Making Square Waves?

[noah katz]

Hi Tom,

What's the coverage angle of that horn? Is it smaller than you've had in that room before?

I wonder if most its goodness is from minimized room reflections.

I've been reading about using waveguides with standard dome tweeters, and I'm sold on the benefits.

Not to hijack the thread, but is it correct to say that a waveguide is a horn with low compression ratio?

Is it just the way the expansion formula works out that waveguides are mostly concave, while horns are convex?

Thanks

[noah katz]

Aren't those Jordan plots a bit suspicious?

How can the top be so flat without response approachiong DC, and why doesn't the slope change for different freq?

What's its driver configuration?

[Tom Danley]

Hi Noah

The horn is a 50 by 50 degree wall angle, the mouth is 28 inches square.
Actually I have never tried this on a big speaker indoors before, all the smaller ones were from much closer. I have a tower for measuring speakers in full space (and so no reflections) but it’s not warm enough to do it outside now.

I would say in audio, horn and waveguide are sort of interchangeable terms. If anything there may be a carryover from the RF world where a waveguide is more for controlling radiation patterns, even sacrificing gain.
A small dome in a waveguide does feel the radiation load from the horn loading, until the dome becomes about 1 wl in circumference. The waveguide also defines the directivity, starting at some lower frequency governed by the horn wall angle and physical size.
A conical horn (round or square like the one in the photo) has a constant radiation angle vs frequency, a curved wall horn has a collapsing directivity pattern with increasing frequency.
The conical horn has a frequency response equal to the acoustic power of the driver (vs frequency).
The curved wall horn has more output at the high end due to the concentration of the radiation angle.
As compression drivers have an acoustic power that falls off starting at about 2500Hz, this must be EQ’d in the conical horn and if everything is perfect, the curved wall horn needs no compensation.
The reverberant sound spectrum in your room is related to the acoustic power and room absorption.
A curved wall horn usually will have a dark murky sound outside the direct pattern, which reflects the true acoustic power.
A constant directivity speaker with a flat power response sounds the same spectrally everywhere, just quieter and no stereo image when your out of the direct pattern.
A soft dome may or may not be loaded by a horn however, it is not a piston radiator over much of its band.

Yes, the 100Hz square wave is suspicious although they may have been able to find a position where it made one.
The 250 Hz square I posted already shows a tilt due to the speaker’s 50Hz low cutoff and phase response.
Cheers,

Tom

[ravon]

Of course a speaker cannot produce real acoustical square waves because a speaker cannot maintain sound pressure when the cone is in a stationary position defined by the DC top and bottom of the input square wave. Try a 0.1 Hz square wave and you'll see what I mean.

The Jordan JX92S in a TL housing (Konus Essence clone) performs very nice square wave lookalikes. After performing a number of tests on a couple of single drivers I saw that the square wave response is just a repetitive step response which may look like a square wave.

Once the step response is known (with high enough accuracy/resolution), the square waveform response can be predicted for almost any audible frequency. If the ringing of the driver caused by the slope transiënt fits nice you get a nice square wave lookalike.
That way I chose a couple of frequencies where the Jordan performs nice. If I would have chosen 80 Hz instead of 100 Hz, there would not be much of a square wave left.

Placing the microphone is critical. With the microphone placed on-axis a Jordan driver does not produce square wave lookalikes.

Problem may be to produce a good low frequency input square wave. My sound cards are not very good at producing low frequency square waves. I use an opamp which switches from rail to rail driven by a sine wave.

[noah katz]

"A small dome in a waveguide does feel the radiation load from the horn loading, until the dome becomes about 1 wl in circumference."

That comes to 3.8 kHz for a 28 mm dome; so it's loaded up until that freq, right? That would agree with what Zaph got with his waveguide.

Can you explain why what are called waveguides have such a shallow concave shape?

Thanks

"Of course a speaker cannot produce real acoustical square waves because a speaker cannot maintain sound pressure when the cone is in a stationary position "

DC would have the cone moving at constant velocity (or maybe it's acceleration).

[ravon]

Have you ever put a small DC on a driver to observe the cone movement?

[noah katz]

"Have you ever put a small DC on a driver to observe the cone movement?"

Yep.

If your point is that the cone is stationary so that corresponds to DC, that doesn't follow, since there is no acoustical output at DC.

Well, I guess there is very briefly, until the excursion limitsa are reached, but I don't think that counts.

[ravon]

DC results in a stationary cone which does not produce sound.

Switching between + and - DC at a very low (0,1 Hz) frequency corresponds to a cone which switches from excursion limit to excursion limit because the time between switching is long enough for the cone to stop moving (ringing).

At higher frequencies switching occurs before the cone has stopped ringing.

And since cones only produce sound when they move it is critical to keep it moving if you want to see an acoustical square wave lookalike.

[kelticwizard]

Originally posted by PB2
B&O was very big on this when they were promoting their filler driver, "the missing link" innovation....The paper is in the AES "Loudspeaker anthology".....title "A Novel Approach to Linear Phase Loudspeakers Using Passive Crossover Networks" Erik Baekgaard

There are oscilloscope traces in there, they're square more or less, much better than most speakers but nothing like an amplifier.

Pete B.

For those interested, I have that paper scanned. Nothing fancy-no pdf. Each page is in GIF, as if it had come from a copy machine. Anyone who wants it sent to to them just Email me.

Here are some oscilloscipe traces for on axis response. As you can see, they are very good.

The article also gives the method to make a third order crossover with the same qualities.

Here is the second order square wave response. This wave form is for a second order crossover at the crossover frequency.

[kelticwizard]

Just thought I would throw in that john k, a member here, has a spreadsheet for calculating the values for the second order version of this crossover.

Here it is.