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(Eric Desrochers)
wrote:

Nousaine wrote:

Many things seem to be surprising at first glance in audio. Like the silly
idea that aiming a subwoofer makes a difference at low frequencies.

(snip)

I've read the whole thread. I have not tried to orient subs in any of
my cars yet (and it's not even an option in my current car) so I won't
comment on its benefits. You seem to be right on a scientific
standpoint but we know that several things in life exists that cannot be
explained by rational means so I won't take a position on this topic at
this time!

You don't need to take a position. If you'd like I'll send you a file that
shows the effects of facing a subwoofer in a car either to the front or rear
(and with the hatch open) both in terms of frequency response and max SPL over
the 10-62 Hz range.

But what happen to the old concept that bass is omnidirectionnal? Or
more exactly, frequencies whose wavelenght are over three time the
diameter of the reproducing driver are non directionnal?

They still hold true. The main difference is that in your home 80 Hz is 2
standard deviations below the frequency at which people can generally locate a
subwooer. In your car the people at Ford Audio found that 150 Hz (an octave up)
was the typical frequency where low frequencies began to become directional.
In a car a 100 Hz or lower crossover always seems to work perfectly and often
200 Hz can be made to work well with regard to direcionality. But recall plain
old frequency response above 100 Hz can become another issue to deal with.

But the basic idea seems logical with a cursory glance. Likewise it seems
strange that a proper listening position EQ might have a downward slope
over the spectrum when the measurement mic is in the listening position.

But that's how sounds occur in nature as well. A trumpet measured at the
bell and at a distant listeners ears will have the far-field slope as
well.

I understand and accept that. It's a well known fact that air absorb
higher frequencies more than lows pver a distance.

Directivity is an issue too.

But in a car, the
near field (which I'd usually describes as 1 meter from the source) is
often *farther* than the actual listening position.

No "near-field" has a specific meaning that in engineering terms means that the
microphone is within a half inch of the diaphram at low frequencies and sees
basically the anechoic response of the speaker.

It is true that the term "near-field" is generally used in way that more
correctly defines what is really the "direct field" of the speaker where the
sound falls off by 6 dB with every doubling of distance which is generally true
@ 1 meter in a room.

At the critical distance where the reflected sound is equal in intensity to the
direct sound we enter the far-field. In your car the radiating surfaces are so
close to the interior surfaces the listener is always in the far field. (the
amount of reflected sound is equal to or greater than direct sound.)

Why a car would
commend (or naturally produce?) a 3 dB per octave slope while a real
listening room (or show venue?) would have a 1.5 dB per octave?

Because there is no 'direct field' in the car and the space is much smaller. In
a similar way the acoustics of the living room differ from a concert hall.

But the effect on system tuning is that many people become irritated with the
equalization process because the try to make the system "flat" in the far field
without taking the slope into account and find it doesn't sound good.

Lots a questions! I'm part of various audio fields since 15 years but
wasn't made aware of this since yesterday!

-- Eric (Dero) Desrochers

Hiroshima 45, Tchernobyl 86, Windows 95

You can prove it to your self easily. Take a small loudspeaker and measure it
in the near-field (mic within a half inch of the cone); then meausre it in the
far field (say @ 4 meters) in a room and note the difference.

Put it in the car (on the dash if you can) and re-measure. Notice the shape of
the curve over the audio bandwidth.