Bass Build-Up
The only portion of a loudspeaker’s response that a room influences is where fractional-space loading occurs.

This is in the long (low-end) wavelengths, where in-phase summation with nearby surfaces is an equalizable situation. 

When a loudspeaker is against a floor, wall, or corner, the normally omni-directional bass energy sums in one direction and adds 6 dB (not 3 dB) for each doubling of surfaces (sources) at the longest wavelengths.

This bass build-up is the one instance where “room EQ” actually exists.

Similarly, multiple low-frequency drivers act just like surfaces and add 6 dB per doubling of sources. This also must be accommodated in the final EQ curve.

However, because of room modes, the mic can’t be placed out in the room where it will receive energy from all the surfaces and/or low-frequency drivers, because the bass response will still be wildly position dependent.

The solution?

Frequency response of a two-way loudspeaker’s high-frequency (HF) and low-frequency (LF) drivers as seen in the transfer function of Smaart. Note the roll-offs of the LF and HF drivers meet at –3 dB.

The ear is still the best way to adjust the bass response. Listen from a few different positions to a variety of bass sources and you should quickly be able to determine a good low-frequency balance. 

I generally adjust the drive (sub-crossover section or power-amp attenuators) to the system subwoofers in a three- or four-way system by taking a gentle, relatively broadband approach to taming low-frequency build-up that changes very gradually with frequency. 

Remember that air also has a very definite absorption characteristic for short high-frequency wavelengths. If the throw of a system is very long (hundreds of feet), consider a high-frequency EQ boost to counter those effects.

Keep in mind that this must be compromised between the closest and farthest listeners.  Don’t make it too bright for those up close. 

Also, choosing a position far from the loudspeakers naturally leads to the expectation of less high-frequency content, so a flat response (to 15 kHz or so) might be disconcerting at 100 feet or more.

Finally, too much high-frequency boost can fry the drivers. Be careful!

Very Consistent
I’ve been using this methodology for several years and have found it highly effective for tuning a system to within 95 percent of the final result, before listening to program source at all.

You’ll also likely find it to work quite well, and keep in mind that it can be applied to any sound system. Sound will be natural and very consistent from system to system, regardless of room acoustics.

The sole exception is “boomy” rooms with very strong room modes. These situations require a system that doesn’t excite the room modes, which can be removed from the system’s response by using notch filters.

Happy tuning!

John Murray is a 30-year industry veteran who has worked for several leading manufacturers, and has also presented two published AES papers as well as chaired four SynAudCon workshops.

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