Horn Expansion (Design)

[oosting]

John,

Sounds like a good idea. Having a 24 inch wide throat might be a problem though. You could fan the sides out but keep them flat. I modified the Excel spreadsheet http://www.ameritech.net/users/oosting/Exponentail_Horn.xls to do that. I also fixed some typos.

Ed

[pzannucci]

John,

I would be concerned about what oosting is talking about. The throat needs to be a good match to the woofer for the horn to be effective. You probably should use a taper out from the woofer to establish the connection to the larger 24in. passages (smooth from the woofer out). Secondly, the box will have to be fairly deep to get to the 2'x2' opening for the mouth if you just use one mouth. You might want to look at dividing into two and using two openings for the mouth. It will give you a larger effective size, lower resonances due to the smaller large areas without bracing and should also keep the box more managable similar to the Klipsch MC series) . Just some other opinions.

Peter Z.

[WMcD]

Gentlemen:

The actual throat area of the K-Horn and other horn loaded speakers using the K-33 is approximately 78 square inches.

It is true that the motor board has a 3" x 13" slot. However, this commmuicates to two 3" x 13" ducts.

The question is thus why PWK uses the slot. It appears to me this is so that a chamber is formed by the concave structure of the woofer. There would not be much of a closed structure without the slot because the throat is approximately half the area of the diaphram.

This allows for a resonant circuit to be formed by the compliance of the front chamber and the mass of the diaphram. This technique allows an improvement in response above about 200 Hz. Leo Baranek discusses this as applied to midranges. I'm quite convinced this is the reason for the slot.

You will probably find that the exponential equations for the K-Horn and LaScala work well if you assume a 78 square inch throat.

Regards,

Gil

[WMcD]

Gentlemen and Ladies:

It is interesting to note that PWK has often expressed exponential growth of the area of his horns in terms of "in what length does the area double."

This is a typical function of exponentials. A bacteria or group of rabbits will double by reproduction in X time. Of course, it works the other way too. Radioactive decay has a half life of X time.

PWK calculates that the area of the exponential horn will double in 1/18.1 of the cut off wavelength. You must convert the cut off frequency to a wavelength.

The result is that the K-Horn cross sectional area doubles (or halves) every 16 inches and has a cut off frequency of about 48 Hz. The LaScala doubles (or halves) every 12 inches and has a cut off frequency of about 70 Hz.

Consider that the LaScala bass horn is 24" by 24" by 24" box. Take the throat area as 78 square inches. The length of the two ducts are as follows.

Going across the back there is a length of about 12 inches. Total area at the turn should be 156 square inches. Going half way to the front is another 12 inches and area should be 312 square inches. Going the next 12 inches to the mouth gives an area of 624 square inches.

Consider that the mouth is approximately 24" by 24" or 576 square inches. So the numbers work out pretty well.

Regards,

Gil

[Ray Garrison]

Gil,

Think I followed that. Just out of curiousity, if you were to take the same funtion to calculate the area / length as used in the La Scala, but shoot for a cut off of 30Hz, what would that look like?

Ray

[oosting]

All,

Mr Gill is right. A cutoff frequency of 62 Hz gives you a doubling rate of 1 ft. I corrected the spreadsheet yet again to accurately calculate the doubling rate. It's at: http://www.ameritech.net/users/oosting/Exponentail_Horn.xls. It's a work-in-progress!

What I find intersting is that the LaScala has a cutoff rate of around 60-70 Hz and responds down to 45 Hz.

It's correct that that the area of the exponential horn will double in 1/18.1 of the cut off wavelength. That number comes from the equation:

Doubling length =

Cutoff_wavelength x ln(2) /(4 x Pi).

For a cutoff of 30 Hz, the area would double every 24.8 inches.

Ed

[pzannucci]

oosting,

I believe that you will get slightly lower response but the response becomes erratic. I would think this is why mid horns usually cut off lower than their crossover (we would notice frequency anomolies in the mids easier) but in the bass sections it is likely that we will not be as atune to the output variations.

Peter Z.

[Randy Bey]

So,

has anyone figured out how large a bass horn would have to be to be flat from 20 Hz to 100 Hz?

I once read about a hi-fi executive back in the days of mono who had a stereo (oops, mono) room where one entire wall was the mouth of a horn that went back 40 ft.

I have fantasies of building such a horn in my basement, starting at one end and culminating in a slot opening near the floor in the rear.

[WMcD]

Regarding scaling up a LaScala it could be done. But consider what happens if you scale it up by two to get an Fc down near 35 Hz. You will have a throat which is four times as big and will have to use four drivers. Also, you will be dealing with a cube which is four feet on a side and will not fit through a standard doorway.

We can look to a Klipsch design of a bass horn with a theoretical 32 Hz Fc. That is shown in US Patent 4,210,223 to Gary C. Gillium and P.W.Klipsch. It looks a lot like the Klipsch theater woofer.

This has a throat area of 172 square inches (using two K-43 drivers). The cross section doubles every 23.3 inches. The mean sound path length is 72.2 inches. The mouth area is 1472.6 square inches. The back chamber is 11,200 cubic inches.

Regards,

Gil

[JohnA]

To All,

Why isn't the 3" x 13" slot in a La Scala the "throat"? Mine begin an immediate expansion from there.

Also, I sold my spare parts to Leonard Frisch (driver & xover). He's still short a K-400 horn. He didn't realize the driver was just the "softball" on the back. If anyone has an extra, he can complete his pair of La Scalas.

John