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Re: Does anything more than 20Hz-20KhZ matter?

Posted by tomservo (M) on November 24, 2005 at 08:22:29

In Reply to: Does anything more than 20Hz-20KhZ matter? posted by WaveMaster on November 16, 2005 at 19:22:27:

Hi

Human hearing does not stop at 20KHz or 20Hz, those points were one set of “limits” attributed to ones ears at one point in history while basic hearing research was being conducted.
Ones ears are nothing like “flat” in there sensitivity, they are only sensitive in the mid range higher and lower, the sensitivity falls off a great deal.
If one used a loudspeaker rating, a good ear would be something like + - 40dB, 20-20KHz.
When one looks at an “equal loudness curve”, one can see that that how loud one judges a sound to be compared to how loud it is actually changes a great deal with frequency.
For example, at 100Hz it takes about 30dB actual SPL just to be detectable, this is about 1000 times more energy than it takes to be detectable in the 2 – 5KHz range.
At 20Hz, it takes about 80dB SPL to reach detection, at 10Hz it takes about 100dB and at 2Hz, it takes about 125dB to be detectable.

What can you hear?
A number of scientific studies have concluded that some people hear inter-aural time delays so short they correspond to 200KHz, Gary Kendall at Northwestern U found high audio signals are associated with stereophonic hearing.
Dolby Labs concluded that sounds below 20Hz were both very common in everyday life and present at high intensities. The down side was that there was essentially no hope of producing them without audible distortion.

Confusing science with selling products.
When I fist got into audio in the late 60’s, the “low cutoff” one “needed” was around 80Hz. There were a few speakers that went down a little lower but loud low bass was unheard of and unnecessary if one asked.
I had heard low bass and pursued it, in the mid 80’s, Dave Merry (sp?) president of a large speaker company came to Intersonics / Servodrive and proceeded to explain that it was unnecessary to reproduce below 75Hz, but they were going to release a subwoofer with a lower cutoff (the MTL-4) anyway. We then gave him a demo than where he heard low bass with a cutoff about an octave lower than the mtl-4 hehe.
There have been countless explanations of where fundamentals are for various instruments and so on arguing one way. On the other hand, the fundamental is the frequency “IF” the signal has a constant amplitude AND has existed sufficiently long in time. Changing the amplitude and or shortening the duration takes that “one” fundamental frequency and turns it into a much broader bandwidth.
For example Don Keele’s has used a pure tone with a gausian amplitude envelope as a test signal since the 80’s. Even thought is a “pure” tone, it has about a 1/3 octave wide bandwidth as a result of the amplitude modulation.
This is music and how it can have a wider BW than the fundamentals alone would suggest.

My conclusion was that if you wanted to know what you can hear, look at the science, if you want to know what is practical to manufacture, what your suppose to buy, look at what is available and what the mfr’s say and what they have to talk about ( like a 1Hz to 10MHz bandwidth, man that’s got to be good, I mean the old one only went to 9MHz)

Remember too that numbers don’t lie but they are used to sell as well as specify.
One of the fellow at work is casually collecting measurements on popular Pro-sound subwoofers and he has found that very often one or more specifications are highly optimistic.
Frequently, when measured in real half space, at a distance large enough to remove the enclosure displacement error, one finds that the specified –3dB frequency may really be –-10dB or more.
For subwoofers, one often finds the sensitivity is rated based on what it does at the high end of its response, in some cases octaves above where it would even be used.
This mfr optimism is especially true when you compare the specified “peak SPL” (normally a calculated “perfect world” number) and what one can actually get with an instantaneous peak hold meter.
I don’t think a serious amplifier company would last long if it claimed 10 or 100 times more output than one could actually get, like is fairly common in the speaker industry.
Compare or measure side by side when you can.

What do you need?
Who knows, like so many things in audio, it depends, this is what you hear not measure.
Also like so many other things, if you’re considering extending the low cutoff, you have no idea if it’s desirable or not until you have a way to switch back and forth between “with and without”.
Keep in mind, if the low extension seems to add “muddiness”, then you have gone well past the point of audible distortion in the subwoofers and that “free sound” being multiples of the drive signal frequency, is the mid bass muddiness.
Unfortunately loudspeakers are by far the weakest link in the reproduction chain.
Pushing the limits either direction past what is easy with today’s driver technology or especially extending the low cutoff downward at high level with acceptable distortion gets something like exponentially difficult / large / costly.

Anyway, it’s a long rambling answer but your question covers a lot of ground.
Happy Thanksgiving

Tom Danley

Danley sound Labs

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