I have seen claims that the steel pan (commonly called steel drum) was the only acoustic instrument developed in the 20th century. Is this true? There are related instruments (the Hang or hand pan) but I'm interested in the history of unique musical instruments that are more than experimental musical instruments. I'm trying to find sources to support or refute these claims, but I cannot seem to find anything definitive.
Plastic aerophones like the Vuvuzela have been around since the 1960's, obviously similar looking brass instruments have been around for a long time, but specifically Vuvuzelas meet your criteria. They're a bit of a gimmic, but were very popular at the 2010 World Cup and have seen widespread use. More
The modern Double French Horn either just misses it with the first prototype being made in 1897, or just sneaks in with Fritz Kruspe patenting it in 1900. But it is not a radically new instrument compared to older horns, it just combined the Bb horn and F horn together into one instrument with some clever plumbing.
Four String Acoustic Bass Guitar (I used to play one, so I know it's new). See http://en.wikipedia.org/wiki/Acoustic_bass_guitar
"The first modern acoustic bass guitar was developed in the mid-1950s by Kay of Chicago but the design did not show up again in a production instrument until the early 1960s when Ernie Ball of San Luis Obispo, California began producing a model. Ball's aim was to provide bass guitarists with a more acoustic-sounding instrument that would match better with the sound of acoustic guitars. Ball stated that "...if there were electric bass guitars to go with electric guitars then you ought to have acoustic basses to go with acoustic guitars." Ball notes that "...the closest thing to an acoustic bass was the Mexican guitarron...in mariachi bands, so I bought one down in Tijuana and tinkered with it.""
Theremin and ondes-martenot are instruments from the 20th century. Anyone who has watched Star Trek has heard the theremin.
The vibraphone - although it may not qualify as a true acoustic instrument, being a hybrid of sorts:
https://en.wikipedia.org/wiki/Vibraphone The vibraphone is similar in appearance to the xylophone, marimba and glockenspiel. Each bar is paired with a resonator tube having a motor-driven butterfly valve at its upper end, mounted on a common shaft, which produces a tremolo or vibrato effect while spinning. The vibraphone also has a sustain pedal similar to that used on a piano; when the pedal is up, the bars are all damped and the sound of each bar is shortened; with the pedal down, they will sound for several seconds.....The first musical instrument called "vibraphone" was marketed by the Leedy Manufacturing Company in the United States in 1921....."
I think the steel pan does qualify as the only acoustic instrument INVENTED in the 20th century because:
Many people experimented with existing instruments and adapted them. E.g. Harry Partch in the 1930s, adapted marimbas, keyboards, violas and woodwind to do different things. He "invented" 10 string guitars and other techniques like slide guitar style. However, none of his instruments were actually "new". They were all adaptations of existing musical instrument, which he altered in some way.
What makes the steelpan absolutely unique is the fact that it is made of one metal surface and "pimples" beaten into the skin of the steel form the notes that are struck.That is totally new as a concept of an instrument. The T heremin and Ondes Martenot are electronic instruments, using electronic means to make notes. therefore they are non-acoustic instruments and do not qualify as a newly invented acoustic instrument.