you familiar with Don Gilmore's "Self-tuning Piano"? The concept is each string is heated, which adjusts the pitch to a pre-set tuning. He would probably have some of the information you are seeking.
_________________________
Eric Gloo
Piano Technician
Certified Dampp-Chaser Installer
Richfield Springs, New York

Top
#2269226 - 04/30/14 06:51 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: A454.7]
DoelKees Offline
1000 Post Club Member

Registered: 05/01/10
Posts: 1766
Loc: Vancouver, Canada
I did the calculation based on these numbers which I happen to have for an A4 string on my piano.
-expansion coeff: 1.6e-5 /C
- Young modulus 1.75e11 N/m^2
- mass density 7850 kg/m^3
- freq 440Hz
- speaking length .375m

I get 2.8 cent pitch drop per degree (assuming only string changes in temp). Seems high, anyone who can check this?

Added: I just tried something on the A4 string. I measured pitch then applied hot air from heater for about 5 sec towards the piano from about 50cm away, and the pitch had dropped by a whopping 13 cent. Maybe 3 cent/degree is correct.

Kees


Edited by DoelKees (04/30/14 06:58 PM)

Top
#2269241 - 04/30/14 07:09 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: RMT]
A454.7 Offline
1000 Post Club Member

Registered: 04/28/12
Posts: 1577
Loc: Manywheres
lol...now, you're cooking with gas!!!

But, this is where I am lost...math and I are not friends. :-(

On a related topic: if the ambient temperature is 20C, what is the surface temperature of the steel wire? Or, what should it be? Same question for the plate: what should the surface temperature be?
_________________________
Masters degree in piano technology, +factory(s) training, etc., blah, blah, yada, yada, yada...[uncensored break-out in song]..."it don't mean a thing, if you aint got that swing."
--Klavierbaukuenstler des Erwachens--
Email: klavierbaukuenstler@gmail.com

Top
#2269267 - 04/30/14 08:17 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: RMT]
Grotriman Offline
500 Post Club Member

Registered: 03/07/04
Posts: 724
Loc: New York City
OK OK OK

I stand corrected - I did the thumb experiment - and to my surprise my unison did go out. Not to the order of magnitude that a humidity change will cause, but still quite audible!

@443 - temperature of the steel wire depends on several factors (like keeping a premature baby warm) - you have radiated heat loss, conductive heat loss and heat loss due to thermal conductivity.

So it actually depends on the temperature of the walls, the temperature of the air blowing by the strings, the humidity of the air (not because of the RH effect on the sound board but due to the thermal conductivity of the air). And the absolute temperature.
_________________________
Regards,

Grotriman

Top
#2269270 - 04/30/14 08:23 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: prout]
Grotriman Offline
500 Post Club Member

Registered: 03/07/04
Posts: 724
Loc: New York City
@prout - you still need to relate moisture change to dimensional change and then to change in crown height (not proportional). But I certainly know when the humidity changes within hours my entire scale is out of whack.
_________________________
Regards,

Grotriman

Top
#2269272 - 04/30/14 08:27 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: DoelKees]
Grotriman Offline
500 Post Club Member

Registered: 03/07/04
Posts: 724
Loc: New York City
Originally Posted By: DoelKees
I did the calculation based on these numbers which I happen to have for an A4 string on my piano.
-expansion coeff: 1.6e-5 /C
- Young modulus 1.75e11 N/m^2
- mass density 7850 kg/m^3
- freq 440Hz
- speaking length .375m

I get 2.8 cent pitch drop per degree (assuming only string changes in temp). Seems high, anyone who can check this?

Added: I just tried something on the A4 string. I measured pitch then applied hot air from heater for about 5 sec towards the piano from about 50cm away, and the pitch had dropped by a whopping 13 cent. Maybe 3 cent/degree is correct.

Kees


Did you insulate the soundboard? You need to keep the two effects separate.

Of course a temperature reading on the string is necessary to answer your question empirically.
_________________________
Regards,

Grotriman

Top
#2269280 - 04/30/14 08:46 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: Grotriman]
prout Offline
500 Post Club Member

Registered: 11/14/13
Posts: 903
Originally Posted By: Grotriman
@prout - you still need to relate moisture change to dimensional change and then to change in crown height (not proportional). But I certainly know when the humidity changes within hours my entire scale is out of whack.


The dimensional change in the length along the radial axis is very roughly about 1% per 4% MC, which is significant.

Question for you. How do you get a large humidity change in environment where the piano resides in such a short time? It has gone from 50% RH outside my house to 99+% in a matter of hours just yesterday, but the change inside the house was from 47% to 55% and the hygrometers in the piano are unchanged at 50%.

Top
#2269287 - 04/30/14 09:04 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: prout]
Grotriman Offline
500 Post Club Member

Registered: 03/07/04
Posts: 724
Loc: New York City
Originally Posted By: prout
Originally Posted By: Grotriman
@prout - you still need to relate moisture change to dimensional change and then to change in crown height (not proportional). But I certainly know when the humidity changes within hours my entire scale is out of whack.


The dimensional change in the length along the radial axis is very roughly about 1% per 4% MC, which is significant.

Question for you. How do you get a large humidity change in environment where the piano resides in such a short time? It has gone from 50% RH outside my house to 99+% in a matter of hours just yesterday, but the change inside the house was from 47% to 55% and the hygrometers in the piano are unchanged at 50%.


My apartment has windows on one wall only and vents in the bathroom (across the living room where the piano is). I have little choice on when the window is open/closed, as my imposition on the family with the piano, the stereo etc. is already overbearing.

So what ever the outside goes from to, the inside does right away - except in the winter when I can have humidifiers running. I am usually not allowed to use AC in the summer. I know I live under strict rules but there are positive aspects...

My humidity can change 15% in an hour some times.

I have the DC and underwear on the piano and it does fairly well I have to say. But if the pads are bad... or the water runs dry... oy vey.

Your hygrometers in the piano are above a dampp chaser system? or without humidity control?

I should purchase a hygrometer for inside the piano to see what's happening there too.
_________________________
Regards,

Grotriman

Top
#2269303 - 04/30/14 09:45 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: Grotriman]
prout Offline
500 Post Club Member

Registered: 11/14/13
Posts: 903
Originally Posted By: Grotriman
Originally Posted By: prout
Originally Posted By: Grotriman
@prout - you still need to relate moisture change to dimensional change and then to change in crown height (not proportional). But I certainly know when the humidity changes within hours my entire scale is out of whack.


The dimensional change in the length along the radial axis is very roughly about 1% per 4% MC, which is significant.

Question for you. How do you get a large humidity change in environment where the piano resides in such a short time? It has gone from 50% RH outside my house to 99+% in a matter of hours just yesterday, but the change inside the house was from 47% to 55% and the hygrometers in the piano are unchanged at 50%.


My apartment has windows on one wall only and vents in the bathroom (across the living room where the piano is). I have little choice on when the window is open/closed, as my imposition on the family with the piano, the stereo etc. is already overbearing.

So what ever the outside goes from to, the inside does right away - except in the winter when I can have humidifiers running. I am usually not allowed to use AC in the summer. I know I live under strict rules but there are positive aspects...

My humidity can change 15% in an hour some times.

I have the DC and underwear on the piano and it does fairly well I have to say. But if the pads are bad... or the water runs dry... oy vey.

Your hygrometers in the piano are above a dampp chaser system? or without humidity control?

I should purchase a hygrometer for inside the piano to see what's happening there too.



That explains it.

My wife is not happy about having the house closed up year around, but she is happy about the piano's tuning stability.

Two hygrometers are in the room, one in the next room open to the first, and two hygrometers are sitting above the soundboard, and a DC is installed in the usual position for a grand. I keep the lid closed, except as necessary. These are cheap mechanical hygrometers (except for the Air conditioning electronic one) and were selected from a large number I bought, tested in a sodium chloride bath, and choose the four most accurate ones and returned the rest. They are adequate for this purpose.

Top
#2269315 - 04/30/14 10:07 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: Grotriman]
DoelKees Offline
1000 Post Club Member

Registered: 05/01/10
Posts: 1766
Loc: Vancouver, Canada
Originally Posted By: Grotriman
Originally Posted By: DoelKees
I did the calculation based on these numbers which I happen to have for an A4 string on my piano.
-expansion coeff: 1.6e-5 /C
- Young modulus 1.75e11 N/m^2
- mass density 7850 kg/m^3
- freq 440Hz
- speaking length .375m

I get 2.8 cent pitch drop per degree (assuming only string changes in temp). Seems high, anyone who can check this?

Added: I just tried something on the A4 string. I measured pitch then applied hot air from heater for about 5 sec towards the piano from about 50cm away, and the pitch had dropped by a whopping 13 cent. Maybe 3 cent/degree is correct.

Kees


Did you insulate the soundboard? You need to keep the two effects separate.

Of course a temperature reading on the string is necessary to answer your question empirically.

I tried. Hard to believe that moisture will get out of the soundboard in 5 secs, so I think it's just a string effect.

I wish I knew how to take the temperature of a string.

Isn't there a patent of some guy who designed a self-tuning piano by sending currents through the strings to change their temperature and hence tension and hence pitch?

Anyways 3 cent per centigrade is my calculation, I think it's right. Maybe that patent (can't find the thread on this) can corroborate that number (for A4 string on upright).

Experimentally you could rapidly lower the room temperature by, say, 5 degrees (open a window), then start taking pitch measurements. Because of the larger surface area to volume ratio of the strings they will drop to room temperature faster than the plate. So the pitch should quickly go up by 15 cents then drop again when the plate cools and shrinks slowly, and then moisture effects will kick in on the soundboard after that.

Theoretically the shrinking of the strings and the plate should exactly compensate with no pitch change overall. You'd have to weld the strings to the V-bar though.

Kees

Top
#2269397 - 05/01/14 01:07 AM Re: Temp/humidity control for Mason and Hamlin BB [Re: A454.7]
phacke Online   content

Gold Supporter until November 11 2014


Registered: 10/18/12
Posts: 602
Loc: CO, USA
A443 Wrote>let's do an experiment together to gather some real data. OK? Are you willing/able to test something like this out?

Well, it looks like you got your confirmation already; but, I'm all for crowd-sourcing experiments like this after the procedure is agreed upon.

Best wishes-

Originally Posted By: A443
Phacke, the short answer is that a c.2 degree increase in temperature results in a measurable lowering in pitch. How much? It is c.0.2 cents in the melodic section, but it depends on--I assume--the length of the string (i.e., in my observations, it is not a consistent amount across the entire register). I don't yet have a formula to predict these amount; I stopped short of that when I realized that if I stayed within a 1/2 degree, I'd get the tuning results I was looking for.

Grotriman, having worked in countless concert hall throughout the world, I can tell you: I often encounter temperature problems via the stage lighting...and the resultant change in the tuning. This is real; it exists; and it is a significant problem not to be dismissed so easily. Lighting technicians often are double booked to make their own adjustments while I'm working. Within a matter of minutes, one can easily experience dramatic changes in pitch level due to swings in temperature, if one is not attentive! You seem to discount my experiences entirely. Which is fine, but let's do an experiment together to gather some real data. OK? Are you willing/able to test something like this out?
_________________________
phacke

Steinway YM (1933)
...Working on:
J. S. Bach, Sonata No. 1 in B minor (BWV 1014) duet with violin
F. Chopin, Prelude 28 (15)

Top
#2269494 - 05/01/14 08:30 AM Re: Temp/humidity control for Mason and Hamlin BB [Re: DoelKees]
Olek Offline
7000 Post Club Member

Registered: 03/14/08
Posts: 7904
Loc: France
Originally Posted By: DoelKees
I did the calculation based on these numbers which I happen to have for an A4 string on my piano.
-expansion coeff: 1.6e-5 /C
- Young modulus 1.75e11 N/m^2
- mass density 7850 kg/m^3
- freq 440Hz
- speaking length .375m

I get 2.8 cent pitch drop per degree (assuming only string changes in temp). Seems high, anyone who can check this?

Added: I just tried something on the A4 string. I measured pitch then applied hot air from heater for about 5 sec towards the piano from about 50cm away, and the pitch had dropped by a whopping 13 cent. Maybe 3 cent/degree is correct.

Kees


I guess I measured about 1.5+- cts with 1 c (not F)
_________________________
It is critical that you call your Senators and Representatives and ask them to cosponsor S. 2587 and H.R. 5052. Getting your legislators to cosponsor these bills


Top
#2270277 - 05/02/14 04:44 PM Re: Temp/humidity control for Mason and Hamlin BB [Re: Olek]
DoelKees Offline
1000 Post Club Member

Registered: 05/01/10
Posts: 1766
Loc: Vancouver, Canada
Another experiment: Lean over the piano as when you change mute. Then breathe one lungful of air into the piano. A4 dropped 2 cents.

Kees

Top
Page 4 of 4 < 1 2 3 4

Moderator:  Piano World 
What's Hot!!
The December Free Piano Newsletter
-------------------
Forums Rules & Help
-------------------
ADVERTISE
on Piano World

The world's most popular piano web site.
-------------------
PIANO BOOKS
Interesting books about the piano, pianists, piano history, biographies, memoirs and more!
(ad) Yamaha CP Music Rest Promo
Yamaha CP Music Rest Promo
(ad) HAILUN Pianos
Hailun Pianos - Click for More
Ad (Seiler/Knabe)
Seiler Pianos
(125ad) Dampp Chaser
Dampp Chaser Piano Life Saver
(ad) Lindeblad Piano
Lindeblad Piano Restoration
Sheet Music Plus (125)
Sheet Music Plus Featured Sale
New Topics - Multiple Forums
Why did my piano come with a chair rather than a bench?
by George Palmer
Today at 04:46 PM
Raising piano strings to the correct tonality
by MEKS
Today at 02:53 PM
Tuning problem or maybe something else...
by Piano RX
Today at 02:35 PM
The end of the year .....blues
by piano_primo_1
Today at 01:57 PM
shortest acceptable upright for a small apartment
by Dennis R in WA
Today at 12:20 PM
Forum Stats
77460 Members
42 Forums
160169 Topics
2352128 Posts

Max Online: 15252 @ 03/21/10 11:39 PM
Gift Ideas for Music Lovers!
Find the Perfect Gift for the Music Lovers on your List!
Visit our online store today.

Visit our online store for gifts for music lovers

 
Help keep the forums up and running with a donation, any amount is appreciated!
Or by becoming a Subscribing member! Thank-you.
Donate   Subscribe
 
Our Piano Related Classified Ads
|
Dealers | Tuners | Lessons | Movers | Restorations | Pianos For Sale | Sell Your Piano |

Advertise on Piano World
| Subscribe | Piano World | PianoSupplies.com | Advertise on Piano World | Donate | Link to Us | Classifieds |
| |Contact | Privacy | Legal | About Us | Site Map | Free Newsletter | Press Room |


copyright 1997 - 2014 Piano World ® all rights reserved
No part of this site may be reproduced without prior written permission