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Teaching and Early Research

Dr. Bose MIT and the 2201: Personal Recollections

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The fundamental problem of loudspeaker design was that the final judgement of quality was subjective. There could not be a scientific method for designing loundspeakers since there was no way of measuring whether one speaker was better or worse than another.

Using science to design a loudspeaker meant eliminating the subjective reactions of a listener in determining how good a speaker was. However, simply using instruments to in some way determine sound quality was also not acceptable since the listener must be the final judge.

This was the conundrum facing Dr. Bose, and his solution was the only one possible: by some method to present the listener with two sounds and to ask if there was a difference. This involved the listener but eliminated the subjective part. This was the crucial insight: a listener could be asked if there was a difference but not whether one sounded better than the other. The answer was to create the sound of a speaker that was perfect and then to create a real speaker that could be listened to and then determine if the listener could hear a difference. At first this seemed impossible but Dr. Bose proposed a solution.

There is one way to make a speaker that could be perfect that was to actually create the ideal wide dispersion speaker. The solution was to bring science into the picture.

A speaker that was truly wide dispersion would need to be a pulsating sphere, otherwise known as a point source radiator and this could be accomplished by building a spherical speaker. Enter the 2201.

Above is the illustration from the patent Bose received for the 2201 loudspeaker. Part of the illustration is for a full sphere (Figure 5), for a half sphere(Figure 4), for a quarter sphere (Figure 3) and for an eighth of a sphere (Figure 1). This eighth of a sphere could fit into a corner of a room. Because the walls act as mirrors to the sound, what a listener would hear is the sound of a full sphere in a room eight times as large. This was the idea for the 2201 pictured below.

But, somewhat unavoidably I have gotten ahead of myself. The question of wheather this was the perfect sounding sphere had to have already been answered and it was.

Creating the Sound of a perfect speaker.

The block diagram above is the heart of the story. This is the experiment that compared the sound of the 2201 to the sound of a perfect 2201. I consider it the most important experiment in the history of sound reproduction. It shows how it is possible to create the sound of a perfect speaker without having a perfect speaker and then comparing it to the sound and then comparing it to the sound of the real speaker. If it works, then we know the 2201 is as good as it can be. To explain this fully requires some serious engineering and mathematical knowledge, so I am going to try to explain the most important parts.

1. The end is to listen over headphones switching between two tracks of a two channel tape recorder.

2. One track is the sound of the real 2201 picked up by a microphone.

3. The other track is the sound, generated by a computer, of what would have been picked up by the same microphone had a perfect speaker created the sound.

4. If the microphone picks up a particular sound it can then process a music signal and the signal will be the music as if it had been picked up by a perfect speaker.

5. The “perfect speaker” is just a tiny spark gap that creates a pulse of sound of the kind that, after the room has done its thing to the pulse just as it had to the real music, the computer can calculate by q process called convolution to create the sound of the perfect speaker. This is pure engineering science and was very difficult to do in the early 1960s for computers were just barely powerful enough to do the calculation. How to get around this limitation was done by Professor Thomas Stockman, an expert in digital signal processing, who overcame the limits of the computer and so made the experiment possible.

6. Now we have the two sounds we need: the sound as if the speaker were perfect and the real sound and so, listening over headphones, we can listen to see if there is a difference and this was exactly the time I joined Dr. Bose’s group.

7. There was no difference no matter what music was played.

8. And so we had, as far as a human ear could tell, a perfect spherical speaker to go onto the market as the Bose 2201 at about $1000 in 1966 dollars. But that is not the end of the story. The 2201 sounded spectacular expect for how it reproduced the sound of string instruments, especially the violin. In a nutshell, finding out why became a two year problem, with the result being the Bose 901 Direct/Reflecting loudspeaker. I summarize this briefly below. The illustrations are from a project I did in 2011.

In addition to the elaboration of the story of the 2201 (above and below), I briefly describe the next pioneering ventures: The Delco-Bose car stereo system and the Acoustic Wave Music System. I was responsible for the introduction of both these products to the press and to the public.

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