10 replies to this topic

[soundminded]

Well guys, I finally did it. I bought a pair of Bose 901's used. They are Series I's with the seldom seen walnut front face. They are, if I may be so bold, the most gorgeous pair of 901's I've ever laid eyes on. They are original and in oiled walnut. They look as if the owner had them stored in a vault for 37 years with not a scratch, scuff or stain on them. I'm glad I waited for a nice Series I as I really wanted that original sound that intrigued me back in 1971. Also, I wanted the original acoustic suspension design as I feel that when Bose redesigned the speaker with a vented approach they got away from the intention which was to provide low bass down to the lowest octave.

Anyway soundminded, I have been reading your thoughts on Bose and have been impressed by your breadth of knowledge on the subject. I'm attaching some pics of the 901's but they hardly do them justice. I was wondering soundminded, could you put up pics of your 901's as well? I'd love to see them as I hear that they're the finest 901's on the planet.
T

My Bose 901 look exactly like yours except the grill cloth on the front is dark green, almost black. They are on black pedistals. There is also a rectangular black plastic plate hanging directly below the top plate from a pair of small invisible L brackets. They hold the 3/8" front poly tweeter just above the front driver. Unless you were on the lookout for them, all you'd see is two silver hex head screws which I will one day get around to painting black. I removed the grill cloths from both the front and back to inspect the drivers and the sealing material. The drivers were in perfect condition but unlike the putty in the AR2as I opened up, the putty in these speakers did not turn into glue, they hardened and cracked. I considered it likely that they were no longer air tight so after speaking with Bose's service department, I decided to decline their offer to trade them in for a pair of Series VI at half price but instead ran a bead of clear GE silicone caulk around the juncture of each driver's metal frame and the wood and over each screw head. It's very easy material to work with as you can use your fingers and I gave it about a day to cure. I was very careful not to get any on the suspensions or the cones. I'm now convinced they are air tight. If I gently push the cone of one driver in slightly, all the others pop out immediately from the increased air pressue and don't return until I let go.

As I said I re-engineered mine. I'm not going to reveal all of the details now but at the risk of getting my patent attorney disgusted with me, I'll say that the mathematical and physical model of an ideal speaker I invented considers not only the directly radiated sound field but ALL of the reflections as well and compensates for the room boundry's differential spectral absorption/reflection characteristics so that the net result is what I call "flat spectral transfer function." This means that all of the sound including the reflections have flat frequency response. (It also explains why speakers which measure the same by conventional means sound different and why the same speaker sounds different in different rooms and in different places in the same room. This is much more than mere room resonance.) The speaker had to therefore be engineered to match the room's acoustics and its location in the room. Because of Bose 901's unique configuration, this is relatively somewhat easier than with most other speakers I've re-engineered and yields even better results because the additionally compensated reflections arrive at nearly the same times and from the same directions as the other reflectons from the main drivers. This is not the same as flat radiated power which was the goal of many early AR designs and is not the same as a pulsating sphere would yield. Both the pulsating sphere model and flat radiated power model are naive imperfect concepts IMO because they fail to take into account room acoustics. All that remains is to tweak the system frequency response for each recording (unavoidable due to variations in the way recordings are made, each having its own spectral balance.) The result is the most accurate speaker for reproducing the sound of acoustic instruments I've ever heard. How do I know? It helps to have real musical instruments you can listen to live any time you want to as they are the only reference which matters. BTW, even knowing what to do, it still took me about 2 1/2 years to get it right to my satisfaction. If I moved the system to another room...I'd have to start all over again.

[Carlspeak]

My Bose 901 look exactly like yours except the grill cloth on the front is dark green, almost black. They are on black pedistals. There is also a rectangular black plastic plate hanging directly below the top plate from a pair of small invisible L brackets. They hold the 3/8" front poly tweeter just above the front driver. Unless you were on the lookout for them, all you'd see is two silver hex head screws which I will one day get around to painting black. I removed the grill cloths from both the front and back to inspect the drivers and the sealing material. The drivers were in perfect condition but unlike the putty in the AR2as I opened up, the putty in these speakers did not turn into glue, they hardened and cracked. I considered it likely that they were no longer air tight so after speaking with Bose's service department, I decided to decline their offer to trade them in for a pair of Series VI at half price but instead ran a bead of clear GE silicone caulk around the juncture of each driver's metal frame and the wood and over each screw head. It's very easy material to work with as you can use your fingers and I gave it about a day to cure. I was very careful not to get any on the suspensions or the cones. I'm now convinced they are air tight. If I gently push the cone of one driver in slightly, all the others pop out immediately from the increased air pressue and don't return until I let go.

As I said I re-engineered mine. I'm not going to reveal all of the details now but at the risk of getting my patent attorney disgusted with me, I'll say that the mathematical and physical model of an ideal speaker I invented considers not only the directly radiated sound field but ALL of the reflections as well and compensates for the room boundry's differential spectral absorption/reflection characteristics so that the net result is what I call "flat spectral transfer function." This means that all of the sound including the reflections have flat frequency response. (It also explains why speakers which measure the same by conventional means sound different and why the same speaker sounds different in different rooms and in different places in the same room. This is much more than mere room resonance.) The speaker had to therefore be engineered to match the room's acoustics and its location in the room. Because of Bose 901's unique configuration, this is relatively somewhat easier than with most other speakers I've re-engineered and yields even better results because the additionally compensated reflections arrive at nearly the same times and from the same directions as the other reflectons from the main drivers. This is not the same as flat radiated power which was the goal of many early AR designs and is not the same as a pulsating sphere would yield. Both the pulsating sphere model and flat radiated power model are naive imperfect concepts IMO because they fail to take into account room acoustics. All that remains is to tweak the system frequency response for each recording (unavoidable due to variations in the way recordings are made, each having its own spectral balance.) The result is the most accurate speaker for reproducing the sound of acoustic instruments I've ever heard. How do I know? It helps to have real musical instruments you can listen to live any time you want to as they are the only reference which matters. BTW, even knowing what to do, it still took me about 2 1/2 years to get it right to my satisfaction. If I moved the system to another room...I'd have to start all over again.

Soundminded:
How would you anticipate a stock Bose 901 would fare in an anechoic chamber frequency response test? That is a test with on axis and multi-off axis measurements using a rotary table?
A debate is ongoing at another forum. I maintain the standard anechoic FR test is useless for a novel speaker like the 901 which relies on reflections which would be damped considerably by the walls of the chamber making the test meaningless.
What are your thoughts on this?

[soundminded]

Soundminded:
How would you anticipate a stock Bose 901 would fare in an anechoic chamber frequency response test? That is a test with on axis and multi-off axis measurements using a rotary table?
A debate is ongoing at another forum. I maintain the standard anechoic FR test is useless for a novel speaker like the 901 which relies on reflections which would be damped considerably by the walls of the chamber making the test meaningless.
What are your thoughts on this?

I've had no success uploading comments lately. I'm using a diffferent computer. Don't know what the problem is.

Funny how the much dreaded Bose flame war never materialized here. I take a rather dispassionate approach to all of this, to me it's just another machine. It is what it is.

I heard Bose 901 Series VI at a shopping mall store in Newport Beach Ca about a month or two ago when I had some time to kill and I must say it was really awful. I didn't spend enough time with it to analyze it carefully but it really had no high end at all and it was not accurate. I listened to some of their HT systems and they didn't have much high end either but they were better than the 901 in this regard. I don't know if those little speakers they use in them are 2" or 3" but they do not make good tweeters. And a 4" driver is an awful tweeter.

As for the original this is how I would describe its frequency response per driver...taking into account the acoustics of my fairly live room and including the Bose equalizer set to its indicated flat position. At the low end, the system resonance seems to have a peak of about 7 to 8 db above the 1 khz output level at around somewhere between 250 and 500 hz. This is in agreement with e/e Magazine's measurement of many years ago which I think they said was 7 db at 500 hz. They said it was inaudible? It seems plainly audible to me. Below resonance, the original drivers falls off at 12 db per octave like any other acoustic suspension speaker but it only gets a 6db per octave boost from the equalizer. Overall system output falls therefore at 6 db per octave reaching the 1 khz output level somewhere around 140 or 150 hz and continues to fall at 6 db per octave. On the high end, it seems to fall off above around 10 to 12 khz pretty sharply. No amount of treble boost can overcome the high inertial mass of the 4" drivers, they just have no high end. But even if they did, dispersion of the forward facing driver would be awful, only audible directly on axis. This is due to the physics of vibrating devices as the wavelength of the frequency approaches the physical dimensions of the moving membrane. So the speaker used as recommended by the manufacturer commits several cardinal sins by audiophile standards. Deep bass is absent without considerable bass boost. There is a disturbing upper bass resonance peak, and there is no high end. The speaker sounds dull, has an upper bass punch but no real deep bass unless driven very hard.

However, it has to be said that there are a large number of very significant advantages and ingenious aspects to this speaker system. It solves a lot of problems other manufacturers haven't even considered. Unfortunately, for audiophiles its weaknesses far outweigh its strengths and correcting them is well beyond their ability.

Then how did I re-engineer it to become the most accurate clearest speaker system I ever heard? I just can't get enough of listeining to it these days. With a great deal of effort, three years of it in fact. All of the problems have been overcome. Lately I have been listening to Earl Wild's recordings of Rachmaninoff's Variations on a theme of Chopin, of Corelli and several other pieces he transcribed and his recording of his Gershwin transcriptions both on Chesky. The sound of the Baldwin SD-10 is stunning. I like it better than the Steinway D. In fact, I'd have to admit better than my own Steinway M. And every time I hear the recording of Heifetz playing Gershwin on his Guarnari del Jesu, I know why people will pay millions of dollars for one of these violins when they come up for auction. Years ago, I had the good luck to hear one live frequently, even occasionally in my own home. It's an experience you don't quickly forget. BTW, even with all of the re-engineering and careful adjustments of the system to the room, further adjustments have to be made for each recording for them to sound accurate. There is no way around the fact that no two recordings are made alike. I have concluded that a sound system which plays one recording accurately will be inaccurate on others unless there are suitable adjustments skillfully used. It is painstaking but worth it.

As I said, I am not passionate about any of this, I just enjoy it. I don't get worked up about it but I really don't like debating audiophiles over tubes, vinyl phonograph records, wires, etc. IMO all of the electrical engineering theory is correct. Any problems in the performance of high fidelity sound systems lie elsewhere. That's not to say that the complimentary shortcoming of one piece of gear in a sound system won't to a degree mitigate the shortcomings of another but that is not engineering. As a result, this project really didn't cost a whole lot of money, that was not an issue. It would be pointless spending far more than I did (not counting what I had lying around this project cost me in the range of $60 for parts.) When you think about it, all you have is your memory of what live music sounds like. Even in a rapid fire A/B switching, all you have to rely on is your memory from one moment to the next. This is why I feel so fortunate to have had the opportunity to hear and play live music from real unamplified musical instruments all of my life. Those are my only references and I get to refresh my memory of what they sound like frequently.

BTW, I looked at the "debate" on Audioholics and it reminded me why lately I spend far more time listeing to music and recordings of music than discussing it on the internet.

[Carlspeak]

Thanks Soundminded for your response. I was getting worried about you there after the few days not getting a response from you since my post.
The EE test you referenced, was that done in an anechoic chamber? From my perspective I believe, in theory, a 901 would fare miserably in a standard anechoic test due to the room's absorbtion of the reflected sound.
You had it right. It was the Audoholics site. I gave up on it also and hoped your detailed knowledge of the 901 would shed some light on the subject to enlighten those who dug thier heels in on the premise the standard anechoic chamber freq. response test we the be-all, end-all test for ANY loudspeaker.

[soundminded]

Thanks Soundminded for your response. I was getting worried about you there after the few days not getting a response from you since my post.
The EE test you referenced, was that done in an anechoic chamber? From my perspective I believe, in theory, a 901 would fare miserably in a standard anechoic test due to the room's absorbtion of the reflected sound.
You had it right. It was the Audoholics site. I gave up on it also and hoped your detailed knowledge of the 901 would shed some light on the subject to enlighten those who dug thier heels in on the premise the standard anechoic chamber freq. response test we the be-all, end-all test for ANY loudspeaker.

34 years ago not even 5 years out of engineering school, I gave serious thought to how sound is propagated by musical instruments, how it interacts with room boundaries (all sorts of rooms, big ones, little ones), and how it arrives at listeners. I had all of the "tools" I needed to study this problem with none of the preconceptions formal training in acoustics would have imposed on me. In other words I had to think the problem through from scratch. And the answer that came to me like a bolt from the blue has proven itself over and over again as I analyze all sorts of acoustic problems but strictly as a hobby. I've spent a lot of time thinking about how musical instruments work and why they sound the way they do. And what has to be done to get the same subjective result from recordings played through electronic components. Needless to say, my ideas don't look or sound like anyone elses and I've had a great deal of fun exploring them. It's also provoked me to give a great deal of thought to how we hear things and how our brains interpret the sound that arrives out our ears. And there also I've come to some surprising conclusions. Since I'm working on some more patents I can't discuss them in any meaningful detail now. But I can tell you that far better sound systems than we now have available to us are entirely possible. And I am not talking about those which measure more accurately but which provide beauty of sound only equaled by live music and not possible at the current state of the art. And interestingly, it has nothing to do with personal preferences, it goes to the heart of the way we hear. My most interesting experiments are with people who have no interest in music or sound reproduction. Right now I'm reading Musicophilia by Dr. Oliver Sach a professor of clinical neurology and psychiatry who teaches at Columbia University. There's a lot of interesting anecdotal mateiral about the interaction of the human nervous system caused by exposure to music, how it affects other body systems, and how there is a unique relationship between human neurology and music which has manifested itself in every known civilization in every era in every part of the world gong back at least 50,000 years. Unfortuntely so far, no hard science in it though. There's another related book by another neurologist I have to get. I've also been reading technical papers I downloaded from Leo Beranek's web site comparing technical measurements of many concert halls and opera houses with conductor preferences and other concertgoing "golden ears" trying to find a correlation between them. As far as I am concerned, with the inventions of the digital compact disc, modern solid state amplifiers, frequency equalizers, and digital sound processors, the electrical problems of sound reproduction have been solved. That was the easy part. Now the real issues have to be worked out and as far as I can tell, there's been just about no progress in a long time. Sorry to be so long winded.

[soundminded]

BTW, in case I left any doubt, the original Bose 901 and Series II are capable of enormous deep bass within their power handling capacity. They each have in aggregate the area of a 15" woofer but their maximum excursion is probably only about 3 mm. This means to equal the output loudness capability of an AR9 for deep bass, several pairs would have to be stacked. Measurements made by Hirsch-Hauk Labs when they first came out showed that at high volume they would respond down to 26 hz and at lower volume to 23 hz. They have about 10% THD, about twice an AR 12" woofer but it's still inaudible. But to equalize the system to be flat at those frequencies the power requirements are staggering. To achieve the same output at 30 hz you'd get with one watt at 1 khz where there are fairly efficient would I think take about 600 watts or more due to an added 10 db of boost. The rated system capacity is only 270 continuous so it would take several stacked to play deep bass loud. Playing deep bass, it eats up my available 138 wpc easily so some recordings require engaging the 10db shelved cut below 40 hz. You also find that with speakers like Bose 901 and AR9 that a surprising number of recordings including CDs have low bass rumble and other noises inherent in them that are not audible on lesser speakers. Fortunately, there are lots of these on the market and usually sell for about $150 to $200 a pair in good condition. Because their surrounds were cloth, if they have been reasonbly cared for, they should be in near perfect condition even after nearly 40 years. All I had to do was seal the gap between the metal speaker frame and the wood cabinets wtih GE silicone to restore their air tightness. So far system LF gain is so great that I haven't been able to operate a turntable in the same room without acoustic feedback. But I haven't tried the Empire 698 or the AR (I still can't find it) with their sprung suspension.