Acoustics & Sound Information

Speech Volume

The average intensity level of a teacher's voice is (65-70 dBA).

How loud should the system be?

	Ask your students or have a peer listen and adjust accordingly.
	Speak with the Mic "Off" and then speak with the Mic "On". Adjust the Speech Volume so there is a noticeable improvement in hearing what is being said.
	The Speech Volume must be comfortable for everyone to listen to. Not too loud and not too soft.
	The Teacher's Voice can be enhanced by 8 to 15 dBA.
	10 db is twice as loud.
	Set your Speech Volume when Students are in your Classroom. Your Room Acoustics change when Students are present.
	Make sure that your amplified classroom audio does not affect the classroom next door.
	Make sure that no one has a headache at the end of the day.

A Higher Teacher Speech Volume makes it Easier to Hear.

Common Sound Levels

Ambient Noise or Background Noise Level

	Background Noise is caused by: heating and ventilation systems, noise from the hallway, and adjacent classrooms.
	Research shows that the average unoccupied noise level in classrooms is 50 dBA.
	The typical occupied classroom noise levels range from (55 - 75 dBA).
	The Background Noise can be louder than the Teacher's Voice.
	Computer rooms range from (73 - 79 dBA)
	The Teacher's Voice is loud at the front of the class but it drops in level from the front to the back of the classroom.
	This results in a higher signal to noise ratio at the front of the class and a lower signal to noise ratio at the back of the class.
	The F-121C Speaker ensures that all of the students have nearly the same signal to noise ratio where ever they are sitting.
	The above effect is referred to as Classroom Sound Equalization. Each student in the classroom has the same Signal to Noise Ratio to listen to the teacher.
	The American Speech-Language-Hearing Association states that the ambient noise should be no louder than (30 - 35 dBA) in an empty classroom.
	A Higher Teacher Message to Background Noise Ratio makes the Teacher's Message Easier to Understand.

Signal to Noise (S/N) Ratio

	Signal to Noise Ratio is the ratio of the Teacher's Speech Level in relation to the Background Noise Level.
	Normally students require a S/N Ratio of at least (15 - 25 dB). The Speech Level should be at least 15 dB higher than the ambient noise in the Classroom.
	Adults working in their mother tongue can get by with less than 6 dB of S/N Ratio where the information is expected and known.
	Children learning another language or adults in ESL (English as a Second Language) need a full 25 dB of S/N Ratio to have a reasonable speech understanding.
	A Higher Signal to Noise Ratio makes the Teacher's Message Easier to Understand.

Signal-to-Noise Ratio & Speech Clarity & Intelligibility

	The Signal-to-Noise Ratio along with the Reverberation Time affect the Speech Clarity & Intelligibility (see the chart below).
	The data below is from the Acoustical Society of America "Classroom Acoustics" Document.

RT (sec)	S/N (dB)	Speech Intelligibility (%)
0.5	+ 10	90 %
0.5	0	55 %
1.5	+ 10	75 %
1.5	0	30 %

Note: Please be aware that Speaker Characteristics also do affect the
Speech Clarity & Intelligibility.

Single Point versus Multiple Point - Sound

It is very important to have a Single Point Sound Source
when you listen to Speech because a Single Point Sound Source Significantly Improves the Clarity and Intelligibility
of the Classroom Audio.

Multiple Sound Sources (multiple speakers in the classroom)
generate sound interference patterns which impair
the Clarity and Intelligibility of the Teacher's Message.

Multiple Sound Sources force you:
to make sense of Multiple Messages from Different Directions.

Why do people use Multiple Speakers in a Classroom
because the Conventional Speakers that they use
Do Not Disperse the Sound Very Well.

	Standard Speakers disperse sound like Flash Lights.
	Wide Dispersion Speakers disperse sound like Light Bulbs.

In trying to fill a Classroom with a Uniform Sound Level by using Multiple Speakers, one problem is solved but another one is introduced.
The Sound Level may be great but now you have Multiple Sound Sources to contend with.

Also these Multiple Conventional Speakers Do Not Disperse
the High Frequency Components very well and significantly
increase the probability of Feedback when the Teacher walks
under the Speaker.

Use only Wide Dispersion Speakers in your Classroom.

Can you imagine what the Sound would sound like if you had Multiple Speakers in a Stadium. Only one central speaker fills the Stadium with Clear and Intelligible Sound.

Commonwealth Stadium - Point Source Sound

For the Best Clarity & Intelligibility:

The Sound Source and the Amplified Sound should emanate from the Same Direction, the Front of the Class.

The Second Best is to use:
One Wide Dispersion Ceiling Speaker.

The Wide Dispersion Ceiling Speaker excels in filling the Classroom with a Uniform Sound Level without
the use of Multiple Ceiling Speakers.

A Single Point Sound Source makes the Teacher's Message
Easier to Listen to.

Flush Mounting Speakers

	Use only Flush Mounting Wall or Ceiling Mounting Speakers because they minimize Wall or Ceiling Reflections.
	Listen to a Flush Mounting Speaker when it is flush with the surface and then move the speaker away from the surface about 6" (to simulate a Box Type of Speaker) and see how the sound degrades as the speaker is moved away from the surface.

Reverberation Time

FM System

	FM stands for Frequency Modulation.
	FM is synonymous with Wireless (in this context).
	FM Systems can be either VHF or UHF Wireless Systems.
	VHF = Very High Frequency
	UHF = Ultra High Frequency

UHF & VHF Wireless Systems

	The UHF & VHF Wireless Systems should have "User Selectable" Channel Frequencies as opposed to "Fixed" Channel Frequencies.
	The UHF & VHF Wireless Systems should have "Two Antennas" to minimize signal drop outs as you walk about the classroom. (Wireless Systems with Diversity)

Boom Microphone

Boom Microphone is synonymous with Headset Microphone.

The Boom Mic dB Advantage

Graph - Key Points

	The output level from the Headset / Boom Mic is about 10 dB higher than the output level from the Lapel Microphone. (This is due to the fact that the Lapel Mic is further away from our mouth than the Headset / Boom Microphone.)
	To get the same Output Level from the Speaker the Amplifier Gain or Volume would have to be set 10 dB higher for the Lapel Microphone than it would for a Headset / Boom Microphone.
	The above additional 10 dB Gain significantly increases the probability of getting Audio Feedback. (the higher the Amplifier Gain, the greater is the probability for feedback)
	The Headset / Boom Mic Graph is nearly identical to the Adult Average Speech Graph except that it is about 25 dB Higher. Basically all the frequencies are reproduced with the same relative intensity.
	The Lapel Mic does not pick up frequencies between 3000 Hz and 6000 Hz as well as the Headset / Boom Microphone. (This is due to the fact that our mouth emits sound more like a Conventional Speaker and not like a Wide Dispersion Speaker.)
	At 4000 Hz, the Headset / Boom Mic has a 10 dB advantage over the Lapel Mic.
	In conclusion, the Headset / Boom Mic delivers a significantly better Audio Signal for Amplification than the Lapel Mic does.

Soundfield FM Systems

The above term is used by some people when they are speaking about Classroom Amplification Systems or Classroom Sound Systems.

A Key Component of any Classroom Amplification System is the Wireless System. The Wireless Headset Microphone (Boom Microphone) along with the Wireless Transmitter and the Wireless Receiver link the Teacher's Voice to the Amplifier and Speaker.

The Wireless System can be a either a VHF or a UHF Frequency Modulated (FM) Wireless System. VHF Wireless Systems currently are being replaced by Cutting Edge UHF Wireless Systems with User Selectable Frequencies and with "True Diversity" so when you walk around the Classroom the probability of having wireless signal drop outs are minimized.
Your best choice is a UHF Wireless FM System.

The Speaker generates a Sound Field which ideally does the following:

	Only one Sound Field is generated. (Only one Speaker is Used) Multiple Speakers generate Interference Patterns.
	The Amplified Sound Field emanates only from One Direction. It is easier to listen to only One Amplified Sound Source than to listen to sound from multiple speakers.
	The One Speaker is a Wide Dispersion Speaker to deliver virtually the same Signal to Noise Ratio for all of the Students.
	The Original Sound Source (the Teacher's Voice) and the Amplified Sound Source emanate from the Same Direction to deliver the Best Clarity & Intelligibility.
	The Sound Field High Frequency Components are Uniformly Dispersed throughout the Classroom via a Wide Dispersion Speaker to minimize the potential for feedback.

The Alphanet Professional - Classroom Amplification Systems address the above issues with the use of High Quality Sound Equipment that has a reputation for High Reliability.

Speech Bandwidth

When we speak we generate frequencies:

from 20 Hz to 20,000 Hz

Telephone Bandwidth

Most of our Speech Energy (Intelligibility) is contained:

	in a 3000 Hz Bandwidth
	from 400 Hz to 3400 Hz

Only the above frequencies are transmitted
by the Telephone Networks in the World.

Request for Comments

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