Frequency response changes when using a fabric screen to cover your hi-fi speakers.

I was suggested that the use of the familiar fabric cloth screens on hi-fi speaker systems had a detrimental effect on the sound, mostly caused by comb-filtering. I decided to check this on my own, so I devised this little experiment.
It turned out it was true, as you can see on the images at the bottom of the page.

Test setup

I only tested the rigth speaker of my "hi-fi" system (Technics SB-LX70), that is connected to my computer, using an old Sansui stereo receiver as amplifier. I generated the white noise test signal using Cool Edit. The sound coming from the speaker was recorded into the computer using a SM57 microphone at 9 centimeters on-axis, with a DIY mic preamp. The sound card was the M-Audio Audiophile 2496. The recorded sound was analized with Spectra Pro software. I tested the tweeter and the midrange separately using the same procedure.

Test procedure

First I recorded the sound without the speaker screen, and used it to calibrate the spectrum analizer. The analyzer now shows the deviation from this calibration instead of absolute readings, allowing us to ignore the characteristic of the microphone, preamp and room that will remain constant during the rest of the experiment.
Then, I recorded again the sound without the screen, and the analizer showed a flat response, as expected.
Last, I recorded the sound with the speaker screen in place and the analizer showed the deviation caused by it.

Let the images speak

Click on the images to enlarge

Frequency response deviation in the tweeter

Frequency response deviation in the midrange

Frequency response of the system, without fabric. These are the absolute readings, used as calibration for each speaker

The comb filter can be seen clearly on both speakers. Though is obviously not as deep as a flanger, it is still too much alteration to be ignored.
Finally, I wouldn't take the numbers of these results as exact, as my setup is not really a laboratory.
If you have comments, corrections or suggestions, please send me an email!
Good luck.
Miguel.

Related stuff

To keep reading:
Examining the Yamaha NS-10M "Tissue Paper Phenomenon", at Bob Hodas site.
Comb filter page at Wikipedia.
The technology of phase shifters and flangers at GEOFEX.

v1.0 13/07/2007

Miguel Canel, Buenos Aires, Argentina bioroids.miguelNOSPAM@yahoo.com.ar (Quitar NOSPAM) http://www.bioroids.com.ar