A different look at horns vs direct radiators???

[jwc]

http://www.volvotreter.de/dl-section.htm

Lately I have been messin around with this guys Excel spreadsheet. If you use the "midbass Fronthorns" tab...this will basically use an exponential flare by default unless you want to modify it to hyperbolic. Anyway, if you put in that you want a 90Hz horn, 1/2 space with a throat of 580 sq cm,....you get a horn that is about 70 cm long with a mouth that is 5745 sq cm. Put that in Hornresp to get the curve. Yes the horn with hornrest will have a cutoff of 90Hz.

Now, in the excel spreadsheet, go to the tractrix rectangular horn defined by throat. Now if you use the same throat size with a 1/2 space horn with a 90Hz flare, you get obviously the same size mouth but with a shorter horn. The horn length about 37 cm. Now putting this into Hornresp to look at the curve, you get a 128Hz horn?

Greg. I have been using that window for tractrix in hornresp as you say. However, I haven't been using the "specify value for FTA". What is that? Tangent angle at mouth?

jc

[jwc]

Look at this image. The tractrix calculation for 580 sq cm (90 sq in) throat for 80Hz at 1/2 space. This is only 46.5 cm long (18.3 Inches). Large mouth. Would be one cool straight horn if you ask me. So (2) K-31's modeled in there????? So would the concept of using tractrix Bomb here? Would I get down to 80Hz?

jc

[DrWho]

Now, in the excel spreadsheet, go to the tractrix rectangular horn defined by throat.

I wonder if that might be the difference? Hornresp is modelling a circular cross-section only.

I also wonder what other assumptions are being made between the programs.

[jwc]

Mike. The spread sheet is using straight forward calculations from the tractrix or exponential thread. Is makes no predections of response.

I have used some other programs before which would agree with this spreadsheet program. It also wouldn't matter if the mouth was circular or rectangular. All you needed was throat size, Fc and what "space" and you would get a horn.

Building a 1/2 space tractrix straight horn isn't an unreasonable size. I just do understand why it wouldn't work down to 80Hz if that is what the tractrix formula churns out.

jc

[Marvel]

Did you read what Erik said about building the midbass with a tractrix?

"You might ask why this guy used the Tractrix contour for his midbass,
where everybody knows, that a horn of such geometry
runs out of gas way before reaching fs. You're
right it's true. However, my idea was to design a horn which should
sound wise match the Tractrix midrange as good as possible. That's
why I decided to use Tractrix instead of a hyperbolic (m<1) or
exponential (m=1) contour. Actually, in my room, these horns measure
140Hz to 700Hz within a range of ± 2dB. Below 140Hz these horns rolls
off with approx 12dB/oct."

[Edgar]

... Now if you use the same throat size with a 1/2 space horn with a 90Hz flare, you get obviously the same size mouth but with a shorter horn.  The horn length about 37 cm.  Now putting this into Hornresp to look at the curve, you get a 128Hz horn?

Greg.  I have been using that window for tractrix in hornresp as you say.  However, I haven't been using the "specify value for FTA".  What is that?  Tangent angle at mouth?

jc

I think that the Excel spreadsheet automatically reduces the mouth area by 1/2 in each of the different radiating angles. That is to say, it gives a full-sized mouth for free (4 PI) response, half-sized mouth for floor (2 PI) response, quarter-sized mouth for double corner (PI) response, and eighth-sized mouth for triple corner (half PI) response. Hornresp does NOT do this. Hornresp uses exactly the same horn regardless of the radiation angle, but it computes the response of that full-sized horn radiating into the smaller angles, as appropriate.

With Hornresp you can specify the tangent angle at the mouth of the horn, the maximum for a full mouth being 90°. I haven't figured out what the program uses for a default value; it doesn't seem to be 90°.

Greg

[jwc]

Did you read what Erik said about building the midbass with a tractrix?

"You might ask why this guy used the Tractrix contour for his midbass, where everybody knows, that a horn of such geometry runs out of gas way before reaching fs. You're right it's true. However, my idea was to design a horn which should sound wise match the Tractrix midrange as good as possible. That's why I decided to use Tractrix instead of a hyperbolic (m<1) or exponential (m=1) contour. Actually, in my room, these horns measure 140Hz to 700Hz within a range of ± 2dB. Below 140Hz these horns rolls off with approx 12dB/oct."

Yea Bruce, I see that. But why? So if I built an 80Hz 1/2 or 1/4 space midbass horn, I wonder if it will get down to 100Hz? Eric's was a 150Hz flare and was + or - 2 db to 140 Hz.

jc

[jwc]

I think that the Excel spreadsheet automatically reduces the mouth area by 1/2 in each of the different radiating angles. That is to say, it gives a full-sized mouth for free (4 PI) response, half-sized mouth for floor (2 PI) response, quarter-sized mouth for double corner (PI) response, and eighth-sized mouth for triple corner (half PI) response.

Yes. Exactly.

But the angle you speak of I'm unfamiliar with. I can put different angle values in there to get the Fc I want but tells me nothing "visually" of what it does to the horn nor how to build it.

jc

[Edgar]

But the angle you speak of I'm unfamiliar with.  I can put different angle values in there to get the Fc I want but tells me nothing "visually" of what it does to the horn nor how to build it.

If you look at Figure 20.2 here, Fta is the angle between the y-axis and the line extending from the y-axis to the horn contour (the lines in the figure with a big dot on each end). For a given horn length, if Fta is 90° then you have a full horn for that given throat and mouth size. If Fta is less than 90°, then you have only part of what would be a longer horn if you could see the whole thing. Longer horn = lower cutoff frequency, but the more you shorten the horn (relative to its full length), the more mouth reflections become a problem. And if you shorten it to something below about 1/4 the wavelength of the design cutoff frequency, you are basically wasting your time.

Greg

[jwc]

Man. That is interesting. I'll have to play with those angles a little when I get home.

Thanks.

Bruce, I noticed that Eric doesn't have the tractrix midbass horns in his system anymore. He is using a 77Hz midbass exponential straight horn.

jc

[DrWho]

Look at this image. The tractrix calculation for 580 sq cm (90 sq in) throat for 80Hz at 1/2 space. This is only 46.5 cm long (18.3 Inches). Large mouth. Would be one cool straight horn if you ask me. So (2) K-31's modeled in there????? So would the concept of using tractrix Bomb here? Would I get down to 80Hz?

Here's what hornresp predicts:

(dark line is 120L rear volume, light line is 60L rear volume)

[DrWho]

And here's the input parameters.

I forced Fta to 90 degrees in order to get the same horn length calculated from your spreadsheet. Apparently hornresp thinks this is a 115Hz horn.

[DrWho]

If I double the mouth size and force Fta to 90, then Hornresp thinks it's an 80Hz horn, but then it's 88.5cm long:

[DrWho]

And then here's the predicted response:

(dark line is the larger mouth, light line is the smaller mouth - both with 120L rear volume)

[DrWho]

And then here's the response with 1/8space loading...

(it's over 105dB from 30Hz to 600Hz) []

[DrWho]

Roy is always talking about a good horn adding loading below Fc, and that a good horn will be the driver's response in a sealed box with extra gain...

Here's the model for two K-31's in a sealed box with 120L:
(dark line is sealed box, light line is the horn)

[jwc]

Mike. All those curves look like what I was getting too. You had to have the full mouth to get the Fc I wanted. That is huge!

Nice visualization with the sealed box scenario. I haven't played with that.

I'm getting leary of tractrix midbass. I may just stick with exponential on my next build.

jc

[DrWho]

Are you interested in an "Fc" or an F3?

Personally, I could care less what the Fc is as long as I have the bandwidth I want. Btw, 1/8 space with 7186sq cm mouth and 60L rear volume:

[jwc]

Are you interested in an "Fc" or an F3?

Personally, I could care less what the Fc is as long as I have the bandwidth I want. Btw, 1/8 space with 7186sq cm mouth and 60L rear volume:

Technically, I don't care. But I use the Fc for calculating and drawing up the curve.

Now what you just have shown looks pretty good. Is that with a 580 sq in throat? What was the length of the horn? Tractrix I assume.

jc

[jwc]

Greg. Weird. Your point was just being discussed on another forum by McBean.

http://www.audioasylum.com/forums/hug/messages/13/134965.html