How To Get Into Hobby RC: Custom Sound Systems

For many hobbyists, the allure of RC flying comes from making their models look like “real” airplanes. Some are happy just approximating the profile of a certain airplane, while others spare no effort or expense to replicate every last detail. Regardless of the level of accuracy a builder pursues, one particular aspect has always been elusive: sound. Most models use screaming glow-fuel engines, growling gasoline engines, or whizzing electric motors. None of those power systems is likely to emulate the sound of the full-scale airplane’s engine. The one notable exception is turbine-powered models, which actually use miniaturized jet engines that sound (and smell) like their big brothers. The necessary flying skills and price point, however, keep turbines out of reach for most modelers.

Recent developments have revamped the sound equation. Several companies now offer sound systems for electric-powered RC models that play audio recordings of full-scale aircraft engines. These system are linked to the throttle control, so the sound revs as you increase power to the model’s motor. I know what you’re thinking: “How can you possibly get convincing engine sounds out of a system that is small and light enough to fit inside a model airplane?” I thought the same thing and ignored the growing popularity of these systems for a while.

My curiosity recently got the better of me and I watched a few YouTube videos of models with sound systems. The videos piqued my interest and I was soon investigating the various available products. One particular system stood apart from the others, the Mr. RC Sound V4.1 Sound System. What is most unique about this sound system is that it does not use speakers–at least not in the traditional sense. This is something I had to test.

What You Get with RC Sound

The heart of the Mr. RC Sound unit is a control board that measures about 1.75” x 2.5”. The board includes a “sound pack chip” with sound files recorded from six popular aircraft engines throughout history. The chip can be swapped for others with different engine options. In addition to the sound of the running engine, each selection on the chip also includes three auxiliary sounds such as chattering machine guns or the whistling of a falling bomb.

The sound board must be connected to the model’s RC receiver via standard 3-wire servo connectors. It is worth noting that male/male wires are needed rather than the male/female wires that are commonly used to extend servo leads. One wire is included, but you must provide others if you wish to use any of the auxiliary sounds. The wire lead for controlling engine sounds is connected in parallel to the model’s Electronic Speed Control (ESC: aka “throttle”) via a splitter, or “y-connector” (not included). Leads for each of the auxiliary sounds require an open port on the receiver.

THE BASIC COMPONENTS OF THE MR. RC SOUND SYSTEM ARE A CONTROL BOARD AND AN ELECTROACOUSTIC TRANSDUCER. THE AIRFRAME OF THE MODEL BECOMES THE SPEAKER.

The sound board can accept input voltage from about 11 to 34 volts. This means that models using a 3S LiPo battery (3 cells in series, 11.1v nominal) to 8S (29.6v) can siphon power from their flight battery to feed the sound system. This allows the vast majority of electric airplanes to avoid the additional weight of a separate battery for sound.

Rather than a speaker, the V4.1 system uses an electroacoustic transducer called the TT-25. It is basically a speaker without the frame or the cone. The TT-25 attaches directly to the airframe of the model, which then behaves similarly to a speaker cone. In essence, the entire airplane becomes a speaker.

The advantages of using a TT-25 over a traditional speaker are numerous. The transducer is smaller and lighter than a comparable speaker. Sound systems that utilize speakers also require sizable holes in the airframe to let the sound escape. Meticulous scale enthusiasts are understandably reluctant to allow a hole saw anywhere near their pride and joy.

My previous experience with transducers is limited to Aura Bass Shakers, which I have used in my car and home theater systems for several years. The point of Bass Shakers is not so much to make an object (usually your seat) project sound like a speaker, but rather to have it vibrate at low frequencies so that you can “feel” the sound. You get the sensation of powerful bass without having to crank up your subwoofer. While I love my Bass Shakers, the intent of their operation is notably different than the transducers in the Mr. RC Sound setup. I was anxious to see if the RC system could perform equally well.

Installing the Mr. RC Sound System

Mr. RC Sound provided a V4.1 system for review and recommended that it be installed in a foam airplane with a 1200-1500mm (47”- 59”) wingspan. I knew that I wanted to use a WWII-era fighter, and there are several quite-accurate scale models on the market that fit this criteria. I already had a model picked for a different project that I thought would also mate well with the sound system, the FlyZone Focke-Wulf FW-190.

I CHOSE THE FLYZONE FOCKE-WULF FW-190 AS THE HOST FOR TESTING THE SOUND SYSTEM. IT CARRIES THE EXTRA WEIGHT WELL AND THE SOUND IS AN ATTENTION-GRABBING FEATURE.

Although this FW-190 is slightly smaller model than recommended (1130mm/44.5” wingspan), I was confident that it could carry the weight of the sound system. I had already flown the airplane in box stock form and felt that it had plenty of excess power and a light wing loading (total flying weight of the airplane/ area of the wing). My kit was almost 2 ounces lighter than advertised (38.3oz actual vs 40.0oz advertised), so I started out with some bonus wiggle room on the weight.

The sound card and a single TT-25 added 3.7 ounces to the FW-190. The sound card was easy to install. I found a cavity in the fuselage below the rudder and elevator servos that was a perfect fit for the card. Looking from the outside, this location placed the card beneath the faux machine guns molded into the top of the fuselage. I cut away the foam gun barrels and replaced them with aluminum tubing. The tubes allow outside air in to help keep the sound card cool.

The TT-25 was a little more difficult to mount since there isn’t much spare space in an airplane this size. I finally decided on flat plot of foam on the bottom of the fuselage just behind the wing. This area is typically closed off, but I had cut a sizable opening to allow cooling air to escape. The opening was just large enough for me to slide the TT-25 through and coax it into place. The patch of self-adhesive tape that comes attached to the TT-25 has done a good job of holding it securely. If it ever breaks free, I can reattach it with superglue or epoxy.

THE SOUND BOARD FIT NEATLY BELOW THE RUDDER AND ELEVATOR SERVOS IN THE FUSELAGE.

I planned to use the FW-190’s stock 3S-1800mAh LiPo battery to also power the sound system. Mr. RC Sound offers an adapter that goes between my existing Deans Ultra Plugs battery connectors. The adapter provides a pigtail with a JST connector that mates with the power leads on the board. This saved me a little bit of soldering and has worked out very well.

Once I had everything installed, I double checked the balance of the airplane. Having the 2.6oz TT-25 behind the wing caused the model’s center of gravity CG to shift too far aft for my comfort. To correct this issue, I replaced the stock 1800mAh LiPo with an ElectriFly 3S-2200mAh LiPo. Since the battery is located in the nose, the additional 40 grams of weight put the CG back into my comfort zone. I had to hog out a little foam on the battery compartment hatch to make the big battery fit easily. Including the sound system and larger battery, the FW-190 gained about 5 ounces of additional weight.

THE FIRST TT-25 WAS MOUNTED ON THE BOTTOM OF THE FUSELAGE BEHIND THE WING. THE AREA WOULD HAVE BEEN INACCESSIBLE IF NOT FOR THE AIR EXIT THAT I HAD PREVIOUSLY ADDED.

Using the Mr. RC Sound System

The version of the FW-190 I used is the Tx-R (Transmitter Ready), which includes a Tactic TR624 6-channel receiver. I linked it to my Tactic TTX650 6-channel computer transmitter. This setup works just fine. The only problem was that the functions of the FW-190 utilized all six channels of the radio (aileron, elevator, throttle, rudder, retractable landing gear, flaps) so I didn’t have an open channel for any of the auxiliary sounds. As a result, my first several flights with the sound system featured engine sounds only.

The real FW-190 was powered by a BMW 801 14-cylinder radial engine. This engine is not one of the six selections on the stock sound chip, so I purchased the “WWII Sound Pack” chip to install on the control board. When you power up the board, a voice informs you which engine is selected. You simply press the button on the board until the engine you want is reported. That engine becomes the default selection each time you activate the board.

The first time I engaged the startup sequence, I was shocked by how loud it was, especially within the walls of my garage workshop.

When I move the transmitter’s throttle stick forward a couple of clicks, the sound system goes into a startup sequence. There’s a little clanging and starter noise, then you hear the engine cough, sputter, and settle into idle. This ESC needs a little more throttle before it begins to turn the prop. So I have to jockey the throttle stick a little if I want the motion of the prop to match the sounds. It’s a fun little challenge.

When the startup sequence is complete, the engine sound then becomes proportional to the throttle position. More throttle equals higher revs, although the RPM range sounds somewhat limited. If you bring the throttle all the way down, it triggers a shutdown sequence. This sounds like, well, like a radial engine shutting down. If you’ve never heard such an event, it is a cacophonous symphony that is difficult to describe.

The first time I engaged the startup sequence, I was shocked by how loud it was, especially within the walls of my garage workshop. I later measured the volume at 97 decibels at six feet away from the airplane. Equally stunning was the sound quality of the running engine: not bad for a hunk of vibrating foam. If I ever build a functioning 1/10-scale BMW radial engine, this is surely what it would sound like. The startup and shutdown seem a bit less convincing to me, but the running sequence has me fooled.

Several other flyers were present the first time I took the sound-equipped FW-190 to the flying field. They paid little attention to me and my foam model, until I “started the engine”. Then, they crowded around and began asking questions. They were clearly impressed, and I hadn’t even flown it yet! Once I did get the model airborne, their opinions only improved. There is always a fair amount of ambient noise at a RC flying field and they all reported that they heard the sound system clearly from their position 15 yards behind the runway. To be clear, the sound does not stay loud as the airplane heads for the horizon, but I can usually hear it as far away as I’m willing to fly it.

Tweaking Your Sound System

After numerous flights with the single TT-25 in place, I began adding more scale details to the FW-190 (the topic of an upcoming article). These changes again shifted the model’s CG rearward, so I began to consider useful ways to add weight to the front of the airplane. The stock sound card is able to drive two TT-25s, so I created a spot for a second transducer just behind the battery. That took care of the balance situation, while also altering the sound characteristics of the system.

I MOUNTED A SMALL PLATE OF FOAM SHEET BEHIND THE BATTERY MOUNT TO CREATE A LOCATION FOR THE SECOND TRANSDUCER.

Measured volume actually changed very little with the second TT-25 (see above decibel chart). The changes were more subjective. The sound now seems more focused and with a broader range of frequencies. Maybe it’s just placebo effect tricking my ears. Either way, I needed the mass of the second TT-25 up front, so it’s staying. It is worth mentioning that with the second TT-25 (2.6oz) and my scale add-ons, the total weight of the FW-190 increased exactly 8 ounces above stock. That’s a 21% increase and shouldn’t be taken lightly. That level of added pork could make an airplane a real pig. This FW-190, however, has taken it in stride. It is still powerful and fun to fly.

One of the coolest aspects of this sound system is the Doppler effect of the engine noise as the airplane zooms by.

I was so impressed with the FW-190’s performance at the heavier weight that I decided to disengage the wing flaps (used to reduce landing speed) and hook up one of the auxiliary sounds. I sampled the three additional sounds in my shop and chose machine guns. The cannon sound was also pretty good, but I found the whistle (like a diving airplane or falling bomb) just annoying. I set up the radio so that my trigger for the machine gun sound is a momentary toggle switch on the TTX650.

Subsequent flights with the 2-speaker system and guns has continued to capture the attention of other flyers at the field. Everybody seems to get a kick out of it. The machine gun effect is a little on the subtle side. I hear it better after the airplane passes me than I do when it’s “shooting” at me. I’ll soon try linking the cannons and guns to the same trigger and see how that fares.

One of the coolest aspects of this sound system is the Doppler effect of the engine noise as the airplane zooms by. That’s the sound we associate with airplanes swooping close to the ground. Perhaps that is because it’s also the sound we all make when we morph our flattened hands into airplanes and twist them to and fro.

Conclusion

As a writer of all things RC, I devote a lot of my effort to opening the minds of people to the possibilities of this hobby–getting them to try new things. So, why was I so closed-minded about sound systems? I just assumed that they could never deliver impressive sound, but I was completely wrong. This thing is way cool. My hypocrisy stings a bit.

I plan to continue exploring the options and possibilities of the Mr. RC Sound System, such as the LED-driver-circuit synched to the machine guns. I will also be trying this system on traditional balsa airplanes with iron-on polyester coverings. Other users of the system are reporting good results with a thin sheet of foam sandwiched between the transducer and the covering.

You will be seeing (and hearing) the FlyZone FW-190 more in the coming weeks as I outline some of the simple detailing and weathering techniques that I have applied to this model!

SOURCE:http://www.tested.com/art/makers/465719-how-get-hobby-rc-custom-sound-systems/

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