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Old 26th November 2009, 02:42 PM   #1
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Default Tapped Horns - Simulation VS Reality

On another forum where I am a moderator, a subwoofer was donated by a sponsor to one of the forum members. He put the sub in a tapped horn, and blew it to pieces in a matter of days.

I'd like to dedicate this thread to that poor sub's memory, and let it know that it did not die in vain.

In that sub's honor, I'm going to take a few minutes on my holiday to explore the behavior of tapped horns in the real world. The simulations and the reality aren't what they seem.

Before I dive into this topic, a few caveats:
  • I've built and measured a pile of speakers over the past couple decades, and I've noticed that the computer simulations of bandpass and vented boxes rarely match reality, particularly in a room. This is because vents do not behave like the simulations would have you believe. For instance, the predicted length of a vent is affected by the location of the mouth and the throat. Tapering a vent improves performance a great deal, and tapering affects the tuning frequency also.
  • Ironically, I've noticed that horns and tapped horns perform closer to the simulations than vented and bandpass boxes do. I believe this is because the helmholtz resonance depends on a lot of variables that aren't factored in by programs like WinISD, while HornResp and Akabak are more complex (and accurate.)
  • Sealed boxes are incredibly forgiving. For instance, modest leaks do little to alter their response. If only other box types were so forgiving! IMHO bandpass boxes are one of the most unforgiving box designs, because leaks screw up the helmholtz resonance. I've seen leaks that were the size of a pinhead that have altered the frequency response! And when you think about the pressure that's being exerted on the chambers in a bandpass box, you can see why a leak that's invisible to the naked eye will have an effect on the output. With a sealed box, not so much, because a small leak is "competing" with the output from a cone that's dwarfs it.

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Old 26th November 2009, 03:22 PM   #2
jbell is offline jbell  United States
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Quote:
Originally Posted by Patrick Bateman View Post
On another forum where I am a moderator, a subwoofer was donated by a sponsor to one of the forum members. He put the sub in a tapped horn, and blew it to pieces in a matter of days.

I'd like to dedicate this thread to that poor sub's memory, and let it know that it did not die in vain.
was it high passed? A pa style tapped horn without a high pass, is like a duck in a shooting gallery...
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Old 26th November 2009, 03:31 PM   #3
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Before we get into the measurements, lets do some CSI work on our victim.

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This is the predicted frequency response of the subwoofer in the manufacturer's recommended enclosure, and in the tapped horn that the victim died in. The murder weapon^H^H^H^H^H^H^H^H^H^H^H^H tapped horn has a nice bump in efficiency, about 10db at 60hz. In the tapped horn, the woofer will sound about twice as loud.

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When the victim's father asked me about putting the victim in a tapped horn, it seemed to be a good fit. High FS, high BL, and low VAS. All are things you want in a TH woofer. The tapped horn is relatively compact, under three cubic feet. The sealed box is VERY small... And for a reason. (We'll get to that shortly.)

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A picture of the victim during happier days

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Note to self - a three cubic foot sub box fits a lot better in an SUV than in my Honda coupe

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Click the image to open in full size.
Hope I didn't upset anyone with these gory photographs.

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Old 26th November 2009, 03:33 PM   #4
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Quote:
Originally Posted by jbell View Post
was it high passed? A pa style tapped horn without a high pass, is like a duck in a shooting gallery...
Q: How can you tell the difference between a good amp for a tapped horn, and a bad amp?
A: The good one has a rumble filter.

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Old 26th November 2009, 05:42 PM   #5
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Only thing I see is there is no loading below 35Hz and displacement gets well into the damage zone with 40 volts.
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Old 26th November 2009, 05:57 PM   #6
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After building the tapped horn, the victim's father noted that "the box design is small, limiting the deeper bass, I knew it should have been twice this size."

All you CSI types know that this is the first hint of the horrors to come. To investigate further, let's fire up hornresp.

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According to our simulations, the tapped horn is capable of generating 100db at 20hz, and 122db at 40hz.
This is one clue to the "lack of deep bass" comments. While the sub can generate lots of deep bass, the 40hz note is over 22db louder than the 20hz note. Factor in cabin gain, and the sealed box will be smoother... But the tapped horn is still more efficient.
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Here's the predicted excursion, with 500 watts into a 3ohm load. If you're listening to rock music, this sub will be rockin' out with 500 watts. Most of the energy is in the octave between 40 and 80hz, and the sub won't exceed it's limits in that octave.

But what if you put on a rap or techno CD? With the sealed box and 500 watts, a bass line at 25hz is exceeding the woofers limits. Not by a LOT though. You might hear a metallic "whacking" sound as the woofer starts to run out of excursion.

With the tapped horn? It's woofer detonation time! A single strike of a drum at 25hz will result in the voice coil jumping nearly TWICE as far as it's supposed to go

The scary part about this crime scene is that you probably wouldn't see it coming. Look at the excursion on the sealed box, and see how it graaaaaaadualy increases. And on the tapped horn?

Voice coil simply jumps out of the gap. With enough power, the woofer is irreparably trashed.

Another thing that I've noticed in my measurements is that a sealed box mounted in the trunk of a car sees an increase in the impedance curve. My hunch is that the air in the trunk "presses" against the cone, raising impedance and lowering excursion.

In other words, the sims predict that a sealed box can only handle 500 watts due to excursion. If my hunch about sealed boxes in trunks is correct, the "real" power handling could be as much as 750watts.

But with the tapped horn, the woofer would still explode, because it's waaaaaaaaaay beyond it's limits. A 25% reduction in excursion isn't going to save it.

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Old 27th November 2009, 06:02 AM   #7
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Hi Patrick Bateman,

Quote:
Originally Posted by Patrick Bateman View Post
The scary part about this crime scene is that you probably wouldn't see it coming.
You would if you used the Hornresp Maximum SPL tool :-). The attached screenprint shows your sealed box example - red indicates displacement limited. For the purpose of the exercise, Xmax is assumed to be 13 mm. (In any analysis such as this, it is important to understand the difference between input power and input voltage).

Quote:
Originally Posted by Patrick Bateman View Post
In other words, the sims predict that a sealed box can only handle 500 watts due to excursion. If my hunch about sealed boxes in trunks is correct, the "real" power handling could be as much as 750watts.
As indicated in the Diaphragm Displacement section of the Hornresp Help file, behaviour at all input voltage levels is assumed to be linear. No allowance is made for low frequency high power amplitude compression.

Kind regards,

David
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Old 27th November 2009, 03:13 PM   #8
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Quote:
Originally Posted by David McBean View Post
Hi Patrick Bateman,



You would if you used the Hornresp Maximum SPL tool :-). The attached screenprint shows your sealed box example - red indicates displacement limited. For the purpose of the exercise, Xmax is assumed to be 13 mm. (In any analysis such as this, it is important to understand the difference between input power and input voltage).



As indicated in the Diaphragm Displacement section of the Hornresp Help file, behaviour at all input voltage levels is assumed to be linear. No allowance is made for low frequency high power amplitude compression.

Kind regards,

David


Oh, I should have mentioned, I didn't design the murder weapon

The dB Drag crowd discovered tapped horns, and a member on a forum that I moderate strapped 6000 watts worth of amplifiers to a ten inside a TH. With tragic results

Here's the person that designed - actually a lot of nice boxes here:
Peter W. Kulicki WorkBlog

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Old 28th November 2009, 03:46 AM   #9
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Wow I feel so Dirty / after reading this i feel like i go around clubing baby seals for there fur
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Old 28th November 2009, 03:48 AM   #10
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you left out that i was trying to build an Sq enclosure, and had no experiance with the fine art of soft bass
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