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(Eric Desrochers) wrote:

Nousaine wrote:

This argument is a red-herring. Yes it's true that a good sounding system
will
have a gradually decreasing frequency response at about 3 dB per octave
over
the spectrum at the listening position.

Sorry to step into this heated thread...

This 3 dB per octave will cause a 20 kHz sound to be 30 dB lower than a
20 Hz sound, is this what you are saying?

This could well be true but it's quite surprising at first glance!

--
Eric (Dero) Desrochers

Hiroshima 45, Tchernobyl 86, Windows 95

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.

There's a web-site and strong advocate that claims 'aming' a subwoofer has
large effects at low frequencies in a car. The reasoning is based on 'standing
wave' theory that appears logical on face but is patently wrong.

The proponent claims that a dual reflection surface (wall and floor) will
produce a standing wave pattern and a large null at 60 Hz if a woofer is placed
3 feet from a wall on the floor. According to the 'theory' a standing wave
cancellation is produced at 60 Hz and bass is compromised.

The idea is simply wrong. There can be a cancellation from a single boundary
cancellation but at 3-feet placement that has to occur at a frequency at least
2 or 3 times the source frequency. This is well know as the "Allison Effect".

Further a standing wave must have at least 2 opposing surfaces to form. In the
wrong example given above the effect cannot be a 'standing wave' because in
order for a standing wave to form we need two sound sources traveling in
opposite directions.
With the single boundary example the 'cancelling' wave (single boundary
reflection) is traveling in the same direction as the source. So it isn't a
standing wave and it doesn't occur at the fundamental frequency of the source.

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.