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General Asylum General audio topics that don't fit into specific categories. |
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In Reply to: Can a square wave test tell something about a speaker ? posted by beppe61 on April 23, 2014 at 00:45:16:
A square wave has been used as a diagnostic tool for electronics for many decades, the idea is for the wave to “looks perfect and square” on an oscilloscope, one needs a bandwidth (flat response and flat phase) that extends from about 1/10 the square frequency to about 10X the square frequency. Theory on the other hand requires bandwidth from DC to microwaves.A concern which is not generally addressed in loudspeakers is the phase response of a multi-way speaker.
To reproduce a square wave in loudspeaker the phase response also be “flat” and hover around zero degrees and so normally a muti-way loudspeaker cannot reproduce this at least over a broad band or in more than one location.
In other words, to reproduce a wide band time variant signal like a square wave (or music) waveform, the sound must come from one point in time and space over a broad bandwidth, something VERY few loudspeakers so.
The reason for the difficulty is normal (all the familiar named types) crossovers impose an “all pass” phase shift which means the low frequency portion is delayed relative to the high frequency section. This dispersion in time can be fixed with DSP but because the upper and lower sources are in different physical locations, the fix generally only applies to one location. Note in the explanation they have the caveat about being close to the drivers as it is unlikely to do this over a range of positions or at even a meter..
On the other hand, if one didn't have the electronic phase shift from a crossover (above 1st order), the outputs of two drivers (say an upper and lower range) can combine coherently (like adding signals with resistors) but that only happens when the sources of sound are less than about ¼ wavelength apart at the highest frequency of interest.
This means the drivers in the vast majority of multi-way loudspeakers, do not add coherently and do radiate as separate self interfering sources (an interference pattern of lobes and nulls).
These issues argue for a single small full range driver on a flat baffle however, these have a different assortment of issues to deal with such as wide bandwidth flat response, lack of headroom, extensions at both ends and lastly that even a single driver has regions of non-zero phase (which equates to a different radiation position in physical depth (time) at different frequencies).
So far as hearing, we did something when we began working on a new type of horn at work which was very useful in evaluating loudspeakers. It removes a fundamental problem, that how our two angular and frequency dependent ears, turn those two inputs into a 3d image in our minds, is not what you get when you use two microphones. Your ears and hearing system have MANY things going on which cannot be replicated via microphone, hence a very “real” sounding stereo recording is rare. . But if you record / listen to just one channel and listen in mono via headphones, you have removed your ears task of seeking information and compiling a conscious stereo image and this makes all the flaws easier to hear.
If one includes a loudspeaker in a generation loss recording with a good measurement mic and 24/96 recorder like we did at work in the old days, one finds that the flaws in even a good loudspeaker prevent you from going more than one or two generations before it becomes unlistenable. Make one that goes three generations and you have a really nice sounding loudspeaker because each generation is an exaggeration of what ever isn’t faithful to the input signal. Do this on a tower outdoors or in a large room like we did at work and you only have he loudspeaker, do this in a room and you have both the room and loudspeaker and here the loudspeakers directivity will also strongly effect how many generations one can go. Ultimately, most of the full range loudspeakers we have at work do reproduce a square wave, over a broad band, over a range of locations without DSP but that is from radiating as a single source in time and space.
For one developing loudspeakers which others will use and seeking faithful reproduction, (they, not having my musical taste), this is a bridge between reading measurements and hearing the warts.
For me /us at work, this often made them easier to identify and fix and unlike evaluating a hifi recording for faithfulness, Using it here, you heard whatever you used as the original live and you hear each generation degrade successively.
Best,
Tom Danley
Danley Sound Labs
Edits: 04/23/14
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Follow Ups
- RE: Can a square wave test tell something about a speaker ? - tomservo 08:22:21 04/23/14 (1)
- RE: Can a square wave test tell something about a speaker ? - beppe61 09:12:12 04/23/14 (0)
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