What is meant by "anechoic?"

[Lee]

1.4 What is meant by "anechoic?" [JSC]Anechoic means not echoing. It usually refers to a style of measuring a speaker's output which attempts to eliminate echoes (or "reflections") of the speaker's output back to the measurement area, which could alter the measurement (positively or negatively).

[Lee]

User contributed notes below:

[geolemon]

In terms of how you might hear this used in car audio...
There are really two ways to think about things..

"In-car" and "Anechoic".

Anechoic being the sort of too-idealistic estimate or measurement of frequency response...
Like the frequency response graph that you might see on the box that your coax speakers came in, that's anechoic!
That response changes quite a bit when you actually install those speakers in the car.

Also, this is primarily discussed on the forums in reference to subwoofers and their performance...
Which brings up another set of terms that are commonly used:
"Transfer Function", or "Cabin Gain", which are really the same thing....
Used to describe the point where you will inherently get a dramatic (but natural) "bass boost" due to nothing other than reproducing sound waves that are longer than your car's interior inside your car's interior....
The lower the frequency, the more dramatic the difference in wavelength compared to the largest interior dimension in your car, and the more "bass boost" you get... up to 20dB quite often!

The response that you might see on a subwoofer's spec sheet...
Or maybe if you are playing around modeling up how a subwoofer's response plot changes as you put it in different enclosure types/sizes, maybe using a tool like WinISD (http://www.linearteam.dk/winisd.html) (which is free, btw!)
...that response shown on the plot is anechoic.
Again... ESPECIALLY in regards to subwoofers, things change DRAMATICALLY when you put a subwoofer actually in a car!

Basically, you can think of "Anechoic" as being the way that something would sound if you set up those speakers in a large, open field, with no walls, and a non-reflective floor...
Not at all like what you face in a relatively tiny car interior, with highly reflective glass and plastic, and highly absorbant seating! ;)