Custom Build · Hertz Flagship System
159.9 dB, recorded at a formal SPL competition. To our knowledge this is the loudest motorcycle in Australia, and Alex still rides it every day. This is his 2017 Road Glide Special, built in-house as a no-compromise Hertz system: three amplifiers, a full DSP, eight speakers, dual 10-inch subwoofers, and a dedicated auxiliary power system.
This is the opposite of an OEM+ sleeper. Where our demo bike keeps the factory wiring and chases clean sound at highway volume, this build goes after extreme output while keeping the sound clean, with the electrical engineering to back it. If you want the top end of what a Harley Touring platform can do, this is what it looks like.
The Brief
When Alex first walked into the workshop, he had one non-negotiable requirement.
"Build the wildest motorcycle sound system Australia has seen, but it has to stay rideable. This bike isn't going to live on a trailer."
He'd been following the huge audio baggers coming out of the United States for years and wanted to build something that could stand alongside them. At the time, he felt nobody was building bikes like that here in Australia.
Naturally, we wheeled out our Road Glide demo bike. At the time it was one of the best systems we'd ever built: SoundStream V2, four speakers and drop-in saddlebag subwoofers, all properly tuned. The sort of bike that usually left riders grinning.
Alex listened. He nodded. Then he looked up and said:
"That's a nice starting point."
You know those moments where everything goes quiet for a second? This was one of them.
Once the ego recovered, we asked him to explain what he actually meant. That's when we realised we weren't building another high-end Harley audio system. We were about to engineer something we'd never attempted before.
The Platform
The base bike is a 2017 Harley-Davidson Road Glide Special with a Milwaukee-Eight 107ci engine, a Freedom Performance 2-into-1 exhaust and an S&S 475 cam. Engine modifications are deliberately mild: almost every dollar and hour of work went into the audio system and the supporting electrical engineering. The bike also runs a full LED lighting package (headlight, DRL indicators, bag-blade lights, signal inserts, and underbody lighting synced to the audio), but this article is about the sound.
The Head Unit: SoundStream HDHU.V2
The system starts with a SoundStream HDHU.V2, the 2014 to 2023 Harley Touring version of SoundStream's V2 platform. It provides wireless Apple CarPlay and Android Auto, a high-brightness weather-resistant display, and 5V preamp outputs feeding the Hertz S8 DSP. The internal amplifier is not used; the HDHU.V2 functions purely as the system source. The unit was updated to the latest firmware and configured for Maestro integration before installation.
The System Brain: Hertz S8 DSP
The Hertz S8 DSP is the heart of the system. The HDHU.V2 feeds six channels into the S8, which handles all crossover, EQ, delay and routing before anything reaches an amplifier. It then sends a processed stereo feed to each SP4.900 (each driving its four speakers as left and right pairs) and a mono feed to the SP1.900. Nothing is summed, and the S8's eight outputs cover the system comfortably. Expert Mode is enabled for full routing and tuning flexibility, with custom crossovers for each driver group (fairing, lowers, rear 6×9s and subwoofers), delay alignment to pull the soundstage forward around the rider, the S8 master output level set to -6 dB with per-channel fine-tuning, and subsonic protection below roughly 25 to 30 Hz applied by both the S8 and the SP1.900's onboard filter. The S8 is mounted beneath the head unit on a custom 3D-printed plate designed in-house for secure positioning, airflow and clean serviceable wiring.
Amplification: Three Hertz Power Stages
Two Hertz SP4.900 four-channel Class-D amplifiers power the eight full-range speakers, one handling the front (fairing and lower fairing) and one the rear (saddlebags and Tour-Pak). Into the build's 4Ω speaker loads each SP4.900 is rated 150W×4 RMS; the amplifier is also capable of 250W×4 at 2Ω. Bass comes from a dedicated Hertz SP1.900 mono amplifier (1,000W RMS at 1Ω), mounted in the Tour-Pak. Driving the two 10-inch subwoofers wired in parallel, it sees a 2Ω load. Both SP4.900 amplifiers are mounted on an XEL Customs 2015-2023 Road Glide dual amp rack under the fairing, the same rack we run on our shop demo bike. That keeps them secure, cooled and serviceable without blocking access to the rest of the bike.
The Speakers: Eight Full-Range Drivers Across Four Locations
Up front, the fairing runs Hertz SX 165 NEO 6.5-inch full-range coaxials for high sensitivity and clean mids that stay efficient at high output, with Hertz SV 165.1 6.5-inch SPL midranges in the lower fairings for strong upper-midbass punch, vocal projection and warmth. The rear runs four Hertz SX 690 H 6×9 3-way coaxials, two in the saddlebags and two in the Tour-Pak. The Tour-Pak placement lifts the sound closer to rider and passenger height, a detail that matters at highway speed.
The Subwoofers: Dual 10-Inch Hertz CS 250 S4
Low end comes from two Hertz CS 250 S4 10-inch subwoofers (300W RMS each, 4Ω) in custom sealed enclosures, wired in parallel for a 2Ω load on the SP1.900. Tight, controlled and felt as much as heard, the subwoofer installation was central to the bike's peak SPL result. Dedicated rear-speaker enclosures help isolate the 6×9 drivers from subwoofer backpressure, reducing unwanted cone interaction and panel resonance at high output, and the custom sealed enclosures provide the air volume and structural rigidity the subwoofers need.
Dedicated Auxiliary Power System
The three amplifiers and the S8 DSP run from a dedicated auxiliary battery housed in the Tour-Pak, which supplies their high transient demand directly rather than placing it on the factory starter battery and charging circuit. A 12V 100Ah / 200A LiFePO4 slimline battery supplies all three Hertz amplifiers and the S8. Charging is managed through a Victron Orion-Tr Smart 12/12-18A DC-DC charger, tuned for LiFePO4 chemistry, which draws a current-limited feed from the motorcycle's charging system and tops the auxiliary battery up while riding. The head unit itself stays on the bike's factory power, so a custom relay-based turn-on was built to trigger the auxiliary-powered amplifiers and DSP cleanly from the head unit's remote turn-on. A dedicated BMS display provides real-time battery monitoring.
The result is that the high transient demand of the audio is drawn from the auxiliary battery rather than placed directly on the starter battery and factory charging circuit. In practice the longest continuous run we have logged is five hours of playback with more than 50% of the battery still remaining.
Wiring throughout is KnuKonceptz: Kolossus Flex tinned OFC power and ground (1/0 AWG main feeds, with 4 AWG runs to each amplifier), Karma SS tinned copper speaker wire in 12 and 16 gauge for the subwoofers and full-range speakers respectively, and Krystal twisted-pair RCA interconnects with Krux right-angle adapters where space is tight. The main 1/0 feed runs into a Bassik fused distribution block: a 120 A branch supplies the two SP4.900s, each run protected by its own 60 A Mini-ANL fuse, while the 100 A branch supplies the SP1.900. The main frame earth connection was upgraded to match, and tinned copper throughout resists the corrosion a motorcycle's wiring lives with. Anderson SB120 connectors provide high-current quick-disconnect, and XT90 and XT60 connectors serve the subwoofer and speaker-pod connections.
System at a Glance
| Component | Power / role |
|---|---|
| SoundStream HDHU.V2 head unit | Source: wireless CarPlay and Android Auto, feeding the S8 DSP |
| Hertz S8 DSP | 8-channel DSP-controlled routing: crossovers, delay, tuning |
| 2× Hertz SP4.900 | Front and rear full-range stages: 150W×4 RMS at the 4Ω speaker loads (250W×4 at 2Ω) |
| Hertz SP1.900 | Mono sub amp: 1,000W RMS at 1Ω, driving the two subs at a 2Ω load |
| SX 165 NEO + SV 165.1 front / 4× SX 690 H rear | Eight full-range speakers across four locations |
| 2× Hertz CS 250 S4 10-inch | Dual sealed 10-inch subwoofers, 2Ω load |
| LiFePO4 100Ah auxiliary power | Dedicated audio battery with DC-DC charging |
| Result | 159.9 dB, recorded at a formal SPL competition |
The Result: 159.9 dB
The headline number, 159.9 dB, was recorded at a formal SPL competition. That figure alone makes a statement, but it was at a separate local bike night that the build really made its point. The organisers at AMX Capalaba ran a loudest-bike competition, and every other entrant was revving their exhaust to chase the crown. Alex turned on the stereo and took the win without the engine making a sound, and the test microphone cut out at 150 dB before he ran out of volume. To our knowledge this is the loudest motorcycle in Australia, and it is also a bike Alex rides every day.
A note on the number. 159.9 dB is an extreme competition SPL, in the range of standing right next to a jet engine at take-off. It is a measurement of peak output, not a listening level. Hearing protection and spectator separation are essential during any high-output testing, and no one should be exposed to levels like this unprotected.
Who This Build Is For
This sits at the very top of the scale, well beyond the Stage 1 to 3 systems most riders build: triple amplification, full DSP, eight full-range speakers, dual subwoofers and a dedicated auxiliary power system. It represents the top end of the systems we currently build. It suits you if you want serious output and clarity together rather than one at the expense of the other, if you are building for competition or serious high-volume touring, if you want a complete engineered system rather than a collection of upgraded parts, and if you are prepared to invest in the electrical infrastructure a system this powerful requires, while still wanting a bike that rides every day.
Behind the Build
Want to see how it all went together? The teardown, lid fabrication, amp rack, fusing and power system are documented photo by photo in the companion article: Inside the Install: How We Built Australia's Loudest Motorcycle.
Frequently Asked Questions
How was the 159.9 dB figure measured?
It was recorded at a formal SPL competition, which uses competition metering rather than our own indoor rider-head reference method. The dB meter was inside the sealed saddlebag for the measurement. It is a peak competition figure, not a like-for-like comparison against the numbers in our dB Reference testing, which are taken at the rider's head on a different scale.
Why does it need a separate battery system?
Three amplifiers and a DSP at competition output draw far more current than a factory Harley system is designed to supply continuously. A dedicated 100Ah LiFePO4 battery with a 200 A continuous output rating, charged through a current-limited DC-DC connection to the bike, supplies the high transient audio demand from the auxiliary battery rather than placing it directly on the starter battery and factory charging circuit, and keeps the audio voltage stable under heavy load.
Can it still be ridden as a normal touring bike?
Yes. That was central to the brief. The longest continuous run we have logged is five hours of playback with more than half the auxiliary battery still remaining, so normal day-to-day riding and demonstration use is well within its capacity, especially with the DC-DC charger topping the battery up while riding.
Do I need to go this far to get great sound?
No. This is the top end of the scale. Most riders get an excellent result from a good head unit, one amplifier, upgraded speakers, and a sub. This build is for when you want the absolute maximum. Tell us your goals and we will map the right stage for your bike.
The 159.9 dB result is the headline, but it is not the whole story. What makes Alex's Road Glide special is that the output comes from a complete engineered system: source, processing, amplification, enclosures, wiring and power all designed together. The bike remains something he can ride every day.
Want a Build Like This?
Tell us your bike, how and where you ride, and how loud you want to go, and we'll design a system to suit, from a clean single-amp setup to a full competition build like this one.
Get in Touch →Shop this system
- SoundStream HDHU.V2 head unit
- Hertz S8 DSP
- Hertz SP4.900 4-channel amplifier
- Hertz SP1.900 mono subwoofer amplifier
- Hertz SX 165 NEO fairing speakers
- Hertz SV 165.1 lower-fairing midranges
- Hertz SX 690 H 6×9 rear speakers
- Hertz CS 250 S4 10-inch subwoofers
Not sure what fits your bike? Contact Motorcycle Audio Australia with your year, model and goals and we'll confirm the right parts →
More reading: the rest of our measured builds in System Tests, and our dB Reference method.
Motorcycle Audio Australia. Sound That Rides.