It is a common misnomer that adding audio to a room will make others in the room hear “better”. While the sound pressure level (SPL) is increased, it does not necessarily make the audio more intelligible especially the human voice and a live microphone. In the past few years, companies have invested heavily in research and development to make the learning environment audibly better. Often times, the nemesis of an intelligible environment are feedback and reverberation. I want to touch on a few items of focus that will make the audio learning experience more enjoyable and give you a few challenges/techniques for improving intelligibility within your room.
- First, here are a few important facts about human speech that are important to know when putting live sound in a room.
- Frequency range of an adult is between 512 Mhz to 2.48 kHz. Obviously, the frequency range will change depending on gender.
- Human speech is a modulated signal with a fundamental frequency in the range of 100 to 400 Hz.
- Intelligibility is imparted by consonants, which average from 10 to 100 ms in duration, and may be as much as 27 dB lower in amplitude than vowels
- The loudness of an adult human voice is about 70 dB SPL at 3 feet.
As any sound engineer will attest, feedback is one of the greatest deterrents to intelligible audio. Feedback is when an audio loop exists between the microphone and loudspeaker. When the loop occurs, you will hear a screeching noise through the amplified speaker system. The noise could occur at almost any frequency, but with a human voice it will most likely occur in the frequency range of the human voice (see above). There are many ways to eliminate feedback, but the most common would be to “notch out” the bad frequency by using and equalizer. When a frequency is notched out its level is decreased so that the feedback will not happen. When using an amplified sound system in a room, fighting feedback without out an equalizer, will force the user to decrease the audio level of the room.
The term “reverberation” refers to sound that is continuously reflected within a large space for an extended period of time. Reverberation time greater than 0.60 seconds is typically too long for a learning environment and will lead to muddled syllables and unclear speech. This problem is common in large rooms with tall ceiling heights (greater than 10ft), and many hard surfaces on walls, floor, and ceiling. Another negative effect of excess reverberation is that it amplifies unwanted noise from intrusive sources. This is why underground concrete subway stations are so much louder than elevated outdoor stops for light rails systems.
Eric Wolfram is the Lead Acoustical Engineer and noise control consultant for Riedel & Associates, an acoustical consulting firm based in Milwaukee Wisconsin. In a 2009 article he shares some excellent points regarding reverberation and why the classroom acoustical environment is critical to student learning and academic performance.
Though it may be difficult to solve all feedback and reverberation issues, there are ways to improve a room’s intelligibility. If you application is having complaints about poor intelligibility, refer to this table for a possible solution (ProAV Magazine, August 2005)
|Primary Causes||What you can do about it|
|Poor Coverage||Check equipment. Use the right loudspeakers for the job|
|Direct-to-reverberant ratio||Point the loudspeakers at the people. Use directional speakers in reverberant spaces. Keep sound off the walls and ceiling. Minimize the loudspeaker-listener distance.|
|Limited sound-system frequency response||Check equipment. Use a good mic and loudspeakers with good bandwidth. Check that they are not covered or obstructed.|
|Low signal-to-noise ratio||Minimize the loudspeaker-listener distance.|
|Excessive loudness||Reduce audio level.|
|Secondary Causes||What you can do about it|
|System distortion||Check equipment, components, and/or gain structure.|
|System Equalization||Be sure the system is capable of reasonably flat response from 200 Hz to 4 kHz|
|Presence of very early reflections||Point the loudspeakers at the people. Use absorbing (or diffusing) materials. Use directional speakers and microphones.|
|Sound focusing or presence of late (or isolated higher-level) reflections||Beware of any curved surface. Treat with absorption/diffusion and use directional speakers. Late sound focusing can also be caused by room corners. Point the loudspeakers at the people.|
|Direction of sound arriving at listener||Ideally, sound should arrive “face on” to the listener. If that’s not feasible, it should arrive from overhead and forward overhead. Avoid aiming sound at the listener from behind.|
|Talker microphone technique||Basic instruction on mic usage. Get the mic as close to the talker’s mouth as possible. Point the mic at the talker (and get him or her to talk into the microphone).|