posted 10-07-2003 02:03 PM                       

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It is indeed interesting that we spend so much time and energy worrying about frequency response and distortion in audio electronics when, given the physical nature of loudspeakers, logic would suggest that they would be the most significant contributors to distortion.

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A very astute and dead on observation Ray. As an extreme example, consider that if any electronic device had the characteristics of common loudspeakers with respect to distortion and the ability to preserve a waveshape, they would likely be deemed BROKEN! That said, remember that marketing departments want you to focus on improvements they can sell you now. There was a time when heads of major speaker manufacturers would boastfully state "there is no need for speaker response below 70Hz" where 6 months to a year later they would be announcing a new subwoofer which extended lower. Of course no one would ever need lower bass than this!

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However I don't recall ever seeing distortion figures quoted in any of the speaker manufacturers' data sheets. A quick check of the spec sheet for JBL's current 3 way cinema system makes only passing reference to distortion (something like "extremely low distortion"). It is almost as if the distortion is so insignificant that it is not worth quoting figures for.

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So in your experience Mark, what degree of distortion are we really talking about here and where or how have you obtained such information on typical cinema loudspeaker systems? Also what degree of variation in frequency response with output power are we talking about?

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Distortion is an interesting matter to examine. The large majority of distortion is tied to increasing excursion in drive units, with the remainder typically caused by extreme intensities of the air in the throat of mid & high frequency horns, or by port turbulence and choking off at low frequencies. All of these effects are limiting/compression effects, somewhat analagous to saturation of an analog tape, or overdriving of an amplifier. One important factor to remember is that once a speaker is overdriven to distortion, anything being produced at the same time by the device will also be distorted. So even if a loudspeaker may have maybe 1% distortion at relatively high levels in the middle of its range, if a signal at its low end drives it to distortion, that 1% distortion figure is gone as any signal present will be similarly distorted.

So far as the distortion levels we typically listen to, most direct radiating subwoofers I have measured reach 5-10% distortion at less than 50W of input, many times with as little as 10W. There certainly are some which will behave much better than others, but at realistic playback levels it is common to see the better direct radiators at a few percent distortion. This is typically in the 105-115dB @1m range. Consider that most cinemas are about 10m deep to the center, and that is 20dB lower than at 1m. I have measured our bass horns as well as a few others and typically distortion is 1/5-1/10th that of direct radiating subwoofers. Where we see the most difference between products is in the onset of distortion. It is not uncommon in many prosound applications where subs are driven at levels resulting in 15-25% distortion.

In main speakers we still have similar problems. At modest levels you will find better speakers with ~0.5% distortion over some bandwidth. Quite often speakers operate yeilding distortion in the 1-5% range. 1% distortion at say 110dB at 1m would be described by most listeners as fairly clean. Certain horns have some interesting distortion mechanisms which show up at moderate levels and higher due to high intensity in the throat, often referred to as "throat distortion."

We have, and you can EQ two different speakers to have almost identical frequency response yet you will still hear the signature of any distortion in the systems. A horn which "honks" badly will still retain that character even after EQing.

So far as level variations, this is mostly due to power compression. As you push more power through a voice coil or diaphram, the wire heats up. As commonly used metals heat up, the resistance increases, which reduces the amount of power delivered to the coil, and hence power compression. Note that many of todays very high power drivers achieve much of this power handling by withstanding much higher temperatures before glues melt and things deform. JBL did a study a few years back which actually proved that at some very high power level you can push more power into some drivers, and because of the heating and resistance rise, you actually get LESS output. To put this in real terms, if the resistance doubles, power halves, resulting in 3dB of power compression.

In the ideal when you increase the signal/power by XdB, you expect to see the speakers get XdB louder. Above about 1/4 rated power for many speakers, this no longer holds true. What you see in measuring a speaker is that this effect is frequency dependent, depending on the design. This effect is seen both from heat as well as from overdriving of a speaker in terms of excursion. Subwoofers are easy examples as they are less complicated devices operating at long wavelengths, but a good example can often be seen in vented boxes where measuring the response at increasing power levels shows significant changes in the shape of the low end roll off. To varying degrees similar effects occur at midbass and midrange frequencies as well.

This got quite lengthy, but I hope someone finds it of interest.

Enough for one post...

posted 10-13-2003 08:49 AM                       

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I'm sure John P can provide the links are methods to obtain the article...he is so good at such fact-finding.

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Steve must think I am just a librarian!

The paper is "Improvements in Motion Picture Sound: The Academy's New Three-Way Loudspeaker Systems" by John Eargle, M. Mayfield, and David Gray, SMPTE Journal, July 1997, Volume 106, page 464.

Here are some links:

http://www.jblpro.com/pub/cinema/cinedsgn.pdf

http://www.aes.org/journal/sample_issue/JAES_V50_11_PG954.pdf

http://www.aes.org/sections/la/meetingnotices/sept_97.pdf

http://www.smpte.org/members_only/library/indexes/ (SMPTE Members only)

http://www.doctorproaudio.com/doctor/bookstore.htm