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Acoustics & Sound Information






Speech Volume

bulletThe average intensity level of a teacher's voice is (65-70 dBA).
bulletHow loud should the system be?
bulletAsk your students or have a peer listen and adjust accordingly.
bulletSpeak 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. 
bulletThe Speech Volume must be comfortable for everyone 
to listen to. Not too loud and not too soft.
bulletThe Teacher's Voice can be enhanced by 8 to 15 dBA.
bullet10 db is twice as loud.
bulletSet your Speech Volume when Students are in your Classroom. 
Your Room Acoustics change when Students are present.
bulletMake sure that your amplified classroom audio does not affect the classroom next door.
bulletMake sure that no one has a headache at the end of the day.
bulletA Higher Teacher Speech Volume makes it Easier to Hear.

Common Sound Levels

Ambient Noise or Background Noise Level

bulletBackground Noise is caused by: heating and ventilation systems, noise from the hallway, and adjacent classrooms.
bulletResearch shows that the average unoccupied noise level in classrooms is 50 dBA.
bulletThe typical occupied classroom noise levels range from (55 - 75 dBA).
bulletThe Background Noise can be louder than the Teacher's Voice.
bulletComputer rooms range from (73 - 79 dBA)
bulletThe 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.
bulletThis 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.
bulletThe F-121C Speaker ensures that all of the students have nearly 
the same signal to noise ratio where ever they are sitting.
bulletThe 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.
bulletThe American Speech-Language-Hearing Association states 
that the ambient noise should be no louder than (30 - 35 dBA) 
in an empty classroom.
bulletA Higher Teacher Message to Background Noise Ratio 
makes the Teacher's Message Easier to Understand.

Signal to Noise (S/N) Ratio

bulletSignal to Noise Ratio is the ratio of the Teacher's Speech Level in relation to the Background Noise Level.
bulletNormally 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.
bulletAdults working in their mother tongue can get by with less than 
6 dB of S/N Ratio where the information is expected and known.
bulletChildren 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.
bulletA Higher Signal to Noise Ratio makes the Teacher's Message
Easier to Understand.

Signal-to-Noise Ratio & Speech Clarity & Intelligibility

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

RT (sec)

S/N (dB)

Speech Intelligibility (%)


+ 10

90 %



55 %


+ 10

75 %



30 %

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

Single Point versus Multiple Point - Sound

bulletIt 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.
bulletMultiple Sound Sources (multiple speakers in the classroom) 
generate sound interference patterns which impair 
the Clarity and Intelligibility of the Teacher's Message.
bulletMultiple Sound Sources force you: 
to make sense of Multiple Messages from Different Directions.
bulletWhy 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

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

UHF & VHF Wireless Systems

bulletThe UHF & VHF Wireless Systems should have "User Selectable" 
Channel Frequencies as opposed to "Fixed" Channel Frequencies.
bulletThe 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

bulletBoom Microphone is synonymous with Headset Microphone.

The Boom Mic dB Advantage

Graph - Key Points

bulletThe 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.)
bulletTo 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.
bulletThe 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)
bulletThe 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.
bulletThe 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.)
bullet At 4000 Hz, the Headset / Boom Mic has a 10 dB advantage 
over the Lapel Mic.
bulletIn 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:

bulletOnly one Sound Field is generated. (Only one Speaker is Used)
Multiple Speakers generate Interference Patterns.
bulletThe 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.
bulletThe One Speaker is a Wide Dispersion Speaker to deliver virtually the same Signal to Noise Ratio for all of the Students.
bulletThe Original Sound Source (the Teacher's Voice) and the Amplified Sound Source emanate from the Same Direction to deliver the Best Clarity & Intelligibility.
bulletThe 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

bulletWhen we speak we generate frequencies:
bulletfrom 20 Hz to 20,000 Hz

Telephone Bandwidth

bulletMost of our Speech Energy (Intelligibility) is contained: 
bulletin a 3000 Hz Bandwidth
bulletfrom 400 Hz to 3400 Hz
bulletOnly the above frequencies are transmitted 
by the Telephone Networks in the World.

Request for Comments

bulletWhat additional Acoustic or Sound Information would you like to see added to this website?
bulletPlease send your comments to: webmaster@alphanet.ca


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Copyright 2001 & 2002 Alphanet Systems Inc.
Last modified: September 02, 2002