Nuts & Volts - May 2007

There are three components to any sound: frequency, amplitude, and

phase. These three factors completely define any and all sounds.

extremely complicated mechanical, biological, and neurological process. This is seen when parts of this system are damaged. (A person may have perfect hearing but not be able to understand speech, for example.)

The best estimate is that hearing is a constructive process. That is, the brain takes data and applies rules and functions to that data to build a representation of the sonic world. These rules and functions are unbelievably subtle and complex. Mother Nature has spent millions of years developing the process of hearing. Unfortunately, she didn’t leave any documentation behind. (One of the more unusual aspects of hearing is “objective tinnitus.” Tinnitus is a ringing or noise heard that has no exterior source. It’s fairly common. Objective tinnitus is a ringing or noise that can actually be heard coming from the patient’s ear by a second person.)

Sound localization is usually attributed to two factors as they apply to both ears: phase and loudness. Localization is poor in humans and is usually limited to a “precision” of 10 degrees of angle or more. (Note, loudness and pitch are the perceptions associated with amplitude and frequency, respectively.) Higher frequency sounds are attenuated by the head so there is an amplitude reduction heard by the ear that is in the sound shadow. This effect starts around 100 Hz and improves at higher frequencies.

Phase, as noted above, is related to the time difference of when the sound reaches the ear. Localization of low-frequency sounds is usually attributed to binaural phase because the shadow effect doesn’t work well at low frequencies. Phase is thought to contribute little to sound localization at 10,000 Hz but it increases at lower frequencies. From about 1,000 Hz and below, most localization is associated with phase.

It is useful to compare the ear’s sensitivity to differences in loudness, pitch, and binaural phase. (Note that normal hearing spans 16 Hz to about 20,000 Hz with a maximum sensitivity

74 May 2007

or threshold of hearing starting at 0 dB and reaching 140 dB, where sounds are painful.) Most people can hear a difference if the amplitude of a sine wave changes by about 0.25 dB or about 3% under ideal conditions. This is quite variable depending upon the frequency and initial amplitude of the sound. But 3% is a fairly large change. It is a similar story for frequency. In this case, most people can hear a frequency shift of about 0.2% (also variable depending on frequency and amplitude). This is better than the amplitude sensitivity but it is still not all that impressive.

Binaural phase is very different. If two clicks are presented at the same time through headphones, the sound appears localized in the middle of the head. This is not surprising to anyone who has ever used headphones. By delaying one click, the sound can be made to move from side to side. This is also not too surprising. What is surprising is that if the click is delayed “by as little as 0.000012 seconds, the sound image moves towards the ear which received the click first” (Ref 1). This is only 12 microseconds! The fact that any organic system can detect a 12 microsecond difference is nearly beyond belief.

This ultra-sensitivity to phase strongly suggests that it is an important aspect in hearing. It seems unlikely that such a system developed by chance.

However, that was binaural phase sensitivity. Is there something similar for monaural phase? Initially, the answer seems to be no because delaying an ordinary sound by about 40 ms is not perceived as an echo. Nevertheless, this initial supposition turns out not to be the case.

In 1978, for my Master’s thesis at the University of Buffalo, I developed a simple procedure that converted any sound into a series of pulses (somewhat similar to a nerve cell). This was a monaural experiment using one small open-air speaker in place of headphones. The pulses were

identical in amplitude and width (frequency components). Therefore, no information could be conveyed by the components of amplitude or frequency. (Information requires a change in the medium in order to be transmitted. This is defined as bandwidth. Any non-varying medium, like a DC voltage, has a bandwidth of zero and cannot carry any information.)

The only parameter that varied was the time interval between the pulses. This is defined as phase. Simply, the machine took sound and removed all of the frequency and amplitude parameters while retaining only the phase parameter. Phase precision was controlled by adjusting the minimum time allowable between the pulses. A short period between the pulses permitted a greater possible number of pulses per second ( depending upon the input signal) and vice versa. The results were consistent with the binaural phase measurements.

In this case, the intelligibility of speech was measured to determine how much information could be carried by phase. This value turned out to be 100% with high pulse rates. While the elimination of amplitude and frequency components distorted the speech, the intelligibility was identical to the control. As the pulse rate was lowered, the intelligibility fell (as did the bandwidth). This allowed the comparison of intelligibility to pulse rate.

The result was that there was a 1% change in intelligibility with a 14 microsecond change in the phase (delay between pulses). It would seem to support the notion that phase is important in the hearing and speech perception processes. (Note that 14 microseconds corresponds to an acoustic path length of about 0.185”. A sine wave with a period of 14 microseconds is equivalent to a signal with a frequency of over 71,000 Hz.)