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Surf Sound Simulator By John Clarke Relax and enjoy the sound of the beach from the comfort of your home. Forget the scorching heat in summer, the cold winds of winter or your bathing suit being full of sand! Ideal for beginners and experienced constructors alike, it’s a fun-filled project. Image Source: https://unsplash.com/photos/birds-eyeview-of-seashore-3P3NHLZGCp8 O ur new Surf Sound Simulator uses standard through-hole components that mount on a blue PCB shaped like a surfboard. It produces a sound that imitates the ebb and flow of the surf rolling up on the beach, including the occasional big wave. It can be used to augment the sound of surf if you live near the beach, or allow you to experience the beach even if you live in Alice Springs. The sound is ideal for masking background noises so that you remain relaxed or for a peaceful sleep. The project uses all standard parts and has a fun surfboard shape that includes graphics depicting waves. It includes an onboard loudspeaker, or you can use the RCA socket to feed the sound to a stereo system or powered speaker for an even more realistic effect. Using large speakers with extra bass will reproduce the deep thumps as the waves crash onto the beach. It’s powered by a 12V DC plugpack, so you don’t have to worry about batteries going flat. It requires no adjustments to work. All you do is switch it on, set the volume and you’re instantly drifting off, imagining a day at the beach. Producing the surf sound The sound of the surf is very similar to white noise, a randomly produced sound that covers the audio 48 Silicon Chip spectrum from 20Hz-20kHz (for humans). White noise has the same intensity level at every frequency. It is similar to the sound coming from an AM radio when it is not tuned to a radio station, or the noise produced by heavy rainfall. Pure white noise needs some changes to sound like the surf. The volume needs to change over time and there needs to be some tailoring of the frequency response to sound realistic. There also must be some randomness to the waves since there is considerable variation in the surf noise as waves come into and crash onto the beach, then withdraw. The volume levels of the surf have a triangular shape over time with some extra details. As a wave comes in, the sound steadily increases, hits its peak and then dies away. To simulate surf sound, we use a white noise source that has its volume varied by triangular ‘envelopes’. By having two such envelopes, we can obtain a degree of randomness to the sound level. With one generator, you only get the same wave crashing at a constant rate, but with two, you get two sets of waves rolling in at more unpredictable intervals. With further shaping of the triangular envelope, we can obtain extra surf sound realism. This design is based on a circuit from October 1990 by Darren Yates Australia's electronics magazine (siliconchip.au/Article/6622). We have kept it based around two lowcost LM324 quad op amps; while we could have reduced the component count using a microcontroller, that would have been less interesting and harder to modify. This version features some improvements to the circuit and it is considerably more compact and appealing on the surfboard-shaped PCB rather than in a plastic box. Block diagram Fig.1 shows the block diagram of the Surf Sound Simulator circuitry. The preamplifier, IC2c, provides the main sound output. It is fed white noise to its non-inverting (+) input, while the volume (or amplifier gain) is altered over time using two triangle wave generators and three modulators, designated MOD1, MOD2 and MOD3. The modulators change the shape of the triangular envelopes. The output of triangle wave generator MOD1 is also fed to a peak amplifier, IC2d. This amplifies just the peak of the triangular waveform, where it increases the triangle wave output level. After feeding this voltage into another modulator (MOD3), it is used to produce a large wave crash simulation for when the wave hits the beach. All three modulators vary the impedance from IC2c’s inverting siliconchip.com.au input to ground, changing the gain and therefore the sound level of the white noise. The output of preamplifier IC2c is fed to a low-pass filter stage comprising IC2b and some passive components. This changes the frequency response of the white noise so that the higher frequencies are reduced, more like water sounds. From there, the signal is available at the CON2 line output for connection to an external amplifier and loudspeaker. This signal is also fed to the volume control (VR1) for the power amplifier, IC2a, that drives the onboard loudspeaker. Circuit details Refer now to Fig.2, which shows all the circuit details. It is similar to the October 1990 version, with some variations. Some changes are simply because DC mains plugpacks these days are switch-mode types that provide a stable voltage under load, so we don’t need a separate regulator. In the 1990s, plugpacks generally comprised a mains transformer, bridge rectifier and filter capacitors. They provided a higher voltage with no load that dropped as current was drawn from the supply. The ripple also increased under load. For a voltage sensitive circuit, regulation was required. Other changes were to isolate the supply between the sensitive circuitry used to produce the surf sound from the amplifier that drives the loudspeaker. This allows a higher volume level, as the 1990 version was a little too quiet. Without the isolation and with higher volume levels, the circuit would oscillate, producing a squealing noise as well as ‘motor boating’. While ‘motor boating’ might seem like a reasonable thing to include in a surf sound simulator, it is actually an electronic term to describe a low-­ frequency circuit oscillation malfunction. This is where a circuit produces its output in bursts, a bit like the putput sound of a single-cylinder motor in a boat. Another change was to prevent click and pop noises when parts of the circuitry suddenly change voltage level, from near 0V to near 12V or vice versa. We will describe those changes as we come to them in the following circuit description. The main part of the circuit is the noise source. This is based on NPN transistor Q1. Its base-emitter junction is connected as a reverse-biased diode. This junction breaks down when the supply is in the reverse direction, allowing current to flow when the voltage across it reaches about 5V. The breakdown is a random process that produces considerable white noise. To avoid damage, the current through the transistor junction is limited to around 200μA using the 33kW resistor to the +12V supply. This noise is capacitively coupled to the non-inverting input (pin 10) of op amp stage IC2c. Two 100kW resistors connected in series across the 12V supply provide a 6V bias for IC2c so that its output can swing symmetrically within the 12V supply range. Triangle wave generators IC1d & IC1c together form the first triangle wave generator, while IC1a & IC1b form the second. The first generator is responsible for a wave that sounds very close (louder), while the second produces a wave that crashes in the distance (lower in volume). Because the two are nearly identical, we’ll just describe how one of them works, then mention the slight differences between the two. IC1d acts a Schmitt-trigger gate, while IC1c is connected as an integrator. IC1d’s output will be either high (around 10.5V) or low (near 0V). It charges or discharges the 33μF capacitor at different rates depending on whether it is high or low. When the output is low, the capacitor charges via the 680kW resistor and series diode (D1) plus the parallel 330kW resistor. When IC1d’s output is high, the capacitor charges only via the 330kW resistor. The 33μF capacitor charge increases in a linear fashion toward the positive supply when the pin 8 output of IC1c is low, while it discharges linearly toward the 0V supply when that output goes high. If you are interested in a more detailed (and complicated) description of how this works, see the panel titled “Triangle wave generation”. The only difference in the second triangle generator based on IC1a & IC1b is that the second generator has some lower-value resistors (100kW & Fig.1: two triangular waveform envelope generators with different periods control the preamplifier gain applied to the white noise source. Three modulators and one peak amplifier tweak the sound to make it more like waves crashing on the shore. The resulting audio is filtered and fed to the line output (CON2) plus a volume control (VR1) and power amplifier to drive the onboard loudspeaker. siliconchip.com.au Australia's electronics magazine November 2024  49 Fig.2: it helps to refer to the block diagram, Fig.1, when trying to understand how this circuit works. Transistors Q2 & Q4, diode-connected in series, produce a bias voltage for current buffer transistors Q3 & Q5 that tracks over temperature, to avoid thermal runaway. 150kW instead of 120kW & 680kW). It helps to make the two waves more random because the two generators run at different speeds. It also provides the second wave with a faster ‘travel rate’ towards the shore. One of the problems with the triangle generators is that the Schmitt trigger outputs (pins 1 & 14) produce a clicking sound whenever the voltage from their output swings between 0V and 10.5V. The 1990 circuit used 100nF capacitors from the outputs to ground to suppress this, but on building the circuit in 2024, we found it wasn’t that effective. Without any capacitors, the rise time of those outputs was 25μs; with the capacitors, it was reduced to 18μs, worsening it! We found that placing the 100nF capacitors at the non-­ inverting inputs of the op amps, at pin 50 Silicon Chip 12 for IC1d and pin 3 for IC1a, significantly increased the output rise time to 75μs. The clicks and pops went away. There are still two clicks that occur when the Surf Sound Simulator is initially switched on, but no more are evident after that. Diode modulators The outputs of the two triangle wave generators drive the diode modulator circuits as shown in the block diagram (Fig.1). These rely on the fact that the conductivity of a diode varies with the voltage across it, ie, a diode with 0.6V across it will conduct more current than one with only 0.2V across it. There are three modulators in the circuit, based on diodes D3 to D6. Diodes D3 & D4 connect to the same IC1c output, so are counted as one Australia's electronics magazine modulator. The first triangle generator drives D3 & D4, the second drives D6, while the third (D5) modulator is driven by the peak amplifier, IC2d. At the cathodes of these diodes is a voltage divider. In the case of D6, for example, there is a pair of 100kW resistors. These set the offset voltage for this modulator to 6V. Different resistance values are used in the voltage dividers of the other modulators. These set the offset levels to different values to ensure the correct switch-on sequence. For diode D6, this means that the output of its triangle wave generator must rise above 6V before the diode has enough forward bias to conduct. This output is coupled to the anode of D6 via a 47kW resistor and also to the inverting input of preamplifier IC2c via a 120nF capacitor. siliconchip.com.au While the voltage from IC1b remains below 6V, D6 is reverse-biased and the 120nF capacitor sees a high-­ impedance to ground. However, when the voltage rises above 6V, the diode begins to conduct, which decreases its AC impedance. The 120nF capacitor thus sees a progressively lower impedance to ground as the voltage across the diode increases. Since op amp IC2c is connected as a non-inverting amplifier, these impedance variations directly control its gain. If the impedance goes down, the gain goes up and vice versa. Thus, the diode modulators control the gain of the preamplifier stage to vary the sound level. When the voltage across D6 reaches 0.6V, the diode appears as a short-­ circuit to the capacitor and the impedance to ground is then set by the 8.2kW resistor connected to D6’s cathode. The 100μF capacitor and 8.2kW resistor form a high-pass filter that rolls off the response below 0.2Hz. D3 and D4 work similarly but have offset voltages of 7.2V and 5.45V, respectively. Note also that D4 controls another high-pass filter, consisting of a 4.7kW resistor and 100nF capacitor, with a -3dB point of 340Hz. Because of their different offset voltages, D4 comes into operation before D3 (which controls lower frequencies), so we get a realistic “whooosshhh” sound as the wave breaks. Peak amplifier The gain of IC2c is also controlled by diode modulator D5, which is driven by peak amplifier IC2d. Its input comes from the output of IC1c. The bias for IC2d’s inverting input (pin 13) is set to about 7V by the 33kW resistor and the two 100kW resistors. Thus, the output of IC2d remains low until pin 8 of IC1c reaches this threshold level. At that point, IC2d amplifies the signal to produce a faster, steeper waveform. This produces the big ‘dumper’ sound of a wave that crashes onto the beach. Triangle wave generation For the first triangle wave generator, IC1d forms a Schmitt trigger gate, while IC1c is connected as an integrator. IC1d’s output will be either high (around 10.5V) or low (near 0V) with different charge and discharge rates for the 33μF capacitor. The capacitor charges when IC1d’s output is low via the 680kW resistor and series diode, plus the parallel 330kW resistor, but only discharges via the 330kW resistor when IC1d’s output is high. The IC1d Schmitt trigger receives the voltage from IC1c’s pin 8 via a 47kW resistor to the non-inverting input (pin 12). Hysteresis is provided by the 120kW resistor from pin 12 to IC1d’s output. When IC1d’s output is low, pin 12 input is pulled lower via the 120kW resistor and the voltage divider formed with the 47kW resistor that monitors the IC1c output. The inverting input at pin 13 is at 6V. IC1d’s output will go low when pin 12 rises above the pin 13 voltage. Knowing that pin 14 of IC1d is low (0V) and that pin 8 is rising, we can find the voltage where pin 8 causes pin 12 to be at 6V. When there is 6V across the 120kW resistor, 50μA flows through it. The voltage at pin 8 must be sufficient to produce a 50μA flow through the 47kW resistor that has its pin 12 end at 6V. The voltage across the 47kW resistor will be 2.35V (47kW x 50μA). So pin 8 would be 8.35V (6V + 2.35V). This means that the 33μF capacitor charges to 8.35V at pin 8. The pin 9 side remains at 6V as IC1c adjusts its output to maintain this 6V. With pin 12 of IC1d just above 6V, its output goes high to around 10.5V. Now the capacitor (and pin 8 of IC1c) begins to discharge toward 0V via the 330kW resistor. Diode D1 is reversed-biased in this case. We can calculate what the pin 8 voltage will be when pin 12 just falls to 6V again. Since IC1d’s output is at 10.5V and pin 12 will be at 6V, the voltage across the 120kW resistor will be 4.5V (10.5 – 6V). So the current through the 120kW resistor will be 37.5μA. This same current flow is through the 47kW resistor, so it will have 1.76V across it, below 6V, giving 4.23V. Once this voltage is reached, the output of IC1d drops again to recharge the capacitor in the positive direction. We ignore any current to the non-inverting input of the op amp, as that will be just 100nA at most. As the two switching levels are 4.5V and 8.3V, that means there is a 3.8V hysteresis provided by the 120kW resistor. Without this, there would be no controlled oscillation. The resulting waveform at pin 8 of IC1c will be a sawtooth, a triangular shape rising faster than it falls. Partly this is because the LM324’s output can pull down to nearly 0V but can only go up to about 10.5V when powered from 12V. The other reason is that there is an extra current path via D1 when IC1d’s output is low. Scope 1 shows oscilloscope traces of IC1d’s output (pin 8) in the top yellow waveform and the triangle waveform output from pin 8 of IC1c in cyan. The triangle wave swings between 4.2V and 8.4V, close to the values calculated above. The faster charge and slower fall time for the triangle wave has the overall effect matches the sound of ocean waves, which come up to shore faster than they run back to the sea. Scope 1: the triangular sawtooth waveform generated at pin 8 of IC1c is shown in the lower (cyan) trace. On the right, the ‘scope indicates that the voltage difference between the peak and trough is 4.16V. The voltage at pin 8 of IC1d that dictates whether the capacitor is being charged (low) or discharged (high) is shown above in yellow. Low-pass filtering As IC2c amplifies the white noise generated by Q1, a 1.2nF capacitor in the feedback loop of IC2c rolls off the response above 130Hz. The 2.2μF capacitor in the feedback network of IC2b also rolls off the low frequency response of this stage below 7Hz. IC2b is a non-inverting amplifier siliconchip.com.au Australia's electronics magazine November 2024  51 Fig.3: assembly of the PCB is straightforward; simply fit the parts as shown here. Make sure the ICs, diodes and electrolytic capacitors (except the non-polarised ones) are orientated correctly. For the polarised electros, the longer lead goes on the side marked +. The speaker goes on the rear of the PCB; it is wired to the CON3 terminals and sound passes through the holes in the PCB. with a gain of 28. The original 1990 circuit used a gain of 11 for this amplifier, but with supply routing changes (described later), a higher gain is possible. It is a significant increase in the maximum volume at just over 8dB. Indicator LED1 is driven from the IC2b amplifier output via a 4.7kW resistor. The LED will light with varying brightness and, to some extent, mimic the sound level. Following IC2b is another low-pass filter stage comprising a 4.7kW resistor, a 10μF coupling capacitor and an 18nF filter capacitor. The 18nF capacitor rolls off the response above 1.88kHz to reduce higher frequencies further, adding realism to the sound. After that, the signal goes to the CON2 RCA socket and also to the 10kW volume control pot (VR1), which feeds the signal to the power amplifier, based on op amp IC2a and transistors Q2 to Q5. Q3 and Q5 buffer the output of the op amp to provide current gain; they are within IC2a’s feedback loop to reduce crossover distortion. Transistors Q2 and Q4 produce a bias voltage for the output transistors (Q3 & Q5). Only the base (B) and emitter (E) terminals of these transistors are connected, using them as diodes to produce a nominal 0.6V bias. These diode junctions match the voltage across the output transistor (Q3 and Q5) base-emitter junctions. The bias voltage ensures the output transistors are always conducting 52 Silicon Chip current and this reduces crossover distortion as signal swing passes the mid (6V) level where the output drive switches between Q3 and Q5. This type of amplifier is called Class-AB. Class-B means that one output transistor conducts for positive excursions and the other conducts for negative excursions. It also has some amount of Class-A operation at low signal levels, where both Q3 and Q5 are conducting, due to a small standing current through both transistors at these low levels. The 1W emitter resistors provide a degree of bias current stability. A higher bias current will cause extra voltage across the 1W resistors that effectively raises the bias required for Q3 and Q5 to conduct, reducing the current through them. The bias voltages from Q2 and Q4 remain more-orless constant unless the temperature changes. The bias current is kept steady with temperature because Q2 is physically touching Q3 and Q4 is touching Q5, so the transistor pairs maintain a similar temperature. This prevents thermal runaway should the output transistors heat up when driving a load like a loudspeaker. Without the thermal matching and with a fixed bias, as Q3 and Q5 heat up and their base-emitter voltages drop, the current through them would increase, causing more heating and thus thermal runaway. In our circuit, Q2 and Q4 will reduce the bias voltage Australia's electronics magazine as they heat up, preventing that. Q3 and Q5 drive the loudspeaker via the 1W resistors and the 470μF coupling capacitor. This capacitor removes the 6V DC offset of the amplifier so that the loudspeaker is driven purely by an AC voltage. Power for the circuit is from a 12V DC plugpack connected to CON1. Switch S1 connects this supply via two paths. One is via the 100W resistor to power the op amps. This supply is bypassed using two 470μF capacitors. The second path is via diode D8 to the loudspeaker amplifier circuitry, bypassed by one 470μF capacitor. Note that apart from the two wires for the loudspeaker, the other components that you can see fitted to this side are not required, and were only needed for our prototype. We have installed a plastic end cap on the back of the loudspeaker to improve its bass response. siliconchip.com.au clamps the reverse voltage applied to the circuit to -0.6V; the current through it is limited to 114mA by the 100W resistor. D8 provides reverse polarity protection for the 470μF capacitor that bypasses the loudspeaker driver supply. It prevents any current from flowing if the supply polarity is wrong. Construction The 100W isolation resistor prevents the circuit from oscillating and motor boating, as mentioned previously. This resistor, along with the two 470μF capacitors, maintains a stable voltage for the op amp circuitry that is separate from the loudspeaker driver supply. Without this isolation, any supply voltage change due to current drawn to drive the loudspeaker would reduce the op amp supply voltage, causing the surf sound generator voltages to vary, leading to motor boating. Reverse supply polarity protection is provided by diodes D7 and D8. D7 siliconchip.com.au All components for the Surf Sound Simulator mount on a double-sided blue PCB coded 01111241 that measures 236 × 80mm. As shown on the overlay diagram, Fig.3, most parts are on the top of the PCB. Only the loudspeaker is on the other side. An end cap is attached to the rear of the loudspeaker to improve its bass response. Begin by fitting the resistors. The colour codes for these are shown in the parts list but it is best also to check the values using a multimeter. Some of the colours can be difficult to discern against the blue background body colour of the resistor. Install the diodes next. D1-D6 are the smaller 1N4148 types, while D7 and D8 are 1N4004s. Take care to fit each with the correct orientation. The two IC sockets can be mounted next. Again, these need to be orientated correctly, with the notched section at the end with pins 1 & 14 as shown. The MKT polyester capacitors can now go in. These are not polarised, so they can go either way around. They will likely be marked with a code Australia's electronics magazine rather than the actual value; the likely codes are shown in the parts list. The transistors can go in next. There are three types: a BC549C for Q1, BC337s for Q2 & Q3 and BC327s for Q4 & Q5. Take care to install each in the correct position. Q2/Q3 and Q4/Q5 have their flat sides facing each other. Ideally, they should touch each other (perhaps with a smear of thermal paste between) so their temperatures track. Install CON1 (the PCB-mounting DC barrel socket) and switch S1 now. S1 can be an Altronics toggle switch or a Jaycar slider switch as specified in the parts list. LED1 can also be fitted now; ensure it goes in with the anode (longer lead) in the hole marked “A”. It can sit close to the PCB. The electrolytic capacitors are next. Most of these are polarised, so they must be orientated with the correct polarity. The plus sign (+) on the PCB shows the positive side, which corresponds to the longer capacitor lead. The striped side of the can is the opposite (negative) side. The two 33μF capacitors are non-polarised (NP) types, so they can be mounted either way. Now install CON2 (the PCB-­ mounting RCA socket) and potentiometer VR1. Insert and solder two PC stakes (PCB pins) at the CON3 speaker connection points. The loudspeaker mounts on the back of the PCB and is connected to those stakes/pins with two short lengths of hookup wire. For the moment, the loudspeaker can be left November 2024  53 Parts List – Surf Sound Simulator off the PCB and connected with wire leads for testing. 1 double-sided plated-through blue PCB coded 01111241, 236 × 80mm 1 76mm 8W 1W loudspeaker (SPK1) [Jaycar AS3006] 1 12V DC 150mA+ plugpack 1 PCB-mount DC barrel socket to suit plugpack (CON1) [Altronics P0621, Jaycar PS0520] 1 vertical PCB-mounting RCA socket (CON2) [Altronics P0131] 1 10kW logarithmic vertical 9mm PCB-mounting potentiometer (VR1) [Altronics R1988] 1 PCB-mounting 90° SPDT toggle or vertical slide switch (S1) [Altronics S1421, Jaycar SS0834] 2 14-pin DIL IC sockets 2 1mm diameter PC stakes (for CON3) 1 65mm uPVC DWV end cap (Iplex D105.65, Holman DWVF0194 or equivalent) [Bunnings 4770359] 2 M3-tapped 25-30mm Nylon standoffs or spacers (25mm for Holman end cap, 30mm for Iplex; use 10+15mm or 15+15mm if you can’t get the required lengths) 2 M3 × 25mm panhead machine screws Semiconductors 2 LM324 quad single-supply op amps (IC1, IC2) 1 BC549 (ideally BC549C) 30V 100mA NPN transistor (Q1) 2 BC337 45V 800mA NPN transistors (Q2, Q3) 2 BC327 45V 800mA PNP transistors (Q4, Q5) 6 1N4148 75V 200mA diodes (D1-D6) 2 1N4004 400V 1A diodes (D7, D8) 1 5mm white LED (LED1) Capacitors (all 63/100V MKT polyester unless noted) 4 470μF 16V PC electrolytic 1 330μF 16V PC electrolytic 3 100μF 16V PC electrolytic 2 33μF 50V NP (non-polarised) PC electrolytic 2 10μF 16V PC electrolytic 1 2.2μF 63V PC electrolytic 1 470nF (code 474) 2 120nF (code 124) 4 100nF (code 104) 1 56nF (code 563) 1 18nF (code 183) 1 1.2nF (code 122) Resistors (all axial ¼W 1%) 1 1MW 2 150kW 3 47kW 3 4.7kW 1 680kW 2 120kW 2 33kW 1 1kW 2 330kW 14 100kW 3 10kW 1 100W 1 270kW 4 68kW 1 8.2kW 2 1W 5% Testing Insert the two LM324 ICs into their sockets with the pin 1 and notched end orientated correctly. Make sure that when you push them down, the pins go into the socket and don’t get folded up under them. When you power the unit up with a 12V DC supply and S1 on, you should see LED1 light and hear the surf sound coming from the loudspeaker. If not, check the setting of the volume potentiometer, VR1. If there is still no sound, check the supplies to IC1 and IC2. There should be around 11.75V between their pins 11 and 4. Also check your construction for correct component locations and orientations. Once you are satisfied with the operation, the loudspeaker can be secured to the rear of the PCB using neutral-cure silicone sealant (roof & gutter sealant), contact adhesive or any other suitable glue. A 76mm-­diameter screen-printed circle is provided on the back of the PCB to show the ideal position. We attached a PVC plumbing end cap (a 65mm DWV [Drain Waste and Vent] type) to the rear of the loudspeaker to provide a baffle for it, giving extra bass. A small notch will need to be made with a round file to allow the speaker wires to enter the bottom edge of the end cap. The end cap can then be secured to the rear of the loudspeaker with the same glue used for the speaker M3 screws and spacers can be attached at the PCB mounting hole near CON1 and switch S1 so that the Surf Sound Simulator can sit horizontally or lean back vertically on the plumbing fitting at the rear of the loudspeaker SC and the lower standoff. The finished Surf Sound Simulator can rest on a shelf, desk or other flat surface. 54 Silicon Chip siliconchip.com.au