Acoustic monitors

Music videos show many musicians (and their sound technicians) wearing electric monitoring systems called in-ear monitors.  These look like hearing aids, smell like hearing aids, and even taste like hearing aids, and to a great extent, they are hearing aids, except that they are not personalized to an individual’s hearing loss.  These in-ear monitors have amplifiers, up to 5 different receivers, and some even have “in situ” microphones that musicians use to monitor their immediate environment.

In-ear monitors have been in common usage for about a decade, and have actually been around for about 20 years.  They are designed to replace the wedge monitors that used to sit up on stage directly in front of a performer, or replace the large loudspeakers that were situated at the sides of the stage to give the musician some “side wash monitoring”.

These allowed performers to roam around on the stage and still have a constant input regardless of their position.  With counseling on proper usage, a musician will even have the benefit of being able to turn down the volume over that which would be received from more traditional wedge or side wash monitors.

In-ear monitoring has been shown to be very useful for amplified live performances.  The in-ear monitor is hooked up either directly (in the case of drummers) or by way of a wireless route to the sound control consol.  If a musician wants more bass, or less lead vocal sound, then that can be accomplished with a judicious use of a slider on the control consol.  In this way the musician gets the ideal mix of instruments and sound, without having to fight against a nearby musical noise source, such as another musical instrument or the monitor of a musical colleague.

But what about non-amplified environments, such as classical and even some jazz that uses upright string bass instruments?  When one goes to a classical concert, check out the cello and bass players- in some cases they are leaning their heads towards their instrument, contacting the tuning pegs or the neck of their instrument in an attempt to hear better.

This is where an acoustic monitor may be quite useful.  It’s really nothing more than a stethoscope for musicians and uses the same simple principles of acoustics that Von Helmholtz or Lord Raleigh delineated over a century ago.  An acoustic monitor is really only 4 feet of #13 hearing aid tubing that is connected to a musician’s left ear.

The connection can be very “low tech”.  For those who are wearing a custom musicians’ earplug such as the ER-15, the filter can be removed and the #13 tubing slid into the main sound bore of the earplug.   To make it look pretty, one can even spend $1.50 and get a 90-degree elbow from any earmold laboratory.  The other end (which is open) is either paper-clipped to the tailpiece of the bass or cello (the black piece where the strings come from) or simply dangled in one of the stylized f-holes on the front of the instrument.  Some of my clients even prefer to use Scotch tape to tape it to a “sweet spot” on the rear of their bass instrument.)

And for those who do not (yet) have custom-made musicians’ earplugs, a yellow foam insert of the type used with insert earphones will be an excellent substitute.  You might need to stretch the #13 tubing over the end of the hard piece of plastic installed in the insert earphone foam tip.

So, let’s do some calculations…  Four feet of #13 tubing that is effectively open at both ends (ear canal and tail piece) functions as a one half wavelength resonator whose resonant frequencies are defined by integer multiples of v/2L where v is the speed of sound and L is the length of the tube.

Doing this in metric (and we will even do this in cm), let us assume that the length is 120 cm (about 4 feet) and the speed of sound is 34,000 cm/second.  The first mode of resonance is therefore 34,000/240 = 141 Hz.  Being a one half wavelength resonator, we also have resonances at 282 Hz (2 x 141 Hz), 423 Hz (3 x 141 Hz), 564 Hz, and so on at multiples of 141 Hz.  Understandably, this, like a physician’s or a nurse’s stethoscope, is only for bass sounds such as a heart murmur.  And like a stethoscope there is only minimal amplification above 1000 Hz.  This is why it is so useful for only the cello and the bass.

Tying a small knot in the bottom end of this length of #13 tubing will turn it into a quarter wavelength resonator and that will generate resonances starting at about 70 Hz (but only have odd numbered resonances).

Whether this low tech device functions as a quarter wavelength resonator or a half wavelength resonator is irrelevant – both “designs” can provide up to 20 dB of low frequency amplification.

A cellist or a bass player can now listen to their own instrument with ease despite the roar of the other instruments around them.

And hearing health care professionals can make one for less than $10 in parts.