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!philabs!cmcl2!harvard!talcott!panda!teddy!rdp From: rdp@teddy.UUCP Newsgroups: net.audio Subject: Re: Speaker Sensitivity Message-ID: <1521@teddy.UUCP> Date: Tue, 29-Oct-85 10:42:49 EST Article-I.D.: teddy.1521 Posted: Tue Oct 29 10:42:49 1985 Date-Received: Fri, 1-Nov-85 01:27:08 EST References: <480@uvaee.UUCP> Reply-To: rdp@teddy.UUCP (Richard D. Pierce) Organization: GenRad, Inc., Concord, Mass. Lines: 77 In article <480@uvaee.UUCP> tet@uvaee.UUCP (Thomas E. Tkacik) writes: >Does anyone know how the speaker configuration (vented, sealed, etc.) >affects a woofers sensitivity? Acoustic suspension speakers are known >for being less efficient than bass reflex speakers :-) >Is this because IT IS an acoustic suspension speaker, or is it because >the drivers used are less efficient than drivers used in vented speakers? >Why would this be the case? > There are a variety of factors affecting sensitivity. In a simple-minded case, take the example of trying to attain reasonably low cutoff frequency in a sealed box enclosure (acoustic-suspension). One is working against a combination of suspension compliance and the compliance of the trapped air. This results in fairly stiff restoring force. Inorder to get a reasonably low cutoff, it becomes necessary to have a correspondingly high moving mass. This combined with the motor assembly, results in a low efficiency. Well, one might say, why not increase the "efficiency" of the magnet-voice coil to increase overall efficiency. Well, that is certainly possible for the mid-band regions, but the reult is a system that would be over-damped in the bass (presuming, of course, it was correctly damped to begin with). Interestingly enough, if one plots amplitude versus frequency for a given system, changing just the BL factor (the combined electro-magnetic "efficiency"), one end up with a family of curves showing expected changes in mid-band efficiency, but the output at and below resonance remains relatively unchanged. This is because below resonance, the voice coil is no longer in charge of the cone. There will be, granted, slight changes, but these will be negligable by comparison to the changes in midband efficiency. Now, in a bass-reflex (or passive radiator) system, the box no longer presents a simple compliance to the driver. It is now a complex reactive load. The designer is now faced with many more degrees of freedom in trying to optimize the system. Such optimization schemes might include maximizing efficiency and bandwidth in exchange for maximum output level (as was done in the BBC monitor series LS1, etc), or trading reasonable bandwidth, moderate efficiency and very low low-frequency distortion for high power handling (as in the KEF RS104aB), or simply sacrificing bandwidth and distortion to get the absolute maximum efficiency and power handling (as JBL had tended to do). THe answer is, unfortunatly, not a simple one. I had spent some time consulting to several companies, attempting to design systems for them. My approach was to specify the system as completely as possible, then use these figures to derive the specifications for the needed bass driver. Looking in driver catalogs from a variety of manufacturers, it was nearly impossible to find anything that would match. Not that I was designing anything wierd, but it seemd that the fascination with magnet size resulted in drivers that were markedly unsuited for closed-box systems. Bass reflex designs were out, primarily because of the stigma associated with such systems ("no, I do not want my speaker to sound like a JBL!") Richard H. Small, in the JAES (see references below) has thoroughly dealt with the efficiency issues of closed-box, bass-reflex, and passive radiator system in his series of articles. I would recommmend the technically minded among you to look these articles up, as they present the entire physical basis of direct-radiator loudspeaker design in a coherent, scientific and unprejudiced manner. Small, Richard H., "Direct-Radiator Loudspeaker System Analysis," Journal of the Audio Engineering Society, vol. 20, no. 5, pp. 383-395, Jun 1972. Small, Richard H., "Closed-Box Loudspeaker Systems," Journal of the Audio Engineering Society, "Part I: Analysis," vol. 20, no. 10, pp. 798-808, Dec 1972. "Part II: Synthesis," vol. 21, no. 1, pp. 11-17, Jan 1973. Small, Richard H., "Vented-Box Loudspeaker Systems Part I: Small Signal Analysis," Journal of the Audio Engineering Society, vol. 21, no. 5, pp. 363-372, Jun 1973, "Part II: Large-Signal Analysis," vol. 21, no. 6, pp. 438-444, Jul/Aug 1973, "Part III: Synthesis," vol. 21, no. 7, pp. 549-554, Sep 1973, "Part IV: Appendices," vol. 21, no. 8, pp. 635-639, Oct 1973. Small, Richard H., "Passive-Radiator Loudspeaker Systems," Journal of the Audio Engineering Society, "Part I: Analysis," vol. 22, no. 8, pp. 592-601, Oct 1974, "Part II: Synthesis," vol. 22, no. 9, pp. 683-689, Nov 1974. Dick Pierce