Audio Test Stimuli: 8 Audio Test Signals and When to Use Them

Electroacoustic measurements rely on many different audio test stimuli. This brief summary of the various options guides you towards the best stimulus for your measurements.

Stepped Sine Sweep
A stepped sine sweep plays a sine wave, one frequency step at a time with a user-defined frequency range and resolution. SoundCheck’s stepped sine sweep, the ‘Stweep’ is optimized with an integer number of cycles at each frequency step and continuous phase between steps for a smooth transition from frequency to frequency. This minimizes transient ringing in the device under test for faster and more repeatable measurements. Stepped sine waves can be configured with different resolutions and averaging times at high and low frequencies to optimize the trade-off between stimulus duration, background noise and therefore, test time and accuracy. The stepped sine sweep is one of the most widely used audio test stimuli for basic electroacoustic testing because it offers good noise immunity and provides acoustic (e.g. frequency response, phase, THD), perceptual distortion, and electrical (e.g. impedance) measurements with a single sweep.

Graph of Stepped Sine Sweep (Stweep) Audio Test Stimulus

Stepped Sine Sweep (Stweep) Audio Test Stimulus

 

Log Sine Sweep (AKA Frequency Log Sweep, Log Chirp or Farina Sweep)
A continuous log sine sweep from low to high covers every frequency in the chosen range. The same time and energy in each octave achieves a good signal-to-noise ratio at all frequencies. The sweep rate is slow at low frequencies for accurate measurement, but increases with frequency for a fast overall test speed. SoundCheck was the first test system to implement simultaneous simulated free field measurements of harmonics using  the Log chirp stimulus, just months after Angelo Farina’s landmark AES paper on the subject in 2000.

The log sine sweep is a good alternative to a stepped sine sweep for production line loudspeaker measurements, and is also common for fast time-selective measurements of fundamental response and distortion. Used with a Time Selective Response algorithm it provides simulated free-field measurements for free-field analysis of a loudspeaker without an anechoic chamber.

Graph of Log Sine Sweep (AKA Frequency Log Sweep, Log Chirp or Farina Sweep)Audio Test Stimulus

Log Sine Sweep (AKA Frequency Log Sweep, Log Chirp or Farina Sweep)Audio Test Stimulus

 

Log Amplitude Sweep
A Log amplitude sweep is similar to a stepped sine sweep except a single frequency is chosen and swept logarithmically through various levels.

It is useful for evaluating how a device performs at different stimulus levels (linearity tests) and for testing devices with compressors, limiters or automatic gain control (AGC). It can also be used with advanced analysis algorithms to evaluate characteristics like THD vs. stimulus level.

 

Graph of Log Amplitude Sweep Audio Test Stimulus

Log Amplitude Sweep Audio Test Stimulus

 

Noise
Both pink and white noise can be used for audio testing. Pink noise has a continuous frequency spectrum and equal power per constant percentage bandwidth, e.g. equal power in any one-third octave band. It is used with constant percentage bandwidth filters such as the RTA. White noise has a continuous frequency spectrum and with equal power per unit bandwidth, for example, equal power in any band of 100-Hz width. It is generally used with FFT analysis.

Noise test stimuli are often used where sine waves cannot be used, such as devices with on-board signal processing e.g. codecs.

Graph of Pink Noise Audio Test Stimulus

Pink Noise Audio Test Stimulus

Graph Of White Noise Audio Test Stimulus

White Noise Audio Test Stimulus

 

Multitone
A multitone stimulus consists of multiple sine tones at discrete frequencies played simultaneously with equal amplitudes and random phase. All the frequencies are present at the same time, in contrast to the stepped sine sweep where the frequencies are present sequentially. All frequencies are covered in a very short stimulus, providing very fast measurements. Multitone is good for measuring frequency response, but the number of simultaneous frequencies in the stimulus makes it impractical for measuring other characteristics such as harmonic distortion, limiting its use.

Multitone test signals have applications in simple, rapid production line tests, for example measuring the frequency response of microphones.

 

Two Tone Stimulus

A two tone stimulus plays two simultaneous tones. In a non-linear system, these generate intermodulation distortion, output frequencies that are not part of the stimulus and have no harmonic relationship to the original signal. There are two commonly-used types of two-tone stimulus. An intermodulation (IM) two-tone stimulus is a stepped sine sweep superimposed on top of a fixed frequency tone. The fixed tone is usually at a low frequency, and the sweeping frequency is generally  five times greater than the fixed tone frequency. A difference frequency (DF) two tone stimulus is composed of two simultaneous sweeping tones separated by a specified frequency interval which may be a fixed difference or a fixed ratio.

Measuring distortion using a two-tone stimulus presents an alternative to harmonic distortion measurement in which the distortion products are kept close to the stimulus frequency and typically fall outside of the masking curve. This makes artifacts more clearly audible and avoids the under/over estimation that can sometimes occur with higher order harmonic distortion methods. IM distortion is generally used for broadband devices such as full range drivers and microphones. DF distortion is used for measuring band-limited devices such as hearing aids.

 

Graph of Two Tone Intermodulation Audio Test Stimulus

Two Tone Intermodulation Audio Test Stimulus

Graph of Two Tone Difference Frequency Audio Test Stimulus

Two Tone Difference Frequency Audio Test Stimulus

 

Speech and Music Based Signals
Speech and music based stimuli enable devices to be tested with real-world test signals. This is particularly important for audio devices with built-in non-linear signal processing. In SoundCheck, any WAV file can be used as the stimulus, making it simple to use music, real speech, or artificial speech. Additional software features such as active speech level control enable these signals to be used for a variety of different purposes including communications testing to industry standards.

Applications include phones, codecs, hearing aid compressors and more. Additionally, these test signals are valuable for creating calibrated background noise for testing smart devices, headphones and wearables.

Graph of Speech (wav file) Audio Test Stimulus

Speech (wav file) Audio Test Stimulus

 

Composite Stimulus Signal
Composite stimuli allow a conditioning signal to warm up the device or open up an activity detector before playing the stimulus signal. They are essential for testing smart devices, where it can be necessary to create a stimulus that consists of a wake word plus a command, or to prepend a stimulus with a trigger tone to enable precise alignment of stimulus and response. SoundCheck offers total flexibility to create customized composite stimuli, and also permits the user to precisely select which sections of the stimulus should be analyzed.

Graph of Compound Audio Test Stimulus consisting of voice activation followed by a test tone

Compound Audio Test Stimulus consisting of voice activation followed by a test tone

All these stimuli are available in SoundCheck. Learn more about SoundCheck’s features and functionality.