Tutorial: Detecting When A User Blows Into The Mic

If, a couple of years back, you’d told me that people would expect to be able to shake their phone or blow into the mic to make something happen I would have laughed. And here we are.

Detecting a shake gesture is straightforward, all the more so in 3.0 with the introduction of motion events.

Detecting when a user blows into the microphone is a bit more difficult. In this tutorial we’ll create a simple simple single-view app that writes a log message to the console when a user blows into the mic.

Source/Github

The code for this tutorial is available on GitHub. You can either clone the repository or download this zip.

Overview

The job of detecting when a user blows into the microphone is separable into two parts: (1) taking input from the microphone and (2) listening for a blowing sound.

We’ll use the new-in-3.0 AVAudioRecorder class to grab the mic input. Choosing AVAudioRecorder lets us use Objective-C without — as other options require — dropping down to C.

The noise/sound of someone blowing into the mic is made up of low-frequency sounds. We’ll use a low pass filter to reduce the high frequency sounds coming in on the mic; when the level of the filtered signal spikes we’ll know someone’s blowing into the mic.

Creating The Project

Launch Xcode and create a new View-Based iPhone application called MicBlow:

1. Create a new project using File > New Project… from Xcode’s menu
2. Select View-based Application from the iPhone OS > Application section, click Choose…
3. Name the project as MicBlow and click Save

Adding The AVFoundation Framework

In order to use the SDK’s AVAudioRecorder class, we’ll need to add the AVFoundation framework to the project:

1. Expand the Targets branch in the Groups & Files panel of the project
2. Control-click or right-click the MicBlow item
3. Choose Add > Existing Frameworks…
4. Click the + button at the bottom left beneath Linked Libraries
5. Choose AVFoundation.framework and click Add
6. AVFoundation.framework will now be listed under Linked Libraries. Close the window

Next, we’ll import the AVFoundation headers in our view controller’s interface file and set up an AVAudioRecorder instance variable:

1. Expand the MicBlow project branch in the Groups & Files panel of the project
2. Expand the Classes folder
3. Edit MicBlowViewController.h by selecting it
4. Update the file. Changes are bold:

#import <UIKit/UIKit.h>
#import <AVFoundation/AVFoundation.h>
#import <CoreAudio/CoreAudioTypes.h>

@interface MicBlowViewController : UIViewController {
	AVAudioRecorder *recorder;
}

@end

To save a step later, we also imported the CoreAudioTypes headers; we’ll need some of its constants when we set up the AVAudioRecorder.

Taking Input From The Mic

We’ll set everything up and start listening to the mic in ViewDidLoad:

1. Uncomment the boilerplate ViewDidLoad method
2. Update it as follows. Changes are bold:

- (void)viewDidLoad {
	[super viewDidLoad];

  	NSURL *url = [NSURL fileURLWithPath:@"/dev/null"];

  	NSDictionary *settings = [NSDictionary dictionaryWithObjectsAndKeys:
  	  	[NSNumber numberWithFloat: 44100.0],                 AVSampleRateKey,
  	  	[NSNumber numberWithInt: kAudioFormatAppleLossless], AVFormatIDKey,
  	  	[NSNumber numberWithInt: 1],                         AVNumberOfChannelsKey,
   	  	[NSNumber numberWithInt: AVAudioQualityMax],         AVEncoderAudioQualityKey,
  	  nil];

  	NSError *error;

  	recorder = [[AVAudioRecorder alloc] initWithURL:url settings:settings error:&error];

  	if (recorder) {
  		[recorder prepareToRecord];
  		recorder.meteringEnabled = YES;
  		[recorder record];
  	} else
  		NSLog([error description]);

}

The primary function of AVAudioRecorder is, as the name implies, to record audio. As a secondary function it provides audio-level information. So, here we discard the audio input by dumping it to the /dev/null bit bucket — while I can’t find any documentation to support it, the consensus seems to be that /dev/null will perform the same as on any Unix — and explicitly turn on audio metering.

Note: if you’re adapting the code for your own use, be sure to send the prepareToRecord (or, record) message before setting the meteringEnabled property or the audio level metering won’t work.

Remember to release the recorder in dealloc. Changes are bold:

- (void)dealloc {
  	[recorder release];
  	[super dealloc];
}

Sampling The Audio Level

We’ll use a timer to check the audio levels approximately 30 times a second. Add an NSTimer instance variable and its callback method to it in MicBlowViewController.h. Changes are bold:

#import <UIKit/UIKit.h>
#import <AVFoundation/AVFoundation.h>
#import <CoreAudio/CoreAudioTypes.h>

@interface MicBlowViewController : UIViewController {
	AVAudioRecorder *recorder;
	NSTimer *levelTimer;
}

- (void)levelTimerCallback:(NSTimer *)timer;

@end

Update the .m file’s ViewDidLoad to enable the timer. Changes are bold:

- (void)viewDidLoad {
	[super viewDidLoad];

  	NSURL *url = [NSURL fileURLWithPath:@"/dev/null"];

  	NSDictionary *settings = [NSDictionary dictionaryWithObjectsAndKeys:
  	  	[NSNumber numberWithFloat: 44100.0],                 AVSampleRateKey,
  	  	[NSNumber numberWithInt: kAudioFormatAppleLossless], AVFormatIDKey,
  	  	[NSNumber numberWithInt: 1],                         AVNumberOfChannelsKey,
   	  	[NSNumber numberWithInt: AVAudioQualityMax],         AVEncoderAudioQualityKey,
  	  nil];

  	NSError *error;

  	recorder = [[AVAudioRecorder alloc] initWithURL:url settings:settings error:&error];

  	if (recorder) {
  		[recorder prepareToRecord];
  		recorder.meteringEnabled = YES;
  		[recorder record];
		levelTimer = [NSTimer scheduledTimerWithTimeInterval: 0.03 target: self selector: @selector(levelTimerCallback:) userInfo: nil repeats: YES];
  	} else
  		NSLog([error description]);

}

For now, we’ll just sample the audio input level directly/with no filtering. Add the implementation of levelTimerCallback: to the .m file:

- (void)levelTimerCallback:(NSTimer *)timer {
	[recorder updateMeters];
	NSLog(@"Average input: %f Peak input: %f", [recorder averagePowerForChannel:0], [recorder peakPowerForChannel:0]);
}

Sending the updateMeters message refreshes the average and peak power meters. The meter use a logarithmic scale, with -160 being complete quiet and zero being maximum input.

Don’t forget to release the timer in dealloc. Changes are bold:

- (void)dealloc {
	[levelTimer release];
	[recorder release];
  	[super dealloc];
}

Listening For A Blowing Sound

As mentioned in the overview, we’ll be using a low pass filter to diminish high frequencies sounds’ contribution to the level. The algorithm creates a running set of results incorporating past sample input; we’ll need an instance variable to hold the results. Update the .h file. Changes are bold:

#import <UIKit/UIKit.h>
#import <AVFoundation/AVFoundation.h>
#import <CoreAudio/CoreAudioTypes.h>

@interface MicBlowViewController : UIViewController {
	AVAudioRecorder *recorder;
	NSTimer *levelTimer;
	double lowPassResults;
}

Implement the algorithm by replacing the levelTimerCallback: method with:

- (void)levelTimerCallback:(NSTimer *)timer {
	[recorder updateMeters];

	const double ALPHA = 0.05;
	double peakPowerForChannel = pow(10, (0.05 * [recorder peakPowerForChannel:0]));
	lowPassResults = ALPHA * peakPowerForChannel + (1.0 - ALPHA) * lowPassResults;

	NSLog(@"Average input: %f Peak input: %f Low pass results: %f", [recorder averagePowerForChannel:0], [recorder peakPowerForChannel:0], lowPassResults);
}

Each time the timer’s callback method is triggered the lowPassResults level variable is recalculated. As a convenience, it’s converted to a 0-1 scale, where zero is complete quiet and one is full volume.

We’ll recognize someone as having blown into the mic when the low pass filtered level crosses a threshold. Choosing the threshold number is somewhat of an art. Set it too low and it’s easily triggered; set it too high and the person has to breath into the mic at gale force and at length. For my app’s need, 0.95 works. We’ll replace the log line with a simple conditional:

- (void)listenForBlow:(NSTimer *)timer {
	[recorder updateMeters];

	const double ALPHA = 0.05;
	double peakPowerForChannel = pow(10, (0.05 * [recorder peakPowerForChannel:0]));
	lowPassResults = ALPHA * peakPowerForChannel + (1.0 - ALPHA) * lowPassResults;

	if (lowPassResults > 0.95)
		NSLog(@"Mic blow detected");
}

Voila! You can detect when someone blows into the mic.

Caveats and Acknowledgements

This approach works well in most situations, but not universally: I’m writing this article in-flight. The roar of the engines constantly triggers the algorithm. Similarly, a noisy room will often have enough low-frequency sound to trigger the algorithm.

The algorithm was extracted/adapted from this Stack Overflow post. The post used the SCListener library for its audio level detection. SCListener pre-dates AVAudioRecorder; it was created to hide the details of dropping down to C to get audio input. With AVAudioRecorder this is no longer so tough.

Finally, this does work on the simulator. You just need to locate the built in mic on your Mac. To my surprise, the mic is located in the tiny hole to the left of the camera on my first generation Macbook.