This may be more of a question for the Physics StackExchange, but I thought I'd post it here since it is in some sense a musical question. The Wikipedia article on the Deep Note includes the description given for it in the trademark registration, which states that

The THX logo theme consists of 30 voices over seven measures, starting in a narrow range, 200 to 400 Hz, and slowly diverting to preselected pitches encompassing three octaves. The 30 voices begin at pitches between 200 Hz and 400 Hz and arrive at pre-selected pitches spanning three octaves by the fourth measure. The highest pitch is slightly detuned while there are double the number of voices of the lowest two pitches.

From this, and the later quote by sound designer Gary Rydstrom,

from a technical standpoint, 'Deep Note' just feels loud because it has a spectrum of frequencies that grows from small to large.

It's clear that the Deep Note's famous loudness comes at least in part from the psychoacoustic fact that the perceived loudness of a sound has a dependency on both its bandwidth and center frequency. In particular, the ear is most sensitive to sounds in the 1-5 kHz range, and as the bandwidth of a sound grows beyond a certain critical band, its perceived loudness increases. In combination, the upward glissando of each of the voices in the Deep Note and their divergence in frequency creates a sound that may seem incredibly loud even at a fairly low total intensity/sound pressure level.

My question on the Deep Note is instead: Do the separate voices actually undergo an increase in sound intensity, or is our perception of increased loudness only due to the frequency factors mentioned above?

Every resource I've found mentions the frequency changes, but I can't seem to find much information about a change in intensity. Any references you can provide would be appreciated.

  • FWIW this probably would not be on topic on Physics.
    – David Z
    Apr 24, 2018 at 2:01

1 Answer 1


Below is a screen shot of a 96 kHz PCM file of the Deep Note with peaks rendered by Wave Editor clearly showing a dramatic change in intensity:

enter image description here

The low area early in the file are about -22 dBFS and the highest peaks in the last third are full scale (0 dBFS).

  • You're fast! I guess I didn't know what I was looking for. Could you provide a reference for the plot, or did you make it yourself? Also, any thoughts as to how much each factor actually contributes to the perceived loudness? There seems to be special importance placed on the bandwidth, so I wonder how much its effect actually contributes given that the intensity change is so great.
    – Luke
    Apr 23, 2018 at 18:11
  • I opened the file in Wave Editor myself and took a screen capture. You can actually do some math on your own to estimate the relative importance of pitch versus intensity WRT loudness by referring to the Fletcher Munson Equal Loudness Contours Apr 23, 2018 at 18:21
  • Thank you for your help. I'll accept the answer shortly. By my understanding though, the Fletcher Munson curves are only valid for pure tones or those of relatively narrow bandwidth, so I'm not sure how to apply them to the Deep Note without doing some sort of listening trial or more in-depth Fourier analysis of pieces of the signal to determine its average bandwidth at different times and adjusting accordingly. Is this what you're suggesting?
    – Luke
    Apr 23, 2018 at 18:29
  • @LukeW Well, I guess that's what I'm suggesting. As far as I know, the Fletcher Munson curves are all we've got in terms of quantitative data on the relationship between frequency, intensity, and loudness. So I think we have to make do. Apr 23, 2018 at 18:32

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