A horn loaded design in Australia's "Silicon Chip" mag

[Grant Bunter]

This thread isn't about promoting another design!

I read this article when it came out in Oct '13, and something just doesn't feel right about some of the anomalies that are attributed to horn loading.

It is about using a 4" full range driver, with HF direct radiated, and bass response being improved by implementing a rear loaded horn.

http://www.siliconchip.com.au/Issue/201 ... +System/30

The above pages are the only ones available online, as a preview, to encourage sales.

So I will type out some quotes which I would like to know more about:

The first can be seen in the preview, all but last paragraph page 31;
"While efficiency is is a big advantage of a horn speaker system, they do not necessarily result in the smoothest bass response."

Page 34;

"Fig 5 impedance curves...multiple peaks resulting from horn loading"

"Fig 6 shows a harmonic distortion plot....note that the distortion is quite low over much of the audible range, but rises at the low end, partly as a result of horn loading"

All thoughts appreciated...

[miked]

Pretty cool stuff. That magazine reminds me of the old company "Heathkit" here in the states. They would sell kits of parts with projects just like that. We used to have magazines like that here in the states too, but I haven't seen anything like that in awhile. Thanks for posting this.

[Bill Fitzmaurice]

I couldn't get it to load, it's as if the source was on dial-up. However:

"While efficiency is is a big advantage of a horn speaker system, they do not necessarily result in the smoothest bass response."

Not true. Horns are as flat as any alignment, if they're large enough or you use enough of them.

"Fig 5 impedance curves...multiple peaks resulting from horn loading"

That's normal.

"Fig 6 shows a harmonic distortion plot....note that the distortion is quite low over much of the audible range, but rises at the low end, partly as a result of horn loading"

A horn only has high THD below the horn corner frequency, as the fundamentals aren't loaded, so they have no gain, while the harmonics do.

[Grant Bunter]

Thanks Bill.

Lowest frequencies THD looks to have been extrapolated from 20 to 80Hz, with the rest of the plot measured.

There was not a scrap of info about how the horn was modelled. The horn corner frequency is also not stated, but I can't imagine it being to low.
3 drivers where called suitable, the lowest Fs was 83Hz.

Response at 40Hz was around -10dB, relatively flat to 60Hz and increasing after that.
Response below 40Hz fell off rapidly. Would that be the corner frequency perhaps?

[sine143]

says at almost 2m in horn length LC of the horn is 50hz. too lazy to do math right now to see what 200cm equates to but 50hz sound decently close. Now, its still displacement limited, and its still gonna have typical backloaded horn midbass black hole.

[byacey]

Pretty cool stuff. That magazine reminds me of the old company "Heathkit" here in the states. They would sell kits of parts with projects just like that. We used to have magazines like that here in the states too, but I haven't seen anything like that in awhile. Thanks for posting this.

Ah,the golden age of electronics. When I was a kid I read every back issue of Popular Electronics that my dad had collected from about 1956 to 1980, when suddenly the electronics world went microprocessor.

They had articles regarding DIY speaker boxes, HiFi, amateur radio, shortwave listening and all manners of experimenter circuits and projects. It was sad to see it all disappear.

[Grant Bunter]

My calculations show absolute maximum horn length to be in the order of 10'4".
Not bad in a box measuring roughly 2'9" x 1'2".

The actual area of the mouth exit is 9 1/2" x 6" (+ some flaring).

Would it be fair to say that mouth area has been traded for horn length?
And that this is not necessarily a good compromise?

[Bill Fitzmaurice]

The actual area of the mouth exit is 9 1/2" x 6" (+ some flaring).

That's perfect... for a 1kHz midrange horn.
That explains the "not necessarily result in the smoothest bass response."

[Grant Bunter]

Thanks Bill.

This has comfirmed the "something isn't quite right" feeling was for good reason, even if the reason was was other than I initially thought lol.

Seems I have learned something after all...

[Bill Fitzmaurice]

I finally got that page to load. The design is typical of many Fostex rear loaders, for example:
http://www.madisound.com/pdf/bk12m-cab.pdf
Note that this one has a corner frequency of 80Hz, with only 95dB sensitivity. That's about average for boxes of this size. IMO there are far better options. These are popular with the single driver crowd, who feel that only single driver systems give the best results. That's totally illogical. Just as orchestras use small instruments for highs, medium size instruments for mids and large instruments for lows, speaker systems get the best results using small, medium and large drivers to cover the full bandwidth of the music spectrum. You can get great results from a single driver, if it's a one inch wide planar sixteen feet or more high, but it ain't gonna happen with a single four.

[Grant Bunter]

Thanks for taking the time to have a look Bill!

They could not get all the pieces for both cabs to fit on one (true) 4 x 8 piece of ply.
You had to get a half sheet for one piece.

Now if they had taken a leaf out of your book and angle cut 2 pieces, there would have been leftover (albeit small) off the one sheet.

And what you say makes perfect sense, right driver for the job.

Just in case others can't get the article to load, here is a direct link to driver and plans, just to see what we're talking about:
http://www.fostexinternational.com/docs ... E103En.pdf

[Bill Fitzmaurice]

Just in case others can't get the article to load, here is a direct link to driver and plans, just to see what we're talking about:
http://www.fostexinternational.com/docs ... E103En.pdf

Note the xmax. No matter what enclosure it's mounted in you won't be able to put more than 10 watts into it anyway. BTW, I have a pair of very old FE103s that I use for my surrounds, high passed at 120Hz. They probably never see more than 5 watts, so for that usage they're fine.