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Re: dam my horns image - me too !

Posted by KerrB on January 8, 2005 at 21:33:52

In Reply to: Re: dam my horns image - me too ! posted by tomservo on January 8, 2005 at 07:17:24:

Hi Tom,

Thanks for your comments.

You wrote ( and I added a few comments in [ ]'s ) :

" If one measured the acoustic power vs frequency of a compression driver
such as on a resistive load (Plane Wave Tube), one sees that all [ improperly loaded ]
compression drivers have a hf roll off which begins in the 2-4KHz area due
to the moving mass and one sees a second knee in the roll off where the
Rdc and Le act (usually around 8-15KHz depending on the driver / size) and
yet another knee is present as one reaches the front volume / throat mass
filter corner. "

However compression drivers may measure driving a " PWT " or CD horn...my 375's on tractrix horns measure as shown on the graph below.

( I hope the RTA graph shows up on this post but if it doesn't, you can see a complete set at http://homepage.mac.com/ikecarumba/PhotoAlbum118.html )

It shows the response of a JBL 375 on a round 500 Hz ( nom. 350 Hz ) Tractrix horn.

As you can see ( I hope ) the response is slightly weak at 500 Hz, rises smoothly to 1 kHz, dips slightly around 1.25 kHz, then is nearly flat to 9 kHz ( I have the active filter set at 9kHz otherwise it would be flat to 10 KHz ).

The system's overall response ( including the 1954 Klipschorn subwoofers w/ JBL 150-C's, straight 100 Hz upper bass horns w/ JBL 130A's and 077 supertweeter ) is quite linear from 40 Hz to 20k (there's a slight dip at 125 Hz and also at corresponding harmonics of 1.25 kHz and 12.5 kHz ).

Both sets of measurements were taken 3 feet away from - and on the center axis of - the midrange horn. I realize it's not the most accurate possible representation of my systems frequency response, but it's good enough ( the Behringer RTA I use is the latest digital technology and very accurate ).

If you want to see a complete set of RTA graphs go to

To continue, you also wrote :

" Comparing the PWT response to the "on horn" response, one finds the
difference is due to the horns directivity and its ability (or not) to
present a proper load to the driver. "

OK... I understand that. You go on to say :

" Assuming one had the driver mounted on a truly "CD" horn, the measured hf
response is the same as in the plane wave tube, that is, the actual [ ? ]
acoustic power response of the driver is what you see and it is the same
anywhere within its coverage angle. "

OK... I get that too [ I guess ]. Then you continue :

" Placing the same driver on an exponential or Tractrix horn, one finds the
response is much  different that the CD as a result in the difference
between the directivity and loading.
With these [ correct ] horns, one finds (on axis) the response is a lot flatter (in the
ideal match, it is flat on axis) because as the drivers acoustic power
falls, the beam width narrows proportionately, focusing the decreasing
acoustic power into a smaller and smaller area. "

OK... sounds reasonable. But then you go on :

" As one can see, if one wants "flat response" from the CD horn, one has to
"fix" the hf roll off inherent in the driver connected to it. "

Which is where you lose me : A driver ain't broke if it rolls off highs early when it's driving an improper " horn; it's only " inherently flawed " if it's on the wrong kind of horn.

You are suggesting sort of a bass-ackwards " fix " seems to me.

Or are you saying one should only use drivers designed to work specifically with waveguides ( or CD drivers ) if one expects good frequency response on axis ?

I would go along with that.

However, I was only discussing CD horns as they exist - not some virtual - theoretically ideal - waveguide/CD horn.

And I think I got it about right. CD horns - as they exist - sound crappy in home hi-fi systems. Especially post ' CD-horn electronic e.q. ' ( which always results in phase distortion ).

I was not talking about "perfect CD horns "

I was talking about exponential horns ( including Tractrix horns ) when I explained why horns beam : A large mouth - necessary for LF reproduction - results in HF beaming unless it's narrow as well. Which is the basic idea behind CD horns.

That's all I was saying.

As far as Dr. Geddes ideas, like I said, who knows ? Certainly not me, as I have not yet read very much about his theories yet.

Best Regards,

Kerry

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