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In Reply to: mics and placement for recording speakers realistically posted by Adriel on December 22, 2010 at 14:01:28:
Hi
At work they have compiled a small library of loudspeaker sounds which can be listened to.
What we do is a variation on the old “generation loss test” used in the old days of analogue tape.
This does not provide a “measured” result but instead allows one to hear an exaggeration of exactly what is wrong with a speaker.
The DSL generation loss test is done this way.
You pick several demanding musical tracks and put them on a CD track about 3 min long or so.
You get a 24/96 multi channel recorder and known quantity measurement microphone.
You place the speaker up in the air (outdoors) with the mic at say 2 meters (also up in the air). This placement insures the direct sound is much louder than any of the reflected sound. If you do this outside, except for the ground, there are no reflections.
You play the music through the speaker at a modest level and record it directly on track 1, on track 2 is the microphone signal.
Then play back the recording except you play track 2 into the speaker and record it on track 4.
You play back track 1 and re-record it on track 3.
Now one has two generations of reproduction, most speakers already sound bad after two generations our best sound funny after 3 to 5. Obviously a “perfect” reproducer, microphone and environment would allow an infinite number of generations.
Each generation exaggerates whatever was wrong with the speaker and becomes more and more obvious.. The farther it is from “perfect” the fewer generations it takes to sound bad. Also, one is recording the original signal as well so if you wish, you can sample the degradation during the repeated AD/DA processes of the direct signal.
One comment about hearing;.
We hear with two ears located at two different points in space.
Not only that, but unconsciously, we have learned to associate the large changes in our ears frequency response with incoming angle as positional information instead, that is how we hear up/ down and behind etc.
Thus, we can hear much more than a two channel system should be able to discern. Not only that but without being aware of it, we automatically ignore a great deal of information as we listen for something in particular.
For example the deep / dramatic looking effect of low frequency comb filtering in a room from reflected sound, is relatively minor to your ears and often it is the delayed signal not the response which one hears more.
As a result, a perfect microphone, picking up pressure variations from one point in space without discrimination, will never sound exactly like what you hear with ears listening from two places, with an aural processor which discriminates the un-important and allows spatial detection in 3D from two points in space.
The condition you describe (hearing you room etc) is real (assuming your mic is proper) but is part of what we automatically / unconsciously reject when listening.
One way this same test can be used to “hear” the room interactions or lack of. You do the same test in a normal room, each generation exaggerates the room and speaker’s flaws. Here, a speaker with much more directivity exhibits much less room coloration.
So far as hearing exactly what your speaker does, you need a reflection free condition, preferably very quiet relative to your test level.
Even then, keep in mind the little mentioned differences between hearing it and recording it.
This all kind of illuminates part of the overall problem making a “good sounding” recording.
Best,
Tom Danley
Danley Sound Labs
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Follow Ups
- RE: mics and placement for recording speakers realistically - tomservo 09:02:04 02/01/11 (0)
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