Intro to this project:
Having acquired a Vespa PX 125 during the summer to cruise to and fro into college and impress the ladies. However after a couple weeks of cruising around I soon realised there were two things missing, blistering sunshine and my favourite tunes. Given that we live in Ireland there was nothing I could do to remedy the sunshine situation, but I could fix the lack of tunes, and with an end of year project to do, the Vespa sound system project was born.
Noise-cancelling technology is based on the fact that a sound is a wave with a particular shape. An external noise-cancelling speaker cancels a sound wave by producing an opposite wave to the sound, dampening the original sound and producing something closer to silence. To produce the opposite sound wave from a sound that is being emitted in a room, the external noise-cancelling speaker begins by acquiring a feed of the sounds that need to be cancelled. The external noise-cancelling speaker acquires the sound feed by recording it through an external microphone. In some setups, this microphone is on the speaker itself, and in other setups, the microphone acquiring the sound feed is a distance away from the speaker.
Once an audio feed is acquired through a microphone, it is processed to determine the best wave to use to cancel the original sound. Sounds that are recurring and static in volume, such as a driving noise, a motor noise or static on a radio, are more likely to be successfully cancelled by a noise-cancellation speaker. Sounds that are spontaneous or have rapid changes in volume are more difficult for the sound- cancelling technology to react to, making them less likely to be successfully cancelled. After the ideal cancellation wave is determined by the noise-cancellation technology, the wave is fed back through an external speaker and should have a significant dampening effect on the original sound. This technology is useful in areas with a lot of noise, such as those near an airport or concert venue. Upon starting the Vespa engine up you realise that you are dealing with a lot of engine noise, and that is before you even hit the road. This ambient noise is constant, which would work perfectly off the theory of Noise Cancelling.
Step 1: Noise Cancelling
In order to understand what we were dealing with it seemed like a good idea to measure the ambient noise generated by the Vespa 125. A recording was made of the noise with the engine running at idle and being revved. The recording was made using the dbMeter Pro App for the Iphone which can record the sound levels and export them as Comma Separated Variable File (CSV). The sound samples are not weighted and are linear db(lin). The dbmeter pro app takes 8 samples per second.
To get a more detailed understanding of the what sounds were going to be present and how they might be overcome a graphical analysis of what sounds were present was undertaken to highlight the various contributing factors. These we then applied to the specific project with the Vespa. Using this information a high level approach was formulated which is outlined in the last picture.
The analysis of the characteristics of the road noise was obtained from a paper entitled The Multi-Coincidence Peak around 1000 Hz in Tyre/Road Noise Spectra by Ulf Sandberg of the ChaImers University of Technology, Department of Applied Acoustics, SE41296 Gothenburg, Swedenand the Swedish National Road and Transport Research Institute (VTI), SE-58195 Unkoping, Sweden.
Step 2: Design and System Layout
POWER SUPPLY CLEAN UP CIRCUIT
This is to protect the system boards from spikes on the Vespa power supply.
Dual Rail Power supply This is needed because of the current configuration of the NCC design it generated a +12,0,-12V dual rail supply, clearly it adds cost and complexity that ideally on could do without and a future task will be to change the design of the NCC to work on a single rail 12Vdc supply
Noise Cancelling Noise cancelling Board to process the ambient noise and produce an inverted signal to be combined with the Music
Preamplifier A preamplifier (preamp) is an electronic amplifier that prepares a small electrical signal for further amplification or processing. A preamplifier is often placed close to the sensor to reduce the effects of noise and interference. It is used to boost the signal strength to drive the cable to the main instrument without significantly degrading the signal-to-noise ratio.
Equaliser / Tone Control Equalisation is the process commonly used in sound recording and reproduction to alter the loudness of pitches and eliminate unwanted noise by adjusting the frequency content of an audio signal. An equalizer is the circuit or equipment used to achieve equalization. Equalisation is used to improve the fidelity of an audio system, so that the tonal balance--i.e. the loudness of various frequencies in relation to each other--as heard through speakers will better resemble that of the original sound.
Equalisation involves using linear filters to alter the frequency response of an audio system. Most hi-fi (stereo) equipment uses relatively simple filters to make bass and treble adjustments.
I have used a 6 channel equaliser to give more flexibility in tailoring the frequency content of an audio signal. The equaliser is used to boost those frequencies in the music stream which suffer most competition from the ambient noise on the Vespa. 2.7.5 Speakers The main the criteria choosing the speakers was suitability for the application, i.e. waterproof, costs, power handling capability, appearance etc. In the end the speakers chosen had an IP rating of IP35, (the second figure indicates the degree of protection from water, in this case 5 equals protection against low pressure jets of water from all practicable directions), Power Handling 40WRMS and 100Wpk, and SPL 87d/W/M, and in black which matches the colour scheme of the Vespa.
AMPLIFIER POWER NEEDED
If one assumes that 90db is required at the listeners ears and that a motorcycle helmet provides attenuation of between 7-10db depending on type it would seem logical that if the chosen speakers have an SPL of 87dB/W/M and the distance from the Speaker to ear is approximately 1m that the input power to the speakers will need to be about 20W. 90dB+10dB (attenuation) = 100dB = 87dB/W/M +13dB 10W = 10dB, 20W = 13dB Thus the output of an inexpensive car audio amplifier is limited by the voltage of the alternator. In most actual car systems, the amplifiers are connected in a bridge-tied load configuration, and speaker impedances are no higher than 4 Ω. High-power car amplifiers use a DC-to-DC converter to generate a higher supply voltage.
I then looked into the Vespa electrical system and debated strongly as to whether or not to run the power for the sound system through the electrical sound system. In the end I decided against it, for a number of reasons:
It involved taking everything electrical off the bike. If I ever wanted to sell the bike on, I want to make sure it goes to the next owner in good condition. I wanted the sound system to be easily installed and removed, Having decided to run the sound system directly from the Vespa Battery, I then measured the voltage of my Vespa battery using a voltmeter. I found that it rarely went above 11.5V.
Step 3: Parts / Printed Circuit Boards
In order to buy all the parts, I compiled an Excel Spreadsheet with a sheet for each with lists of each components and catalogue references.
The parts were then procured from a variety of sources, depending on price and availability.
PRINTED CIRCUIT BOARDS
First thing first is making the PCB design. The package I ended up using is Cadsoft Eagle. A number of PCB fabricators will accept their CAD files directly. It's a really great package. The Board Editor looks a lot like the schematic editor, with some different commands. When you first go to create a board design, all the components will be in a clump over to the left of the origin, and there will be a frame that marks the allowed size of a board when using the freeware or "Lite" versions of EAGLE (80x100mm). All the component pads will have to be inside that outline when you move them around, although you can cheat a bit and have traces or board outlines that exceed the board size limit. This has the annoying side-effect that if you pick up a component from its original location, you can't put it back down outside the outline (however, you can use ESC to abort the move, and the component will revert to its original location.)
All the signals you created in the schematic are currently AIR WIRES; thin yellow lines that are drawn in the shortest possible way, crossing each other as needed. They stay connected to component pins even when you move the component around. The RATSNEST command recomposes and redraws these after you move things around (and, say, make two connected pins closer together than they used to be.) The Freeware version of Eagle only supports a TOP and BOTTOM layer, A signal can transition from one layer to another using a via, which is a conducting hole. Creating the PCB design consists of placing all the components in logical places, and routing all the air wires in a way that allows the design to work. In general, you can start by placing the components similar to how they appear on the schematic. One way to get hints on part placement is to look at some significant signals to see whether they have nice straight paths, or whether they zigzag all over the board. First use the RATSNEST icon/command to have EAGLE recomputes the airwires. After the rest of the components are placed in ok-looking relative locations, I can squeeze them together again (manually, moving them one at a time; no magic command for this!) and shrink the board outline some more. Since my PCB’S where going to be etched at home I had to make boards with wide traces and big spaces. Using the Design Rule Check icon I was able to modify values manually and individually. Pads are bigger, and they're all round.
There are other ways to enter a "change" command in the text command entry area that you'll want to look into if you need to change a lot of devices to a particular package, so you can skip going through the list for each one.
The EAGLE auto router isn't the best in the world, but even when it does a bad job; it still gives some general hints on how things need to look, or where the trouble spots are. Click the AUTOROUTE icon, and a dialog box will pop up. The default parameters will produce a double sided board. Select the ROUTE tool and click on an endpoint of an unrouted (yellow) airwire, and you can position a trace pretty much anywhere you want, selecting width, layer, and type of bend from the menu bar as you go along. Once this was done I ran a design rule check to make sure that none of the manual editing I had done violates the rules. Now that I was done designing the boards, I wanted to customise my PCB’s. I wanted to add a picture of a Vespa to each board. In EAGLE you are only allowed to be only importing one colour. First I found a completely outline image of a Vespa PX, I saved it at as a bitmap (BMP) at 8-bits at the most, as this is what works with Eagle.
This is imported using a script called “import-bmp.ulp”; with this I was now able to move the Vespa PX to the appropriate spot on the PCB.
Step 4: Etching PCB's
As I was making double sided boards I had to make sure that both sides would match up. I made sure the front image was flipped/mirrored so it is the right way around when I transferred it onto the copper. As the back has been done as if you were looking straight through I didn’t have to flip that side.
Once my design was complete, I did a test print on normal paper to check and make sure everything was OK. I was now ready to feed a page of the magazine paper through the laser printer and print it out (I found that thin, flimsy magazine paper worked best). When it was printed I then measured out the corners for placing onto the copper board,
Next the copper board needs to be prepped for the ink to transfer. I first gave it a quick clean with washing up liquid, then I used a really fine grade of sand paper to take the top layer off, rinsed it under a cold tap, then using a scouring sponge and some alcohol I give it a last go over before rinsing it off again and drying with a clean towel. The key to applying the PCB design is to make sure your copper circuit board is absolutely clean.
Now I had to transfer the toner on the magazine paper to the copper board. Making sure the copper board is as clean as possible and don't handle it too much and also make sure the iron is as hot as possible with the steam function turned off. I have started etching one side at a time because I think it's easier to line the two sides up. So I start with putting the cut out from the magazine face down on the copper and holding one side start pressing the iron down hard. Once the paper starts sticking to the copper you can then iron outwards from that spot until it's all stuck tight.
I found that it takes about 15 minutes to iron the design on, and making sure it's totally stuck. Once I was happy that it wasn’t moving around/creasing. I pressed really hard and rubbed all over with the tip of the iron to make sure it’s on. Once it's done (you can usually see the design through the paper) I ran it under cold water and leave to soak for a minute or two. Then start rubbing the paper gently until it starts to come off. I preferred not to peel it off as I think the rubbing worked a lot better.
Before I started to etch, I double checked all the traces and made sure everything worked. If it didn't all come out, as it didn’t with in some parts of my boards I used a permanent marker to fill in what tracks or pads were missed.
Now it's time to etch; using ferric chloride in an etching tray/glass. I placed the board face up in the solution and using a cloth gently wiped the solution-saturated sponge over the surface of the board over and over. I added more etchant as I went along to help speed up the process. The board background becomes more green when the copper becomes eroded.
When it's finally etched, I gave it a rinse and clean under a cold water tap.
I then drilled out all the holes in the board. I drilled all the boards in my old school engineering room. I marked each hole with a centre punch and eased a 0.8mm bit into the groove, this very time consuming and requires a steady hand and a lot of patience as these drill bits can break off very easily.
Now all that was left to do need is for was to solder your parts on and connect it up and party. I used this method for making the Noise Cancelling Board, the Pre-Amp Board and the Equaliser/Pre-Correction Board.
Step 5: SYSTEM COMPONENTS
POWER SUPPLY FILTER
This is to protect the system boards from any spikes or overvoltage’s on the Vespa Electrical system. I inserted a series inductor in addition to normal decoupling capacitor to form a LC filter, so that it can resist transient voltage. It also prevents noise emission, smoothing the Vespa’s noisy DC consumption. This Zener diode forms a clamp to prevent the voltage going above 20V is designed especially for car electronics, to protect rest of the circuits from high voltage spikes on the DC.
DUAL RAIL POWER SUPPLY
This power supply circuit provides +12V and -12V required for the noise cancelling circuit to operate. Due to time constraints I decided to buy in this circuit ready made this offer the best trade off of time cost and quality.
The Pre-Amp amplifies the signal. It is not strong enough to power speakers, but ready to be amplified by the power amplifier. In this case, the final amps have no controls, just input / output, so the pre-amp has also volume control
6 CHANNEL EQUALISER/PRE-CORRECTOR
With this circuit you can control and blend frequencies and tones as desired. Essentially, the circuit consists of an IC 741 whose gain at various frequencies is determined by corresponding potentiometer setting. The audible frequency spectrum is covered in six steps: 50Hz, 160Hz, 500Hz, 1.6kHz, 5kHz, and 16kHz. All potentiometers are of 100kΩ linear type. The circuit provides adequate boost / cut for normal use.
This circuit can be placed either at the start of sound system as a pre-corrector or after the noise cancelling circuit as an equaliser.
NOISE CANCELLING BOARD
The NCC is based on two microphones one for each channel left and right and then 3 op-amp circuits which perform the following functions: • Amplify the very low level noise signal from the electret condenser mic’s.
• Invert the noise signal
• Sum and amplify the noise signal and the music signal A switch is included to allow the inverted on non-inverted noise signal to be connected to the sum and amplify stage and two potentiometers provide level control for the music input and the noise signal.
POWER AMPLIFIER STAGE
These are off the shelf items, which come in Kit form, which I decided early on provided the best way forwards. I had to source the heat sinks and heat conducting paste, because getting the heat away from the IC’s is the biggest problem. Later in the project I discovered another IC from Philips TDA1516BQ 24 W BTL or 2 x 12 watt stereo car radio power amplifier designed for automotive use which with the addition of just 3 capacitors provided the same functionality. The circuit is based on the Philips TDA2050 32w hi-fi audio power amplifier, which Philips suggests is suitable for both HiFi and TV audio applications and not specifically automotive applications.
The Loudspeakers were chosen for their suitability for the project and the pair I ordered are black to blend in with the Vespa and be inconspicuous.
The Speakers have the following characteristics:
Water resistant multi-purpose speaker IP 35 the speaker can be used outdoors and is ideal for damp environments
SPL 87dB /W/M
Power Ratings 40w rms 100 watts pk
Frequency response 80 Hz - 20 kHz
Impedance 8 ohms
ENCLOSURE- GLOVE BOX DOOR
I purchase a new glove box door off eBay to enable me to work on the project with the Vespa intact and in daily use. I used a plasma cutter to cut the holes for the speakers and sanded down the new glove box door and sprayed it to match the colour of my Vespa.