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Old 29th July 2011, 10:19 AM   #2041
Mark Kravchenko --- www.kravchenko-audio.com
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Well David you are indeed one productive dude!

What I can contribute towards measurement and verification I will.
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Old 29th July 2011, 10:40 AM   #2042
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by jbell View Post
Air speed may be one thing to be aware of, but I'm thinking that air friction (if that's the right description) may be even more relevant in small horns.
I'd like to refer to that as different "damping" or "Q".
The same when one packs a vent tube with straws. The Q has been changed.
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Old 29th July 2011, 10:45 AM   #2043
Mark Kravchenko --- www.kravchenko-audio.com
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Quote:
I'd like to refer to that as different "damping" or "Q".
The same when one packs a vent tube with straws. The Q has been changed.
Good thinking.
Andrew you are always a source of inspiration.
I have tried the straw trick on ports. The change is usually subtle but there is a change.
Are you thinking the Q idea is a change in the air's compliance?
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Old 29th July 2011, 10:45 AM   #2044
AndrewT is offline AndrewT  Scotland
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Default Is this Off Topic?

Quote:
Originally Posted by mwmkravchenko View Post
.........thingy magigy...........
lovely phrase, but can I be pedantic and suggest the middle g be replaced with a j?
But then some other will tell us that there is a g missing. The frivolous discussion might never stop.
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Old 29th July 2011, 10:47 AM   #2045
Mark Kravchenko --- www.kravchenko-audio.com
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Falling off of chair laughing!

Definitely far off topic but really appreciated!
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Old 29th July 2011, 10:57 AM   #2046
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Josh Ricci View Post
........ the real world results the air velocity doesn't seem to match up that well either. In horns there is a pattern of high velocity and low velocity movement that will follow the driver displacement roughly. .............
Rather than referring to high velocity and low velocity in the horn, I have seen reference in many horn discussions to High Pressure and Low Pressure.
If this a more accurate term then we can expand on it's meaning.
At zero signal the pressures at the throat and the mouth and everywhere else in and around the horn are all equal to atmospheric pressure (atmos) at that altitude, at that time.

Now apply a signal.
The variation in pressure at the throat propagates along the horn to the mouth.
The pressure variation at the (low area) throat are reputed to be much higher than the pressure variations at the (high area) mouth.

Let's assume that the Mouth pressure variations are indeed much less than the throat pressure variations.

Increase the signal level and the throat pressure variations will increase.
At zero signal we have throat pressure = atmos +-0signal.
At normal level output we have atmos +-normal signal.
At high level output we have atmos +-high signal.

I can see no problem with atmos + normal signal nor with atmos + high signal.

I do see a problem with atmos - high signal.
The difference between atmos - normal signal and atmos- high signal could be that the air is changing. Water vapour condensing out, temperature changing or worst case reaching absolute zero pressure.

I really do think it is reduction in pressure limitation that causes the distortion resulting from overdriving the horn.
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Last edited by AndrewT; 29th July 2011 at 11:01 AM.
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Old 29th July 2011, 11:03 AM   #2047
Mark Kravchenko --- www.kravchenko-audio.com
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So a high power stepped sine wave test should get to the bottom of some of this. Say 1/24th octave short duration bursts.

It will excite the horn to a high pressure point and be traceable as to what frequency is of greatest concern in terms of output distortion.

If these things can be correlated to air speed then we ( not me I'm a math imbecile ) may have something useful to model with mathematically.
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Old 29th July 2011, 03:42 PM   #2048
Kolbrek is offline Kolbrek  Norway
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Distortion in horns has been researched for a long time. If you search for "horn distortion" in the AES E-Library you'll get lots of hits. (You'll have to pay for the papers if you are not an AES member and have paid for online library access).

The air itself is nonlinear. When the pressure is high, the temperature rises, and the sound travels faster. When the pressure is low, temperature falls, and the sound speed decreases. The result is that the wave peaks gain on the throughs. A sine wave will begin to look more like a sawtooth wave.

I think there are also issues wiht high particle velocity; that the velocity of the signal adds to the velocity of sound, so that the wave "carries itself forward". The result is the same as above, the peaks will move faster than the throughs.

At some point the peaks will overtake the throughs, and you get shock waves. But that is at extremeley high SPLs.

It is possible to model, and that has been done in many of the papers you'll find in the AES library search. But the equations are quite complex.

In the horn throat you'll easily get 130-160dB SPL. To measure distortion at these levels you need a good high-level mic. I use the LinearX M52 Microphone - High Level Low Distortion Measurement Microphone. It's good to 170dB.

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Bjørn
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Old 29th July 2011, 05:25 PM   #2049
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Kolbrek,
The phenomenon I am interested in most is one where the horns output seemingly plateaus in a few narrow frequency ranges and further increases in power produce no further output gains. This does not happen at areas of high driver excursion but higher up in response. In tapped horns the prominent first spike that is always present (5th harmonic) is one area that always seems to exhibit large amounts of compression and ultimately a complete halt on increased output. Any thoughts?
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Old 29th July 2011, 06:02 PM   #2050
Kolbrek is offline Kolbrek  Norway
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Do you mean the first response peak above the pass band? Tapped horns are quite complex electroacoustic systems, so it's not easy to see intuitively where the problem originates. But the internal pressure and volume velocity in a tapped horn is much higher than in the last part from the driver and out. This may be a cause.

Have you noticed similar things in ordinary front-loaded horns? Have you looked at the throat impedance to see if there are any peaks or dips at the frequencies in question? There may be standing waves that build up high internal pressure or velocity values, that you don't see outside the horn.

Just some thoughts.

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