[QA] Classroom PA 講堂擴音系統

> 最近我在為我們講堂買一些設備來改善錄音效果,
> 我們需要接多個麥克風(預計4個)作為講課,翻譯,問問題用。
> 我看了一些混音器及麥克風的資料並有下列問題想請教您:

(translation of the above)

I ran into your site when looking for solutions for setting up our auditorium for classes. We need multiple microphones (four) for lecturing, interpretation, and questions. After checking out some documents of mixers and microphones, I have the following questions:

> Subject: 請教錄音問題
> 你好,
> 無意間拜訪了您的網站, 知道您對錄音非常有研究,
> 若蒙不棄想請教您一些問題。最近我在為我們講堂
> 買一些設備來改善錄音效果,
> 我們需要接多個麥克風(預計4個)作為講課,翻譯,問問題用。
> 我看了一些混音器及麥克風的資料並有下列問題想請教您:
>
> 1. Dynamic 和 condenser microphone 哪一個比較適合講課錄音
> 2. Balanced 和 unbalanced microphone 的差別, (看到一個 Mixer
> 文件說只能插 balanced microphone 到XLR孔, Why?)
> 3. 常常麥克風使用時會產生嗡嗡聲, 請問要如何避免
>
> 僅此先致上我誠摯的謝謝
>
> <刪>

您好，

現場人聲的演講和音樂要求不同，您在預算上可以省下一些。建議如下：

1. Dynamic microphones 便宜耐用，很適合您的情形。但建議使用 cardioid，甚至 supercardioid 或 hypercardioid 以減少回授 (feedback) 噪音。

2. 現代的麥克風只要是 XLR 大部份是 balanced microphone。三孔 XLR 有一條接線是用來平衡地線(參考零伏)以減低 ground loop 的問題。如果走線不遠，可以以便宜的 unbalanced microphone 替用 (要另外接個轉接頭)；如果走線稍遠，使用balanced microphone能改善訊號強度與品質；如果再遠一點，最好使用 preamp，把訊號轉成 line level。

另外，如果麥克風得在聽眾手裡傳來傳去，我建議使用無線麥克風。雖然價位稍高，但如果買多頻的接收器，可以一次支援多支麥克風，在總體成本上也許會比佈線成本低 (比方說您必須挖地破牆)。

另外請注意 phantom power 的問題。condenser microphone 收音原理要求給電，但 dynamic 不需要。如果您用 unbalanced microphone 當替代，要注意 mixer 上的接腳相容性與 phantom power 開關。

3. 減少回授問題：

• 喇叭不要正對麥克風；
• 使用 cardioid，supercardioid 或 hypercardioid等麥克風；
• 另外，有一種 feedback destroyer 可以自動找出回授的頻率再以電子方式消去。

在錄音室裡我們用耳機避免回授問題；在現場表演時，改變喇叭擺位常常能解決問題；如果以上不易辦到，再使用 feedback destroyer (大部份時候問題都能解決，不過，偶爾有些音樂訊號會被吃掉)。

璨光

　

以下是上文翻譯

(Translations of the above text)

Question:

I ran into your site when looking for solutions for setting up our auditorium for classes. We need multiple microphones (four) for lecturing, interpretation, and questions. After checking out some documents of mixers and microphones, I have the following questions:

1. Do dynamic microphones or condenser microphones better suit our need?

2. What's the difference between a balanced and an unbalanced microphone? A mix manual says it only supports a balanced microphone via an XLR port. Why?

3. How does one avoid the high frequency screaming noises when using a microphone?

Many thanks.

　

Answer:

The consideration for speech and music is different. Fortunately you can save some of your budget. Here's my suggestion:

1. Dynamic microphones are cheaper and studier, thus recommended. However, I suggest getting those with cardioid, supercardioid, or even hypercardioid polar patterns to minimize feedback problems.

2. Modern microphones with XLR ports are usually balanced. One of the XLR line is to balance the zero volt ground line, thus minimize ground loop humming. If you are running very short cables, it's possible to connect an unbalanced microphone via an adaptor; if you are running slightly longer length, a balanced microphone will greatly improve the signal quality; if you are running long cables, a preamp is highly suggested to level your signal to line standard.

In addition, if some of the microphones are to be passed among the audience, wireless microphones may be a good direction to go. You can get a multi-band or multi-frequency receiver. In sum, you might save more this way than having to dig your floors when cabling.

Also, please take some caution in phantom power switches and line layout. Condenser microphones need the phantom power to operate but not dynamic ones. If you use unbalanced microphones, be sure to check the line layout and the switch status.

3. These are common ways to minimize the feedback problem:

• Avoid pointing the microphones to the speakers;
• Use microphones that support cardioid, supercardioid, or hypercardioid polar patterns;
• Use a device called feedback destroyer (or something alike); it detects the problematic frequency and automatically EQs it out.

In the studio setting, we simply use a pair of headphones to avoid leakage; in a live setting, changing the direction of the speakers or the signal paths usually helps; feedback destroyer is usually my last choice when other means fail (mostly it leaves my music sigal intacted but not always).

Tsan-Kuang