[net.audio] Snake oil and demanded apologies

rdp@teddy.UUCP (08/15/85)

[]

The following I decided to post because of it's wide potential interest.


>>>I have just ordered a pair of sub-woofers from a private low-yield speaker
>>>and electronic manufacturer in New Jersey.  Each is about a 2-foot cube.
>>>With all the internal baffling and channeling, the speaker (actually a 
>>>self-made 15-inch woofer) produces an effective horn length of 38 FEET !
>>>This speaker can be driven to as low as 2 Hz !  This is not a low peak, 
>>>but has actually driven pink noise at 2 Hz for short periods of time.
>>>This set-up has forced the maker to re-inforce his listening room.
>>>
>>>Mark Nevar
>>
>>Boy, I hope whoever you ordered them from has a cancellation policy or a
>>money back policy, because, methinks, you just bought some snake oil.
>>
>>Why? The cutoff frequency of a horn is dependent upon the size of the mouth
>>of the horn, and somewhat less upon it's length. Also, the efficiency is
>>dependent upon the taper characteristics of the horn. It is, I assure you,
>>impossible to fit a horn capable of doing what is claimed inside a 2 cubic
>>foot enclosure. Look, for example, a Klipsch's. For abvout a 50 Hz cutoff,
>>the have an enclosed volume of (I estimate) about 8 cubic feet and, while
>>they have some severe problems, they really do make it to 50 Hz or so, where
>>they die like a lead balloon.
>>
>>The above is a great simplification of course, but there is no way that
>>the physics will allow the speaker your refering to to do the job that is
>>claimed for it.
>>
>>Dick Pierce
>>
>Dick,
>
>The person I ordered them from has a lifetime return policy on all his
>audio equipment (amps, pre-amps, speakers, sub-woofers).  If at any time
>I wish to upgrade my equipment I get either what I payed or the new
>market value of the equipment(whichever is higher).  But read my statement 
>again.  Nowhere do I say the subwoofer is a horn !  All I state is 
>that the self-made 15 inch woofer prooduces an effective horn length
>of 38 feet.  It works beautifully in conjunction with his electronics 
>and other drivers.  Do you think I would buy a piece of equipment without
>first auditioning it ?  I would not pay $2200 for the subwoofer setup 
>unless I thought it was the best available.  I chose to keep this flame off
>the net because the egos on there do not permit a calm conversation.
>I wish you had chosen to do the same.  A public apology on the net would
>be nice, however.  If you have any further questions concerning my foolish
>purchase, don't hesitate to ask.
>

First of all, I am no more enamored of the egos on the net then you are. I
attempted to work as an honest, ethical proffessional in the consumer and
proffesional audio business for seven years before fleeing the bullshit
and taking refuge in a real profession. 

IN the realm of serious, hopefully scientific query, please explain to me
what is meant by "self-made woofer produces effective horn length of 38
feet". SOrry, but having been quite involved in the research and design of
acoustic transducers, this statement makes very little sense. 

The science of the acoustics of loudspeakers was well established by the
late 30's (see N. W. McClaughlen, et al, "Loudspeakers", for example).
Reviewing this literature, and comparing it to the current theoretical
work (not in "The Amatuer Audio", or any of the effite popular rags, but
things like the Journal of the Acoustical Society, etc.), reveals precious
little "new" knowledge gained in that time, with the possible exception
of materials research. WHy, all of a sudden, has an unknown, "private
low yield" company sunddenly come across something unknown to researchers?
What is it, REALLY?

After doing a bit of exploration, I can't find any way the speaker you
describe is workable. First of all, a 15 inch woofer with INFINITE compliance
placed in a 2 cubic foot enclosure has a much higher resonance than you
describe. Secondly, any realizable woofer cannot be expected to have a
free-air resonance as low as described, without absolutely gross inefficiency.
Thirdly, asking any woofer, even a "self made" woofer (which I guess means
the guy makes it himself), can't be asked to be driven to the frequencies
you describe without 1. gross distortions and 2. severely reduced output
relative to its real pass band. If you are describing some folded labyrinth,
then you still loose, because such a structure in the volume you describe
is far from sufficient to deal with the signal from a cone with that area,
given the volumes of air needed to be moved, and it still suffers from the
limitations of fundamental resonance.

Once, in Harvard Square, Acoustic Research had a showroom. In the front
window was a cut-away of an AR-3A, being driven by a low frequency 
oscillator, with the result of the woofer wagging back and forth at 1/2
Hz!. It was most impressive. If the showroom door was sealed, yes, the
walls might creak as a result. But so what? The linear excursion needed
to maintain flat response to that frequency (assuming it was about 1 inch
at 50 hz, probably something of an over-estimate) was in the neighborhood
of 50 FEET! (since, for a given output, the excursion needed goes as the
inverse square of the frequency!) 

I'm sorry, but I still think your buying, at the very least, black magic.

If there are fundamental, supportable, and REAL physical principles in
action in this sub-woofer, then you are doing a disservice by not letting
the network know. In the meantime, I stand by my first reply. It appears
that there is, so far, only one ego trampled upon.

Regards, 


Dick Pierce

michaelk@azure.UUCP (Mike Kersenbrock) (08/24/85)

> []
> 
> The following I decided to post because of it's wide potential interest.
> 
> 
> >>>I have just ordered a pair of sub-woofers from a private low-yield speaker
> >>>and electronic manufacturer in New Jersey.  Each is about a 2-foot cube.
[everything else edited out]                                     ~~~~~~~~~~~~

I might point out that the original poster specified a 2-foot cube ^^^^^^
not "2 cubic feet" as the retort postings mentioned in both instances.
.

A 2-foot cube as specified is 2^3 = 8 cubic feet of volume.
I'd be interested in the calculations with this new volume (minus some
for wall thickness). A 2-foot cube is pretty good sized.

Remember the new Bose Yuppie boom box that was discussed a few
months ago?  The one that is supposed to have "decent" bass yet
is "the size of a bread box"?  What if you made one
of those with 8 cubic feet of box?  Remember the Bose was also supposed
to be ultra efficient.  What if you traded off efficiency as well?  Whatever.

Mike Kersenbrock
Tektronix Software Development Products
Aloha, Oregon

P.S.- My AR-9 speakers are rated to 28Hz at -3dB, and to
      18 Hz DIN spec (-8dB from "average" 100-4000hz). The internal
      volume of the AR-9 is 120 liters (4.24 cu. ft) with (2) 12" woofers.