diff --git a/JoshHeaps.Net/Hubs/EchoHub.cs b/JoshHeaps.Net/Hubs/EchoHub.cs
new file mode 100644
index 0000000..c67c4e3
--- /dev/null
+++ b/JoshHeaps.Net/Hubs/EchoHub.cs
@@ -0,0 +1,52 @@
+using JoshHeaps.Net.Models;
+using JoshHeaps.Net.Services.Interfaces;
+using Microsoft.AspNetCore.SignalR;
+
+namespace JoshHeaps.Net.Hubs;
+
+/// <summary>
+/// Membership and peak reporting for acoustic ranging rooms. Nothing that arrives here is audio:
+/// a report is a handful of sample indices, and the geometry is solved on the clients.
+/// </summary>
+public class EchoHub(IEchoRoomStore rooms) : Hub
+{
+    /// <summary>Join (or create) a room and receive an id plus the current roster.</summary>
+    public async Task<EchoJoinResult> JoinRoom(string roomCode, string displayName, int sampleRate)
+    {
+        var result = rooms.Join(roomCode, Context.ConnectionId, displayName, sampleRate);
+        await Groups.AddToGroupAsync(Context.ConnectionId, GroupFor(result.RoomCode));
+        await Clients.Group(GroupFor(result.RoomCode)).SendAsync("RoomChanged", result.Room);
+
+        return result;
+    }
+
+    /// <summary>File this device's arrival indices for the room's open round.</summary>
+    public bool ReportRound(EchoPeakReport report) => rooms.Report(Context.ConnectionId, report);
+
+    /// <summary>
+    /// Server clock, for estimating each device's offset from it. Only needs to be good to a few
+    /// tens of milliseconds: it decides which chirp is whose, never how far away anything is.
+    /// </summary>
+    public long ServerTime() => DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
+
+    public async Task<bool> LeaveRoom() => await RemoveFromRoom();
+
+    public override async Task OnDisconnectedAsync(Exception? exception)
+    {
+        await RemoveFromRoom();
+        await base.OnDisconnectedAsync(exception);
+    }
+
+    internal static string GroupFor(string roomCode) => $"echo:{roomCode.ToUpperInvariant()}";
+
+    private async Task<bool> RemoveFromRoom()
+    {
+        var (roomCode, room) = rooms.Leave(Context.ConnectionId);
+        if (roomCode is null) return false;
+
+        await Groups.RemoveFromGroupAsync(Context.ConnectionId, GroupFor(roomCode));
+        if (room is not null) await Clients.Group(GroupFor(roomCode)).SendAsync("RoomChanged", room);
+
+        return true;
+    }
+}
diff --git a/JoshHeaps.Net/Models/EchoModels.cs b/JoshHeaps.Net/Models/EchoModels.cs
new file mode 100644
index 0000000..d18a751
--- /dev/null
+++ b/JoshHeaps.Net/Models/EchoModels.cs
@@ -0,0 +1,84 @@
+namespace JoshHeaps.Net.Models;
+
+/// <summary>A device taking part in a ranging room.</summary>
+public sealed class EchoDevice
+{
+    public required string DeviceId { get; init; }
+    public required string ConnectionId { get; init; }
+    public required string DisplayName { get; set; }
+    public int SampleRate { get; set; }
+}
+
+/// <summary>
+/// One chirp cycle: every device plays in turn, and every device listens to the whole thing.
+/// </summary>
+public sealed class EchoRound
+{
+    public required string RoundId { get; init; }
+    public required string[] SlotOrder { get; init; }
+    public int SlotMilliseconds { get; init; }
+    public DateTimeOffset Deadline { get; init; }
+    public Dictionary<string, EchoPeakReport> Reports { get; } = [];
+}
+
+/// <summary>
+/// What one device heard. Peaks are fractional sample indices into that device's own continuous
+/// recording, one per slot, null where a chirp was not detected. Nothing else is ever uploaded.
+/// </summary>
+public sealed class EchoPeakReport
+{
+    public required string DeviceId { get; init; }
+    public required string RoundId { get; init; }
+    public int Slot { get; init; }
+    public int SampleRate { get; init; }
+    public double Epsilon { get; init; }
+    public double?[] Peaks { get; init; } = [];
+}
+
+public sealed class EchoJoinResult
+{
+    public required string DeviceId { get; init; }
+    public required string RoomCode { get; init; }
+    public required EchoRoomSnapshot Room { get; init; }
+}
+
+public sealed class EchoRoomSnapshot
+{
+    public required string RoomCode { get; init; }
+    public required EchoDeviceSnapshot[] Devices { get; init; }
+}
+
+public sealed class EchoDeviceSnapshot
+{
+    public required string DeviceId { get; init; }
+    public required string DisplayName { get; init; }
+    public int SampleRate { get; init; }
+}
+
+/// <summary>Tells every device when to chirp: its own slot index and how long a slot lasts.</summary>
+public sealed class EchoRoundSchedule
+{
+    public required string RoundId { get; init; }
+    public required string[] SlotOrder { get; init; }
+    public int SlotMilliseconds { get; init; }
+    public int TailMilliseconds { get; init; }
+
+    /// <summary>
+    /// When slot zero should sound, in server time, set far enough ahead that every device can
+    /// receive the message and book the playback before it arrives. Devices schedule against this
+    /// rather than against message arrival, so one slow client cannot drag its chirp into another
+    /// device's slot and invalidate the round for everybody.
+    /// </summary>
+    public long StartsAtUnixMs { get; init; }
+}
+
+/// <summary>
+/// Every device's peaks for one round, broadcast unchanged. Each client solves the geometry itself
+/// so the server never needs the ranging maths.
+/// </summary>
+public sealed class EchoRoundResult
+{
+    public required string RoundId { get; init; }
+    public required string[] SlotOrder { get; init; }
+    public required EchoPeakReport[] Reports { get; init; }
+}
diff --git a/JoshHeaps.Net/Program.cs b/JoshHeaps.Net/Program.cs
index cd006d7..b665273 100644
--- a/JoshHeaps.Net/Program.cs
+++ b/JoshHeaps.Net/Program.cs
@@ -1,6 +1,7 @@
 using JoshHeaps.Net.Hubs;
 using JoshHeaps.Net.Services.Implementations;
 using JoshHeaps.Net.Services.Interfaces;
+using Microsoft.Extensions.Options;
 
 var builder = WebApplication.CreateBuilder(args);
 
@@ -30,6 +31,12 @@ builder.Services.AddSingleton<IGameStore, GameStore>();
 builder.Services.AddSingleton<ISelfPlayCoordinator, SelfPlayCoordinator>();
 builder.Services.AddSingleton<AutoTrainingSettings>();
 
+builder.Services.Configure<EchoRoundSettings>(configuration.GetSection(EchoRoundSettings.SectionName));
+builder.Services.AddSingleton(provider =>
+    provider.GetRequiredService<IOptions<EchoRoundSettings>>().Value);
+builder.Services.AddSingleton<IEchoRoomStore, EchoRoomStore>();
+builder.Services.AddHostedService<EchoRoundService>();
+
 if (!builder.Environment.IsDevelopment())
 {
     builder.Services.AddHostedService<AutoIpUpdateService>();
@@ -72,4 +79,6 @@ app.MapControllers();
 
 app.MapHub<ChessHub>("/chessHub");
 
+app.MapHub<EchoHub>("/echoHub");
+
 app.Run();
\ No newline at end of file
diff --git a/JoshHeaps.Net/Services/Implementations/EchoRoomStore.cs b/JoshHeaps.Net/Services/Implementations/EchoRoomStore.cs
new file mode 100644
index 0000000..9ed5f48
--- /dev/null
+++ b/JoshHeaps.Net/Services/Implementations/EchoRoomStore.cs
@@ -0,0 +1,189 @@
+using System.Collections.Concurrent;
+using JoshHeaps.Net.Models;
+using JoshHeaps.Net.Services.Interfaces;
+
+namespace JoshHeaps.Net.Services.Implementations;
+
+/// <summary>
+/// In-memory ranging rooms. Singleton: rooms are process-wide and short-lived, and the site runs
+/// as a single instance, so there is nothing to persist and no backplane to coordinate.
+/// </summary>
+public sealed class EchoRoomStore : IEchoRoomStore
+{
+    private sealed class Room
+    {
+        public required string Code { get; init; }
+        public List<EchoDevice> Devices { get; } = [];
+        public EchoRound? Round { get; set; }
+        public int RoundCounter { get; set; }
+        public DateTimeOffset LastActivity { get; set; } = DateTimeOffset.UtcNow;
+    }
+
+    private readonly ConcurrentDictionary<string, Room> _rooms = new(StringComparer.OrdinalIgnoreCase);
+    private readonly ConcurrentDictionary<string, string> _roomsByConnection = [];
+
+    public EchoJoinResult Join(string roomCode, string connectionId, string displayName, int sampleRate)
+    {
+        var room = _rooms.GetOrAdd(roomCode, code => new Room { Code = code });
+        var device = new EchoDevice
+        {
+            DeviceId = Guid.NewGuid().ToString("N")[..8],
+            ConnectionId = connectionId,
+            DisplayName = displayName,
+            SampleRate = sampleRate
+        };
+
+        lock (room)
+        {
+            room.Devices.RemoveAll(existing => existing.ConnectionId == connectionId);
+            room.Devices.Add(device);
+            room.LastActivity = DateTimeOffset.UtcNow;
+            _roomsByConnection[connectionId] = room.Code;
+
+            return new EchoJoinResult { DeviceId = device.DeviceId, RoomCode = room.Code, Room = SnapshotLocked(room) };
+        }
+    }
+
+    public (string? roomCode, EchoRoomSnapshot? room) Leave(string connectionId)
+    {
+        if (!_roomsByConnection.TryRemove(connectionId, out var roomCode)) return (null, null);
+        if (!_rooms.TryGetValue(roomCode, out var room)) return (roomCode, null);
+
+        lock (room)
+        {
+            room.Devices.RemoveAll(device => device.ConnectionId == connectionId);
+            room.Round = null;
+            room.LastActivity = DateTimeOffset.UtcNow;
+
+            if (room.Devices.Count == 0) _rooms.TryRemove(roomCode, out _);
+
+            return (roomCode, SnapshotLocked(room));
+        }
+    }
+
+    public EchoRoomSnapshot? Snapshot(string roomCode)
+    {
+        if (!_rooms.TryGetValue(roomCode, out var room)) return null;
+
+        lock (room) return SnapshotLocked(room);
+    }
+
+    public IReadOnlyCollection<string> MeasurableRooms =>
+        [.. _rooms.Values.Where(room => room.Devices.Count >= 2).Select(room => room.Code)];
+
+    public EchoRoundSchedule? StartRound(
+        string roomCode,
+        int slotMilliseconds,
+        int tailMilliseconds,
+        TimeSpan lead,
+        TimeSpan grace)
+    {
+        if (!_rooms.TryGetValue(roomCode, out var room)) return null;
+
+        lock (room)
+        {
+            if (room.Round is not null || room.Devices.Count < 2) return null;
+
+            var slotOrder = RotatedSlotOrder(room);
+            var startsAt = DateTimeOffset.UtcNow + lead;
+            var duration = TimeSpan.FromMilliseconds(slotOrder.Length * slotMilliseconds + tailMilliseconds);
+
+            room.Round = new EchoRound
+            {
+                RoundId = Guid.NewGuid().ToString("N")[..12],
+                SlotOrder = slotOrder,
+                SlotMilliseconds = slotMilliseconds,
+                Deadline = startsAt + duration + grace
+            };
+            room.LastActivity = DateTimeOffset.UtcNow;
+
+            return new EchoRoundSchedule
+            {
+                RoundId = room.Round.RoundId,
+                SlotOrder = slotOrder,
+                SlotMilliseconds = slotMilliseconds,
+                TailMilliseconds = tailMilliseconds,
+                StartsAtUnixMs = startsAt.ToUnixTimeMilliseconds()
+            };
+        }
+    }
+
+    public bool Report(string connectionId, EchoPeakReport report)
+    {
+        var device = FindDevice(connectionId, out var room);
+        if (device is null || room is null) return false;
+
+        lock (room)
+        {
+            if (room.Round?.RoundId != report.RoundId) return false;
+            if (!room.Round.SlotOrder.Contains(device.DeviceId)) return false;
+
+            room.Round.Reports[device.DeviceId] = report;
+            room.LastActivity = DateTimeOffset.UtcNow;
+            return true;
+        }
+    }
+
+    public EchoRoundResult? TryCloseRound(string roomCode)
+    {
+        if (!_rooms.TryGetValue(roomCode, out var room)) return null;
+
+        lock (room)
+        {
+            var round = room.Round;
+            if (round is null) return null;
+
+            var everyoneReported = round.SlotOrder.All(round.Reports.ContainsKey);
+            if (!everyoneReported && DateTimeOffset.UtcNow < round.Deadline) return null;
+
+            room.Round = null;
+
+            return new EchoRoundResult
+            {
+                RoundId = round.RoundId,
+                SlotOrder = round.SlotOrder,
+                Reports = [.. round.SlotOrder.Where(round.Reports.ContainsKey).Select(id => round.Reports[id])]
+            };
+        }
+    }
+
+    public int PruneIdle(TimeSpan idleFor)
+    {
+        var cutoff = DateTimeOffset.UtcNow - idleFor;
+        var stale = _rooms.Values.Where(room => room.LastActivity < cutoff).Select(room => room.Code).ToList();
+
+        foreach (var code in stale) _rooms.TryRemove(code, out _);
+
+        return stale.Count;
+    }
+
+    private static EchoRoomSnapshot SnapshotLocked(Room room) =>
+        new()
+        {
+            RoomCode = room.Code,
+            Devices =
+            [
+                .. room.Devices.Select(device => new EchoDeviceSnapshot
+                {
+                    DeviceId = device.DeviceId,
+                    DisplayName = device.DisplayName,
+                    SampleRate = device.SampleRate
+                })
+            ]
+        };
+
+    private static string[] RotatedSlotOrder(Room room)
+    {
+        var offset = room.RoundCounter++ % room.Devices.Count;
+        return [.. room.Devices.Skip(offset).Concat(room.Devices.Take(offset)).Select(device => device.DeviceId)];
+    }
+
+    private EchoDevice? FindDevice(string connectionId, out Room? room)
+    {
+        room = null;
+        if (!_roomsByConnection.TryGetValue(connectionId, out var roomCode)) return null;
+        if (!_rooms.TryGetValue(roomCode, out room)) return null;
+
+        lock (room) return room.Devices.FirstOrDefault(device => device.ConnectionId == connectionId);
+    }
+}
diff --git a/JoshHeaps.Net/Services/Implementations/EchoRoundService.cs b/JoshHeaps.Net/Services/Implementations/EchoRoundService.cs
new file mode 100644
index 0000000..bef55c0
--- /dev/null
+++ b/JoshHeaps.Net/Services/Implementations/EchoRoundService.cs
@@ -0,0 +1,107 @@
+using JoshHeaps.Net.Hubs;
+using JoshHeaps.Net.Services.Interfaces;
+using Microsoft.AspNetCore.SignalR;
+
+namespace JoshHeaps.Net.Services.Implementations;
+
+public sealed class EchoRoundSettings
+{
+    public const string SectionName = "Echo";
+
+    /// <summary>
+    /// Slot length. Wide enough that no device's chirp can land in another's search window once
+    /// unknown output latency (tens of milliseconds, different per device) and message jitter are
+    /// accounted for.
+    /// </summary>
+    public int SlotMilliseconds { get; set; } = 500;
+
+    /// <summary>Time after the last chirp for propagation, detection and reporting.</summary>
+    public int TailMilliseconds { get; set; } = 700;
+
+    public int GraceMilliseconds { get; set; } = 1500;
+    public int GapMilliseconds { get; set; } = 250;
+    public int IdleRoomMinutes { get; set; } = 10;
+
+    /// <summary>
+    /// How far ahead a round is announced. Must exceed the worst message delivery plus client
+    /// stall, or a device will find its slot already gone and sit the round out.
+    /// </summary>
+    public int LeadMilliseconds { get; set; } = 600;
+}
+
+/// <summary>
+/// Drives continuous ranging: opens a round for every room that has two or more devices, closes it
+/// once everyone has reported or the deadline passes, and broadcasts the raw peak table.
+/// </summary>
+public sealed class EchoRoundService(
+    IEchoRoomStore rooms,
+    IHubContext<EchoHub> hub,
+    EchoRoundSettings settings,
+    ILogger<EchoRoundService> logger) : BackgroundService
+{
+    private readonly Dictionary<string, DateTimeOffset> _nextRoundAllowedAt = [];
+
+    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+    {
+        var lastPrune = DateTimeOffset.UtcNow;
+
+        while (!stoppingToken.IsCancellationRequested)
+        {
+            try
+            {
+                await TickAsync();
+                lastPrune = PruneIfDue(lastPrune);
+            }
+            catch (Exception error)
+            {
+                logger.LogError(error, "Echo round tick failed");
+            }
+
+            await Task.Delay(50, stoppingToken);
+        }
+    }
+
+    private async Task TickAsync()
+    {
+        foreach (var roomCode in rooms.MeasurableRooms)
+        {
+            await CloseFinishedRoundAsync(roomCode);
+            await OpenRoundIfDueAsync(roomCode);
+        }
+    }
+
+    private async Task CloseFinishedRoundAsync(string roomCode)
+    {
+        var result = rooms.TryCloseRound(roomCode);
+        if (result is null) return;
+
+        _nextRoundAllowedAt[roomCode] = DateTimeOffset.UtcNow.AddMilliseconds(settings.GapMilliseconds);
+        await hub.Clients.Group(EchoHub.GroupFor(roomCode)).SendAsync("RoundComplete", result);
+    }
+
+    private async Task OpenRoundIfDueAsync(string roomCode)
+    {
+        if (_nextRoundAllowedAt.TryGetValue(roomCode, out var earliest) && DateTimeOffset.UtcNow < earliest) return;
+
+        var schedule = rooms.StartRound(
+            roomCode,
+            settings.SlotMilliseconds,
+            settings.TailMilliseconds,
+            TimeSpan.FromMilliseconds(settings.LeadMilliseconds),
+            TimeSpan.FromMilliseconds(settings.GraceMilliseconds));
+
+        if (schedule is null) return;
+
+        await hub.Clients.Group(EchoHub.GroupFor(roomCode)).SendAsync("RoundStarting", schedule);
+    }
+
+    private DateTimeOffset PruneIfDue(DateTimeOffset lastPrune)
+    {
+        if (DateTimeOffset.UtcNow - lastPrune < TimeSpan.FromMinutes(1)) return lastPrune;
+
+        var pruned = rooms.PruneIdle(TimeSpan.FromMinutes(settings.IdleRoomMinutes));
+        if (pruned > 0) logger.LogInformation("Pruned {Count} idle echo room(s)", pruned);
+
+        return DateTimeOffset.UtcNow;
+    }
+}
diff --git a/JoshHeaps.Net/Services/Interfaces/IEchoRoomStore.cs b/JoshHeaps.Net/Services/Interfaces/IEchoRoomStore.cs
new file mode 100644
index 0000000..8b90548
--- /dev/null
+++ b/JoshHeaps.Net/Services/Interfaces/IEchoRoomStore.cs
@@ -0,0 +1,45 @@
+using JoshHeaps.Net.Models;
+
+namespace JoshHeaps.Net.Services.Interfaces;
+
+/// <summary>
+/// Process-wide registry of ranging rooms. Holds only device identity and the sample indices each
+/// device reported, so the server is a scheduler and a relay — it never sees audio.
+/// </summary>
+public interface IEchoRoomStore
+{
+    /// <summary>Add a device to a room, creating the room if this is the first arrival.</summary>
+    EchoJoinResult Join(string roomCode, string connectionId, string displayName, int sampleRate);
+
+    /// <summary>Remove whichever device owns this connection, returning the room it left.</summary>
+    (string? roomCode, EchoRoomSnapshot? room) Leave(string connectionId);
+
+    /// <summary>Snapshot of a room's roster, or null if the room is gone.</summary>
+    EchoRoomSnapshot? Snapshot(string roomCode);
+
+    /// <summary>Room codes with at least two devices, which is the minimum for a measurement.</summary>
+    IReadOnlyCollection<string> MeasurableRooms { get; }
+
+    /// <summary>
+    /// Open a new round for a room, rotating which device chirps first so no single device is
+    /// permanently the slot-order anchor.
+    /// </summary>
+    EchoRoundSchedule? StartRound(
+        string roomCode,
+        int slotMilliseconds,
+        int tailMilliseconds,
+        TimeSpan lead,
+        TimeSpan grace);
+
+    /// <summary>File a device's peaks against the room's open round.</summary>
+    bool Report(string connectionId, EchoPeakReport report);
+
+    /// <summary>
+    /// Close the open round if every device has reported or its deadline has passed, returning the
+    /// reports to broadcast.
+    /// </summary>
+    EchoRoundResult? TryCloseRound(string roomCode);
+
+    /// <summary>Drop rooms that have had no activity for longer than <paramref name="idleFor"/>.</summary>
+    int PruneIdle(TimeSpan idleFor);
+}