Total harmonic distortion in music and taking HA’s out to hear it all

A man was telling his neighbor, “I just bought a new hearing aid. It cost me $4,000, but it’s state of the art. All my hearing problems are over.”

“That’s great,” answered the neighbor. “What kind is it?”

“Twelve-thirty.”

Hearing aids are most often problematic.

When I want to truly hear a good piece of music, I must take the hearing aids out of my ears. Only when the music reaches my ears directly, have I really heard the music in all its fullness. Why?

Lots of reasons, but lets focus on total harmonic distortion. Its a shortcut to describing the sound quality from equipment. As a percentage, I think it can be thought as a measurement of “scrambled” tones. Anything over 3% is obvious. I (used to) window shop a lot for stereos and I learned what distortion was from reading many many datasheets and from hands on with the hi-fi equipment.

In the days of tube amplifiers, 0.5% total harmonic distortion (THD) was a common number to advertise. Later, as transistors and semiconductors replaced those , 1/10 that much distortion, or 0.05% became a common standard.

Hearing aid manufacturers have a hard time keeping THD below 5%. They think they are doing good if they can hit that mark.

I would not excuse the manufacturers because the devices are small. This is 2009, for goodness sake!

Even in 1982, the Sony Walkman, had a 0.05% rating! (didn’t it?) I remember comparing my hearing aids to the Walkman, and there was just absolutely no comparison, the Walkman blew the doors off anything else I put in my ears.

Heres what came up at the top of a Google search on total harmonic distortion in hearing aids.

This study examined the harmonic distortion levels of 1170 hearing aids submitted to the Veterans Administration for contract evaluation during the last six fiscal years. Measurements were made, with a 75 dB SPL input, at 500, 700, and 900 Hz and at the frequency at which maximum distortion was found. The gain of each aid was set with a 62.5 dB SPL input to that point at which distortion at all amplified frequencies was less than 10%. Based upon average gain and average saturation sound pressure level, each aid was placed in one of three power categories. The results show that the levels of maximum distortion appear to be inversely proportional to the rated power of the aids. Also, at all frequencies, the levels of harmonic distortion found in aids during the last two years generally equaled or exceeded those found in 1962, the first year for this study.
Roger N. Kasten, Stephen H. Lotterman. 1967 article found for sale on the ASHA web site (Journal of Speech and Hearing Research) which I do not endorse.

Does not seem so wonderful, to find the highest result on Google is a 42 year old article?

Another Google result is this 1982 commentary from a researcher , where it appears that between 32% and 43% of the hearing aids made FAILED to meet industry standards. Yikes!

Performance of new hearing aids using the ANSI S3.22-1976 standard.

One hundred new hearing aids were tested to determine their compliance with ANSI S3.22-1976 specifications. Thirty-four models representing eight manufacturers were included. Estimates of the test equipment’s accuracy were utilized as required by the standard to correct the tolerances permitted for the 11 measurements made. Results revealed that 68% of the instruments tested met all specifications, when the accuracy of the test apparatus was accounted for, while 11% fewer hearing aids passed all tests when it was excluded. No greater than a 10% difference was found in the performance of various types of aids, such as automatic gain control, directional, linear, or nondirectional.

by TH Townsend. Abstract is for sale at National institute of Health web site which I do not endorse, but i am curious about

Tags: Hearing