Fundamentals of Audio and Acoustics – Human Hearing
It is beneficial for sound practitioners to have a basic understanding of the way that people hear and perceive sound. The human auditory system is an amazing device, and it is quite complex.
Its job is to transduce fluctuations in the ambient atmospheric pressure into electrical signals that will be processed by the brain and perceived as sound by the listener. We will look at a few characteristics of the human auditory system that are of significance to audio practitioners.
The dynamic range of a system describes the difference between the highest level that can pass through the system and its noise floor. The threshold of human hearing is about 0.00002Pascals (Pa) at mid frequencies. The human auditory system can withstand peaks of up to 200Pa at these same frequencies. This makes the dynamic range of the human auditory system approximately.
The hearing system can not take much exposure at this level before damage occurs. Speech systems are often designed for 80dB ref. 20μPa and music systems about 90dB ref. 20μPa for the mid-range part of the spectrum. Normal conversation is about 60dB ref. 20 μPa, which is a good target for the playback level of a speech system if it produces adequate signal-to-noise ratio.
Audio practitioners give much attention to achieving a flat spectral response. The human auditory system is not flat and its response varies with level. At low levels, its sensitivity to low frequencies is much less than its sensitivity to mid-frequencies. As level increases, the difference between low- and mid-frequency sensitivity is less, producing a more uniform spectral response. The classic equal loudness contours, Image 1 below, describe this phenomenon and have given us the weighting curves, Image 2 below, used to measure sound levels and represent them numerically in a way that correlates with human perception.
Modern sound systems are capable of producing very high sound pressure levels over large distances. Care must be taken to avoid damaging the hearing of the audience.
The time response of the hearing system is slow compared to the number of audible events that can occur in a given time span. As such, our hearing system integrates closely spaced sound arrivals (<50 ms) with regard to level. This is what makes sound indoors appear louder than sound outdoors. While reflected sound increases the perceived level of a sound source, it also adds colorations. This is the heart of how we perceive acoustic instruments and auditoriums. A good recording studio or concert hall produces a musically pleasing reflected sound field to a listener position. In general, secondary energy arrivals pose problems if they arrive earlier than 10ms (severe tonal coloration) after the first arrival or later than 50ms (potential echo), Image 3.
Image 3. The time offset between sound arrivals will determine if the secondary arrival is useful or harmful in conveying information to the listener. The direction of arrival is also important and is considered by acousticians when designing auditoriums. Courtesy SynAudCon.
The integration properties of the hearing system make it less sensitive to impulsive sound events with regard to level. Peaks in audio program material are often 20dB or more higher in level than the perceived loudness of the signal. Program material that measures 90dB A-weighted (slow response) may contain short term events at 110dB A-weighted or more, so care must be taken when exposing musicians and audiences to high powered sound systems.
The eardrum is a pressure sensitive diaphragm that responds to fluctuations in the ambient atmospheric pressure. Like a loudspeaker or microphone, it can be overdriven and damaged. The Occupational Safety and Health Administration (OSHA) is responsible for assuring that the workplace is in compliance regarding sound exposure. Sound systems are a major source of high level sounds and should work within OSHA guidelines. Tinnitus, or ringing in the ears, is one symptom of excessive sound exposure.
Excerpt from Handbook for Sound Engineers, 5th Edition edited by Glen Ballou © 2015 Taylor & Francis Group. All Rights Reserved.
About the Author
Glen Ballou is a graduate of General Motors Institute, now Kettering University, with a bachelor’s degree in Industrial Engineering and a minor in Electrical Engineering. He has been a Syn-Aud-Con representative, has served as governor, convention chairman, papers chairman, and facilities chairman of the Audio Engineering Society (AES), and has been a member for the Society of Motion Picture and Television Engineers (SMPTE). He has been a contributor to S&VC, Sound and Communications, and Church Production magazine on a variety of subjects. Glen also wrote the chapter on capacitors and inductors for the CRC Press publication The Electrical Engineering Handbook. Glen is owner of Innovative Communications, a company that specializes in room acoustics and sound system design.