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Old 24th February 2004
  #14
Barefoot Sound
 
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From a speaker designer's perspective soffit mounting is vastly superior to free standing.

GP you're right about early reflections and comb filtering being a major issue - especially in a small room where you can't pull the speakers far away from the walls. Check out my 2D Wall Bounce Calculator over at the Recording Studio Design Forum. It paints a rather bleak picture of the response anomalies associated with front and side wall reflections. Setting the distance of the monitors to the front wall to zero simulates soffit mounting. Since the calculator simulates free standing speakers you'll notice the +6dB bass response step that oudplayer mentioned.

Freestanding speakers have an issue called "baffle step" or "diffraction loss" which must be compensated for. Imagine the woofers and tweeters radiate omni directionally within their particular frequency bands. However they are mounted in the front baffle of a box. At high frequencies, where the wavelengths are small compared to the dimensions of the speaker baffle, the otherwise omni directional sound is forced to radiate only in the forward hemisphere. The sound that would have gone towards the rear of the speaker is added to the forward radiation, resulting a +6dB increase in output. However, at low frequencies where the wavelengths are large compared to the speaker baffle dimensions, the sound radiates omni directionally. So there is a 6dB mismatch in the on-axis response between the low end and the high end of the spectrum.

Freestanding speakers compensate for this mismatch with a -6dB high shelf filter (typically this is designed into the crossover) to flatten the on-axis response. That's fine, but you wind up with a power response mismatch. In order to get a flat on-axis response the total power emanating into the room must be +6dB higher in the bass. Now, when you take a free standing speaker and flush mount it, you force ALL wavelengths to radiate in the forward hemisphere. This results once again in a 6dB response mismatch, only this time it's heavy in the bass.

This mismatch can be compensated for with a specifically designed low shelf filter. I have posted a design for a DIY Baffle Step Decompensation Filter over at the Recording Studio Design Forum.

All in all, soffit mounting yields markedly superior results. It avoids many problems with wall reflections, comb filtering and edge diffraction. Additionally, both the on-axis and power responses are flat.

Hope this helps!

Thomas