Dear Fellow Horn Players,

I am a 13-year-old junior high student (so please be patient with me, please) and have enjoyed playing the horn for 6 years. Recently our science teacher gave us an assignment requiring us to come up with an experiment, do trials, make tables and graphs, etc. I decided to use my horn (since I love it so much) in my experiment.

Still, in order to conduct a successful experiment, one must obtain information on the subject/s which you are experimenting. I've surfed the web near and far, looked in encyclopedias, and still am unable to find the needed information. Can anyone help me? I need to basically have detailed answers to the following questions:

  • How do sound waves behave when a note is sharp or flat?
  • How do sound waves behave when a note is changing from sharp to flat or vice versa?
  • How do sound waves behave in rooms of different volume (say, a concert hall and a practice room, for example)?
I might come up with more questions later. Thanks for your help!
-Z
Dear Z,

I suggest going to your local library and look for books regarding acoustical physics. They should be able to help you find general information on how any instrument cuz in a sense all instruments work very much the same in that respect. But to quickly answer your questions in a laypersons way, I guess you would have to think of it this way, the slower the air travels, the flatter the note is going to be, the longer the pipe is, the lower and more flatter the note will be and vice versa. That's usually why trumpets and flutes have a tough time flattening their sound and a tuba has problems sharpening a sound. Sound waves go smaller and faster when notes are sharpened and go bigger and slower when they are flattened. For you're third guestion it all depends on where the sound bounces off, a small room will have a quicker response time due to the fact that the waves have a shorter distance, but it can really make you sound bad and sharp, kinda like a mute or when you stop the horn with your hand, in a way the room is like an extension of this pipe thing I was talking about. But the best thing for you is to go get a book on acoustical physics and try to apply it to your horn project in how this would affect the horn's sound.

M Coco
M Coco wrote some stuff which is partially quoted further down in this message.

IT IS WRONG, WRONG, WRONG!!!

The only good advice there is to look in a book on acoustics. The speed of sound does NOT depend on the pitch or amplitude of the sound. For our purposes it is constant.

Yesterday, I contacted our young friend Zyta with some information and suggestions, and will not repeat that here on the list. I suggested that Zyta find and read Arthur Benade's book, "Horns, Strings and Harmony." I very strongly suggest that M Coco do the same. It is an excellent book

For our younger listers, here are some basic facts:

  1. Sound is waves of pressure traveling through the air.
  2. Higher pitch (sharper) corresponds to higher frequency/shorter wavelength (frequency is the number of vibrations per second, and wavelength is the the distance between two successive peaks or troughs of pressure).
  3. Lower pitch (flatter) corresponds to lower frequency/longer wavelength.
  4. frequency times wavelength (F * L) = speed of sound, about 1087 feet/sec.
  5. Louder corresponds to greater difference in pressure between minima and maxima.

I quote here two excerpts from M Coco's message:

I suggest going to your local library and look for books regarding acoustical physics. They should be able to help you find general information on how any instrument cuz in a sense all instruments work very much the same in that respect.
That is a GOOD SUGGESTION, and the statement is absolutely true.
But to quickly answer your questions in a laypersons way, I guess you would have to think of it this way, the slower the air travels, the flatter the note is going to be...
and so on. That is WRONG!!!

Said layperson is incorrectly informed.

M Coco, sorry if I've hurt your feelings, but this egregious misinformation had to be put right. I will be glad to answer questions which you might have. In addition, there are others on the horn list who are more qualified than I to explain the physics of acoustics.

Richard Berthelsdorf, Ph.D. (physics)
Richard, nowhere in the quote you provided does M Coco say anything about SOUND traveling faster or slower. All I saw was a reference to AIR travelling faster or slower, resulting in a sharper or flatter pitch. Seems perfectly reasonable to me.
Jerry Houston
Still, in order to conduct a successful experiment, one must obtain information on the subject/s which you are experimenting. I've surfed the web near and far, looked in encyclopedias, and still am unable to find the needed information.
Here are some publications that you may be able to find in your library:
----------------

Fasman, Mark J.  

Brass bibliography : sources on the history, literature, pedagogy,
performance, and acoustics of brass instruments
ML128.W5 F3 1990 
------------------

Backus, John

The acoustical foundations of music
ML3805.B245 A3 1977 
-----------------------

Benade, Arthur H. 

Fundamentals of musical acoustics
ML3805 .B328 

Horns, strings, and harmony
ML3805 .B33 
--------------------------

These days your library probably has some kind of computerized card catalog that you could search. For example, I just searched for 'Architectural Acoustics' in my library and got 48 hits.

Charles Turner
I would suggest getting in touch with Bruce Heim at Louisiana State University.
He did is dissertation on Acoustics.

just a thought. If I find his e-mail I'll post it.

much regards

darrel dartez
I received a private message in response to my last post, saying:
Your description implies that sound is a transverse wave, rather than a lateral wave. I know the definitions still apply, but you might wanna clear that up ;-)
OK, I'll try to clear this up, since apparently more than one person is confused.

Sound is not a transverse wave, nor is it a lateral wave.
Sound is a longitudinal mechanical wave.
What does this mean?

Here's a loose quote from an introductory physics book, which might help: Imagine a piston at one end of a long tube filled with air. If we push the piston forward, the layers of air in front of it are compressed. These layers in turn will compress layers further along the tube, and a wave of compression travels down the tube. If we quickly withdraw the piston, the layers of air in front of it expand and a pulse of rarefaction travels down the tube from layer to layer. If the piston oscillates back and forth, a continuous train of compressions and rarefactions will travel along the tube. This is a longitudinal wave train - sound. The particles of the medium (air) are traveling back and forth along the direction of sound propagation, that is, in a longitudinal direction.

Let's do another little thought-experiment. Suppose there is a source of sound producing a concert A (440 Hz). Now freeze time, and measure the air pressure along a line between the sound source and a listener. Starting at some point where the pressure is highest, as you move either backward or forward from that point, you will find that the pressure decreases until it is lowest about 15 inches from the high pressure point. From there, the pressure will increase again, until it reaches a high point just under 30 inches (the wavelength of 440 Hz from the original starting point.

So what's a transverse wave?
Imagine a vibrating violin string. In this case, the particles of the medium (sections of the string) travel from side to side at right angles to the direction of wave propagation (which is along the length of the string), that is, in a transverse direction. The waves you see on the string are transverse waves. As the string vibrates, it pushes on the surrounding air just like the piston mentioned above, generating the longitudinal pressure waves of sound.

You can make both kinds of waves with a Slinky.
Shake one end from side to side, and you will see transverse waves travel down the Slinky. Push and pull one end back and forth, and you will see longitudinal waves travel down it.

If there is still some confusion, perhaps a well-known physics teacher on the list could state things more clearly than I, who have never taught acoustics. However, I am always willing to answer questions.

Richard
Keep answering! It's fascinating.

Now, how about the effect of temperature??

When it's a chilly room, I always have to push my tuning slide in - right? (or am I untalented and desperate for an honest teacher?)? Is it that cooler air is denser and the waves are closer together?

John Pirtle
I don't want to bore the list too much, but here we go.

Yes. Cooler air is denser, so sound travels slower, so the wavelength corresponding to a given frequency is shorter.

For every drop in temperature of 1 degree C, the speed of sound in air decreases by about 2 ft/second. Suppose your horn is 12.48 ft long, corresponding to frequency (1090 ft/sec) / (12.48 ft) = 87.31 Hz (F). Now cool the air in the horn by 10 C. The speed of sound is now 20 ft/sec less, or 1070 ft/sec. Let's assume that the length of the horn doesn't change (is thus true?). The new frequency is 1070/12.48 = 85.7 Hz. By golly, you're way flat!

How much will you have to compensate with your tuning slide? You want to bring the frequency back up to 87.31 Hz. The wavelength of 87.31 Hz in the cooler air is (1070 ft/sec) / (87.3 Hz) = 12.26 ft. So, the horn needs to shorten by 12.48 ft - 12.26 ft = 2.6 inches. The slide needs to go in 1.3". That's a lot!

Now let's test the assumption. How much will the horn change in length? The coefficient of thermal expansion for brass is about 1.9 x 10^-5. That is, for every 1 degree C drop in temperature, the horn shortens by about 19 parts per million. So, if we cool the instrument by 10 C, it will shrink by 190 ppm, or by 12.48 ft * 190/1000000 = 0.03 inches. Far less than the effect of air temperature (and in the other direction).

BTW, if you check the numbers above, you'll find rounding errors here & there which won't affect the basic results. Someone else (Chris?) can explain how sound actually resonates inside a horn.

Richard
Of course when the air enters the horn it is at body temperature, and cools as it progresses through the wrap until it reaches 'horn temperature'. I'm not sure how far in this would be, but it would moderate the effect to some degree. 10 degrees C is about 18 degrees F, and I've certainly played over that temperaure range (seasonal variation) without having to move my slide 1.3".
Chris
Herb Foster very kindly pointed out a stupid error I made in my last message regarding sound waves:
I hope you didn't confuse the issue with the violin statement. Of course, very little sound is produced by the string pushing the air. The vibrations travel down the bridge and down to the back, where most of the sound is radiated, both directly and into the body and through the f holes.
Of course, Herb is absolutely correct. I don't know where my mind was, but it must not have been on the right subject.

The moral is - don't believe everything you read.
A second one is - proofread everything you write.

Richard
Thanks to Chris Stratton for pointing out an important omission in my description of the effect of room temperature on horn pitch:
Of course when the air enters the horn it is at body temperature, and cools as it progresses through the wrap until it reaches 'horn temperature'. I'm not sure how far in this would be, but it would moderate the effect to some degree. 10 degrees C is about 18 degrees F, and I've certainly played over that temperaure range (seasonal variation) without having to move my slide 1.3".
I have to admit that I was somewhat surprised at the 1.3" also, but didn't follow through with thinking about it. If we make a guess that the air reaches "horn temperature" only by the time it reaches the bell, then we'd need to push the tuning slide in half as far, or 0.6". That seems much more reasonable, and I'll bet even that's an overestimate.

During our intial warm-up, we're also changing the temperature of the horn itself, so by the time we're ready to tune to others, the slide will likely not have to go in nearly as far as it would while the instrument was dead cold.

Richard
To give a contribution to the comments of Richard and Chris: in stead of having to pull out the tuning slide 1.3" isn't it more logical that the other players will be in a lower pitch as well? Therefore the difference in pitch would only be to an absolute A=440 but not as dramatic in the orchestra.
Erwin Bous
That is probably true for brass, but I *think* that strings (and woodwinds?) go sharp in a cooler room. ?? So intonation havoc is truly wrought. Am I right oh physics learned ones?

I do know for certain that woodwinds squeak more because they can use the "cold room" excuse. :)

John Pirtle
Richard,

Thanks for the numbers and the conversion to slide length. I thought it was very interesting. I don't think that the rounding will be a problem as five out of four people have problems with fractions anyway.

Later,

Stephen Pearce
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