How can I get rid of a 4khz-8khz room spike?

Last year I consistently measured a treble frequency response spike in my system in the range of 5kHz to 6.3kHz. The treble frequency spike remained in the range of 5dB to 8dB (measured with the Radio Shack analog SPL meter with measurements adjusted to compensate for the inaccuracy of the meter) despite various room treatment experiments, component changes and cable changes. Throughout this period, the spike was most apparent when listening to string sections in orchestral music where the violins would exhibit a coloration that made them sound "hot" and forward.

I was convinced that my problem was inherent in the treble response of my speakers until about six months ago when I was surprised to discover that the problem was primarily an issue with my AC electric. Apparently, there was high frequency noise riding along on my AC electric lines. I managed to solve the problem and revert the treble frequency response to flat with the upgrade of my AC electric system, including the addition of a 7.5 kVA isolation transformer.

If you find that your problem is not inherent in your speakers and is also not related to room reflections, you might find that your AC electric supply is the culprit. If you are interested in more detail, check my virtual system thread.

Good luck.

Nanotweeter, if you launch the link in Herman's post above, you will see a 2001 thread that was initiated by Sean and that includes a compensation chart of adjustments that need to be made at each 1/3 octave interval in order to adjust for the inaccuracies of the Shack meter. Those compensation adjustments are summarized below:

10Hz.....+20.5
12.5Hz...+16.5
16Hz.....+11.5
20Hz.....+7.5
25Hz.....+5
31.5Hz...+3
40Hz.....+2.5
50Hz.....+1.5
63Hz.....+1.5
80Hz.....+1.5
100Hz....+2
125Hz....+0.5
160Hz....-0.5
200Hz....-0.5
250Hz....+0.5
315Hz....-0.5
400Hz....0
500Hz....-0.5
630Hz....0
800Hz....0
1kHz.....0
1.25kHz..0
1.6kHz...-0.5
2kHz.....-1.5
2.5kHz...-1.5
3.15kHz..-1.5
4kHz.....-2
5kHz.....-2
6.3kHz...-2
8kHz.....-2
10kHz....-1
12.5kHz..+0.5
16kHz....0
20kHz....+1

Add the above numbers to your SPL meter readings in order to obtain the actual SPL level in your room at each 1/3 octave interval. Given the -2dB compensating adjustments required at your problematic treble frequencies, I think you will find that you still have a reduced but still very sizable 8dB or so frequency response spike throughout those frequency bands.