What Is a Sonogram?

A sonogram is a two-dimensional representation of a signal in which time appears on one axis, frequency on another, and amplitude or energy is encoded as intensity or color. The term is used most consistently in acoustic phonetics, bioacoustics, and audio engineering, where it refers to the time-frequency plot of a sound signal derived from the short-time Fourier transform (STFT). In these fields, "sonogram" and "spectrogram" are often used interchangeably, though the two terms carry a technical distinction described in ScienceDirect's overview of the sonogram: a spectrogram displays frequency content across time-domain slices of a signal, while a sonogram as formally defined in optics and ultrasonics literature represents the arrival time of frequency-filtered components as a function of filter center frequency. In everyday usage within acoustics, both terms describe the same visual artifact.

A related but distinct use of the word appears in medical imaging, where "sonogram" sometimes refers to an ultrasound image produced by reflected sound waves from tissue, an entirely different modality. The acoustic signal-processing sense of the word, which is the subject of this article, is the dominant meaning within the electrical engineering and signal processing communities.

Short-Time Fourier Transform and Computation

The STFT is the mathematical engine behind the sonogram. Rather than computing a single Fourier transform across an entire signal, the STFT divides the signal into overlapping short windows, typically 20 to 30 milliseconds for speech analysis, and applies the discrete Fourier transform to each window independently. The result is a sequence of spectra indexed by time. The Introduction to Speech Processing textbook from Aalto University describes this process in detail and explains the fundamental trade-off it introduces: a narrow time window gives good temporal resolution but poor frequency resolution, while a wide window gives the opposite. Selecting window length therefore depends on the temporal dynamics of the signal under study; consonant transitions in speech demand narrow windows, whereas the pitch structure of vowels benefits from wider ones.

The resulting time-frequency matrix is typically displayed with a color or grayscale mapping. Low-energy regions appear dark, high-energy regions bright. Formant bands in speech, frequency sweeps in bird calls, and harmonic overtones in musical notes all appear as distinct visual features, making the sonogram a powerful tool for qualitative inspection of acoustic structure.

Acoustic Phonetics and Bioacoustics

Sonograms became foundational in acoustic phonetics in the 1940s and 1950s when researchers at Bell Telephone Laboratories developed the Sound Spectrograph, a dedicated instrument that printed sonogram traces on paper. The device made it possible to compare speech sounds visually and to identify acoustic correlates of phonemes, prosody, and speaker characteristics that were otherwise hidden in the audio waveform.

In bioacoustics, sonograms serve as the primary representation for analyzing animal vocalizations. Researchers studying cetacean communication, bird song, bat echolocation, and insect calls rely on sonogram analysis to classify call types, measure frequency contours, and document repertoire variation across populations. The IRCAM AudioSculpt documentation on sonogram analysis demonstrates how modern software extends these capabilities to interactive editing and resynthesis of sounds from their time-frequency representation.

Applications

Sonograms have applications in a range of fields, including:

• Speech analysis and phonetics research for characterizing vowels, consonants, and prosody
• Bioacoustics and wildlife monitoring for classifying animal calls and vocalizations
• Music analysis and audio engineering for visualizing harmonic content and onset detection
• Medical ultrasound imaging in the sense of B-mode acoustic tissue imaging
• Condition monitoring of machinery using vibration and acoustic emission spectrograms
• Seismology for visualizing earthquake wave arrivals and frequency content