The sound of "Head Like a Hole" is quite characteristic of NIN's general approach to music-making. Though production values and technology have changed, Trent Reznor today remains in a sonic zone close to what he began doing decades ago.
He relies on the combination of seemingly irreconcilable elements: pop format and sonic experimentation. "Head Like a Hole" has a mostly traditional structure, with only the presence of a double-chorus deviating in any way from normal patterns of pop music construction. The double-chorus is exactly what it sounds like, a chorus of two distinct progressions. The first begins with the line "Head like a hole," and the second begins with "Bow down before the one you serve." The song begins with—what else—an intro, an follows the verse-chorus format, repeats itself, proceeds to the bridge, and then repeats the chorus until the song ends.
Reznor demonstrates pop sensibilities on a smaller scale, too. His melodies are traditional in the way that they bounce between lower and higher notes. You might not notice all this conventional stuff, though, since the content of the lyrics tends to be attention-grabbingly different. At the same time, Reznor has always been an experimental musician who embraces computers and technology in his music. Written, sung, played, arranged, and programmed entirely by Reznor, NIN is effectively a one-man band. Reznor requires the aid of technology for rhythm parts, and the sampling and looping that characterizes the album's sound.
He has said that Pretty Hate Machine "was recorded on a old-school Mac, which was about fifteen hundred bucks then, a sequencer program and one sampler that you could buy in the paper for three hundred bucks right now" (source).
Sampling, which is the musical practice of taking the actual audio of something else and using it in your music (like M.I.A. does in "Paper Planes," for example, basing the song off of the Clash's "Straight to Hell") is a huge part of NIN's sound. "Head Like a Hole," for example, begins and ends with samples of tribal African music. Of this Reznor said, "I get tired of listening to 600 African albums to find cool samples. That gets a bit dull." (Source)
Where these particular African ladies singing come from is thus hard to say. Unlike artists like M.I.A. and Kanye West, who sample in part for the novelty of the artist that they're sampling, Reznor has said that he is "more interested in the textures than the novelty of who or what I've appropriated" (source). Reznor has moved away from sampling of human voices in his more recent work, becoming more interested in distorted guitar samples and computerized sounds.
The bridge of "Head Like a Hole" contains some early examples of these kinds of sounds. He said of this, "I'll do a few twenty- or twenty-five-minute sessions of me just playing guitar. Then I'll listen back to it say, 'Around ten minutes in I did something cool.' I'll cut maybe twenty parts out that way and put each one in the right place. It's not so much avoiding having to play the whole song as it is a tool to flesh out an arrangement." (Source)
In the case of "Head Like a Hole," he loops these cool-sounding guitar bits for an entirely different effect. The result of the use of technology in the recording of the song is that the musical side is very icy and precise canvas over which Reznor lays his intentionally imperfect vocals. The complete package then has a certain "man versus machine" quality, as Reznor himself has called it. Left to his own devices, Reznor's use of technology and his inherent pop sensibilities have created a sound that many have attempted to copy but none have quite mastered.
The NIN sound remains, quite distinctly, the NIN sound.