Why smartphones need external microphones to improve the sound quality of voice recordings in lecture halls or school classrooms

There are two microphones on most Android devices and three on the iPhone. On the Android device there is one at the top edge of the phone and one on the bottom edge next to the charging port. On an iPhone there are three microphones, on the front panel at the top, one on the rear of the phone at the top and one on the bottom edge. These microphones are omni-directional microphones and pick up sound from all around the device. If you are trying to record the teacher or lecturer there will be a number of issues with just using the smartphones own microphone.

   1. Distance from the speaker- sound intensity drops according to the inverse square law- if a phone is twice as far away from the speaker as another phone, the sound intensity at the second phone is one quarter of the first phone.

   2. Poor room acoustics will distort any sound received at the phone’s microphone. Sound from the speaker will bounce off the walls, ceiling and floors and arrive a fraction of a second later than sound received directly from the speaker. It has had further to travel and thus arrives later at the phone’s microphone. So instead of getting a clear sound, there will be a spectrum of sound arriving at the microphone making the recording distorted.

   3. Noise between the speaker and the microphone will be picked up by the phone’s microphone. So the more people between the phone and the speaker the more noise will be picked up by the phone. Because the interfering sound is closer to the phone than the speaker it will be competing strongly in terms of volume as a result of the effect of the inverse square law.

    4. Noise to the side of the phone and behind the phone will also be picked up creating more interference.

To reduce the effect of this interference there are two solutions. 

   1. Using a directional microphone can greatly help the sound quality as it can be pointed at the speaker focussing on the direction from which the speaker’s voice is coming from. This will eliminate interference from the side and rear of the phone but will still be subject to noise between the speaker and the phone, and also to the effect of room acoustics. Directional microphones are relatively inexpensive so they are a cost effective solution which will improve the audio quality in a lecture hall or school classroom.

    2. Using a wireless microphone. A wireless microphone comprises a transmitter microphone and a receiver. The transmitter microphone can be placed next to or worn by the speaker and the receiver plugged into the phone using a special connecting cable. This largely overcomes the issues of distance between speaker and phone, the effect of poor room acoustics as the sound has only a short distance to travel before it reaches the wireless microphone, and interference from noise around the phone and between the phone and the speaker is eliminated. They are however, a lot more expensive than the standard directional microphone.

To connect a standard directional or wireless microphone to a smartphone requires an additional cable as the standard stereo plug supplied with most microphones does not work. The smartphone has a 4 pin connection which enables users to connect both audio out to a headset or earphones and a microphone input. However, a 4 pin or TRRS plug is required to make the connection. Conversor manufacture a range of cables and connectors which enable the connection of a range of directional and wireless microphones to a smartphone.

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