Why AVSpeechSynthesizer object method speakUtterance only worked when object declared by @property

Question

@interface MineViewController ()

@property (nonatomic, strong) AVSpeechSynthesizer *speechSynthesizer;

@end

@implementation MineViewController

• (void)speak {

//version1

self.speechSynthesizer = [[AVSpeechSynthesizer alloc] init];
self.speechSynthesizer.delegate = self;

AVSpeechUtterance *utterance = [[AVSpeechUtterance alloc] initWithString:@"12345678"];
AVSpeechSynthesisVoice *voice = [AVSpeechSynthesisVoice voiceWithLanguage:@"zh-CN"];
[utterance setVoice:voice];

//(worked speakUtterance successed)
[self.speechSynthesizer speakUtterance:utterance];

//version2

AVSpeechSynthesizer *speechSynthesizer = [[AVSpeechSynthesizer alloc] init];
speechSynthesizer.delegate = self;

AVSpeechUtterance *utterance = [[AVSpeechUtterance alloc] initWithString:@"12345678"];
AVSpeechSynthesisVoice *voice = [AVSpeechSynthesisVoice voiceWithLanguage:@"zh-CN"];
[utterance setVoice:voice];

//(not worked speakUtterance no response)
[speechSynthesizer speakUtterance:utterance];

}

@end