70V/100V constant-voltage distribution vs Low-impedance (direct) distribution.
Constant-voltage 70V/100V distribution puts a transformer at each loudspeaker so dozens of them can share one long cable run and be tapped to the level their zone needs — the practical architecture for distributed speech and background music across a building.
Quick answer — when to choose 70V/100V constant-voltage distribution or Low-impedance (direct) distribution
Quick answer
Constant-voltage 70V/100V distribution puts a transformer at each loudspeaker so dozens of them can share one long cable run and be tapped to the level their zone needs — the practical architecture for distributed speech and background music across a building. Low-impedance distribution connects loudspeakers directly to amplifier channels, keeping full bandwidth and damping for rooms where sound quality is the point. Most real buildings use both: constant-voltage through the distributed areas, low-impedance in the rooms that have to sound right.
When to use
You are deciding how loudspeakers will be distributed across a building or venue and need to know which architecture each area should use.
When not to use
The complaint is intelligibility in a reverberant room — that is an acoustic and coverage problem the distribution architecture will not fix.
Option A
70V/100V constant-voltage distribution
Loudspeakers sit on a stepped-up line through transformers, so many of them share one long run and each is tapped to the level that zone needs.
Choose 70V/100V constant-voltage distribution when
- Many loudspeakers spread over long distances — corridors, concourses, back-of-house, campus areas
- Zones need to be balanced individually by tap rather than by re-cabling
- The system carries paging or announcement duty where even, intelligible coverage matters more than dynamic range
Option B
Low-impedance (direct) distribution
Each loudspeaker or pair connects directly to an amplifier channel, with no transformer between them and full bandwidth and damping preserved.
Choose Low-impedance (direct) distribution when
- Sound quality and dynamic range are the point — cinema, listening rooms, performance and monitoring
- Runs are short and the loudspeaker count per amplifier channel is small
- The design needs amplifier damping control over the driver, which a line transformer blunts
Side by side
The fields that decide it
| Dimension | 70V/100V constant-voltage | Low-impedance (direct) |
|---|---|---|
| Loudspeakers per run | Many, limited by the amplifier's available power | Few — set by the amplifier's minimum load |
| Cable run length | Long runs practical with modest conductor size | Short runs; losses and damping degrade with distance |
| Level setting per device | Tapped individually at the loudspeaker | Set by amplifier channel or passive attenuation |
| Bandwidth and damping | Transformer limits low-frequency extension and damping | Full bandwidth, amplifier damping preserved |
| Fault behaviour | One failed device does not take the line down | A short can take the amplifier channel down |
| Typical use | Corridors, concourse, back-of-house, paging, background music | Cinema, performance, monitoring, critical listening |
Tap settings, line loading and transformer insertion loss are product-specific — confirm against manufacturer data and a coverage design before committing a layout.
Before the drawings freeze
What changes at design stage
What changes at design stage
- Zone the building by how each area is used and heard before choosing distribution — the zoning plan, not the cable, is the design decision.
- Identify the rooms where sound quality is the point; those come out of the distributed line and onto their own amplifier channels.
- Where the system carries life-safety announcement duty, that scope is engineered and approved separately against the building's approved fire design.
- Plan headroom on the constant-voltage line so later additions do not require re-engineering the whole run.
Infrastructure implications
- Constant-voltage runs are long and shared, so containment routes and pull-in sequencing have to be agreed with the cabling design.
- Amplifier and rack space, its ventilation and its power provision belong in the design early — amplifiers are a real heat load.
- Low-impedance zones need their cable runs kept short and their conductor size chosen against the run, not from habit.
- Where announcements must survive a mains failure, the supply arrangement follows the approved design for that system, not general practice.
Getting it built
Coordination & integration
Coordination for architects & consultants
- Agree ceiling type, void depth and finish with the architect before loudspeaker types are fixed — the ceiling constrains the device, not the reverse.
- Coordinate zone boundaries with the operator, because a zone that spans two operational areas will always be wrong for one of them.
- Where the venue also has an acoustic scope, agree who owns treatment; distribution architecture cannot compensate for an untreated room.
- Confirm with the fire consultant which zones carry announcement duty, so those are engineered to the approved design from the start.
Integration considerations
- Where both architectures coexist, the routing and priority logic must be designed so an announcement reaches the right zones regardless of what is playing.
- Life-safety announcement priority is enforced in hardware at the zone amplifier where the approved design requires it, not only in software.
- Source routing, paging inputs and background-music zones belong in one documented matrix rather than being patched in during commissioning.
- Monitoring of amplifier and line faults belongs in the operations view so a failed zone is known before a user reports it.
Typical project fit
Where each is the right answer
Decision summary
- Hotel corridors, lobby, back-of-house and pool deck
- Constant-voltage throughout the distributed areas, tapped per zone, with weather-rated devices where the area is exposed.
- Dedicated cinema or performance room
- Low-impedance on its own amplifier channels — full bandwidth and damping preserved, with the room treated to match.
- Auditorium with a performance system and a paging layer
- Both, deliberately layered — low-impedance for the performance system, constant-voltage for the distributed announcement and overflow areas.
From the field
Common mistakes
- Loading a constant-voltage line to its limit with no headroom, so the first added loudspeaker forces a redesign of the whole run.
- Putting a critical listening room on the distributed line and then trying to recover its low-frequency extension with processing.
- Setting every tap to the same level and calling it a zone design, when the areas have completely different noise floors.
- Treating an intelligibility complaint as a distribution problem when the room's reverberation is the actual constraint.
- Specifying indoor devices for covered outdoor areas, which lasts until the first monsoon.
TechnoGuru engineering perspective
How we specify it
We zone the building first and let the zoning choose the architecture. Distributed speech and background music go on a constant-voltage line with real headroom left on it, tapped per zone against each area's noise floor; rooms where the sound is the point come off that line onto their own amplifier channels. Where the system also carries announcement duty, that scope is engineered to the building's approved fire design with priority enforced where required, and the coverage is proven by measurement at handover rather than asserted.
Where this connects
Services, sectors, tools & proof
Relevant services
Relevant tools & calculators
Related comparisons
Further reading
Selected proof
Owner-approved, publicly-verifiable project references. No quantities, pricing, layouts or restricted detail are shown.
Brand ecosystems that appear in this decision
TechnoGuru works with, specifies, supplies, installs or integrates these brand ecosystems depending on project fit, availability and client requirements.
· How we approach this on a live brief
On a real av & audio brief the choice between 70V/100V constant-voltage distribution and Low-impedance (direct) distribution usually turns on a few of the fields above, not every row in the table. Share the drawings, the BOQ or the constraints and a project lead will name the one we would specify for the building — with the engineering reason set out in writing.
· Frequently asked
70V/100V distributed audio vs low-impedance audio —
what people ask first.
Can one system use both 100V and low-impedance zones?
Yes, and most real buildings do. Distributed areas — corridors, concourse, back-of-house — sit on the constant-voltage line, while rooms where sound quality is the point come off onto their own amplifier channels. What matters is that the routing and priority logic are designed as one matrix rather than patched together at commissioning.
Does a 100V line always sound worse?
It is constrained rather than simply worse. The line transformer limits low-frequency extension and blunts the amplifier's damping control over the driver, which matters a great deal in a listening room and very little in a corridor where speech intelligibility and even coverage are the goal. Match the architecture to what the area has to do.
How many loudspeakers can go on one 100V run?
It is set by the amplifier's available power against the sum of the tap settings, with headroom deliberately left for later additions — not by a fixed device count. Tap settings, line loading and transformer insertion loss are product-specific, so the run is sized against manufacturer data and the coverage design rather than a rule of thumb.
What do you need to design distributed audio?
The zoning plan and how each area is used, ceiling type and void depth, the noise floor in the noisier areas, whether any zone carries announcement duty under the approved fire design, and which rooms are critical listening spaces. From those we advise the architecture per zone and route the commercial question to a written estimate after drawings.
· Begin
Review a distributed-audio scheme
after a look at the drawings.
Share drawings, a BOQ, a project brief or site information and a project lead will return an engineering reading — not a sales pitch — within two working days.