Dr. Amar Gopal Bose (1929-2013) passed away earlier this month leaving the helm of a corporation that he helped guide for almost 50 years. In a message from Bob Maresca of Bose, he states that “Dr. Bose…. was more than a chairman. He was our teacher – always encouraging us, always believing that we could do great things, and that anything was possible”. Dr. Bose had a distinguished career in the private sector as well as at the Massachusetts Institute of Technology (MIT) in Cambridge where he taught for a number of years.
The technology behind the Bose Wave Systems, which is what people normally think about when they think of Bose is remarkably simple. And it is something that we have used in hearing aids since the inception of earmolds to couple them to a client’s ear. The technology is based on a principle called “bass reflex”.
“Bass reflex” should be the name of a rock group waiting to happen. I haven’t checked the Internet about this, but I am positive that there probably are a number of rock groups called “Bass Reflex” already.
Other than having a cool-sounding name, the bass reflex principle is an acoustic method to enhance the lower frequency sound transmission. We have acoustic horns that enhance the higher frequency sound transmission, but due to the long wave lengths of lower frequency sounds, acoustic horns don’t have any real effect for the bass notes (unless the horn is several meters in length).
Bass reflex is based on the “inertance” of the acoustic system. In English this means that a mass of air can be forced to oscillate and act as a single unit. This vibration results in a broadly tune low-frequency resonance that acts as an acoustic amplifier. In hearing aid technology with vented earmolds, the mass of the air in the vent acts as a single oscillating mass and has a “vent associated resonance”. This is typically around 400-500 Hz, but depends on the volume of air in the vent as well as its shape. The explanation of this can be found in almost any acoustics textbook published after 1962… so this idea has been around for about half a century.
For hard of hearing people with relatively good low-frequency hearing acuity, when fit with hearing aids, we sometimes find a paradoxical result. When they are fit with a vented earmold they may report that the lower frequency environmental sounds are louder, rather than less loud. Venting is supposed to “reduce” the low-frequency transmission of sound that gets to a person’s eardrum and not “increase” it, but of course in this blog we are talking about the bass reflex principle. It is this bass reflex principle that creates a low-frequency resonance that works in the opposite direction for some vents and for some hard of hearing people the vent enhances the lower frequency sound that ultimately gets through to the eardrum.
I am sure that Dr. Bose didn’t know what was happening in the hearing aid field, but having an understanding of the bass reflex principle allowed him to enhance the low-frequency sound that could be transmitted through a home stereo system. (For those who are old enough, in the 1950s and 1960s we used to call this our “hi-fi”). You could recognize a stereo system with a bass reflex port since these openings were usually adjacent to the loud speaker port.
Of course, Bose did a few other things (that we are also now seeing in the hearing aid field). If you ever get to see a Bose Wave system (those little boxes that have a big box sound), when nobody is looking, take out your trusty screwdriver and open up the box (or simply smash it on the ground and blame the dog or the goldfish for doing it). You will see that there is a long tube that connects the loudspeaker port to the outside of the stereo box. This tube adds acoustic length to the system such that the tube can flare, which serves to enhance the mid and higher frequency sound transmission as well.
I know that you are dying to just do the math with me, so let’s get at it:
Everyone knows the important equation that we live our lives by…. F = v/2L.
In this life-changing equation, F is frequency (in Hz), v is the speed of sound (34,000 cm/sec), and L is the length of the resonating tube (in cm). If the length of the Bose system tubing is 20 cm, then F = 34,000/40 = 850 Hz. Neat, eh? What’s really neat is that this equation also gives us the first mode of resonance of a half wave length resonator of a tube that has a length L, but this is really only a coincidence…
This means that all sounds above 850 Hz will be enhanced by the 20-cm long tubing inside the Bose System. And then of course we have the Bass Reflex principle to take care of the lower frequency enhancement.
Dr. Bose has made some amazing contributions to our knowledge of sound re-enforcement systems, and every one of his contributions simultaneously was found in the hearing aid industry.
