This is the Review Notes in Acoustics as one topic in ECE Board Exam taken from various sources including but not limited to past Board Examination Questions in Electronic System and Technologies (EST), Communications Books, Journals and other Communications References. This particular Coaching Notes in Communications Engineering has random Questions and Answers in random topics. Make sure to familiarize this review notes to increase the chance of passing the ECE Board Exam.
Summary of Important Acoustics Review Notes
Sound level meter – an instrument designed to measure a frequency-weighted value of the sound pressure level
Noy – a unit of noisiness related to the perceived noise level
Noise rating curves – an agreed set of empirical curves relating octave-band sound pressure level to the center frequency of the octave bands.
Natural frequency – the frequency of a free vibration
Flanking transmission – the transmission of sound from one room to an adjacent room, via common walls, floors or ceilings
Hearing level – a measure of threshold of hearing, expressed in decibels relative to a specified standard of normal hearing
330 m/s – velocity of sound in air
Speaker is a device that converts current variations into sound waves
Carbon type of microphone operates on the principle that the electrical resistance of carbon granules varies as the pressure on the granules vary
Bass response is bypassing high audio frequencies
Pure tone of sound used as standard on testing is 1000 Hz
Echo is early reflection of sound
Dolby – noise reduction system used for film sound in movie
Sound intensity is the sound energy per unit area at right angles to the propagation direction per unit time
Phon is the unit of loudness level of a sound
Sound intensity is the average rate of transmission of sound energy in a given direction through a cross-section area of 1 sq. m. at right angles to the direction
Mel is the unit of pitch
Decibel – a measure of the intensity of sound in comparison to another sound intensity
Sound wave has two main characteristics which are pitch and loudness
Dynamic type of microphone operated by electromagnetic induction that generates an output signal voltage
Supersonic – speed that is faster than speed of sound
Sound waves travel in water at a 5000 ft/sec speed
Wavelength – crest-to-crest distance along the direction of wave travel
Sound intensity level is 10 log I/Iref
Sound pressure level is 20 log P/Pref
The most important specification of loudspeakers and microphones is frequency response
Fundamental – lowest frequency produced by a musical instrument
Diffraction – tendency of a sound energy to spread
When waves bend away from straight lines of travel, it is called refraction
Reverberation time – required time for any sound to decay to 60 dB
The intensity needed to produce an audible sound varies with frequency
Ultrasonic – sound that vibrates at frequency too high for the human ear to hear (over 20 kHz)
Crystal microphone will be damaged if exposed to high temperature above 52°C
Spider – a thin springy sheet of bakelite or metal that permits the voice coil in a dynamic loudspeaker to move back and forth along the core of its magnet
One hundred twenty μbars of pressure variation is equal to 115.56 dBSPL
Proximity effect is a microphone characteristic that results in a boost in bass frequencies for close microphone spacing
20 Hz to 20 kHz is the audio frequency range
10 Hz to 20 kHz is the bass frequency range
5,000 Hz to 10 kHz – high frequency range of audio signals
65 is the dB SPL of a voice paging in an office
90 – 100 is the dBdb SPL of an auditorium with contemporary music
80 – 85 is the church db SPL with speech reinforcement only
Intensity can also be called as loudness
The loudness of a sound depends upon the energy of motion imparted to vibrating molecules of the medium transmitting the sound
Loudness is affected by the distance between the listener and the source of the sound and its intensity varies inversely with the square of this distance
If the distance between the listener and the source of the sound is doubled, the intensity is reduced to ¼
If the distance between the listener and the source of the sound is decreased to ½ the original amount, the intensity of the sound would be 4 times as great
At a sensation level of 40dB, 1000 Hz tone is 1000 mels
If the sound waves are converted to electrical waves by a microphone, the frequency of the electric current will be 25 to 8000 Hz
For a music lover concert “A” is 440 Hz. If a musical note one octave higher 0were played, it would be double that frequency
In a 220 Hz, if a note was played one octave lower it would be 110 Hz
Much of music generally referred to in octaves
Distortion is an undesired change in wave form as the signal passes through a device
Distortion enhances intelligibility when an exciter is added
Exciters – a class of signal processors
Hall construction and internal finishes affect the final sound quality significantly
Positioning a loudspeaker near a wall can dramatically alter its frequency response in two distinct ways namely hump and notch
The acoustics of most auditoria are very different when the room is full compared to the empty condition
A microphone transducer converts acoustical energy
All microphone have two basic components namely, diaphragm and generating element
When the average absorption is greater than 0.2, Norris-Eyring formula is used to compute the actual reverberation time
At room temperature, the velocity of sound in meters/seconds is 341.8 m/s
The ratio of frequencies is termed as interval
6 dB is the increase in sound pressure level in dB, if the pressure is doubled
Pitch – a term which is subjective but dependent mainly on frequency and also affected by intensity
Masking – an effect that occurs in the ear where a louder sound can reduce or even stop the nerve voltage generated by a weaker sound
For computation of ideal reverberation time, Stephen and Bate is applicable
The loudness of sound is a subjective effect which is a function of the ear and brain
Reverberation time – defined as the time taken for the intensity of sound energy in the room to drop to one millionth of its initial value
Sound intensity is the sound energy per unit area at right angles of the propagation direction, per unit time
Phon – the unit of loudness level
Flutter echo – consists of a rapid succession of noticeable echoes
W.C Sabine – laid the foundations of acoustic theory of buildings
Sound – an aural sensation by pressure variation in the air which are always produced by some source of vibrations
10-12 W/m2 is considered to be as the threshold of hearing
The average absorption for a person is 4.7 units
Frequency – the number of vibration or pressure fluctuations per second
Sound intensity – defined as the average rate of transmission of sound energy in a given direction through a cross-sectional area of 1 m^2 at right angles to the direction
The minimum sound intensity that can be heard is termed as threshold of hearing
Mel – the unit of pitch
3 dB is the increase in sound pressure level in dB, if the intensity is doubled
The velocity of sound is considered to be constant at 330 m/s for the purpose of building acoustics
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