The Compression Handbook Fourth Edition An overview of the characteristics and applications of compression amplification Table of Contents Chapter 1: Understanding Hearing Loss .................................................3 Essential Terminology ............................................................................4 Chapter 2: Basics of Compression ........................................................10 Characteristics of a Compressor .........................................................12 Visualizing Compression ......................................................................20 Chapter 3: Applications of Compression ..............................................21 Avoiding Distortion, Discomfort and Damage .....................................21 Optimizing Use of the Residual Dynamic Range and Restoring Normal Loudness Perception ......................................22 Maintaining Listening Comfort .............................................................24 Maximizing Speech Intelligibility ..........................................................25 Chapter 4: Fitting Compression ............................................................27 Factors to Consider ...............................................................................27 Prescribing Amplification .....................................................................29 Chapter 5: Expansion ............................................................................31 Issues Resulting from WDRC ...............................................................31 What is Expansion? ...............................................................................31 Characterizing Expansion .....................................................................33 Measuring Expansion ...........................................................................35 Chapter 6: Troubleshooting & Fine-Tuning ...........................................37 A Systematic Approach .........................................................................37 Understanding the Compression Architecture ...................................38 Common Complaints and Solutions ....................................................38 Recommended Reading ......................................................................... 41 References .............................................................................................42 2 Chapter 1: Understanding Hearing Loss According to the World Health Organization (WHO), For an individual with normal hearing, average there are 360 million persons in the world with conversational speech falls approximately midway disabling hearing loss (WHO, 2012). At the present within the dynamic range of hearing and coincides time, hearing aids are the most common first step with the most comfortable loudness level (MCL). in [re]habilitation. On the surface, this seems fairly straightforward because much is known about The most common complaint associated with the physiology of the auditory system and the hearing loss is the inability to hear; specifically, psychoacoustics of perception. However, hearing loss soft sounds. Figure 1-1B depicts a person with impacts an individual in numerous ways, making sensorineural hearing loss. Once again the white the fitting of a hearing aid a complex process. bar is the range of sounds in the environment, while the blue bar represents the individual’s The intact auditory system is capable of perceiving a dynamic range of hearing. You will notice three wide range of sounds, from the quiet pitter-patter of things immediately. First, average conversational rain to the loud boom of explosives. In Figure 1-1A, speech is now barely audible to the individual. the white bar represents the entire range of Second, soft sounds (for example, gentle rain) are sounds, from extremely soft to extremely intense, below the threshold of hearing and, therefore, that may occur in an individual’s environment. The too soft to be heard. And, finally, intense sounds softest sounds that are audible lie at the threshold (for example, the boom of an explosion) are still of hearing sensitivity. At the opposite end is the perceived as being loud. As a result of the threshold loudness discomfort level, representing the most increasing and the LDL remaining the same, the intense sounds that can be tolerated without pain. dynamic range of hearing is considerably reduced In between these two extremes is the dynamic compared to that of a person with normal hearing. range of hearing (shown by the blue bar). Intense Too Loud Too Loud Sounds Moderate Sounds Soft Too Soft Too Soft Sounds A B Range of Environmental Sounds Dynamic Range of Hearing Figure 1-1 The relationship between the range of sounds in the environment and the dynamic range of hearing for persons with: (A) normal hearing, and (B) sensorineural hearing loss. Figure 1.1 The relationship between the range of sounds in the environment and the dynamic range of hearing for persons with: (A) normal hearing, and (B) sensorineural hearing loss 3 Human communication is arguably the single This handbook is designed to provide the reader most important function of the auditory system. with a working knowledge of compression Indeed, reduced ability to hear speech is a amplification: what it is, how it works, and how major reason for seeking help. In addition to it is applied. Also included is some discussion a reduced dynamic range, the communication on the principles of fitting compression difficulties of a person with hearing impairment systems, as well as troubleshooting problems. are further complicated by the dynamic nature of speech itself. As shown in Figure 1-2, average conversational speech spans a range Essential Terminology of 30dB (Beranek, 1947). Note that, in general, Before launching into the detailed workings of vowel sounds (for example /a/, /u/, and /i/) are a compression circuit, it is important to have low-pitched, relatively intense, and primarily some general knowledge of amplification. responsible for making speech audible. On the Despite the variety available, all hearing aids other hand, consonants (especially unvoiced have some of the same basic components: sounds such as /th/, /f/ and /s/) are high- a microphone, an amplifier, a receiver, pitched, relatively soft and carry most of the and a battery (Figure 1-3). The microphone information that aids in speech understanding. picks up the incoming acoustic signal and converts it to an electrical signal. The amplifier Frequency [Hz] then magnifies the electrical signal. Like a 125 250 500 1K 2K 4K 8K loudspeaker, the receiver converts the amplified 0 electrical signal back into an acoustic signal that is delivered to the ear. Finally, the battery 20 f th z v s provides the power for the circuit. p k j m d b i a h g 40 n ng o r ch e l It is essential that the reader understand the u 60 following terminology that relates to the signal entering the hearing aid, the amplification, and 80 the sound that is delivered to the ear. 100 ANSI 120 Hearing Level in decibels (dB) re: ANSI 1996 American National Standards Institute has created ANSI 3.22. The ANSI S3.221 is the Figure 1-2Figure 1.2 Frequency and level of various speech sounds hearing aid standard that has been the defining atFrequency average conversational and level levels. of various speech sounds at average conversational levels document for hearing instrument performance parameters in the United States since 1977. It defines the terminology around hearing aids In light of these considerations, the hearing and how manufacturers test the hearing aids. healthcare professional is faced with squeezing an elephant into a suitcase when fitting hearing Input aids – soft sounds must be made audible Input refers to the acoustic signal entering the without loud sounds becoming uncomfortable, hearing aid. Specifically, the American National and speech should remain at a comfortable Standards Institute (ANSI, 2014) defines input level. Compression amplification is a means level as the sound pressure level (SPL) at the for fitting the world of sound (the elephant) into microphone opening of a hearing aid. Input the narrow dynamic range of the individual with level is expressed in dBSPL. hearing impairment (suitcase). 4 Input Amplifier Output Microphone Receiver Battery Figure 1-3 FigureBasic components 1.3 of a hearing aid: microphone, amplifier, receiver and battery. Basics components of a hearing aid: microphone, amplifier, receiver, and battery Output Gain Output refers to the amplified signal that Gain refers to the amount of amplification is delivered to the ear. The output level is applied to the input signal. Specifically, expressed in dBSPL. ANSI (2014) defines gain as the difference between the output SPL in a coupler and Input/Output Function the input SPL. Gain is expressed in dB. The An input/output (I/O) function is a graphical mathematical relationship between input, gain representation of the output of a hearing aid and output is given by the simple formula: at various input levels. According to ANSI (2014) the I/O graph has the output SPL on the Gain = Output – Input ordinate (Y axis) with the input on the abscissa (X axis). The scales for both axis should be linear and of equal spacing. I/O curves are run at individual frequencies. Figure 1-4 shows a sample I/O function of a hearing aid. In this example, an input of 50 dBSPL results in an output of 80 dBSPL, while an input of 90 dBSPL results in an output of 110 dBSPL. It can also be seen