Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/3/84; site teddy.UUCP Path: utzoo!linus!decvax!genrad!panda!teddy!rdp From: rdp@teddy.UUCP Newsgroups: net.audio Subject: Re: RAW SPEAKERS Message-ID: <1098@teddy.UUCP> Date: Mon, 5-Aug-85 17:53:32 EDT Article-I.D.: teddy.1098 Posted: Mon Aug 5 17:53:32 1985 Date-Received: Wed, 7-Aug-85 02:27:02 EDT References: <3177@decwrl.UUCP> <28400013@smu> <1036@teddy.UUCP> <9570@ucbvax.ARPA> Reply-To: rdp@teddy.UUCP (Richard D. Pierce) Organization: GenRad, Inc., Concord, Mass. Lines: 131 Summary: [] In reply to my discussion about small woofers (see referenced article: In article <9570@ucbvax.ARPA> klein@ucbvax.UUCP (Mike Klein) writes: >It is generally recognized that smaller woofers have more distortion >than larger ones, all other things being equal. And this is indeed why: > "Generally recognized" does not constitute "proven in fact", as we shall see.. > >Fundamental resonance is determined mainly by the cone and voice coil mass >and the suspension compliance. Note that small woofers usually have very soft >suspensions and larger ones usually have stiffer suspensions. On the whole, >smaller woofers have a higher fundamental resonance, but the difference may >not be nearly as much as you'd expect. > Yes, fundamental resonance is a function of total mechanical stiffness and the inverse of total moving mass, but, I'm afraid, the assumption that small woofers usually have soft suspensions is not so. If you have specific product data to back your claim, fine. I do, on the order of about 800 samples of drivers. One way of measuring true effective compliance is to specify what is refered to as the "equivalent volume of compliance". This is the restorative force presented by an enclosed vloume of air that is the same as the mechanical stiffness of the woofer. Let's take two relatively well known drivers from KEF in England. The 5 inch B110 (the same driver used in the BBC LS3-5A) has an equivalent compliance volume of about 7.5 liters, whereas the B200, an 8 inch driver having the same spider assembly, the same magnet and voice coil, and the same suspension cross section, as a figure more like 25 liters. There are, in my archives, nearly 150 other examples that I refuse to bore the network with. Most driver manufacturers do not make their suspensions, but buy them from a variety of sources such as Mueller, Seas, and so forth. Their own product data do not support the above claims. Note also, that within a given manufacturers line, the same spider assembly is used, and that is often the overriding source of stiffness. Let's also look at this from another viewpoint. If you look at a range of suspensions from someone like Mueller, you find that the cross-sections are the same, regardless of diameter. This would imply that the stiffness per linear run of suspension is uniform. This further implies that the stiffness of a suspension is proportional to its diameter. However, the mosing mass is proportional to the square of the diameter. This would then mean that for a given range of drivers (say from KEF or Dalesford or what have you) that the fundamental resonance is a function of the square root of the diameter, because of the compliance, and the inverse of the diameter, because of the mass. Low and behold, it turns out the the mass becomes the more important term. Interestingly enough, the manufacturers product data supports this fully. I have this data for about 45 manufacturers, and have confirmed it for most, have you? > >Small woofers are massively overdamped because they are designed to go >in a small box which raises the system's overall Q more than a larger box would. Wrong, small woofers, in the vast majority of cases, are overdamped because it is more economical for a driver assembler to stock fewer diverse magnet assemblies. This I know because I once was intimately involved with several of them. To go one step further, most small high-quality drivers are to far overdampled for any practical sized enclosures. Case in point. The Dalesford d110 has an equivalent compliance volume of about 8.3 liters, and a system Q of about .22 with a 40 cycle resonance. Now, the final system aparameters can be determined by square root of the ratio of the driver compliance to box compliance. Let's seek a target system Q of .7, which is the best compromise of maximally flat, minimal phase error, etc, etc (i.e. Butterworth characteristics) In order to achieve this, we need a driver Q to system Q ratio of .7/.22 or roughly 3. Refering to the experts (Small, JAS about 78 or so), we find that that requires a ratio between driver compliance and box compliance on the order of 8 or 9, meaning that the box must be about 1 liter in size. This is smaller than the driver itself. On the other hand, one variant of the KEF b110 has a system Q of .5, which when combined with a box of about 7 or 8 liters, results in a system whose Q is around .7, cutoff is about 70 Hz, etc. In practice, I have found through manufaterers data sheets and my own extensive testing, that small, high quality woofers are far to overdamped to be usable as bass drivers. Ain't nuthin like REAL data, is there? :-) >This is not a design defect! It is also not the real reason why efficiency >suffers. Efficiency, system cutoff frequency, and box volume are the three >fundamental factors that must be traded off, and it's not very fair to >the frequency: > > best efficiency = (constant) * (box volume) * (cutoff frequency ^ 3) > >A woofer in a small box trying to have a low cutoff frequency is >*fundamentally* constrained to have a low efficiency. It actually >does not depend on the woofer size. (this equation holds for free-field >radiation.) > Why is it independent of diameter? Because your asking this little tiny woofer to do far more excursing to produce a given sound level, thus driving it farther into it's non-linear operating regions! > >>re: use of non-paper materials in loudspeakers to prevent breakup, etc. > >This is really true! Recent research has shown that newfangled materials >can actually behave very well through the breakup region. Paper is not >one of them. Bextrene and polypropylene are very good. > Recent? KEF was using Bextrene (polystyrene with a rubber over-coat) in the 60's. My experimentation with polypropylene was done in 1972. Ho-hum, more breakthroughs.... > >Also -- The "tightness" of the bass has nothing to do with size of the >woofer, but with the overall damping of the woofer system. However, >some people may assign the word "tightness" to a frequency response that >emphasizes the 60-100 Hz range, because that's the frequency range that >hits your gut hardest. The typical smaller woofer in a smaller box >may very well have this kind of frequency response, and may appear >to have a "tight" bass. Gee, I hope your not accusing ME of saying that!!! :-) The upshot of all of this is that is seems that a lot of so-called information seems to be rumor, guessing and so forth. I am not necessarily accusing the above respondant of such, but my original remarks are base od quite a bit of real research. I have probably 10 feet of experimental data to extract my conclusions from. I do not expect results, I measure them. Only after there is a considerable body of experimental data to prove or disprove something do I (and should anyone) make a conclusion. In the above case, some of the respondants conclusions might well be intuitively correct, but real product data proves otherwise. Sorry. Dick Pierce