Coming up with a suite of measurements that can give you a hard answer to a product’s performance is not an easy task; near impossible I would hazard to say. And I think you are witnessing this first hand. There are just too many factors in play. Added to this you have the desired goal of boiling it down to as few measurements as possible to make it something a large number of people can quickly evaluate. One number would be best for that goal, a sort of “this one measured 8.3 and that one measured 8.9 therefore the 8.9 will sound better than the 8.3”. Given this conflict you can see what a monumental task it is for reviewers to come up with a useful measurement set for their reviews and the importance of balancing their objective findings with their subjective ones.
I don’t want to get into too much detail in this post since I sense a larger future newsletter piece is more appropriate, but for an example of one such complication of boiled-down measurements let’s look at the distortion level commonly used. The norm is to use a grab number of 10% THD (Total Harmonic Distortion). It creates a benchmark, but does it at the same time skew the real world performance of the results? I would say the answer is a resounding yes. For example the maximum real world output of a subwoofer is the point at which at any frequency it makes some sort of objectionable noise. The fact that the subwoofer may have MUCH higher output at other frequencies is not really relevant in your living room because you cannot exceed the level of the objectionable noise occurring at some single frequency. It should be noted that objectionable noises occur at very low percentages of distortion at much higher frequencies and would not even be picked up in the 10% THD number.
On top of this is the simplicity of 10% THD. At what frequency and which harmonic are we talking about here? If the frequency is very low and it is the 2nd harmonic then it would not become an audible distortion until you were many times above the 10% level. If we are talking about the upper end range of frequencies from your subwoofer and higher harmonics, like any above the 4th, then 10% would be much too high a percentage to use.
In our DSP subs the design parameters include that the subwoofer must never make any audible distortions at any frequency at any level you choose to set the volume, and that the subwoofer needs to be very linear in a 4pi environment. We feel this gives our customers a much higher usable overall output in their living room compared to chasing down a few spot frequencies. The linearity gives our subwoofers what is commonly termed a “very musical” sound; though I would prefer to call it realistic.
On a related and quite exciting note, it looks like Gene and I are going to collaborate on putting together a larger piece on this subject.
President & Chief Engineer