An impulse response can be thought of as a graphic representation of audio energy, akin to the unique fingerprint of the tested device(s). That energy can originate from a microphone or electrical probes. Within an impulse response lies a wealth of data, with one particularly valuable aspect being the initial arrival time of that audio energy. As an example, imagine a high-performance audio system set up in a room and a sudden burst of sound comes blaring through the system. That burst of sound can be measured with a microphone and compared to the original signal that was sent into the system. Using that data, calculations can be made and display on a graph, the impulse response. The graph not only reveals the arrival time of the signal, but also captures other audio energy before and after the initial arrival. Simply put, an impulse response represents the measured reaction to a signal sent into a system, and this can be measured electrically or acoustically.
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TüN® 4 and MAX™: What is an Impulse Response?
So, what exactly does an Impulse Response look like?
The graph in Figure A displays time (measured in milliseconds) on the X-Axis and amplitude (measured in decibels full scale) on the Y-Axis. In the center of the X-axis, 0.00 represents "time zero" or the signal at the time it started. Here, the Impulse Response looks like a single upward pulse displayed at time zero. The graph displayed above is a Linear Impulse Response graph and shows that the measured signal is arriving at the same time as when the signal started. If this signal is sent into an electrical device such as an amplifier, and the signal is measured at the output of the amplifier, the Impulse Response will display the amplifier’s effect on that signal. If that signal also passes through a speaker, a microphone could be used to measure the result acoustically.
There is a lot of information that can be gathered from an Impulse Response such as arrival time, level, and frequency range of a signal. Some other things include latency, time of flight and even reflections of sound in an environment or lack thereof. When using the Linear Impulse Response graph, the relative polarity of the signal is also indicated.
What is required to measure an Impulse Response?
In order to measure Impulse Response, a user will a need software that has dual FFT analysis capabilities and a USB audio interface that has multiple inputs and outputs. JL Audio’s TüN® Software combined with the MAX™ Audio Measurement System is a great way to perform these types of measurements. Signal types that are used for impulse response measurements must have a large range of frequencies present and ideally contain most frequencies between 20 hertz and 20 thousand hertz. In TüN® Software, periodic pink noise (20Hz-20kHz) can be used while the impulse response is measured in real-time.
This signal is first generated in software and is output via a USB audio interface (such as MAX™) usually connected via RCA wires into the device being tested. Then, the output of the device being tested can be input into the audio interface via an open input channel. This creates a measurement loop that an impulse response can be created from. The signal that was sent into the device being tested is referred to as the “Reference Signal” and the signal that output via the device is referred to as the “Measure Signal”.
Figure B displays an impulse response captured from the output of a JL Audio VXi amplifier. Here, the measurement appears a bit different than the previous ‘perfect’ impulse response measurement in Figure A. Notice that the new signal (blue) is further to the right, indicating a later arrival time. This is due to the electrical latency of the amplifier as it takes time for the signal to pass through the components inside. The internal components have also caused the shape of the impulse response to change.
Figure C displays an impulse response captured acoustically with a microphone from a speaker connected to the output of the amplifier. Notice this impulse response looks different than both Figures A and B. It is arriving later than the other measurements because it also includes the ‘time of flight’. The time of flight begins when the sound changes from electrical signal to mechanical movement in the speaker and the sound travels from the speaker to the microphone capsule.
Figure C also displays something else that is not present in Figures A or B. When performing acoustical measurements, the sound from the speaker is not the only sound present during that measurement. The environment also influences the way the speaker sounds (and measures) due to not only ambient noise, but also the sound from the speaker reflecting off objects and arriving at the microphone later in time. In order to capture data without the environment, the measurement would need to happen in an anechoic chamber where there is no added echo or influence from the environment. Acoustic impulse response measurements are very useful for a variety of tasks, such as finding the arrival time of speakers that don’t have a direct unobstructed path to a microphone (such as a subwoofer in a trunk), aligning multiple speakers in time, understanding how audio reflections are displayed within the data, and can help with finding positions for sound treatment as well.