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What are The Biggest Problems With Speakers Today?

Hi Tubeguy

Interesting question that I have given years of thought to.

If you measure a loudspeaker in anechoic conditions and you compare what the microphone picks up to the drive signal, what you see is bewildering.
As Richard Heyser (I think) said once “ if you looked at a set of loudspeaker measurements in all the domains available, without knowing it was a loudspeaker, one would conclude it is irreparably broken” (or something close to that).
Put the speaker in a room and then not only what the speaker does on axis counts but what comes out in every direction also counts.

O speaker o speaker in what ways are you broken?
Everyone has been taught what an impulse response is, it is also the same thing as the speakers amplitude and acoustic phase response as seen in the time domain, hence one can derive amplitude and phase from an impulse or an impulse from amplitude and phase (via inverse FFT and FFT).

You can measure with an actual impulse as the signal too, you can create an impulse which has flat energy from 10Hz to 20KHz and burn it to a cd if you want.
In this case, it is a signal, which contains equal energy at all in band frequencies, and it happens at one instant in time (sounds like a “Tic”).

Feed that to even a single driver and examine the sound which comes out and you find the time of radiation depends on frequency to a degree, spread out in time.

Unfortunately, the requirements of the opposite ends of the bandwidth are mutually exclusive of each other and level being adverse to both.
Nearly all of the loudspeaker driver linearity problems are level dependent, that is they get worse considerably faster than the level is increased.
The solution when one wants 9 or 10 octaves BW is go with more than one range or go with a driver design you can make suitable for high frequencies BUT large enough to produce low frequencies.
These speakers have the advantage of physical size which produces a larger “near field”, the golden zone where direct sound is 10dB or more above the reverberant field. Each driver design, ESS, planar etc has limitations and issues however.

Alternately, look at a typical cone / dome multiway speaker and one finds in addition to the drivers individual amplitudes and phases, one has the amplitude and phase response of a passive network loaded by a driver and not a perfect resistor.
Now, one normally has a phase shift (time) going from well above to well below crossover, 90 degrees times the crossover order when above first order crossovers.
This means that the difference in time between the upper and lower driver is now dependent on the driver it self, its relationship to the mating driver and the crossover delay. All passive filters, anything you can do with parts to change amplitude causes delay. A high pass filter at frequency X has less delay than a low pass filter at the same F.
These sources are typically not coincident so the actual distance to each range to the ear or microphone is dependant on where it is “out front”. Being located to the right, above or below isn’t a problem….Unless the spacing is “large enough” acoustical to cause self interference, that distance being about 1/4 wl at what ever F. While horn systems can produce large outputs with low audible distortion, they are physically large and this makes self interference more of an issue.
Self interference may not show up on axis but is evident when you measure the speakers radiation over the band in all directions. .
This self inference related acoustic signature (I think) is what makes the speakers physical location more or less pronounced (when you have one on) and a speaker with a strong spatial identity (I think) competes / harms the stereo image when two are on. You don’t want to be drawn to the speaker at all, just interpret the recording that arrives at your ears.
DSP can be used to ‘fix” problems in time and amplitude / phase but cannot fix problems related to excessive driver to driver spacing in the X and Y planes in more than one listening location.
Then when in a room, now one wants to avoid close reflections so the less pattern control a speaker has the farther it should be from the side walls and the closer you need to sit to have a proper image.
A wonderful demonstration of how harmful the room is to the recorded stereo image is to take a set point source speakers outside and listen, usually with domes and cones it is amazing.

So, to summarize a gloomy list, Loudspeakers transmogrify the signal (at least) in the following ways;

Even a single driver stretches out impulsive events in time and produce longer events ahead of or behind the input signal according the drivers acoustic phase (nicely described by Heyser). Both acoustic phase and energy storage can be issues here.
A single driver also produces “free sound” not part of the input signal, in the form of harmonic distortion and uncorrelated noise and does so at a rate, which increases rapidly with level. It’s a good thing we don’t hear distortion well at all in transient signals.
Acoustically, a single driver operated over a wide band, has a rapidly collapsing radiation pattern as one gets up high, to the extent one desires the reverberant field to be about the same spectrum as the direct, then this variable pattern is undesirable but still directivity is an advantage here.
Then you have the large phase shifts crossovers impose added to the usually much too large driver to driver spacing making the result position dependant too.


For the speakers at work, I tried to resolve all of these issues, I will be curious to hear what you guys think.
On that note, I have a pair of SH-50’s on the demo schedule and after they go to Oswald’s mill, they will be going to Fla.
Best,
Tom



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