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How to properly measure subs?

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Douglas Cyr:
Hello,

To measure the frequency response of a sub, is there a minimum distance away from the cabinet to place the mic?

Is it something like a full wavelength away at high pass frequency?

Paul G. OBrien:
Outdoors well away from all structures
Sub and mic at ground plane 10m apart
28.3v output from the amplifier.
band limit the test sweep to avoid damaging the driver below cabinet tuning.

Art Welter:

--- Quote from: Douglas Cyr on December 27, 2022, 01:38:23 PM ---Hello,

To measure the frequency response of a sub, is there a minimum distance away from the cabinet to place the mic?

Is it something like a full wavelength away at high pass frequency?

--- End quote ---
The wavelength of the frequency is not an issue, the size of the radiating surface compared to measurement distance is. Two meters is generally far enough away for most subwoofers, both the mic and cabinet on the ground, outdoors, well away from reflective surfaces.

Pat Brown, a guru of measurement wrote :

"It is often thought that a remote measurement position is necessary for low frequencies since their wavelengths are long. Actually the opposite is true. It is more difficult to get into the far-field of a device at high frequencies, since the shorter wavelengths make the criteria in Item 4 more difficult to satisfy."

Item 4:

"4. The distance from the source where the path length difference for wave arrivals from points on the device on the surface plane perpendicular to the point of observation are within one-quarter wavelength at the highest frequency of interest ."

This is an important distinction between high frequency and low frequency measurement, criteria #4 can be satisfied at 95 Hz and below for a subwoofer of one square meter mouth area measured at one meter.

The distance to reflective structures becomes more problematic with a longer distance between the source and measurement mic.

Art

Helge A Bentsen:
I've used a rule of thumb that seems to work reasonably well for repeating measurements outdoors.
Distance between mic and speaker = 2x largest dimension of speaker.
So, if a 2x18" is 1m wide, I use 2m.

John Schalk:
The folks at Data-Bass use 2 meters, ground plane for their tests.  Here is a link to their full testing procedure:

https://data-bass.com/#/articles/5cc0bc36a75a260004255c88?_k=ifhmvu

I have a follow up question regarding the amount of free space required.  Let's say that the lowest frequency you want to test is 25 Hz, which has a wavelength of 45.2 feet.  How far away do any large boundaries need to be for a good test?  Is 45' feet in any direction enough?  Can it be just over 1/2 a wavelength, so 25' in any direction?

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