Eddie Runner wrote:


Nousaine wrote:

Now let's clear up a few mistaken comments about what I've said. In a
small
space like a car (or a room) below the lowest modal frequency the driver
just
pressurizes the interior space and the location of the source isn't a
factor.

This is because the wavelengths involved are so long that standing waves
won't
form. In a 12 x 22 foot room that frequency is 26 Hz.

Its not that they WONT form, the wave still propogates the listening area
there are UPS and DOWNS to the sine waves... The difference and
definition of the pressure wave is that reletivly ALL of the listeing area is
a positive pressure or negative pressure at once because the wave is so
long...

Now to find out what size space can be subject to a pressure wave
the whole place must be about the same pressure, so for a 18 ft long
wave (about 60Hz) and 18 ft space would NOT be a pressure wave!
Because of the simple fact that there is a whole wave in there at any time,
the UP and the DOWN of the sine wave (360degrees).

Lets change the space to 1/2 the size, a 9 ft space... Still it cannot be
a pressure field because there is 180 degees of change in the space..
Still not relativly all the same pressre is it??? Nope....

Now lets cut it in half again Tom... 4.5ft.... Much smaller than most
cars I think.... And still we have 90 degrees of phase shift.... Even at
this point I would have a hard time calling it a pressure field....

Cut it in half again and its about 2ft.... Were sure getting close to a
pressure field now, but I dont know anyone with a 2ft car interior..
And we still have 45 degrees of the wave in the car... so is it really
a pressure field??? At this point it could be argues for or against
about equally (IMO)....

Cut it in half again and at about 1ft and only 1/16th of a wavelength
I might agree with you now.... Still 22.5 degrees of phase difference
between one end of the space and the other though... Still not a perfect
pressure field is it??

Now to your 26Hz you mentioned above...
Were talking about Car Stereo TOM... My paper that talks
about standing waves in a car is writen and referenced to 60Hz.

And as one can see from your "graphs" there's no standing wave patterns below
60 Hz in a Neon. Plus IF you would do a close-mic frequency response
measurement of a small sealed subwoofer system and then measure that system
in-car and match levels you'll see that there will be a 12 dB per octave
reinforcement as fequency below 60 Hz, give or take depending on car size,
which shows that the system is operating in the pressure region.

Or didn't you know that?



Certainly the pressure wave of a 26Hz wave would be much larger
than a 60 Hz pressure wave... It could happen in a larger space at
that frequency....
Lets run through it anyway
50ft = 360 degrees
25ft = 180 degrees
12ft = 90 degrees
6ft = 45 degrees
3ft = 22 degrees

Which one of those is a true pressure wave???

Finally Tom wants to talk Tech....

Eddie Runner
http://www.installer.com/tech/

I love to talk tech. But when you have to deal with an amateur it's sometimes
difficult. Again Eddie please explain to me how a given woofer system manages
to generate more sound pressure when its lying on its back ot faced to the
front or the back?

Oh you say that it'll generate less SPL with the cone faced into the ground?
Well only if the come motion is impeded in some way. But you'll "hear" less
SPL. Sure, but only at higher frequencies.