A monitor advertised at several hundred metres can struggle to reach two rooms away in a Victorian terrace or a converted stone cottage. That isn’t a defective product or a dishonest claim — it’s the predictable result of a number measured with nothing in the way being applied to a building full of masonry.
The range figure printed most prominently on a baby monitor is almost always open-field range — measured outdoors or in a clear space with nothing between the two units. It’s a legitimate measurement of the radio’s maximum reach under ideal conditions, and it tells you very little about a house. Indoors, a signal is attenuated by every material it passes through, and the amount varies enormously by construction: modern plasterboard partitions absorb relatively little, while solid brick, stone, poured concrete, and metal lath behind old plaster absorb a great deal. A building’s construction, not its floor area, is what determines whether a monitor reaches — which is why two houses of identical size can produce completely different results with the same product. The specification worth reading is indoor range, stated separately, and the practical approach is to buy with headroom rather than to the exact distance you need. Where a manufacturer publishes only one number, it’s the optimistic one, and the honest way to read it is as an upper bound that your house will reduce by an amount nobody can predict from a spec sheet.
§01Two measurements, one usually printed
Open-field range answers the question “how far can these two units communicate with nothing in between.” It’s measured in a clear space, and it’s a real property of the radio — a monitor with a higher open-field figure genuinely has more transmit capability than one with a lower figure, all else equal.
Indoor range answers a different question: how far can they communicate through a building. That number is always lower, sometimes dramatically, and it depends on a variable the manufacturer has no access to — what your walls are made of. Which is precisely why many products state only the first figure: it’s reproducible, comparable, and flattering. The second requires either a stated test condition or an honest acknowledgement that it varies.
§02What actually absorbs a signal
Three factors do most of the work, and understanding them lets you predict roughly how your own building will behave.
Denser materials absorb more radio energy. A modern stud partition — plasterboard over a timber frame with an air gap — is close to transparent by comparison with solid masonry. Brick attenuates significantly more; stone and poured concrete more again. And thickness compounds it: a single-brick internal wall is a different obstacle from a double-thickness exterior wall or a load-bearing stone wall in an older building.
WHY OLDER BUILDINGS ARE HARDER: pre-war construction typically used solid load-bearing walls internally, where modern building uses lightweight partitions. The room count is the same; the material between rooms isn’t.Metal is the most effective attenuator of the common building materials, and older buildings contain it in unexpected places. Metal lath behind lime plaster — a common pre-war wall construction — puts a fine metal mesh across an entire wall surface. Foil-backed insulation, increasingly common in renovations, does something similar. Steel-reinforced concrete floors, radiators, large mirrors, and metal-framed glazing all contribute.
This is why a wall that looks like ordinary plaster can perform far worse than expected, and why signal behaviour in a renovated older building is genuinely difficult to predict from appearance alone.
PRACTICAL SIGN: if a wall in an older building blocks mobile phone signal noticeably, it will affect a monitor too.Each wall a signal crosses subtracts from what remains, so three thin walls can cost more than one thick one. Floors count too, and often more than walls — a floor is typically a denser structure, and in older buildings may contain steel or substantial timber.
Angle matters as well: a signal crossing a wall at a shallow angle passes through more material than one crossing perpendicular. A camera diagonally opposite the parent unit through a corner is travelling through more wall than the straight-line distance suggests.
PRACTICAL FIX: repositioning a camera a metre sideways to give the signal a straighter path through fewer surfaces sometimes helps more than any specification.FIG.01 — Why one figure can’t describe both. The open-field measurement is real and reproducible; the indoor path depends entirely on what a specific building puts in the way, which is why it varies between houses of identical size.
§03Rough guide to construction types
Not precise figures — building materials vary too much for that — but a usable ordering of what tends to cause trouble:
| Construction | Typical effect on signal |
|---|---|
| PLASTERBOARD PARTITION | Modest. Modern internal stud walls are among the least obstructive common constructions. |
| SOLID BRICK | Noticeable, and cumulative across several walls. Common as internal walls in older housing. |
| STONE | Substantial, and typically thicker than brick. Common in older rural and period properties. |
| POURED / REINFORCED CONCRETE | Substantial, with the steel reinforcement adding significantly. Common in apartment floors. |
| METAL LATH BEHIND PLASTER | Disproportionately obstructive for its thickness — a fine metal mesh across the whole wall. |
| FOIL-BACKED INSULATION | Similar effect to metal lath, and increasingly common in renovated properties. |
TABLE.01 — Relative rather than absolute. The point isn’t to calculate a number but to know which of these your building contains, since that predicts far more about performance than floor area does.
§04How range figures should be published
A single number invites misreading, because a buyer naturally assumes it applies to their house. Two numbers, stated separately and labelled, remove that ambiguity — one describing the radio’s capability in clear conditions, one describing realistic indoor performance.
For reference, our own products are published as 280m open field and 40m indoor. Those figures sit far apart deliberately, because that gap is real for every monitor on the market — most just don’t print the second number. The indoor figure is the one a buyer should plan against, and it still describes typical rather than worst-case construction: a building with dense masonry or metal lath throughout can reduce it further.
The general principle is worth applying to any product: if only one range figure is given, treat it as the open-field maximum and assume indoor performance is a fraction of it. That assumption is safer than the alternative, and it’s what the physics supports.
§05Testing before you commit
The only reliable way to know how a specific building behaves is to test it — and that’s worth doing during a return window rather than discovering the answer at 2am in week three.
Camera where it’ll actually live, parent unit where you’ll actually be — not a convenient approximation. Signal behaviour changes noticeably over a metre or two.
The furthest point you’d realistically take the parent unit — a far bedroom, the garden, the basement. If it holds there, everywhere closer is covered.
Intermittent dropouts don’t show up in a two-minute check. A link that’s marginal will fail occasionally rather than constantly, which is harder to notice and more annoying to live with.
Moving the camera to give a straighter path through fewer or thinner walls sometimes solves what looks like a range failure. A metre sideways can change the number of surfaces the signal crosses.
§06If the signal genuinely won’t reach
Some buildings defeat some monitors, and it’s worth knowing the realistic options rather than assuming a more expensive product automatically solves it.
Reposition first. The cheapest fix, and more often effective than expected. Both units can move — the parent unit’s usual resting place is often as adjustable as the camera’s position, and moving it away from a metal-heavy area or to the other side of a doorway can be enough.
Reconsider the path, not just the distance. A route through a doorway or along a hallway crosses less material than one straight through two masonry walls, even if it’s longer in metres. Radio doesn’t travel only in straight lines through walls — signal also finds its way around obstacles through openings.
Understand that a higher open-field number may not help proportionally. If the problem is a metal-lath wall, a monitor with a larger open-field rating faces the same obstacle. More transmit capability helps, but not in proportion to the headline figure, because that figure was measured without the thing that’s actually causing the problem.
Worth noting that connection architecture matters here too, though not always in the direction people assume. A monitor using its own dedicated link between two units has a signal path you can reason about and reposition. A monitor depending on a home network inherits that network’s coverage — which in a thick-walled house may have exactly the same problem, plus a router position you may not be free to change. The architectural comparison covers what each approach depends on.
§07Questions worth asking
- Is an indoor range figure published separately from the open-field one, or is there only a single number?
- What are your internal walls actually made of — not what they look like, but what’s behind the plaster?
- Does the signal path cross a floor as well as walls? Floors typically attenuate more than internal walls.
- Is there a metal-heavy element in the path — lath, foil insulation, a large radiator, reinforced concrete?
- How much headroom does the indoor figure give over the distance you actually need, including the worst-case position?
- Can you test within a return window, over several hours rather than a few minutes?
§08Frequently asked questions
Why doesn’t my baby monitor work through thick walls?
Because the range figure quoted on most products is open-field range, measured with nothing between the two units. Indoors, every material the signal passes through absorbs some of it, and the amount varies enormously by construction — modern plasterboard partitions absorb relatively little, while solid brick, stone, poured concrete and metal lath behind old plaster absorb a great deal. A monitor rated for several hundred metres in clear air can genuinely struggle across two rooms in a building with dense masonry walls.
What’s the difference between open-field range and indoor range?
Open-field range measures how far two units can communicate with nothing in between — a real property of the radio, measured in a clear space, and reproducible for comparison between products. Indoor range measures communication through a building, which is always lower and depends on what the walls are made of. Manufacturers frequently publish only the open-field figure because it’s flattering and comparable; the indoor figure is the one that describes what you’ll actually experience.
Which building materials block a baby monitor signal most?
Metal is the most effective attenuator among common building materials, and it appears in places that aren’t obvious — metal lath behind lime plaster in pre-war construction, foil-backed insulation in renovations, steel reinforcement in concrete floors. Among non-metal materials, density and thickness drive the effect: plasterboard partitions are relatively transparent, solid brick noticeably more obstructive, and stone or poured concrete more again. A practical sign is mobile phone signal — a wall that noticeably degrades it will affect a monitor too.
Will a monitor with a bigger range number solve my thick-wall problem?
Not necessarily in proportion to the headline figure. More transmit capability does help, but the open-field number was measured without the thing causing your problem — so a product rated at twice the distance doesn’t necessarily get through twice as much masonry. If a metal-lath wall is the obstacle, both products face the same barrier. Repositioning to give the signal a shorter or less obstructed path is often more effective than upgrading, and it costs nothing to try first.
How should I test a baby monitor’s range in my house?
Set up both units in their real positions rather than convenient approximations, since signal behaviour changes over a metre or two. Test the worst-case location you’d realistically use — the furthest bedroom, the garden, the basement — because if it holds there, everywhere closer is covered. Leave it connected for several hours rather than checking for two minutes, since a marginal link fails intermittently rather than constantly. And do all of this within a return window rather than discovering the answer weeks later.
What range figures does True Bond publish?
Both, stated separately: 280m open field and 40m indoor. Those figures sit far apart deliberately, because that gap is real for every monitor on the market — most simply don’t print the second number. The indoor figure is the one to plan against, and it describes typical rather than worst-case construction, so a building with dense masonry or metal lath throughout may see less. Publishing one number invites a buyer to assume it applies to their house, which it doesn’t for any product.
Ask for the indoor figure, not the headline one
Ours are 280m open field and 40m indoor, published separately because the gap is real for every product. Tell us your building’s construction and the distance you need, and we’ll tell you honestly whether there’s enough headroom.
Ask about range in your building → info@truebondtech.com · WhatsApp +86 189 2846 4489 · View productsThis series