The Psychoacoustics of Headphone Listening.

by HeadRoom Corporation

Listening to music on headphones is different, and acoustically less satisfying, than listening to speakers. The reason for this has to do with a human’s ability to locate sounds in space. This ability is described by the science of psychoacoustics, and specifically by the set of formulas called “Head Related Transfer Functions” (HRTF).

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For the sake of our argument, we will use the special condition of two speakers set 30-degrees to either side of the listener.

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Now let’s say you turn off the left speaker. Both ears will continue to hear the right speaker. Because the left ear is slightly farther away than the right ear, it hears the sound slightly later. This time difference between ears is called the inter-aural time difference (ITD), and is the primary cue with which your brain figures out left-to-right position of sounds.

Now imagine you are wearing a pair of headphones, and you turn off the left channel. Only the right ear hears the right channel. The left ear hears nothing. This never happens in nature (except maybe if you have a fly buzzing in your ear ), and leads to that annoying blobs-in-your-head audio image normally associated with headphone listening. The reason it’s annoying is because your brain doesn’t have enough information to correctly localize sounds, and it keeps struggling to figure things out. In the end (usually after about two hours), your brain starts screaming, “Let me outta here!”, and your headphone istening is done for the day.

Enter HeadRoom. We employ as little circuitry as possible in an attempt to fix the localization problem without mucking up the signal. We won’t take all the credit; a guy named Ben Bauer figured a lot of this stuff out way back in the 60’s. He came up with a basic theory of time delays and other factors to compensate for headphone listening.

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The state of the art in electronics today permits the effective design of Bauer’s headphone compensation circuit. All HeadRoom amps have such a circuit. A little bit of each channel is crossfed through a time delay over to the other channel. This circuit, which we call our Audio Image Processor models the basic inter-aural time differences found when listening to a pair of speakers.

Although the audio image remains inside your head, it now has a sense of continuity from left to right. The most common customer comment is that the music just sounds more natural with HeadRoom. The correct time delays are heard by the ears, and your brain is able to process the audio information in a normal manner.

But what about something more complicated like surround sound? For that you would need a five channel processor. On pages 32 and 33 of this catalog you will see home theater products called the Auri and the Lucas. These are much more complex digital processors that perform a five channel head related transfer function mapping to headphones.

These processors take into account inter-aural time differences between ears for five channels. They also calculate acoustic properties for elevation and add room acoustic reverberances. All of these parameters are calculated for common head sizes and shapes. The resulting audio presentation remains in (or slightly outside) your head and, for home theater applications, is quite an involving and effective synthetic acoustic environment. Unfortunately, because of the extraordinary amount of processing, we feel these amps lack the precision and clarity needed for excellent music reproduction.

You can stop reading right here if you’re not interested in the technical details associated with the HeadRoom Audio Image Processor.

There are a number of other phenomena that the brain uses to aid in localization. One of these is pinna reflections. The ear nearest a sound source shows increased high frequency response. Reflections off the outer part of the ear (the pinna) cause comb filter effects that aid in the determination of elevation.

Reflections off the walls of the room and foreknowledge of the difference between the way various floor coverings sound also contribute to localization.

The most important cue of all is the way all these factors change as your head moves relative to sound sources. It was found in experiments that when a subject’s head is clamped, the number of times a sound coming from the front was mistaken as coming from behind went up by a factor of ten. It was also found in homing experiments that the blindfolded subjects would precisely guide themselves to sound sources.

Still with us? OK, here goes with the numbers: With the speaker 30 degrees off axis, you get about a 300µs delay between left and right ears.

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There is an amplitude difference between ears, too. The far ear is in the shadow of the head, so it hears the sound at a slightly lower volume.

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This amplitude difference also varies with frequency. Though the lows are not attenuated by the head much, the highs begin to roll off rather quickly around 2kHz.

Then a funny thing happens: the highs start reaching the far ear by traveling along the skin surface. This effect causes a broad rise centered around 5kHz. The result is a hump in the far ear curve response.

The near ear experiences better high frequency sensitivity, and hence sees a smoothly rising frequence responce curve.

The described phenomena are the primary left-right localization cues that the HeadRoom Audio Image Processor tries to model, although it accomplishes it in a completely different way.

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The HeadRoom Audio Image Proccessor circuit consists simply of two-stage active filter which controls both the Frequency response and phase of the crossfeed circuit. The complex phase delay this circuit introduces creates a delay very similar to the ITD up to about 2 kHz.Because ITD information becomes less meaningful above 2 kHz, the circuit also rolls frequency responce off above that point. The crossfed signal is simply summed in with the opposite channel at a 10 dB lower amplitude.

Because of the strange and complex way these two signals sum together, the frequency response of the HeadRoom process reasonably matches the hump in the far ear response curve.

When a signal is delayed then sum it back on itself, a cancelation results every time the signal is 180 degrees out of phase. This causes a series of notches up the frequency response curve which is commonly called a comb filter. In the HeadRoom processor, we take a bit of right channel, delay it, then sum it with the left. The resulting comb filter effect is weak because we sum the delayed signal in at a lower level and with rolled off highs. Because the delay has its first 180 degree out point at 2.5 kHz, the HeadRoom processed mono signal has a gentle dip centered at 2.5 kHz.

As a result of the factors described above, there is an overall warming of the mono signal. The Filter switch on the front of HeadRoom amps is designed to compensate for this warming by slightly boosting the high frequencies. This boost is roughly equivalent to the near ear response.

The real question is, “What does all this sound like?” You’ll notice a number of things when you listen to a HeadRoom amp. First, the processor is very subtle. Because your ears can’t move relative to the sound sources in headphones, it takes a long time for your brain to “learn” this new and synthetic acoustic environment. However, you will notice right away that you can listen for much longer periods of time without any listening fatigue.

After about 40 hours of listening, your brain will have learned the new acoustic trick and will become adept at localizing sounds very much like with a pair of speakers. There is a smooth left-to-right continuity, and the audio image will have reasonably good depth. HeadRoom amps simply give your brain a more natural headphone listening experience

c. 1998, HeadRoom Corporation.
From HeadRoom Corporation. (Republished with permission.)

 

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