Build A Mixing Bowl Harmonic Reverb

Words, photo,and sounds by: Telegraphy

Reverberation is the most common and fundamental form of audio effect. This natural occurring effect has audio engineers painstakingly devising better ways to be able to harness natures "audio plug-in". Whether  it's with software or hardware, one thing is certain. Mimicking nature is hard work. With all of today's advanced software reverbs out there, you still can not beat the hardware version.

  A while back I wrote an article describing how to build a D.I.Y. Plate Reverb The response I received from the D.I.Y. community  regarding this article was beyond my wildest dreams. I had no idea there was so much keen interest in hardware based effects out there. That same interest which got me started building the plate reverb, (more recently) persuaded me to take that very same concept and bring it to a whole new level. Taking advantage of a well known side effect all stringed instruments suffer from (wanted or not) a condition known as Sympathetic Resonance would become the foundation of my Mixing Bowl Harmonic Reverb.

 Mixing bowl ? More on that later.

 Before we begin, one has to understand what makes a plate reverb....Well, reverberate. Quoting from D.I.Y. Plate Reverb  
        The concept of a plate reverb is quite simple. An electromagnet, like the one found on a audio speaker, is directly or indirectly coupled to the center of a piece of sheet of metal. Audio from a sound source is fed into this electromagnet (voice coil) which will physically vibrate the piece of sheet metal (plate). These audio vibrations are echoed many times, echos which are in fact persistence of audio. The amount of persistence (reverberation) is determined by  the physical characteristics of the plate. These characteristics many include  length, height, and stiffness of the piece of sheet metal. Once reverberation has been set off in the plate itself, it then needs to be detected. This is accomplish by directly or indirectly coupling microphones to the plate. The micophones pickup the reverberations and sends them back to to be mixed  with the original "dry" audio.
 Now that we know how it works, let's take it to a higher level. Keeping this concept in mind, let's swap the plate with a pair of ordinary kitchen variety salad mixing bowls.......Not satisfied enough? O.K., we'll tie in between them, old used base guitar strings. This will produce some exciting harmonic reverberations. Yes I love excitement. That's why I have my nephew leave lego parts by my bed when I wake up in the morning. Bear foot of course. Exciting !!!  So, why use base strings? The strings act like frequency amplifiers. We can pick and choose what portions of the audio spectrum we want to enhance by way's of using different tuned strings. It works like this... The base strings are acted upon by sympathetic resonance. This condition is best described as when a loud sound directed toward a stringed instrument (ie.guitar or piano) it's strings will vibrate along with that sound. The most familiar exploit is singing into a guitar and hearing it's strings sing back at you.       

 The following article is a step by step description of a home built harmonic reverb.   

 Step 1 :

  The cost of building this reverb was around  $100. All materials needed (excluding electronics) were obtained at my local Lowes home hardware store. Electronics on the other hand, were from RadioShack.
materials needed

Some of the parts needed for this project were made in house. Having basic machine shop tools makes a big difference. If your not fortunate to have facilities to make your own parts, don't be afraid to ask your local Ma and Pop hardware store clerk on how they can improvise making parts for you. Why Ma and Pop hardware stores? Because, they have more experience and to put it simply: THEY HAVE TIME TO SERVE YOU!!

  The mixing bowls I purchased at my local Salvation Army second hand store. One thing you can count on at these stores (besides used underwear and soiled bed mattress) is having a great selection of kitchen gear. Perusing through the kitchen isle, a man feels out of place. Woman instinctively know your up to no good - Especially if your tapping each and every single pot and pan with your finger and then bristly putting it up to your ear ( just to hear how they vibrate) . It's just a hint for woman that this man isn't cooking quiche for dinner tonight.
The used base strings I "borrowed" from my brother who plays base guitar in his band "Air Base Guitar With No Amp Listening To Deathmetal"  Why base strings? I wanted this reverb to have better resonance down low. The small size of it's construction prohibits lower frequency response. To alleviate this problem, thick strings that resonate well at low frequencies are employed. Yea, we'll get you new strings one day bro. Your not using them now. Right? 


                                                                 Let the construction begin.
 Step 2:  Cutting the perforated angle iron into six 13 inch lengths plus two 9 inch lengths. Bolt them using 1/4-20 nuts and bolts. These measurements are somewhat critical as the points of the triangle has to clear the diameter of our mixing bowls. I this case it's a 8-1/4 inch diameter bowl. As for the three horizontal spreaders; I search high and low for an affordable alternative to which you would find in the metal stock section (they really rob you with highly priced "cheap" metal) So I found myself in the shelving and closet isle, because I have a closet full used close bought at Salvation Army store. And it's messy ! Looking around, I couldn't help but notice the 6 foot metal pipe used for closets. Priced at $9, this was 70% less then what you would find in the metal stock isle. Fantastic !   
Every band has a pyramid on their album.
This should be one of them.

 Now before we strap on those mixing bowls to the triangles, we need to punch some holes through the bowls. I used a 3/16 sheet metal punch but you can use the good old fashioned drill. Six evenly spaced holes along the top lip and one big 1/2 hole down in the center of the bowls base. Now before you go and mix someones salad - Tune those base strings  with homemade tuners.

 A 1/2 inch aluminum rod kept rolling under my foot. Thinking it was a safety hazarded, I cut it up into six 1-1/4 inch sections. "There, now I have six safety hazards" " Wait a minute! I can tune base strings with these. Just drill and tape 8-32 screws into them and make a through hole at the other end.    
Holes punched in bowls.
Lashing a base string through the hole at the other end of the tuner and then doing the same at the mixing bowl holes located around the lip. We now have the main component of the Mixing Bowl Harmonic Reverb. You can use as little or as many strings as you choose. Just keep in mind the tension of the strings will have to be spread out around the circumference of the mixing bowls evenly. Equal tension is what we're aiming for. I just inherently went for an even number of six. Because - Well because my brother had six. 


The finished tuner

Attached bowl
Pulling it taut!
Step 3: The reverb is starting to come together. Time for us to kick back, have an adult beverage ( under age folks please drink your Ovaltine) Reviewing on what was built :
 1:   Super structure made with perforated angle iron and closet tubing. 
 2:  Homemade tuners made from aluminum rod for tuning the base strings.
 3:  Lashing base strings to tuner and bowl
 4:  DRINK ........Why? Because no good project ever started with a bowl of salad! (no pun intended)

  Now the tuner(s) attaches to the bowl with the 8-32 screw running  through the holes punch around the lip of our mixing bowl. When all of the equally length strings are lashed to the other bowl, it is then time to attach both bowls to the triangles. This is done by using a method my father suggested. Threading hefty nylon cord through the hole drilled in the center of our mixing bowls,the nylon cord is wrapped around a dowel rod to keep it from slipping back out. Use nylon or some other material that's taut and won't stretch as much. It's really annoying having to re-tune your base strings every minute because of a stretchy cord.  

Now we're getting somewhere
Step 4:  Oh, you might have noticed that both mixing bowls do not look the same......I of them is a sauce pan. Sorry mom !

 Beyond being a bad boy and not knowing the difference between a fork and a spoon (Spork) , we have something that resembles a Mixing Bowl Harmonic Reverb. After struggling to slip our 32 inch closet metal tubes in place with all of the tension provide by the base strings. It is now time to transition over to making a transducer.

 Around here, we use good old fashioned car speakers as our transducer. Why? Well for one, I'm dirt poor and can't afford a $20 audio surface transducer and second, I have tons a old speakers just laying around (I'm a man - We keep the stupidest things)  So how do you intend on using an old Chrysler car speaker from the 1970's ?  For that I will once again be quoting from D.I.Y. Plate Reverb 

How to disembowel a speaker:
(the ionosonde way)
           1: Ripe out cone body. Use hobby knife to cut away cone right down to the dust cover.  "Making first incision"
           2: Cut away dust cover (being careful not to cut into the voice coil or the spider. DOING SO WILL RESULT IN COMPLETE DEATH. ) "nurse Mary, please hand me the forceps".
           3: Using what ever tool at your disposable, cut away speaker basket taking care not to cut away the connection terminals.(I used sheet metal nippers) "this limb needs to be removed, cut off saw nurse Mary"     
           4: Remove terminals leaving the metal mounting tabs( I'll use these as the voice coil mounts) "O.K. sutcher up the patient nurse Mary - My bill will be in the mail"

Ah, the Chrysler Deluxe speaker.

1) Cut diaphragm out
2) Trim access.
3) Use sheet metal nippers or what ever is handy to cut housing
away from core.
4) Cut out dust cover
exposing voice coil.
5) Make linking shaft out of dowel rod.
6) Glue in place. Making sure
not to glue voice coil to magnet. BAD!

Attach !

 A few notes about the transducer installation. Depending on how firm your bowls are attached to the super structure, you may need to screw the tip of the linking shaft firmly onto the bowl. Drilling a small hole through the side of the bowl where the linking shaft will rest upon, screw the tip of the shaft through that hole to firmly hold it in place. This firm connection of the linking shaft to the side of the bowl will ensure better prorogation of sound energy into the bowl. Secondly, the 8 ohm impedance of this transducer is to little for mixer boards to handle. So I put in line a step up audio transformer which gives it a better match so more sound energy can flow through.

 Lastly, the pick-up's . Again I shall quote D.I.Y Plate Reverb
Piss...hey you....What if I told you that Radio Shack has had a secrete only known to musicians for many years. You would say to yourself, "Telegraphy your crazy!!", has you search through those pro-audio gear catalog's looking at $100+ acoustic pickups. Well just between you and me (and keep this on the down-low) Radio Shack sells pickups for $4.49 each. They don't label them as such but instead their called Piezo Elements 1500-3000 Hz model: 273 073  The sound quality of this devices are great for the price.
Taking those $4.49 acoustic pickups (Piezo Elements) out of their packages, I realized that these pickups didn't look like your average piezo element.Well for starters, the elements are buried inside a blast proof, geek proof, flame resistant, and radiation proof  -  black plastic case. To extract this rare element, you need a good sharp hobby knife, a screw driver, and most of all - patience. All you have to do is pry open the top with a screw driver (sounds easy enough. Boy are you in for a treat). Down inside of it's impregnable case is the piezo element. It's almost press fitted down in there, so your going to need a hobby knife to gently pry it loss. 

Gluing the pick-ups on opposite sides of the "non-driven bowl" and then running the cabling from both pick-ups and driver transducer to a nice junction box with some 1/4 phone jack plugs. I can finally say it's done. Have another Ovoltine on me!

                                                                               Listen to the Harmonic reverb