I wonder how we'd measure the intermodulation distortion at the listening position, do you have any ideas? This is about way more than frequency response.
I don't think you mean IMD. IMD comes from an amplifier driving a bass signal (amps) and a midrange signals (milliamps) at the same time. The huge demands from the bass signals will cause new frequencies to be intermodulated around a vocalists voice or other midrange instruments (called sidebands). These intermodulations are totally unrelated to the voice of the vocalist and are very intrusive. For example, a kick drum at 80 Hz played at the same time as 500 Hz, 1000 Hz and 2 and 4 Khz harmonics from a vocalist creates all kinds of additional frequencies....920 Hz, 1080 Hz, 2080 Hz, 1920 HZ, 4080 HZ and 3920 Hz and many more multiples of the 80 Hz. such as 1160 Hz and 840 Hz and so on and so forth. This results in a loss of clarity and makes it harder to hear detail - it can also be fatiguing. Think of it as a blurry lens...or camera shake on a photo...the amplifiers huge efforts at big power hungry kick drum bass is "shaking" or "blurring" the midrange vocalist.
However, this is not what I think you mean. I suspect you are referring to
Comb Filtering. This is where one reflected wave interferes with another. For example: bass frequencies are omnidirectional up to about 600 Hz so these frequencies radiate backwards from your speakers, bounce off the wall behind them and selectively reinforce or destroy all of what you hear in the lower midrange.
There are only three known solutions to properly hear the lower midrange primary signal;
1) Place speakers at least 6 feet from the wall behind them and sit close to the speakers (this reduces the effect dramatically and it is exactly how near-fields are used in studios to get an accurate mix)
2) Soffit mount the speakers (eliminates the issue altogether)
3) Headphones
Above about 600 Hz this is a non issue as the rear wall behind the speakers no longer has any effect as sound only raidates forward from typical box speakers at these frequencies. So this issue is most noticeable on male vocals. If you find female vocalists very tightly placed in the soundstage and beautifully transparent and crystal clear but male vocalists do not sound quite as clear, transparent ot tihtly focussed in the same way => then "comb filtering" or rear wall quarter wave cancellation is typically the problem....move your speaker well out into the room if you observe this!
