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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]

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?

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?

[Frank Koenig]

The frequency resolution of the measurement (in Hz) is 1 / the window length (in seconds). For example, a 50 ms window results in 20 Hz frequency resolution. To achieve a pseudo-anechoic measurement the window must close before the first reflection arrives at the mic. So the distance from the speaker to the nearest reflective object plus the distance from the object to the mic minus the distance from the speaker to the mic must correspond to a time-of-flight no less than the window duration. As a result, the further the mic is from the speaker so too the reflective objects must be further from the speaker-mic axis.

Someone on DIYaudio did a rather thorough, and very interesting, comparison of the effect of different ground surfaces on ground-plane measurements. The short takeaway is that the harder and smoother the surface the higher the frequency to which the measurement is valid. You basically want the surface to have low absorption and be smooth at wavelength-scale so as to cause a specular reflection (just like a mirror in optics). A big, paved parking lot is ideal.

--Frank

[Douglas Cyr]

This is great info, thanks everyone!

[Ivan Beaver]

Just to add to the other comments, as the physical size of the sub get larger, the further you need to be away in order to minimize the close effects of the boundary of the cabinet.

There have been all sorts of "standards" presented by various people over the years.

We use 10m, as that is easy to back calculate (add 20dB), and minimizes any close boundary gains.

We drive the signal at 28.3V (a 20dB gain over 2.83V), so they distance loss and the voltage gain cancel each other out.

[Frank Koenig]

Someone on DIYaudio did a rather thorough, and very interesting, comparison of the effect of different ground surfaces on ground-plane measurements.

Turns out I saved the link:

https://www.diyaudio.com/community/threads/measuring-response-how-close-is-close-enough-to-anechoic.353347/page-4

Look at the posts by "bikinpunk".

--Frank

[John Schalk]

Turns out I saved the link:

https://www.diyaudio.com/community/threads/measuring-response-how-close-is-close-enough-to-anechoic.353347/page-4

Look at the posts by "bikinpunk".

--Frank

That is a great thread Frank, thanks for sharing the link.  Do you happen to know what software Bikinpunk was using that has gating (windowing?). Many of his screen shots were from REW, but I get the feeling that he was importing data into REW to make the comparison plots.  I've looked for that feature in REW, but didn't find it.

[ProSoundWeb Community]