Michael Turner
Drivers and enclosure: Aim for frequency response from 60 Hz to 18 kHz for full-range units; add a 6–8″ down-firing submodule if you need sub-40 Hz extension. Target 100–110 dB SPL peak at 1 m for outdoor intelligibility; limit continuous output to 95 dB to protect hearing. Use sealed or ported enclosures with internal damping foam and rubber gaskets. Driver mounting angle 10–20° forward improves directivity when worn on the torso.
Power and battery management: Specify Li-ion 21700 cells with BMS supporting 30 A continuous current and overcharge/discharge protection. For 20,000 mAh (74 Wh) battery, expect full recharge via USB-C PD 45 W in ~2–2.5 hours; with 10,000 mAh plan for 1–1.5 hours at 30 W. Recommend operating at ≤80% depth-of-discharge to reach 500+ charge cycles. Include a fuse (10–20 A slow blow) and thermal cutoff; label max input charge current on the unit.
Ergonomics and build: Keep total system weight ≤3.5 kg for all-day wear; use padded shoulder straps 40–60 mm wide, a sternum strap with 25–35 mm webbing, and an optional hip belt to transfer 40–60% of load off shoulders. Choose water-resistant 600D polyester or Cordura, YKK zippers, and reinforced stitching at 10 mm stitch length for load points. Place the heaviest components close to the spine to reduce torque on the shoulders.
Connectivity, DSP and measurement: Provide Bluetooth 5.2 with aptX Adaptive, wired 3.5 mm aux, and one balanced XLR input for external mixers. Implement a basic DSP chain: high-pass at 50–60 Hz, low-latency limiter at −6 dBFS threshold, compressor ratio 3:1 with 10 ms attack and 150–250 ms release. Calibrate using pink noise and a measurement mic (94 dB @ 1 kHz calibrator): measure at 1 m, adjust EQ for ±3 dB flatness across target band. Include a quick-reference label with max SPL, battery capacity, charge time and safety warnings.
Maintenance and accessories: Provide a removable rain cover, replaceable strap pads, a spare fuse, and contact cleaner for connectors. Recommend cleaning contact points every 100 hours of use, inspecting stitching and foam seals every 200 hours, and storing batteries at 40–60% charge between uses for longevity.
Selecting transducers and speaker enclosures for outdoor use
Choose horn-loaded compression drivers (1″ or 2″ throat) paired with constant-directivity waveguides for mid/high coverage if you need throw beyond 20–30 m; specify sensitivity ≥100 dB (1 W/1 m) and power handling 200–1000 W RMS for each high-frequency module to reach peak SPLs of 125–138 dB without severe distortion.
For low frequencies, select 12–18″ woofers with Xmax ≥8–12 mm and thermal power ratings 500–2000 W RMS depending on target SPL; sealed enclosures yield tighter bass and smaller volume but require higher excursion and power, ported (front- or rear-loaded) cabinets extend LF down to 30–40 Hz with enclosure volumes typically 60–200 liters for 12–15″ drivers–design ports for airspeed <17 m/s to avoid port noise at high SPL.
Use active DSP crossovers with 24 dB/octave slopes and time-alignment delays between LF and HF to prevent combing on-axis; set high-pass at 30–40 Hz for single full-range enclosures, or 20–25 Hz for subwoofer systems. Match amplifier continuous power to speaker AES ratings and provide 1.5–2× headroom for transient peaks; clip protection and limiters in the amplifier/DSP chain protect transducers and improve reliability.
Specify enclosure materials by environment: 18–21 mm marine-grade plywood with epoxy seal and polyurethane finish for best acoustic stiffness and serviceability; rotomolded polyethylene or fiberglass for high-impact, waterproof cases that resist corrosion; anodized/aluminum enclosures for flown arrays with integrated cooling fins. Use stainless-steel fasteners, powder-coated grille, and foam-backed stainless mesh to protect drivers from debris and bird pecking.
Weatherproofing details: IP54-rated cabinets are minimum for damp climates; for heavy rain or salt air, aim for IP65 components and IP67-rated connectors. Fit GORE-type breathable vents on each driver chamber to equalize pressure while blocking water ingress; silicone gasket all panel joints and include drainage channels under the driver to route incidental water away from electrical parts. Use desiccant packs inside sealed cavities for long-term storage and replace yearly.
Connector and mounting choices: use Neutrik Speakon NL4 or waterproof XLRs with cable glands for speaker feed; provide M10/M8 threaded inserts and 35 mm pole sockets for flexible rigging. For portable systems balance weight vs acoustic performance–rotomolded enclosures reduce weight but may require larger internal volumes for the same LF response. For event planners also consider logistics and food pairing–see which green vegetables have protein for complementary menu planning on outdoor deployments.
Designing the power system: battery capacity, charging strategy and runtime calculation
For a 4-hour run at ~60 W average draw, select a 320–400 Wh pack (≈25–31 Ah at 12.8 V LiFePO4); increase capacity by 30% if frequent peaks exceed average or if you need AC inverter output.
Estimating consumption and calculating runtime
- List every load and use real-world averages, not peak ratings. Typical component draws:
- Portable amplifier: 30–200 W RMS per channel peak; use an average power factor of 0.15–0.35 of RMS rating for rough estimates (e.g., 2×100 W amp → average 30–70 W depending on program material).
- Digital signal processor / preamp: 3–8 W.
- Wireless receiver / Bluetooth: 1–4 W.
- LED indicators or lights: 5–20 W.
- DC-DC converters/regulators: add 5–10% overhead.
- Runtime formula (Wh basis): Runtime (h) = (Battery Wh × usable DoD) / System average W.
- Example: 320 Wh LiFePO4, usable DoD 0.9 → usable = 288 Wh. With 60 W average → 288 / 60 = 4.8 h.
- Account for conversion/inverter losses: divide usable Wh by total system losses (e.g., 0.9 for converters × 0.9 for inverter = 0.81 net) before dividing by average W.
- Allow for peak headroom: size battery to handle surge currents without hitting BMS cutoff. Use a headroom multiplier 1.2–1.5 on capacity when program material has frequent transients.
Battery chemistry, pack design and charging strategy
- Chemistry selection:
- LiFePO4: nominal 3.2–3.3 V per cell, 4S = 12.8–13.2 V nominal; safe DoD up to 90%, 2000–5000 cycles at 80% DoD, stable thermal behavior, recommended for regular outdoor use.
- Li‑ion (NMC): nominal 3.6–3.7 V per cell, 4S = 14.4–14.8 V nominal; higher energy density but shorter cycle life and stricter charging parameters.
- Pack voltage: match nominal pack voltage to device input range. For equipment specified as 12 V nominal, use 4S LiFePO4 (12.8 V nom) or confirm amplifier supports 14.8 V if using 4S Li‑ion.
- BMS and protections:
- Include BMS with cell balancing, overcurrent protection, short-circuit protection and temperature cutoff. BMS continuous current rating must exceed expected continuous draw by 25–50%.
- Place an HRC fuse or auto-resetting breaker at the pack positive terminal sized ~1.25× expected maximum current draw.
- Charging protocol and charger selection:
- Use CC‑CV chargers matched to chemistry: Li‑ion 4.2 V/cell, LiFePO4 3.6–3.65 V/cell. Charger voltage = cell voltage × series cells.
- Recommended charge current:
- LiFePO4: 0.2C for routine charging (long life), 0.5C acceptable for faster cycles, up to 1C only if cell spec allows and temperature management present.
- Li‑ion NMC: 0.2–0.5C typical; avoid >1C unless cells are rated and cooled.
- Balancing: active or passive balancing required for multi‑cell packs; ensure charger supports balance leads or the BMS performs balancing during charge.
- Storage charge: store LiFePO4 at ~50% SoC; store Li‑ion at ~40–60% SoC. Avoid long-term storage at 100% or at very low SoC.
- Topology and practical tips:
- Prefer modular packs for hot-swap: two identical 160–200 Wh modules in parallel give redundancy and easier charging in the field.
- If AC loads are needed, use a pure sine inverter sized 1.25–1.5× expected continuous AC load; account for inverter no-load current (1–2 W for small units, up to 10–20 W for larger) when computing runtime.
- For brief amplifier peaks, add a local capacitor bank (electrolytic or polymer) or a small supercap module rated for the supply voltage to reduce stress on pack and BMS.
- Wire sizing: calculate max continuous current I = P/V. Use these guidelines: up to 30 A → AWG 10, up to 50 A → AWG 8, up to 70 A → AWG 6. Keep cable runs short to limit voltage drop; target <3% drop on main positive lead.
- Environmental and lifecycle factors:
- Derate usable capacity at low temperature: expect 15–30% loss at 0°C and larger losses below −10°C for standard cells. Provide thermal insulation or active warming for cold deployments.
- Cycle planning: if you need >1000 cycles at high DoD, favor LiFePO4; budget replacement after 2–5 years for heavy daily use depending on cycle depth.
Implementing directional beamforming and array geometry for targeted sound zones
Use a multi-band array with element spacing ≤ λ/2 at the highest frequency you intend to actively steer; split the system into low-, mid- and high-frequency subarrays to avoid grating lobes and keep driver count practical.
Array geometry guidelines
Element spacing: d ≤ λ_min/2 where λ_min = c/f_max (c = 343 m/s). Example: f_max = 6 kHz → λ_min ≈ 0.057 m → d ≤ 0.028 m. If physical spacing cannot meet that, confine active steering to frequencies ≤ f_break where spacing condition holds, and use compact tweeters or horns for f > f_break.
Main-lobe width: use first-null approximation θ_null ≈ λ/L (radians) and half-power beamwidth HPBW ≈ 0.885·λ/L. Example: target center frequency 1.5 kHz (λ≈0.229 m), array length L = 0.6 m → HPBW ≈ 0.885·0.229/0.6 ≈ 0.34 rad ≈ 19.4°. To achieve a target zone width w at distance R, angular requirement θ_target = w/R; choose L ≥ 0.885·λ/θ_target. Example: w=1 m at R=5 m → θ_target=0.2 rad → L ≥ 1.02 m at 1.5 kHz.
Geometry trade-offs: linear arrays give high azimuth directivity with minimal elevation control; planar arrays (MxN) enable 2D steering at the cost of element count (~M·N). Arc or circular arrays deliver more uniform steering around a center point and simplify beamforming delays when the listener distribution spans large azimuth angles.
Sidelobe control: apply amplitude tapering (Hann, Blackman, or Chebyshev). Hann reduces first sidelobe to ≈ −31 dB but increases HPBW by ~1.4–1.6×. Chebyshev allows specifying sidelobe level (e.g., −40 dB) at the expense of wider main lobe; pick taper based on acceptable trade between leakage and resolution.
Beamforming algorithms, DSP and practical parameters
Algorithm selection: use delay-and-sum for low-complexity steering and predictable latency. Use MVDR (Capon) or LCMV for interference suppression and null placement; regularize covariance with diagonal loading α·I where α ≈ 0.01·trace(R)/M for robust inversion. For moving targets or changing environments, update covariance estimates with exponential averaging (time constant 0.5–2 s) and limit weight updates to 5–20 Hz to avoid audible fluctuations.
Implementation domain: frequency-domain (STFT) beamforming is preferred for multi-band control. Example parameters: Fs = 48 kHz or 96 kHz; frame length 512 samples @48 kHz → 10.7 ms frames with 50% overlap → 5.3 ms hop; choose frame size so per-bin steering phase resolution meets latency and spectro-temporal trade-off. For fine fractional delay in time-domain FIR, use 48–128 taps for sub-sample delays at 48 kHz; at 96 kHz, 32–64 taps often suffice.
Compute budget example: N = 12 channels, STFT-based beamforming with 512-bin FFT (complex multiply per bin per channel), Fs=48 kHz, 256 active bins → roughly 12·256·(complex multiplies per frame)·(Fs/frame_hop) ≈ tens of millions of MAC/s. Offload heavy complex-matrix ops to an FPGA or DSP core (Xilinx Zynq, TI C66x, or a dedicated audio DSP). For mobile battery-powered rigs choose hardware that sustains required MAC/s while keeping power draw within system budget.
Calibration and measurement: measure per-driver frequency response and array steering vectors in anechoic conditions or use ground-compensated outdoor sweeps. Store complex per-frequency corrections and apply inverse filters to equalize amplitude and phase before beamforming. Validate zone SPL maps at target distances and adjust tapering or add null constraints until off-axis SPL meets required attenuation (e.g., −20 dB outside target zone).
Practical constraints and tips: limit active steering bandwidth where element spacing causes grating lobes; combine physical directivity (waveguides/horns) with DSP steering to reduce required element count; choose update rates and STFT latency balanced against acceptable audible artifacts (aim total system latency ≤ 20–30 ms for interactive use; higher latencies acceptable for broadcast-like content).
Ergonomic mounting, weight distribution and ventilation for all-day wear
Mount the unit so the primary mass sits 100–150 mm above the posterior iliac crest with the center of mass ≤50 mm lateral from the spine; keep total carried mass ≤3.5 kg for continuous daily use and ≤5.0 kg only for intermittent duties.
Mounting, attachment points and load transfer
Use a four-point mounting plate (approx. 120 × 200 mm) that spreads contact over ≥600 cm² to keep interface pressure below ~4 kPa (30 mmHg). Transfer at least 60% of the static load to a padded hip belt with 40–60 mm wide webbing and 25–35 mm closed-cell foam; reserve the shoulders for stabilisation only. Incorporate load-lifter straps angled 15–20° from the vertical to tune fore-aft balance and keep the device centerline within 50 mm of the wearer’s sagittal plane to reduce moment at the lumbar pivot.
Shoulder straps: 35–50 mm contour width, 12–18 mm open-cell padding, foam density 30–40 kg/m³. Sternum strap: adjustable height spanning 90–140 mm below the clavicle, quick-release rated ≥2 kN. All attachment hardware should be positioned for symmetrical left/right load and use low-profile recessed fasteners to avoid point pressure under clothing.
Thermal management, ventilation channels and sweat handling
Provide a continuous air channel between the torso and shell: spacer mesh thickness 8–15 mm with channel height 10–20 mm and channel pitch 25–40 mm yields measurable convective flow during walking (typical 1.2–1.6 m/s). Select 3D spacer textile with 60–75% porosity and hydrophobic face fabric to move moisture laterally; recommended surface breathability (MVTR) >10,000 g/m²·24h for liners in contact zones.
Position heat-generating modules low and lateral (near hips) to couple thermal mass to body areas with better heat-sinking and avoid concentrated hot spots on the upper back. Integrate removable, machine-washable liners and perforated foam inserts for cleaning; add small drainage channels and a modular drain port for rapid water egress (example part reference: best umbrella pop out drains for vessel sink in chrome).
Route cables and quick-disconnects along the shoulder strap spine with strain-relief loops and lockable connectors to prevent torque transfer to the harness. For long stints, offer hip-mounted battery or mass-transfer modules that dock to the main assembly, keeping peak shoulder loads under 25% of total carried weight and maintaining wearer mobility.
Compliance and safety: SPL limits, hearing protection and local noise regulations
Set the maximum A-weighted equivalent continuous SPL at the operator’s ear to 85 dBA LEQ for an 8-hour period using a 3 dB exchange rate; treat 140 dB(C) peak as an absolute upper limit for impulsive sound and require immediate shutdown if exceeded.
Regulatory reference table (use this to set device software/hardware limits and to prepare permit applications):
| Standard / Agency | Continuous limit (LAeq) | Exchange rate | Peak / impulsive limit |
|---|---|---|---|
| NIOSH (recommended exposure limit) | 85 dBA (8 hours) | 3 dB | 140 dB(C) peak (guidance) |
| OSHA (permissible exposure) | 90 dBA (8 hours) / action level 85 dBA | 5 dB | 140 dB(C) peak |
| EU Directive (workers) | Lower action 80 dBA; upper action 85 dBA; exposure limit 87 dBA | 3 dB (commonly applied) | 137–140 dB(C) (member-state specifics) |
Use the following exposure-time table (3 dB exchange) for permissible continuous exposure planning, staff rotation and automatic shutdown thresholds:
| LAeq (dBA) | Allowed continuous exposure (approx.) |
|---|---|
| 85 | 8 hours |
| 88 | 4 hours |
| 91 | 2 hours |
| 94 | 1 hour |
| 97 | 30 minutes |
| 100 | 15 minutes |
| 103 | 7.5 minutes |
| 106 | 3.75 minutes |
| 109 | ~1.9 minutes |
| 112 | ~57 seconds |
Measurement protocol: use a Class 1 sound level meter or calibrated Type-2 meter with microphone at the ear position of the wearer and at the nearest receptor (property line or representative residence). Record LAeq (integrated), LAmax (fast), and Lpeak (C-weighted) for each operating mode and archive CSV logs. Calibrate the meter before each session and repeat spot checks every hour.
Hearing protection selection and practical rules: foam earplugs NRR 25–33 dB for routine exposures up to ~95 dBA; earmuffs NRR 20–30 dB when wearing headgear is possible; dual protection (earplug + earmuff) when measured LAeq exceeds 100 dBA–expect roughly +5 dB additional real-world attenuation over the higher-rated device. For impulsive sources or transient peaks >140 dB(C), remove personnel from exposure and redesign the source signal.
Conversion and estimation tips: estimate SPL drop using free-field inverse-square law: SPL2 = SPL1 − 20·log10(r2/r1). Use a 6 dB reduction per doubling of distance as a conservative design rule for outdoor placement. When setting DSP limiters, measure at 1 m from the cabinet/array, convert to the ear position using the distance rule, then set the limiter at the SPL that guarantees the worker/public exposure stays within limits plus a 3–5 dB safety margin.
Local permit and nuisance requirements: query municipal codes and event permits before deployment. Common permit metrics: daytime property-line limits often range 55–75 dBA A-weighted (jurisdiction dependent), curfew hours, and complaint response times. Document pre-event baseline measurements, on-site Lmax hits and post-event logs. Retain meter files for 2–3 years to support enforcement reviews.
Operational controls and mitigations: implement hardware/software hard-limiters, real-time SPL display for operator, schedule staff rotation per exposure-time table, provide HPD with clear labeling and training, orient directional transducers toward target zones and away from sensitive receptors, and use local attenuation (barriers, angling) when property-line levels approach municipal thresholds.
Equipment transport and measurement kit recommendation: keep the meter, calibration kit, spare mics and ear protection in a rugged carry case or best messenger bag to buy for work sized to avoid crushing the microphone capsule; label the kit and store calibration records with the device.
Minimum compliance checklist to deploy with a wearable audio rig:
1) Pre-check: Class 1 meter calibrated, baseline LAeq at planned receptor points.
2) Limits: DSP/limiter set to ensure ear position ≤85 dBA (adjust for shift length using exposure table).
3) HPD: issue rated protection, enforce fit/training, dual protection when LAeq >100 dBA.
4) Documentation: continuous logging, permit on-site, complaint response plan.
5) Emergency: automatic shutdown on Lpeak >140 dB(C) or repeated exceedance above set thresholds.
