Question
I am working with a 3-year old with a hearing loss of 40 dB at 250, 65 dB at 500 and the rest of the frequencies (750 - 6000 Hz) are at 100 dB and NR. She is wearing a power digital hearing aid. To meet 1 and 2kHz DSL targets, 250 and 500 are overamplified by about 10 dB. Her behavioral aided scores are 20 at 500, 30 at 1kHz, 30 at 2kHz and 60 dB at 3kHz. Another audiologist recommended that I reduce the low frequency gain to guard against 'upward spread of masking'. But when I reduce the lows, her functional aided thresholds drop. My question is, what is an example of an audiogram that would alert me to the dilemma of upward spread of masking? I didn't think that it would be an issue with a child with a predominantly severe to profound hearing loss."
Answer
Regardless of the degree of hearing loss, upward spread of masking is always a possibility but is not easily predicted from aided or unaided threshold measures. Of course, the only direct way to know if upward spread of masking is a problem for a given listener is to perform sufficiently sensitive speech perception measures under different frequency response conditions. This is exactly what Margo Skinner demonstrated now, almost 20 years ago, in her text Hearing Aid Evaluation (Skinner, 1988). Through a series of speech perception measures carried out under a range of filtering conditions, she was able to demonstrate that when the low frequency output of the speech stimuli, in dB SPL, exceeded the high frequency output, performance scores decreased. Within this section of the text (pp.27-30), she discussed the importance of achieving an 'appropriate balance' between the low and high frequency amplified speech energy for individuals with hearing loss. It is unlikely that this 3-year old will happily participate in the extensive speech perception testing that Dr. Skinner performed with her research subjects. However, something can be learned from this work and applied in our clinical work in 2006.
We can now use electroacoustic measures to know something about the 'balance' between high and low frequency amplified speech. Specifically, it is now possible, using modern hearing instrument test systems, to carefully study the relationships among the child's unaided thresholds, in ear canal SPL, and the output of hearing instruments in response to real speech inputs ranging from very low to very high speech input levels (i.e., using an SPLogram format). In this way, we can make predictions regarding the audibility of a range of speech input levels and we can also study the relationship between amplified speech output in the low frequencies to the amplified speech output in the high frequency region. As a general rule (derived from Dr. Skinner's exquisite studies), the low frequency speech output of the hearing instrument should be less than or equal to (but not exceed) the high frequency output. In this way, you can ensure that you have achieved an 'appropriate balance' between the high and low frequency components of amplified speech.
This approach has offers several advantages to one that uses aided threshold measures to predict how a hearing instrument might operate in response to real speech. Several advantages are as follows:
- Electroacoustic measures that are performed in a test chamber are less variable relative to aided threshold measures.
- Electroacoustic measures provide a description of hearing instrument performance for the complete range of frequencies (at multiple input levels) and not at a very few discrete frequencies as measured in aided threshold testing.
- Electroacoustic measures of hearing instrument performance can now be performed directly with the signal of primary interest (i.e., speech). Why is this important? It is important to keep in mind that aided sound field thresholds are performed with relatively low-level narrow band test signals. Thus, aided threshold will tell us something about the lowest level of sound that a child can detect in the aided condition as a function of frequency. However, because we are now for the most part using multi-channel compression technology, the aided threshold obtained to low-level narrow band test signals is of very limited utility in predicting how the hearing instrument will operate in response to real speech inputs. Both gain and frequency response characteristics will likely change as the type and level of the input signal changes.
Reference
Skinner M.W. (1988) Hearing Aid Evaluation. Englewood Cliffs NJ: Prentice Hall.
Dr. Richard Seewald holds a Canada Research Chair in Childhood Hearing at the National Centre for Audiology in London, Ontario, Canada. He is also a Professor in the School of Communication Sciences and Disorders, Faculty of Health Sciences, University of Western Ontario. For the past 25 years, Dr. Seewald's work has been focused on issues that pertain to the selection and fitting of amplification in infants and young children and is known internationally for his work in developing the Desired Sensation Level (DSL) Method for pediatric hearing instrument fitting.