Voice. Most powerful and natural instrument.

Do you agree?
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Try and imagine the Doors without Jim Morrison.

Voice can, rarely does, but can carry the show. Is opera natural singing? If so what some, say, Placido Domingo can/could do is enough to move you just by the voice.

How many great songs have been made by the particular singer and to such an extent that for a generation or more they have the definitive version. Nat "King" Cole and the Christmas Song.

The voice may not be the most powerful or natural but it is the most magical as to what it can do.
I don't like Domingo's voice, older Carreras's is much more appealing to me.
Natural? Yes, for obvious reasons. Powerful? Yes. Just listen to Azam Ali from 'Portals of Grace'.

I saw Rickie Lee Jones live back when she did 'Pirates' at the Pantages and her voice simply soared over the horn section like they weren't even there. She hit notes with such power that the whole place went slackjawed. No one could believe that someone who was still getting high back then and rambled on incoherently between songs (with one audience member yelling shut up and sing-she just laughed) could suddenly transform her little self into this vocal dynamo. It was quite a show.

Some performers can really belt it out.
On the 'db' end of this scale, it'll come up short.

'Suppoe we could have a yell off... a SAX and Tennor, maybe. I'll bet on the brass for db content....

In either case though, it's not the insturment so much as it is the nut playing it... Same way with voice.

I'd soon as hear plenty of insturments over plenty of voices.... as a rule the instruments will sound better... great singers are far harder to find than instruments. Brass and reeds don't need as much tuning care as do strings, or voices. They appear more the honest pitch and timber appliances than are human voices.

When both are on top of their game, I'll give the nod to the singer over the accompaniement or side men... usually. Alone? Instrumentalists? Well it's all about an emotive connection and music is exactly that... an emotive event.
Belting out a song is one thing "Jeniffer Hudson" singing is another. Singing a song is an art that so few know what to do? The current crop that claim to have the title singer need to go back and reevaluate what it takes to sing.
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After reading Elizabeth's response I think I'll take the Blue Whale over the current crop of diva's who simply yell as loud as they can. I could only watch two minutes of the Grammys and it was so nauseating to see all of them singing together trying to out diva each other. I was waiting to hear the gong but no one hit it.
I should also clarify that when some can really belt it out that the great ones do it without apparent strain. They don't bend over like they're having a seizure, mouth agape, eyes tightly shut, doing the Joe Cocker thing.
They simply sing way better than the rest of us.
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Friends of my parents were opera singers. She was soprano while he was bass/baritone. I heard her practicing ones in the room and it was unbearably loud. It sounded like regular opera voice 5x louder. I've never heard him singing but when he coughed once next to me it was like an explosion.

In Indian classical music voices sound like instruments almost like imitating them. It is the other way around - instruments imitate perfection of voices.
Handel probably more than any other opera composer I've heard used the voice as an instrument, competing and complementing other instruments in their settings, especially in Julius Caesar.
Nonoise: I could not agree with you more. The current crop of singers all come from the new school of vocal nonsense.The school of vocal hysterics...
There is a legend, or truth, that one very famous Russian opera singer could break a crystal glass with his voice. In any case, he is considered to have had the most powerful voice of all singers of that time.
I guess I thought my response was obviously about volume, rather than emotional expression. Of course, the human voice is the most expressive of all instruments, this goes without saying. But as far as sheer power in the sense of volume - not even close. The human voice is simply not capable of the decibel levels of brass and percussion instruments, unless of course it is electronically amplified.

Trained human voice can get up to 124dB and in opera often does (recording eng of MET says 120dB: http://www.metoperafamily.org/guild/diva/question.aspx?id=15).

Shouting Guinness Record is 128dB.

Not many instruments can do that. Loudest string instrument is the smallest one - violin=86dB
French Horn (do you play it?) goes up to 114dB while trumpet might go around 140dB because sound is very concentrated. Drum set is about 115dB.

124dB that opera singer can produce is on the threshold of pain. Opera singers have sung with an orchestra without microphones. Many damaged their own hearing (150dB inside of the head). Pain threshold and loss of hearing was probably the reason why they haven't been trained for even louder voice - today we have microphones.
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And in the future life?
What kind of system would one need to reproduce blue whale's singing realistically, I wonder?
Anyway, it appears that everyone more or less agrees with my original statement about human voice.
Hi Kijanki - yes, I do play the French horn. I tried looking up the decibel levels of various instruments quickly, but I would have to dig deep to find it. I know I have it somewhere. Anyway, the actual decibel level possible is not always that relevant to how loud something is perceived. The ear hears the amplitude of different timbres differently. The sound power of the trombone, for instance, is many times that of the French horn, even if both are playing at the same dynamic level. The sound power of a bass drum is more than four times again that of a trombone. And of course, the sound power of all of these instruments exceeds that of the voice. They can also sustain their maximum volumes for a longer period of time, especially in the case of percussion, where they don't have to stop to breathe, and are using much stronger muscle groups to play. Only if we are talking about a relatively short period of time (a long high note at the end of an aria, say) would the unamplified voice be perceived as loudly as a brass instrument, regardless of the actual decibel level.

That's true, voice cannot sustain peak loudness for a long time but rating of over 120dB is more than peak of your French Horn (114dB). As I said before, I witnessed soprano practicing scales in small room and it was scary loud. It wasn't just very loud human voice but rather sounded like painfully loud amplified voice. I just could not believe that the same person can at one time speak regular voice and the other time produce this incredible loudness, and it was only practice and not even peak.

Talking about effort required - how hard it is to play/blow French Horn. Trumpet players often get very red on the face or end up getting big overinflated cheeks (Dizzy Gillespie comes to mind). If I remember correctly, there is more than one French Horn in the orchestra (possibly 2) and they are located in the very back left side to the left of percussion. That placement would suggest that they posses very loud sound.
Kijanki is absolutely correct. Issues of perception aside, I will offer this example:

I have sat in the clarinet section of the Metropolitan Opera Orchestra during rehearsals of many operas. It is typical during the sitzprobe (first time that the orchestra rehearses with the singers; usually in the rehearsal room, not in the pit/stage), for the singers to stand behind the orchestra, also facing the conductor. They stand further back than both the horns, and the trumpets/trombones. The sheer volume that a great singer can produce is astounding. It is sometimes unbearable in it's power (undistorted, of course), and can definitely be more powerful than a horn, trumpet, or trombone playing full tilt.

Believe it, or not.
Hi Frogman - as usual, you are correct. Indeed, I have had that experience many times myself with opera. I had been thinking more along the lines of an audience perspective. Even opera voices sometimes have difficulty carrying to the back of the hall, though not of course at the Met, where they have the very highest caliber, and a very well designed hall. However, even the average opera voice is not remotely comparable to the average human voice - the average jazz, folk, or pop singer relying on amplification would be incapable of such amplitude. It takes very well trained voices indeed to reach those volumes. I suppose the sound power of such a voice could indeed exceed that even of a trombone, though I doubt it would approach the bass drum. Unfortunately, my reference only gives the relative sound power of the orchestral instruments, not including the voice.

Kijanki, the very top professional orchestras (perhaps 15 or so in the US) will employ 6 horn players. Many others will employ 5, and almost all will employ 4. A great deal of the orchestral/opera/ballet rep calls for four horn parts. Their location on stage or in the pit varies quite a bit from orchestra to orchestra, but is usually towards the back, often in the very back. It also depends on the space. The placement of the horn section is always an acoustical issue, due to the fact that our bells point "the wrong way."

As far as decibel level compared to the trumpet, the horn is capable of more decibels, but the trumpet's sound power is slightly higher, because of the higher register. That is a good example of how our perception is involved.

As far as how hard is it to blow, it does take considerable physical effort. Learning to use the airstream (involving particularly the muscles of the diaphragm) properly is only part of it, however - there are also the muscles of the lips, which are as highly developed as any set of muscles a professional athlete would use. In general, all musicians are using much weaker muscle groups than athletes, putting just as much stress on them, and they have to last us for much longer careers. Very few brass players make it through their careers without major "chop problems" at some point. Though usually it is just a case of fatigue resulting from overuse, small tears or even nerve damage can result, and these type of injuries can end careers. I have unfortunately had several close friends that this has happened to.
Why wouldn't a vastly more complex instrument with a seamless interface have the edge in overall musicality?

We're close on the interface issue: the kazoo. What we need is a better kazoo.