The acoustics of musical instruments: Part C#

Marshall Chasin
May 25, 2011

Welcome to part 3 of the acoustics of musical instruments (or as musicians would say, part C#)… actually I am sure that they wouldn’t say that- just trying to be funny.

In the first of this blog series on the acoustics of musical instruments we discussed quarter wavelength resonator instruments such as the clarinet, and the brass section.  Clarinets have register keys and not octave keys because like all quarter wavelength resonators, the first harmonic is at 3 times the fundamental (or as musicians would say, a 12th, or as really nerdy acousticians would say, an octave an a half).  The second part of the blog series (part B flat), we discussed half wavelength resonators such as the strings and vocalists.  When something (a string or a vocal chord) is held tightly at both ends the harmonics occur at integer multiples of the fundamental- 1, 2, 3,….  such that the first mode of resonance is double (2) the fundamental, which is one octave.  Instruments such as the oboe, bassoon, and saxophone- all half wavelength instruments- have octave keys (and not register keys).

This part of the blog series will be about where sound emanates from the instrument and how this may alter the way we “mic” the instrument for amplified sound.

The sound from the clarinet (and all other woodwind instruments) emanate from the first uncovered hole and not from the bottom or flare, which in some cases is essentially ornamental.

In this clarinet, the note C is being played (three fingers down) or if you happen to be a smart ass, it can also be a G (if its in the higher register… an octave and a half higher).  Placing a microphone near the bottom of the clarinet or in front of the lips of the player would not be as optimal as placing it near the bottom of the line marked “L”, indicating the first non-covered hole.  In the case of the clarinet, the flair at the bottom is ornamental and has no really value other than to help it stand up on the floor.

This is also the case with the other woodwinds such as the sax, flute, bassoon, and oboe.  Note that I never talk about the piccolo since its so hard to spell (picollo?) and is too loud for anyone (about 126 dB SPL at the right ear drum).

Strings are a bit different in that the greatest excursion of the string from the rest position is at the center between where the finger articulates the note and the end of the fret board.  Placing a microphone over the center area will probably give you the best sound.  It also depends on ergonomics of whether the musician would tolerate having a microphone placed, so sometimes negotiations will be in order.

My final comment concerns microphones.  There is a long history about whether one should use a dynamic microphone or a condenser one.  With speech analysis it doesn’t matter because the real difference is in the dynamic range or more specifically the highest sound level that can be transduced.  With dynamic microphones, the maximum is typically 112-115 dB SPL whereas the condenser microphone (also called a capacitor microphone for those over the age of 300) can handle 135 dB SPL.  Suffice it to say that when recording percussive sounds a condenser should be used.  For most other instruments, either a dynamic or a condenser should be fine.

Oh, by the way, there is one other difference between a condenser microphone and a dynamic one.  The diaphragm on the condenser is much lighter than the one of the dynamic microphone such that because of inertial issues, the condenser diaphragm can start to move quicker (and stop quicker) than a dynamic microphone.  Subsequently the condenser microphone is better for “sudden” instruments such as the percussion.

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