Spectrogram Reading for SLPs: A Visual Guide to Voice Quality

A spectrogram is a three-dimensional representation of sound compressed into two dimensions. The horizontal axis represents time, the vertical axis represents frequency, and the darkness or color intensity represents amplitude (energy) at each time-frequency point.

Think of it as taking thousands of frequency snapshots of your audio and stacking them side by side. Dark regions indicate strong energy at that frequency and time; light or white regions indicate little energy.

The distinction between wideband and narrowband spectrograms is the most important concept for clinical spectrogram reading. Each reveals different aspects of voice production.

Wideband Spectrograms: See the Vocal Tract

Wideband spectrograms use short analysis windows (typically 4–5 ms in Praat). This provides excellent temporal resolution—you can see individual glottal pulses as vertical striations. The dark horizontal bands represent formants, the resonant frequencies of the vocal tract.

Use wideband for: Analyzing articulation, tracking formant transitions during connected speech, visualizing voice onset time, and seeing rapid temporal events. For clinical formant work, see our formant analysis guide.

Narrowband Spectrograms: See the Voice Source

Narrowband spectrograms use longer analysis windows (30–50 ms). This provides excellent frequency resolution—you can see individual harmonics as distinct horizontal lines. The spacing between harmonics equals F0 (fundamental frequency).

Use narrowband for: Voice quality assessment, signal typing, identifying subharmonics, detecting F0 instability, and evaluating periodicity.

Before you calculate jitter, shimmer, or any perturbation measure, you must determine your patient's voice signal type. This classification—Types 1–3 proposed by Ingo Titze in the 1995 NCVS Workshop on Acoustic Voice Analysis, Type 4 added by Sprecher and colleagues in 2010—determines which acoustic measures will produce valid results. A 2026 systematic review of three decades of signal-typing research describes it as exactly that: the gatekeeper for selecting valid analysis methods (Hegde et al., 2026).

This step is often missed in clinical practice—yet it's essential. Calculating jitter on a Type 3 voice is meaningless; the algorithm cannot reliably track pitch periods in an aperiodic signal. The full argument, including the software-dependence problem, is in our jitter and shimmer guide.

In a narrowband spectrogram of a healthy voice, you'll see a series of horizontal lines stacked vertically. Each line is a harmonic—an integer multiple of the fundamental frequency (F0).

What Harmonics Tell You

• Harmonic spacing = F0: If the first harmonic is at 150 Hz and the second at 300 Hz, the fundamental frequency is 150 Hz.
• Number of visible harmonics: A healthy voice shows many well-defined harmonics across a 0–4 kHz narrowband view. A visibly reduced count of clear harmonics suggests increased noise or reduced harmonic energy.
• Harmonic clarity: Sharp, well-defined harmonics indicate good periodicity. Fuzzy, smeared, or undulating harmonics suggest instability.
• Interharmonic noise: Energy between harmonic lines indicates turbulent airflow (breathiness) or aperiodic vibration (roughness).

Subharmonics: The Period-Doubling Pattern

Subharmonics appear as additional horizontal lines between the regular harmonics. If you see lines at F0, 1.5×F0, 2×F0, 2.5×F0, etc., you're observing subharmonics at half the fundamental frequency—indicating the vocal folds are completing two different vibratory cycles in alternation.

Subharmonics are common in:

• Vocal fold lesions (nodules, polyps) affecting vibratory symmetry
• Unilateral vocal fold paralysis
• Pubertal voice change
• Intentional vocal effects (vocal fry, growl)

Different voice quality dimensions produce characteristic spectrographic appearances. Learning to recognize these patterns accelerates your clinical interpretation.

While spectrograms alone cannot diagnose specific pathologies (that requires laryngeal examination), certain patterns are commonly associated with specific conditions. These associations can guide your clinical hypothesis and inform your referral decisions.

Here's a systematic approach to incorporating spectrogram analysis into your voice assessment:

For those using Praat, here are optimized settings for clinical voice spectrogram analysis:

Note for high-pitched voices: a wideband display only merges harmonics into formant bands when its analysis bandwidth exceeds the speaker's F0. The standard 5 ms window gives a bandwidth of roughly 260 Hz, so for women and children with F0 near or above that, the “wideband” display starts resolving individual harmonics and looks like a hybrid. The fix is a shorter window (try 0.003–0.0035 s) to widen the bandwidth. Narrowband display is the opposite case and is unaffected: high F0 spaces the harmonics further apart, which only makes them easier to see.