Wednesday, October 19, 2016

Trashcan Monster, Part 2

Moving to 1/2" tubing and fittings definitely sped up the prop.  I think as a general rule of thumb you should go up a size or two on the fittings and tubing.  Even though the cylinder, valve and accumulator all have 1/4" NPT connections, look at the difference between these two fittings:

Two push-to-connect fittings with 1/4" NPT threads.
Left: 1/4" O.D. tubing.  Right: 1/2" O.D. tubing.
Even though these two fittings both screw into a component that has 1/4" NPTF connectors, the orifice on the right is twice as big, allowing greater air flow.

Initially I thought the Automation Direct NITRA valve was not performing to its specs, but it turned out the 1/4" tubing and fittings were choking the air flow.  1/2" tubing and fittings fixed the problem.

The prop uses my standard pneumatic design where each prop has its own accumulator tank to buffer air from the main supply line so as to not affect other props.

I like to use Insteon dual-band relay switches to trigger my props which allows me to use either a wireless remote control or a battery-powered Insteon motion sensor to trigger each prop.

The Insteon relay originally turned a 120VAC relay on and off.  The relay's normally-open contacts were connected to one of the Arduino's analog inputs through a pull-down resistor.  While testing the control box I discovered the Arduino was resetting every time the relay shut off, so I suspect the relay coil was discharging a spike which was affecting the AC to DC power supply.  I'm sure I could have fixed this with a diode or capacitor or something but I didn't want to deal with it and set off in search for another way for the Arduino to tell if the Insteon relay was turned on.

One technique would be to use a photodiode sensor aimed at a small light turned off and on by the Insteon relay.  I used a similar method in my Big Blue Clear-Com Call Light project. 

Another technique would be to use some kind of AC voltage or current sensor.  Since a current sensor would require a load - which could have been a light anyway, I searched for a voltage sensor.  Several articles on the internet mentioned the "zmpt101b" and I discovered it is sold on Amazon.

This AC voltage sensor board is a little tricky to use - while no AC voltage is applied the initial reading is stable, but when AC voltage is present the analog input reading sweeps from plus to minus around 60 from the initial reading.  This means that the reading crosses through the "off" value even when "on" which can lead to a trigger loop.  Code was needed to watch this reading and wait for it to stop sweeping to determine that it was truly "off."

Update:  After finishing the prop and writing this post, I discovered a couple of even smaller components to translate Insteon commands into an Arduino trigger:  A High Voltage Phototransistor Optocoupler ($0.49) and the INSTEON 2443-222 Micro On/Off Module ($40), which is apparently Dual-Band.  Again, next prop...

I've also come to realize that the sound quality of the ELK-120 board is not great.  Research into the specifications of its ISD 4004 chip showed it uses an audio sampling rate of 8khz.  Compare that to the 44.1khz rate of a regular audio CD.  Turns out it would have been cheaper to use a Raspberry Pi and a 20w amplifier board than the Arduino Leonardo, 4 channels of relay board and an ELK-120.  Not only cheaper, but better sound quality, and I'd save around 11 square inches of space which would let me use a smaller enclosure for all the electronics.  I'll do that on the next prop.

This video shows the control box internals.

This video shows the completed prop in operation.

I posted a parts list with pricing on Google Docs.

Monday, October 17, 2016

Halloween Yard Haunt Sound System

I have always had sound for my yard haunts.  Even going back to my childhood I had a basic system - a Halloween record playing through an outdoor speaker.  Over the years it has evolved. 
  • Early 1980s - record player and outdoor speaker
  • Late 1980s - two cassette decks crossfaded using a small Radio Shack mixer and several outdoor speakers.  There were different cassettes for each "scene" that the people would walk through and as they transitioned I would crossfade to the next cassette then change the cassette in the other player.
  • 2005 - A boombox playing a CD that loops and a digital recorder playing back through amplified speakers.
  • 2007 - Sound Cue System running on a laptop connected to a multichannel USB sound card, a couple of stereo consumer audio receivers and several 8 ohm speakers.
  • 2012 - 5 hour, 13-channel recording playing from an Alesis HD24 through a Crown 8-channel 70v amplifier and a 70v transformer on each speaker.

I've seen articles about using radios tuned to a weak FM transmitter, wireless speakers (which need power run to them anyway) or small self-contained sound systems with their own playback and amplifier components.

I still feel that the components used outside in the haunt should be inexpensive and easy to replace if stolen or damaged by weather.  So since 2012 I've been pretty happy with a centralized system. 

I use 70-volt constant-voltage wherever I can.  There are still a couple of channels that drive 8-ohm speakers over heavy cable to reduce loss, but all the 70-volt stuff runs over cheap 18-gauge lampcord.  Any 8-ohm speaker can be converted to use 70-volt by adding a transformer of the appropriate wattage.

This $13 70-volt transformer
is available from Amazon.

To compare standard 8-ohm mode against 70-volt, check out this chart:

Power available to system in watts using 18-gauge cable
 100 feet500 feet
70-volt / constant-voltage95.0878.63
8-ohm / constant-current74.3630.93

70-volt systems are far more efficient.

I purchase the sound effects from - once you own the sound effects you can legally use them in a public performance. 

Nero has a couple of free software utilities that were very useful when putting together the soundtrack.

Nero SoundTrax - great for layering sound effects
Nero WaveEditor - great for preparing individual sound effects

Once the individual tracks are completed they are played in a continuous loop while recording to the Alesis HD24.  I bought the HD24 on eBay years ago.  They are probably even cheaper now.

Recording the soundtrack

On the HD24 you can arm individual tracks separately so you can make one pass with the first set, move on to the next set, and so on.  I figured the longest Halloween night could ever run is five hours, so the "song" on the HD24 is that long.

The Alesis HD24

I felt the mix on a couple of the scenes would be best tweaked on site rather than premixed, so I kept those as separate channels and fed them through a Gentner AP800 mixer.  I hacked the Gentner using an Arduino to make a remote mix control.

The Gentner AP800 can be found on eBay for around $25

The output of the Gentner and the other channels from the HD24 connect to a Crown CTs-8200 8-channel amplifier. 

The Crown CTs-8200 can be found on eBay for around $400. 
That's $50 per channel!

Each pair of outputs on the Crown can be set to run in 70-volt or standard 8-ohm mode.

I found it handy to keep track of the patch and levels to make future setups easy.
System Configuration Record
How the sounds are used:
  1. Crickets - there are three speakers on the left channel and three speakers on the right channel, hid at ground level in ivy and bushes.
  2. Regular & Spooky Wind - balanced via the Gentner, there are three speakers on the left channel and three speakers on the right channel set along the edge of the roofline all across the front of the property.
  3. Tree Critters - one speaker per channel, hidden up in trees at the far left and right of the yard haunt.  The tracks cycle through things like bats, owls, birds, and other spooky sounds with a bit of silence in between each one.  They also alternate which side they play on.
  4. Zombie Scene - a story in sound about two zombies.  The speaker for this has a 12" woofer for good bass and is hidden at the rear of the scene area.
  5. Atrium Wind & Music - balanced via the Gentner, there are one to two speakers on the roof facing the atrium area.  The wind in here is different because it is an enclosed space.
The system fits into a 19" rolling rack:

Monday, October 3, 2016

Trashcan Monster, Part 1

For Halloween 2016 I'm going to try to build a "trashcan monster" shock prop, using a metal trash can from Home Depot.

After getting tired of hooking up a laptop to my Jack in the Box prop to tweak settings, I decided to see what I could do with a small backlit 16x2 display and a pushbutton menu design.  Anything that needs to be adjustable could be done through this interface, simplifying things.

And I am going 1/4" on all air components to increase the flow rate so the prop can open and close really fast.

Here's the initial pneumatic linkage design:

The Arduino code can be found here and I've uploaded a video showing how the menu navigation works.

Here are some photos of the completed linkage:

Linkage1 Linkage2

And here's the mask frame where the speaker is, and after the mask is placed over the frame:

TCM Mask Support TCM Mask with gauze

A quick test of the pneumatics using 1/4" tubing in this video seemed too slow for a shock prop. I'm going to upgrade all the tubing and fittings to 1/2" and see how that performs.

Continue to Trashcan Monster, Part 2

Tuesday, April 5, 2016

Use C-weighting to monitor levels in live sound

I continue to encounter live sound "professionals" who have not fully educated themselves on the science of sound and the frequency response of the human ear.  The topic evokes responses of borderline religious or political fervor, where discussion invariably devolves into personal attacks when defenders cannot produce technically accurate rebuttals. It's understandable that someone who has been doing sound for a long time would become defensive when learning they may have been doing something wrong the entire time.

I have seen all sorts of explanations about the correct application of A- and C-weighting.  Some professionals think that the difference has to do with the sampling rate, or think that C-weighting is a completely flat measurement.  Many people continue to use A-weighting to measure loud performances which is incorrect.

All this in the face of what is abundantly clear:  Many live sound "professionals" (and particularly the "DJ" subset) permanently raise the hearing thresholds of their audiences by running at high levels.  Each generation of concert-goers, once their hearing has been damaged, goes on to demand high levels from the next generation of live sound professionals, and the cycle never ends. Maybe we need one or two "reasonable sound level" shows during a run for audiences that don't want their hearing damaged.

I have personally resigned from productions when forced to exceed what I have set as my maximum sound level based on my own research. Not surprisingly, the individuals requesting higher levels had on prior occasions disclosed they had some form of hearing damage.  I urge you to do your own research and monitor your levels with an SPL meter. 

I don't want to hear excuses like "even though I'm at the mixing console I'm not the one who decides how loud the levels will be" - this sounds eerily similar to the excuse "I was just following orders" - and we know that whether your finger is on the trigger of a gun or on the master fader of a mixing console that you ARE responsible for the damage you cause.  As professionals we absolutely have the right to negotiate our contract however we want, and to exercise our right by walking off a production.

Here is a list of what facts you need to accept in order to reach my conclusions:
  1. The frequency response of the human ear is not flat
  2. The frequency response of the human ear changes at different overall sound levels.  At low sound levels, such as those of a quiet room, the frequency response curve is different than when in a loud concert.  (Fletcher and Munson)
  3. The A and C curves closely match human hearing at 40 and 100 phons, respectively.
  4. OSHA's inappropriate selection of the A curve for measuring loud performances happened prior to 1983. Once a standard is created, it is difficult to change to something better
  5. The appropriate curve to use on your SPL meter is based on the loudness of the performance (quiet lecture or loud concert)
My argument is that if you use the C scale and stay under Yamaha's limits you will certainly stay under OSHA's limits using the A scale, with the added benefit of causing LESS hearing damage.


Given the sensitivity characteristic of the ear, the 'A weighted' curve is most suitable for low level sound measurement....In the presence of loud sounds, such as rock concerts, the ear has a "flatter" sensitivity characteristic...In order for the measured sound level to more closely correspond to the perceived sound level, one would want a flatter response from the SPL meter.  This is the function of the B and C weighting scales.  In apparent conflict with this common-sense approach, O.S.H.A. and most government agencies that get involved with sound level monitoring continue to use the A scale for measuring loud noises.  Since this causes them to obtain lower readings than they otherwise would, the inappropriate use of the A scale works in favor of those who don't want to be restricted...It is beyond the scope of this book to detail the 'danger' levels, but anyone responsible for sound reinforcement should be cautious about delivering levels above 95 db(C) SPL to an audience for any prolonged period.
Davis & Jones. pp. 30-31 Yamaha's Sound Reinforcement Handbook. 2nd ed. Milwaukee: Hal Leonard, 1989.
C-weighting is optimized for full-bandwidth sources at levels exceeding 85dB. A-weighting filters out the high and low frequencies and is optimized for lower volumes.
Gibson, Bill. p.46 The Ultimate Live Sound Operator's Handbook. 2nd ed. Milwaukee: Hal Leonard, 2011.
Pro audio equipment often lists an A-weighted noise spec − not because it correlates well with our hearing − but because it can "hide" nasty hum components that make for bad noise specs.
...the C-weighting correlates better with the human response to high noise levels.
Gracey & Associates
On a sound level meter (SLM)...and we are forced to choose between A and C, we should pick C.
Doctor ProAudio
When dealing with the uninformed, live sound folks can "cheat" the rules (or some poser's cursory knowledge) by using the A-weighted filter so that measurements appear to be lower in SPL than a standard or rule that is based on a Flat or a C-weighted limit.
Pro Sound Web