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Small IEC like Baffle for LS up to 80mm
Small IEC like Baffle for LS up to 80mm
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Old 14th August 2018, 08:08 AM   #1
karsten21 is offline karsten21  Germany
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Default Small IEC like Baffle for LS up to 80mm

Dear Experts,
I would like to build a small anechoic box to measure small loudspeakers up to 80mm diameter and less than 2W. Unfortunately I have only reduced space so the box canīt be more than 70x70x70cm outer dimension. I plan to use acoustic damping material with height arround 10cm so the inner volume will be ~ 50x50x50cm.
Now I need an IEC like baffle for this small box. Calculation the cutoff frequency I use fu=c/(4xl). c= sonic speed (343m/s), l is length of baffle ( here 50cm ). So I get 171.5Hz wich is O.K. for this small loudspeakers.

The question is if there is an offical IEC like dimension for such small baffle available. It is necessary to poition the loudspeaker not in center like the big ones or itīs O.K. to put the loudspeakers in the middle?


Thanks a lot for your tips and comments.


Best regards
Karsten
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Old 15th August 2018, 11:46 AM   #2
onni is offline onni  Sweden
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I don't think that will work very well. The damping material won't absorb all sound, so the box will influence the measurement heavily.

Why not just use gated measurements? Link.

/Anton
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Old 18th August 2018, 05:10 AM   #3
TMM is offline TMM  Australia
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As above, 10cm will probably not be enough to adequately absorb the frequencies that an 80mm driver produces. To effectively avoid reflections down to say 200Hz you need an acoustic absorber which has a thickness of 343/(200*2) = 86cm which is impractical.

It will be far more practical to instead measure on an 'infinite' baffle (open baffle in reality) and use gated measurements to avoid reflections. Theoretically an open baffle of around 1mx1m with the microphone placed close (~0.3m) should achieve this as the extra path length from the driver to the edge of the baffle and then to the microphone will be around 0.8m longer than the path length directly from the driver to the mic. In reality, gating is not perfect and does not have 1 wavelength of rejection so the baffle dimensions (and distance to walls, floor, furniture etc) need to be even larger than this, or the microphone distance significantly closer such that the amplitude of the sound coming directly from the driver 'drowns out' any reflections.

Personally I use an open baffle 1.2m tall and 0.9m wide with the driver slightly offset from centre. You can read more details here: Timothy Feleppa's Pages: Speaker Measurement Setup
With the mic at 30cm I see significant (about +/-3dB amplitude) ripple below 2kHz when using a frequency dependant gate (e.g. 'Complex Smoothing' in HolmImpulse). By moving the mic to 10cm the ripples below 2kHz reduce to about +/-1dB but the high frequencies become inaccurate as path length differences from the mic to different parts of the cone become significant. As a result I take measurements at both 10cm and 30cm and cross them over about 3kHz which achieves a smooth and accurate measurement from a few hundred Hz up to over 20kHz.

You can use The Edge to experiment with baffle sizes and driver offsets.

Last edited by TMM; 18th August 2018 at 05:21 AM.
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Old 18th August 2018, 01:53 PM   #4
xrk971 is offline xrk971  United States
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Small IEC like Baffle for LS up to 80mm
A 3-sided or 5-sided Dagger (tall aspect ratio pyramid) type rear chamber progressively stuffed with polyfill or fiberglass works very well as an acoustic black hole. It only needs to be about 30-40cm deep. The spiral Nautaloss also works very well and is even more compact but more time consuming to make. Both measure the same as if they were an infinite baffle from standpoint of no measureable rear wave reflection.

Also, using a trapezoid gets you very close to a much larger IEC baffle from standpoint of elimination of baffle diffraction artifacts.

iec baffles vs cabinet measurements

3-sided Dagger:
10F/8424 & RS225-8 FAST Ref Monitor

5-sided Daigger:
10F/8424 & RS225-8 FAST Ref Monitor
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