BiquadFilterNode
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`BiquadFilterNode` 接口表示一个简单低阶滤波器（双二阶滤波器），通过 `AudioContext.createBiquadFilter()` 方法创建。它是一个能表示不同类型的过滤器，声调控制设备，图形均衡器的 `AudioNode`。一个`BiquadFilterNode`（双二阶滤波器）总是恰好有一个输入和一个输出。

|  |  |
| --- | --- |
| Number of inputs | `1` |
| Number of outputs | `1` |
| Channel count mode | `"max"` |
| Channel count | `2` (not used in the default count mode) |
| Channel interpretation | `"speakers"` |

属性
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*继承属性自父级的* *`AudioNode`*.

`BiquadFilterNode.frequency` (en-US)

Is an a-rate `AudioParam`, a double representing a frequency in the current filtering algorithm measured in hertz (Hz).

`BiquadFilterNode.detune` (en-US)

Is an a-rate `AudioParam` representing detuning of the frequency in cents.

`BiquadFilterNode.Q` (en-US)

Is an a-rate `AudioParam`, a double representing a Q factor, or *quality factor*.

`BiquadFilterNode.gain` (en-US) 只读 

Is an a-rate `AudioParam`, a double representing the gain used in the current filtering algorithm.

`BiquadFilterNode.type` (en-US)

节点实现定义不同过滤算法的一个字符串值。

| `type` | Description | `frequency` | `Q` | `gain` |
| --- | --- | --- | --- | --- |
| `lowpass` | Standard second-order resonant lowpass filter with 12dB/octave rolloff. Frequencies below the cutoff pass through; frequencies above it are attenuated. | The cutoff frequency. | Indicates how peaked the frequency is around the cutoff. The greater the value is, the greater is the peak. | *Not used* |
| `highpass` | Standard second-order resonant highpass filter with 12dB/octave rolloff. Frequencies below the cutoff are attenuated; frequencies above it pass through. | The cutoff frequency. | Indicates how peaked the frequency is around the cutoff. The greater the value, the greater the peak. | *Not used* |
| `bandpass` | Standard second-order bandpass filter. Frequencies outside the given range of frequencies are attenuated; the frequencies inside it pass through. | The center of the range of frequencies. | Controls the width of the frequency band. The greater the `Q` value, the smaller the frequency band. | *Not used* |
| `lowshelf` | Standard second-order lowshelf filer. Frequencies lower than the frequency get a boost, or an attenuation; frequencies over it are unchanged. | The upper limit of the frequencies getting a boost or an attenuation. | *Not used* | The boost, in dB, to be applied; if negative, it will be an attenuation. |
| `highshelf` | Standard second-order highshelf filer. Frequencies higher than the frequency get a boost or an attenuation; frequencies lower than it are unchanged. | The lower limit of the frequencies getting a boost or an attenuation. | *Not used* | The boost, in dB, to be applied; if negative, it will be an attenuation. |
| `peaking` | Frequencies inside the range get a boost or an attenuation; frequencies outside it are unchanged. | The middle of the frequency range getting a boost or an attenuation. | Controls the width of the frequency band. The greater the `Q` value, the smaller the frequency band. | The boost, in dB, to be applied; if negative, it will be an attenuation. |
| `notch` | Standard notch filter, also called a *band-stop* or *band-rejection* filter. It is the opposite of a bandpass filter: frequencies outside the give range of frequencies pass through; frequencies inside it are attenuated. | The center of the range of frequencies. | Controls the width of the frequency band. The greater the `Q` value, the smaller the frequency band. | *Not used* |
| `allpass` | Standard second-order allpass filter. It lets all frequencies through, but changes the phase-relationship between the various frequencies. | The frequency with the maximal group delay, that is, the frequency where the center of the phase transition occurs. | Controls how sharp the transition is at the medium frequency. The larger this parameter is, the sharper and larger the transition will be. | *Not used* |

方法
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*继承方法自父级的 `AudioNode`*.

`BiquadFilterNode.getFrequencyResponse()` (en-US)

From the current filter parameter settings this method calculates the frequency response for frequencies specified in the provided array of frequencies.

示例
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参见 `AudioContext.createBiquadFilter` 以获取示例代码，其中展示了如何使用 `AudioContext` 创建一个 Biquad 滤波器节点。

规范
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| Specification |
| --- |
| Web Audio API # BiquadFilterNode |

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参见
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* Using the Web Audio API (en-US)