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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