IMD not THD is where we should focus

Reading the 30 page (!) preamble to a 1976 book containing in depth reviews on receivers by the late Angus McKenzie and it made some interesting points.

The mid to late 70s saw companies in a spec war to produce the lowest THD, but McKenzie points out that several percent of THD, particularly at the frequency extremes is sonically pretty benign and often passes undetected. However, much smaller amounts of IMD are subjectively far more objectionable.

This was interesting since in the 70s, many designs routinely produce IMD at 0.1-0.5% even for quite expensive models of amplifier. McKenzie found that invariably his blind listening panel results when reviewing would tally with the nature of IMD produced by the amplifiers or receivers on test. The same correlation could not be made for varying degrees of THD, and the case was made that IMD consists of artefacts not harmonically related to the original signal, unlike THD and that this is far more sonically irritating.

I'll scan and upload the piece later but it does make a strong case that many manufacturers spent an awful lot of time looking at the wrong numbers, and perhaps still do.

Today it's rare to find SS equipment with audible distortion of any kind, but certainly loudspeakers still produce very audible distortions, and especially IMD.

 

I've heard it said that in loudspeakers the same non-linearities in the driver movement are the cause of THD and IMD. So often if a speaker has low THD it will have low IMD as well.

It is certianly a good argument for line arrays I think. Loads of drivers hardly putting in any effort = low distortion.

 

There are good reasons why all the loudspeakers I like best have at least 12" bass drivers.

 

Why? It would seem to me the most important thing to reduce IMD is going for a multi-way design keeping bass modulation away from the mid. I think if you separate the system at say 200Hz then you could push the woofer rather hard and not notice particularly bad sounds.

 

Interesting thought. Do you know a speaker that crosses over to the mid at 200 Hz and has a woofer smaller than 12"? I couldn't think of one.

Anyway, I think a large woofer just couples to the room better.

 

Kensai + Advantage. The mid is a 4" magnesium cone and crosses at 120Hz and 2.2KHz. Woofers are 7" but there are two per speaker.

Audio Physic do a few speakers with this setup too. Genelec do two mid-field monitors with a 5" mid and two 8" woofers per speaker but they cross at 400Hz I think.

My point was not really about crossing at 200Hz as I'm sure crossing at 400Hz would be a great advantage too, over a 2-way system.

 

A true great. I really despair at what passes for hi-fi reviewing from the present day hacks.

( there are, of course, some honourable exceptions.)