虽然每台Mac设备都配有内置于产品中的麦克风，但您还会发现另一个用于音频输入和输出的端口——耳机插孔。当然，在Apple决定将其作为一个昂贵的附加功能之前！
不过，在那之前，您可以使用此端口将外部麦克风等线路输入音频输入设备连接到您的Mac。macOS 允许您切换耳机插孔的用途以接受输入设备，以及将任何连接的设备设置为默认输入设备。
下面介绍如何在 macOS 上使用音频线路输入音频输入设备。
什么是音频线路？
设备上的线路输入音频插孔允许您向该设备输入声音。这与典型的音频输出（或线路输出）插孔形成对比，您可以使用它在扬声器系统等外部设备上播放来自 PC 的声音。
您的 PC 上可能还有一个麦克风输入插孔。这通常是普通线路输入插孔的较弱版本，因为麦克风上的输入电平通常比音频放大器等其他类型的输入设备低得多。标准(Standard)线路输入插孔也可以标记为麦克风输入——它们之间没有太大区别。
在Mac上，无论哪种方式，您都不需要担心区别。较旧的Mac设备有一个单独的线路输入插孔供您用于所有音频输入设备（包括外部麦克风）和一个用于耳机和扬声器的音频输出插孔。在现代Mac(Macs)中，这已简化为一个可切换的插孔，您可以使用其中任何一个。
您需要使用 macOS的“系统偏好设置”(’ System Preferences)菜单来告诉 macOS 使用此音频端口进行声音输入，而不是使用内置麦克风等内部设备。
在 macOS 上使用 Line In 音频输入设备(Using Line In Audio Input Devices On macOS)
正如我们所提到的，现代Mac设备只有一个音频插孔，可用于类似的输入和输出设备。要将其用于线路输入音频输入，您需要访问 macOS系统偏好设置(System Preferences)。
- 为此，请单击Mac显示屏左上角的Apple 图标。(Apple icon)从那里，单击System Preferences。您也可以单击Dock中的(Dock)系统偏好设置(System Preferences)图标来启动它。
- 在“系统偏好设置”(System Preferences )窗口中，单击声音(Sound)图标以输入您在 macOS 上的声音设置。
- 默认情况下，声音(Sound)窗口将默认为输出(Output)选项卡。您需要单击“输入(Input)”选项卡来更改音频输入设置。
- 要开始使用您的线路输入设备，您需要在“输入(Input)”选项卡中将其切换为音频输入设备。这可能看起来不同，具体取决于您的Mac设备型号。例如，在 2019 MacBook上，使用(MacBook)麦克风端口(Microphone port)检测到外部麦克风，但在旧设备上这可能被列为音频输入端口。(Audio line-in port)
macOS 也可能已自动切换到此设备，但如果没有，请单击该设备以在“选择声音输入设备”(Select a device for sound input)列表中选择它。
在 macOS 上调整音频输入设备中的线路(Adjusting Line In Audio Input Devices On macOS)
激活外部音频线路输入设备后，您就可以开始使用它了。但是，在此之前，您可能希望调整音量以及其他设置，例如音频采样率。
输入音量设置显示在声音(Sound)设置菜单中选择声音输入设备(Select a device for sound input)列表的下方。您可以通过单击Apple 图标(Apple icon )> System Preferences > Sound > Input来访问它。
- 输入音量(Input volume)滑块允许您更改输入声音的音量。例如，降低外部麦克风的音量会降低任何录制声音的音量。使用触控板或鼠标向左移动滑块以降低音量或向右移动以增加音量。任何(Any)检测到的声音都将以可视形式显示在其下方的输入电平(Input level)栏中。
- 您还可以使用音频 MIDI 设置(Audio MIDI Setup)应用程序对外部音频输入设备进行更高级的更改。这是一个核心 macOS 应用程序，您可以通过单击 Dock 上的Launchpad图标，单击其他(Other)文件夹，然后按音频 MIDI 设置(Audio MIDI Setup)图标来启动应用程序来找到它。
- 要更改音频输入设置，请单击“音频 MIDI 设置(Audio MIDI Setup)”屏幕左侧列表中的外部设备。
- 在右侧，您会看到各种音频选项。您可以在格式(Format)下拉菜单下更改音频采样率（以赫兹显示） 。您还可以在此处的Master Stream(Master Stream)部分下更改设备的输入音量。使用鼠标或触控板向左移动滑块以减少或向右移动以增加。
您将能够在您的设备的主(Master)音量幻灯片旁边看到当前音量值（以分贝列出）以及输入音量（十进制数字，最大为 1） 。
您可能还可以使用其他控件和设置，具体取决于您使用的设备类型。某些(Certain)音频输入设备也可能带有自己的控制软件，允许您进一步校准输入设置。
在现代 macOS 设备上使用外部设备(Using External Devices on Modern macOS Devices)
无法保证耳机插孔在未来的Mac(Macs)中仍将是内置功能，但尽管它仍然存在，但您可以使用它来连接外部音频输入设备，而无需昂贵的适配器。例如，您可能希望使用GarageBand 中的两个不同麦克风进行录音(record using two different microphones in GarageBand)。
您还可以使用线路输入音频插孔在 macOS 中录制乐器(record an instrument in macOS)。如果您对音乐制作感兴趣，那么天空就是极限。在下面的评论部分让我们知道您对外部音频线路输入设备的使用。
Use the Line In Audio Input on a Mac
While every Maс device cоmes with a microphone that’ѕ built into the product, you’ll also find another port for audio input and output—the headphone jack. That is, of course, until Apple decides to make it a pricy add-оn fеature!
Until that time comes, however, you can use this port to connect line-in audio input devices like external microphones to your Mac. macOS allows you to switch the purpose of the headphone jack to accept input devices, as well as set any attached device as the default input device.
Here’s how to use audio line-in audio input devices on macOS.
What Is Line In Audio?
A line-in audio jack on a device allows you to input sounds to that device. This is in contrast to the typical audio-out (or line-out) jack, which you might use to play sounds from your PC on external devices like a speaker system.
You may also have a mic-in jack on your PC. This is typically a weaker version of a normal line-in jack, as the input levels on a mic are usually at a much lower level than other types of input devices like audio amplifiers. Standard line-in jacks can also be labeled mic-in—there isn’t much difference between them.
On a Mac, you don’t really need to worry about the distinction, either way. Older Mac devices had a separate line-in jack for you to use for all audio input devices (including external microphones) and an audio-out jack for headphones and speakers. This has been reduced to a single, switchable jack in modern Macs that allows you to use either.
You’ll need to use macOS’ System Preferences menu to tell macOS to use this audio port for sound input, rather than an internal device like your built-in mic.
Using Line In Audio Input Devices On macOS
As we’ve mentioned, modern Mac devices only have a single audio jack that you can use for input and output devices alike. To use it for line-in audio input, you’ll need to access your macOS System Preferences.
- To do this, click the Apple icon in the top-left of your Mac display. From there, click System Preferences. You can also click the System Preferences icon from the Dock to launch it instead.
- In the System Preferences window, click the Sound icon to enter your sound settings on macOS.
- By default, the Sound window will default to the Output tab. You’ll want to click on the Input tab to change your audio line-in settings.
- To start using your line-in device, you’ll need to switch to this as your audio input device in the Input tab. This may look different, depending on your model of Mac device. On a 2019 MacBook, for instance, an external microphone is detected using the Microphone port, but this may be listed as an Audio line-in port on older devices.
macOS may also have switched to this device automatically, but if it hasn’t, click on the device to select it in your Select a device for sound input list.
Adjusting Line In Audio Input Devices On macOS
With your external audio line-in input device activated, you can begin using it. However, before you do that, you may wish to adjust the volume, as well as other settings such as the audio sample rates.
The input volume settings are shown just beneath the Select a device for sound input list in your Sound settings menu. You can access this by clicking the Apple icon > System Preferences > Sound > Input.
- The Input volume slider allows you to change the volume of your inputted sound. Reducing the volume on an external microphone, for instance, would reduce how loud any recorded sound would be. Use your trackpad or mouse to move the slider left to reduce the volume or right to increase it. Any detected sounds will appear in visual form in the Input level bar below it.
- You can also use the Audio MIDI Setup app to make more advanced changes to your external audio input devices. This is a core macOS app, which you can find by clicking the Launchpad icon on your Dock, clicking the Other folder, then pressing the Audio MIDI Setup icon to launch the app.
- To change your audio line-in settings, click the external device in the list on the left of the Audio MIDI Setup screen.
- To the right, you’ll see various audio options. You can change the audio sample rate (shown in hertz) under the Format drop-down menu. You can also change the input volume of your device here, under the Master Stream section. Use your mouse or trackpad to move the slider left to decrease or right to increase.
You’ll be able to see the current volume value (listed in decibels), as well as the input volume (a decimal figure, up to a maximum of 1) next to the Master volume slide for your device.
You may also have additional controls and settings available to you, depending on the type of device you’re using. Certain audio input devices may also come with their own control software, allowing you to further calibrate your input settings.
Using External Devices on Modern macOS Devices
There’s no guarantee that the headphone jack will remain a built-in feature in future Macs, but while it’s still there, you can use it to connect external audio input devices without needing an expensive adapter. You might want to use it to record using two different microphones in GarageBand, for instance.
You can also use the line-in audio jack to record an instrument in macOS. If you’re interested in music production, the sky’s the limit. Let us know your uses for external audio line-in devices in the comments section below.
