Metal and Wood Horns

[Marco Gilardetti]

So if I understand this correctly the walls of the cello are designed to reproduce the frequency of and vibrate in sympathy with the vibrations generated by the strings

Yes, it is more or less so. The string itself, vibrating in free air, would put very little air particles in motion. The cello's body, with its wide surface, helps to put in motion many more air particles all around, generating strong sound waves that are easily hearable.

the horn is designed to be a dead thing and to not vibrate but simply direct the sound.

Well, not exactly... or better "no longer exactly". This is basically what the "ancient" theory said: that megaphones and similar objects worked because they concentrated the sound in one direction, but this is not what really happens.

The acoustical impedance is to some degree similar to electric impedance (hence the similar name), should you be somewhat more familiar with it. Very shortly, it can be mathematically demonstrated that the power that can be transmitted from one device to another device is maximised if the two devices have the same impedance. For example, in tube radios or amplifiers, as tubes have an inherently very high impedance, if the output is connected directly to a loudspeaker (which conversely has tipically a very low impedance, around 8 Ohm or so) almost no sound can be heard. This is why tube radios have output transformers in order to connect the output tube to the loudspeaker: the transformer is calculated so that the impedance of the tube and of the loudspeaker are matched. The loudspeaker "sees" the tube through the transformer as if it had a low impedance, and on the other side the tube "sees" the loudspeaker through the transformer as if it had a high impedance. If the two impedances are exactly matched, the power transmitted reaches its maximum.

With gramophones, the vibrating diaphragm is the "high impedance" device, which generates in a small area in its surroundings sound waves of compressed air. The huge quantity of air filling the space of the room has "low impedance" and the tiny soundwaves surrounding the soundbox would not be able to put in motion all the huge quantity of air particles filling the room, and as a result only a feeble, almost un-hearable sound is generated. If a horn is applied to the diaphragm, then the air particles set in motion by the diaphragm will not interact directly with the entire room, but they will progressively transmit their motion to increasing and increasing surfaces inside the horn, while the waves expand as they travel from the small throat of the horn to the wide mouth of the horn. When the wavefront reaches the mouth of the horn, there is not too much impedance mismatch left, and the sound is delivered to the room efficiently.

[Daithi]

no longer exactly ..... This is basically what the "ancient" theory said: that megaphones and similar objects worked because they concentrated the sound in one direction, but this is not what really happens.

With gramophones, the vibrating diaphragm is the "high impedance" device, which generates in a small area in its surroundings sound waves of compressed air. The huge quantity of air filling the space of the room has "low impedance" and the tiny soundwaves surrounding the soundbox would not be able to put in motion all the huge quantity of air particles filling the room, and as a result only a feeble, almost un-hearable sound is generated. If a horn is applied to the diaphragm, then the air particles set in motion by the diaphragm will not interact directly with the entire room, but they will progressively transmit their motion to increasing and increasing surfaces inside the horn, while the waves expand as they travel from the small throat of the horn to the wide mouth of the horn. When the wavefront reaches the mouth of the horn, there is not too much impedance mismatch left, and the sound is delivered to the room efficiently.

Thanks Marco you have gone to huge efforts on my behalf and I doubt I deserve them.
I still don't understand what impedance means. My understanding of the word itself is that its creating an obstruction to an action or getting in the way of physical movement or taking energy from that movement and slowing it down as a result. Sound engineers talk like electricians and electricians talk like plumbers. Each discipline uses another as an analogy instead of discussing the specific meaning itself as it relates to the specific discipline. Its very confusing and circular thinking for me. I wonder what plumbers think about impedance. Is a tap or faucet high impedance since it represents a consriction in the pipe. Is a drain low impedance when it flows freely but high impedance when its blocked by a dead rat.
If we say that "the vibrating diaphragm is the "high impedance" device", what exactly does that mean. Why is it high impedance. What makes it high impedance and what exacty is it impeding(needle vibrations?) or stopping or slowing down and how is it doing that?
I think I can see how the whole room is low impedance because there is plenty of volume space in the room and therefore its like the unblocked drain. See even I can't talk about it without resorting to an analogy from another discipline.

I really like your description of how the horn transmits sound and if I understand it correctly, when you say "they will...transmit their motion to increasing and increasing surfaces inside the horn" is this a reference to the surface of a wave front moving towards the mouth of the horn in the cavity created by the horn or a refernce to the movement of molecules within the material of the horn itself?

[Daithi]

I have been reading and reading and came across the following from a guy in India talking about headphones.

Wood is not so great thats why its great....due to irregular pore design, density etc.. It colours the sound, but in a way that it sounds tonal to people plus due to pores have large surface area increasing soundstage and sub bass.

Metal on other hand avoid colouration on mids and bass, but deeply colour treble....because metal density is high so it avoids shaking colouration....but it reflects thing back to your driver....where treble goes harsh

Plastic is perfect balance between these two

[Marco Gilardetti]

My understanding of the word itself is that its creating an obstruction to an action or getting in the way of physical movement or taking energy from that movement and slowing it down as a result.

That's quite correct.

Sound engineers talk like electricians and electricians talk like plumbers. Each discipline uses another as an analogy instead of discussing the specific meaning itself as it relates to the specific discipline. Its very confusing and circular thinking for me. I wonder what plumbers think about impedance.

The problem is that impedance can be efficiently described only in terms of complex formulae (that is: built up by complex numbers) and using the math language of differential algebra. I know very few people who vaguely remember what complex numbers are, and among those, those that are fluent in differential algebra I can count with the fingers on one hand (and some fingers remain unused). So I suppose this is why most engineers/phisicists try to exemplify these concepts through similarities that a broader number of people should be more or less familiar with.

If we say that "the vibrating diaphragm is the "high impedance" device", what exactly does that mean. Why is it high impedance.

As said, an exact definition would imply transcribing complex differential formulae, which are useless in this context in my opinion. As a side note, I've never seen an exhaustive description of impedance in near field scenario, even though I'm 100% sure that someone addressed the problem at a point in history (usually, the far field scenario is described, with many semplifications, and impedance is treated as a constant). Whatever. Acoustic impedance is by definition a pressure divided by a volume. Intuitively, try to figure out that in the immediate surroundings of the soundbox there is a local high pressure generated by the movement of the diaphragm, thus there's a local high impedance environment. When a soundwave passes abruptly from a high impedance to a lower impedance (and vice-versa) environment, what happens is that only a part of the wave is transmitted forward, the other part being reflected back. The more abrupt the transition is, the more the wave is reflected back. The horn acts in such a way that the soundwaves don't hit immediately the whole air of the room, but the energy of the wave is transmitted to progressively increasing surfaces until, when the wave finally leaves the horn's mouth and enters the room, the change is not so abrupt as to reflect back a relevant portion of the wave.

I really like your description of how the horn transmits sound and if I understand it correctly, when you say "they will...transmit their motion to increasing and increasing surfaces inside the horn" is this a reference to the surface of a wave front moving towards the mouth of the horn in the cavity created by the horn or a refernce to the movement of molecules within the material of the horn itself?

No, the molecules of the horn should stay inert. The less they move, the better it is. However, you can think about it in terms of a wavefront progressing and expanding smoothly, as well as in terms of air particles (particles of the air included within the horn) hitting each other. It can be demonstrated that if few particles immediately hit a huge number of particles (that fill a room, in our case) there will be a very feeble effect, while if the same few particles first hit a not-so-few group of particles, which in turn hit more particle, which finally hit a huge number of particles, the energy is transferred much more efficiently.

Hope this helped to some degree.

[Marco Gilardetti]

I have been reading and reading and came across the following from a guy in India talking about headphones.

Aside from the fact that what this folk from India says is questionable per se, headphones and the way in which they are built have absolutely nothing to do with horns and horn-loaded speakers.

[Daithi]

Whatever. Acoustic impedance is by definition a pressure divided by a volume. Intuitively, try to figure out that in the immediate surroundings of the soundbox there is a local high pressure generated by the movement of the diaphragm, thus there's a local high impedance environment. When a soundwave passes abruptly from a high impedance to a lower impedance (and vice-versa) environment, what happens is that only a part of the wave is transmitted forward, the other part being reflected back. The more abrupt the transition is, the more the wave is reflected back. The horn acts in such a way that the soundwaves don't hit immediately the whole air of the room, but the energy of the wave is transmitted to progressively increasing surfaces until, when the wave finally leaves the horn's mouth and enters the room, the change is not so abrupt as to reflect back a relevant portion of the wave............
......the molecules of the horn should stay inert. The less they move, the better it is. However, you can think about it in terms of a wavefront progressing and expanding smoothly, as well as in terms of air particles (particles of the air included within the horn) hitting each other. It can be demonstrated that if few particles immediately hit a huge number of particles (that fill a room, in our case) there will be a very feeble effect, while if the same few particles first hit a not-so-few group of particles, which in turn hit more particle, which finally hit a huge number of particles, the energy is transferred much more efficiently. ........Hope this helped to some degree.

It really does help a lot in fact you are starting to make sense to me. Your statement that "Acoustic impedance is by definition a pressure divided by a volume" makes total sense to me and clears up a lot of confusion for me.
So getting back to horn materials. Presumably wood resonates a large range of frequencies from bass to treble otherwise why make a violin from it? Whereas metal, if the guy from India is anything to go by, tends to only resonate the treble(depending on the thickness perhaps)?
Your statement that "the molecules of the horn should stay inert" points the way to the ideal horn material in my view. A material that remains inert in the presence of a pressure wave. Presumably it should not absorb too much energy from the wave and should not vibrate in sympathy with the wave. So what in your opinion is an inert material in this context?
And finally thanks very much for your explanations and your patience.

[Daithi]

Perhaps there is no such thing as a material that does not resonate so failing that how about a material that resonates well outside the range of normal human hearing?

[epigramophone]

This discussion has become far too technical for me to follow, but I would suggest that the "papier applique" external horns fitted to EMG and Expert hand made gramophones are as acoustically inert as it is possible for a horn to be. Therein lies their superiority over all other gramophones.

[Daithi]

This discussion has become far too technical for me to follow, but I would suggest that the "papier applique" external horns fitted to EMG and Expert hand made gramophones are as acoustically inert as it is possible for a horn to be. Therein lies their superiority over all other gramophones.

I'm out of my depth too. Is this Papier Mache? Its kind of an amorphous material which might tend to render it inert to vibrations especially if its made with the paper pulp method. I was wondering if a laminate of differnt materials would have noise canceling properties?.

[Lucius1958]

This discussion has become far too technical for me to follow, but I would suggest that the "papier applique" external horns fitted to EMG and Expert hand made gramophones are as acoustically inert as it is possible for a horn to be. Therein lies their superiority over all other gramophones.

I'm out of my depth too. Is this Papier Mache? Its kind of an amorphous material which might tend to render it inert to vibrations especially if its made with the paper pulp method. I was wondering if a laminate of differnt materials would have noise canceling properties?.

"Papier appliqué" uses layers of sheet paper glued together, rather than the pulp used in "papier mâché".

- Bill