diff --git a/modules/juce_audio_devices/native/juce_Audio_ios.cpp b/modules/juce_audio_devices/native/juce_Audio_ios.cpp
index fa0ba27793..dac1d9b706 100644
--- a/modules/juce_audio_devices/native/juce_Audio_ios.cpp
+++ b/modules/juce_audio_devices/native/juce_Audio_ios.cpp
@@ -257,6 +257,81 @@ private:
     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (iOSAudioIODeviceType)
 };
 
+class SubstituteAudioUnit
+{
+public:
+    /* Returns true if the audio callback was called. False if a timeout occurred. */
+    bool waitForAudioCallback()
+    {
+        if (audioUnit != nullptr)
+        {
+            AudioComponentInstanceDispose (audioUnit);
+            audioUnit = nullptr;
+        }
+
+        AudioComponentDescription desc;
+        desc.componentType = kAudioUnitType_Output;
+        desc.componentSubType = kAudioUnitSubType_RemoteIO;
+        desc.componentManufacturer = kAudioUnitManufacturer_Apple;
+        desc.componentFlags = 0;
+        desc.componentFlagsMask = 0;
+
+        AudioComponent comp = AudioComponentFindNext (nullptr, &desc);
+        AudioComponentInstanceNew (comp, &audioUnit);
+
+        if (audioUnit == nullptr)
+            return false;
+
+        {
+            AURenderCallbackStruct inputProc;
+            inputProc.inputProc = audioUnitCallback;
+            inputProc.inputProcRefCon = this;
+            AudioUnitSetProperty (audioUnit, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Input, 0, &inputProc, sizeof (inputProc));
+        }
+
+        {
+            AudioStreamBasicDescription format;
+            zerostruct (format);
+            format.mSampleRate = [AVAudioSession sharedInstance].sampleRate;
+            format.mFormatID = kAudioFormatLinearPCM;
+            format.mFormatFlags = kAudioFormatFlagIsFloat | kAudioFormatFlagIsNonInterleaved | kAudioFormatFlagsNativeEndian | kLinearPCMFormatFlagIsPacked;
+            format.mBitsPerChannel = 8 * sizeof (float);
+            format.mFramesPerPacket = 1;
+            format.mChannelsPerFrame = 2;
+            format.mBytesPerFrame = format.mBytesPerPacket = sizeof (float);
+
+            AudioUnitSetProperty (audioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input,  0, &format, sizeof (format));
+            AudioUnitSetProperty (audioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Output, 1, &format, sizeof (format));
+        }
+
+        AudioUnitInitialize (audioUnit);
+        AudioOutputUnitStart (audioUnit);
+
+        const auto result = audioCallbackOccurred.wait (1000.0);
+
+        AudioComponentInstanceDispose (audioUnit);
+        audioUnit = nullptr;
+
+        return result;
+    }
+
+private:
+    static OSStatus audioUnitCallback (void* object,
+                                       AudioUnitRenderActionFlags*,
+                                       const AudioTimeStamp*,
+                                       UInt32,
+                                       UInt32,
+                                       AudioBufferList*)
+    {
+        static_cast<SubstituteAudioUnit*> (object)->audioCallbackOccurred.signal();
+
+        return noErr;
+    }
+
+    AudioUnit audioUnit{};
+    WaitableEvent audioCallbackOccurred;
+};
+
 //==============================================================================
 struct iOSAudioIODevice::Pimpl final : public AsyncUpdater
 {
@@ -311,6 +386,16 @@ struct iOSAudioIODevice::Pimpl final : public AsyncUpdater
     {
         JUCE_NSERROR_CHECK ([[AVAudioSession sharedInstance] setActive: enabled
                                                                  error: &error]);
+
+        if (@available (ios 18, *))
+        {
+            if (enabled)
+            {
+                SubstituteAudioUnit au;
+                [[maybe_unused]] const auto success = au.waitForAudioCallback();
+                jassert (success);
+            }
+        }
     }
 
     int getBufferSize (const double currentSampleRate)
@@ -334,8 +419,20 @@ struct iOSAudioIODevice::Pimpl final : public AsyncUpdater
         NSTimeInterval bufferDuration = currentSampleRate > 0 ? (NSTimeInterval) (newBufferSize + extraOffset) / currentSampleRate : 0.0;
 
         auto session = [AVAudioSession sharedInstance];
-        JUCE_NSERROR_CHECK ([session setPreferredIOBufferDuration: bufferDuration
-                                                            error: &error]);
+
+        // According to the apple docs, it's best to set preferred sample rates and block sizes
+        // while the device is inactive, and then to query the real values after activation.
+        // Unfortunately, on iOS 18.0, the real block size isn't immediately available after
+        // a call to setActive, so we also need to wait for the first audio callback.
+        // This will be slow!
+        // https://developer.apple.com/library/archive/qa/qa1631/_index.html
+        if (@available (ios 18, *))
+            setAudioSessionActive (false);
+
+        JUCE_NSERROR_CHECK ([session setPreferredIOBufferDuration: bufferDuration error: &error]);
+
+        if (@available (ios 18, *))
+            setAudioSessionActive (true);
 
         return getBufferSize (currentSampleRate);
     }
@@ -347,7 +444,7 @@ struct iOSAudioIODevice::Pimpl final : public AsyncUpdater
         auto newBufferSize = tryBufferSize (sampleRate, 64);
         jassert (newBufferSize > 0);
 
-        const auto longestBufferSize  = tryBufferSize (sampleRate, 4096);
+        const auto longestBufferSize = tryBufferSize (sampleRate, 4096);
 
         while (newBufferSize <= longestBufferSize)
         {
@@ -377,9 +474,7 @@ struct iOSAudioIODevice::Pimpl final : public AsyncUpdater
     double trySampleRate (double rate)
     {
         auto session = [AVAudioSession sharedInstance];
-        JUCE_NSERROR_CHECK ([session setPreferredSampleRate: rate
-                                                      error: &error]);
-
+        JUCE_NSERROR_CHECK ([session setPreferredSampleRate: rate error: &error]);
         return session.sampleRate;
     }