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Acoustic analysis of slat blinds?
Old 25th July 2014
  #1
Acoustic analysis of slat blinds?

Newbie alert. Have been intensively lurking the acoustics threads here for awhile, building some bass traps, etc.

This is a theoretical/curiosity question. As best I can gather from web searches, slat blinds on windows appear to be considered acoustically neutral, having not much acoustic effect one way or the other. However have not been able to find much authoritative written on the topic. Web search on "acoustic diffraction grating" mostly retrieves links to lasers and ultrasonics.

Does anyone have links to online accessible pdf's which discuss the "theoretical or practical acoustic behavior" of slat blinds?

Intuitively it seems that slat blinds might behave similar to optical diffraction gratings, which among other things can spatially spread the spectrum of light according to wavelength. However, optical spectrum spread via diffraction grating usually involves coherent light beams or collimated beams (parallel rays), wheras audio waves in a room might be more likely spherical wave-fronts?

For specific instance, in my "audio/computing office" are 3 windows and an exterior door with wood slat blinds. By sheer accident they are located very symmetric to the speakers-- Two side-by-side windows/blinds directly behind/in-between the monitor speakers, one blinded window in the middle of the left wall and the blinded door in the middle of the right wall. So whatever effect they have ought to be symmetrical.

The slat width is 2.5" and slat spacing is 2", so one might guess that whatever effect they have would be most noticeable beginning in the vicinity of 5 KHz?

The window blinds sit 6" in front of vinyl-frame double pane windows. Isolation is surprisingly good. I can run 100 dB spl sine sweeps inside, and barely hear anything standing a few feet outside the house. Any leaked audio is completely masked by summer crickets. A full-tilt rock band playing in the house would most likely be much louder outside the house of course.

Anyway the 6" depth between blinds and glass might be another turnover frequency of note, for whatever acoustic effect they may have. Perhaps in the ballpark of 2 KHz?

For instance, imagining crude ray-tracing, with the blinds partially open, adjusted "angled up" so that only little stripes of window are visible between slats--

* Audio striking the bottom region of each slat would be reflected toward the ceiling, with a spread-- A spherical wave-front would hit each slat at slightly different angle and presumably be reflected toward a different part of the ceiling?

* Audio striking the upper region of each slat would reflect to the inner side of the slat above, then reflect at coincident angle to the glass which is spaced about a half-ms behind the blinds. The ray would bounce one or more times at "random" time delays between slats and glass before making it back out into the room thru a gap between blinds?

* Rays near-perpendicular to the glass, which shoot thru the slat gaps, would be reflected back out with about a half-ms delay compared to reflections off the slats. Rays that make it thru the slat gaps, but at larger angles, would make one or more delay bounces before exiting back into the room?

Perhaps silly speculation and perhaps ray-tracing is an invalid way to consider the effect. Though intuitively the regularly spaced angled slats seem that they ought to have a fairly complex effect in scattering audio in both space and time, at least in the top couple of octaves?
Old 25th July 2014
  #2
Old 26th July 2014
  #3
Thanks, Jens. Somehow didn't find that thread when searching.

The thread mentions what I had already guessed, that the 2.5" slats in my blinds would probably only affect the top couple of octaves. Since I won't be changing the blinds in the office, was just curious for understanding, if there had been formal analysis of this kind of regularly spaced acoustic diffraction grating.

From reading about equally-spaced slats for midrange diffusors, gathered that equal sized spacing of flat planes would be a bad idea compared to other configuration of equivalent materials. But in the case of high freqs, mental ray-tracing seems to indicate a fairly broad scattering, but perhaps I am imagining it all wrong.

An equivalent of my slat blinds in a window frame, effective at 1.2 kHz, would seem to require ten inch angled slats sitting in front of a two foot deep box, installed in a fairly huge room in which to be effective?
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