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by John L. Murphy

Subject: Vehicle Cabin Effect (windows up/down)
(posted 18Mar99 to Bass List)

Justin wrote:

>I am about to upgrade the stereo in my BMW- probably with some Vifa P13
>woofers and LPG 25 tweeters.  How is sticking the Vifa in a door
>modeled? Should it be considered free-air?

It strongly depends on whether the vehicle is sealed or not (windows
up/down, top up/down).  But assuming we are talking about a reasonably well
sealed vehicle the speaker will experience cavity loading which will
introduce a 12 dB per octave rise in the bass below  some corner frequency.
The corner frequency is determined by the major dimension of the vehicle
cabin.  I think Tom Danley commented on this in a recent post.  In my
WinSpeakerz/MacSpeakerz software I recommend 48, 66 and 87 Hz respectively
for modeling the vehicle transfer function of Vans, Midsize and Compact
cars.  These frequency recommendations are the result of speaker design
consulting I did for a major auto sound manufacturer.

In short, I suggest modeling the auto environment starting with a half space
load and then modifying that response for the bass boost due to cavity

A closed box with corner frequency at the cavity effect frequency
theoretically can result in a flat response to DC!  The 12 dB / octave
cutoff slope of the closed box is, ideally, exactly compensated by the 12 dB
/ octave rise from the cavity effect.  If you doubt the possibility that ANY
speaker can EVER respond down to DC (0 Hz) consider this:  When the cone of
a closed box speaker moves forward into a well sealed vehicle cabin it
actually pressurizes the cabin.  That constitutes DC response!  In general
the same is true of our home listening rooms except that the corner
frequencies shift lower and the rooms trend to be more leaky thereby
limiting the ultimate bass boost.  This cavity effect is one of the
components of what we know collectively as "room gain".



John L. Murphy
Physicist/Audio Engineer
True Audio
Check out my new book "Introduction to Loudspeaker Design" at Amazon.com



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