Tutorial FFT measurement

Small-room acoustics is a complex physical phenomenon that influences the way we listen and perceive any musical material played inside them. In this tutorial you will guided to quickly know the acoustical characteristics of your room, starting from simple tests that can be performed directly with an Internet-connected computer and a microphone.

INTRO

Nowadays recording studios, listening rooms and home theatres often have surfaces below 30-40 m2 (300-400 ft2) and heights below 4m (13 ft): with these geometric features, the influence of resonance modes is dominant. With this well-known acoustic effect, some frequencies (those whose wavelength is a multiple of the distance between two walls) are amplified at some points of the room and are less perceptible at other points in the same environment.
The room impulse response is the main tool for analyzing the internal acoustic response of a room. This particular signal contains all the spectral and temporal information of the room: through its processing, can be measured the steady state frequency response with traditional FFT (Fast Fourier Transform) analysis, reverberation time analysis and more advanced and innovative analyzes obtainable with Tune Your Studio services (for more details, click here).

The foundations of our analysis are described in scientific papers downloadable here.

If there are problems in the low frequency range, the low-end of a music track (kick drum, bass, male voices) will not be played properly. There will be problems in the low-end reproduction also moving to other listening systems and to other rooms.
Often, traditional FFT performed in one-third octave frequencies band, is the main tool used in acoustic response analysis. But the FFT spectra have the embedded limitations of showing the steady state response of the room, i.e. valid while listening long lasting sounds and notes. This analysis method is not suitable to take into account the transient nature of music. The FFT analysis has to be considered the starting point, useful to highlight the most macroscopic issues in the acoustics of an environment.

FREQUENCY RESPONSE ONLINE MEASUREMENT

This tutorial will show you how to get the low frequency response of your room directly from your browser. The First Step is the free registration to the online tool. Then you will be provided with a direct link towards the analysis tool.

In order to perform this measurement, you need:

  • - an Internet connected computer
  • - a sound card with microphone input
  • - a microphone (a linear, condenser and omnidirectional mic is the best choice, if possible)
The microphone has to be positioned at ears height, in the vertex of the equilateral triangle connecting the two speakers and the listening point (the so-called “sweet-spot”).
In the Second Step, the size of the room is required:

This information is needed to calculate the position of resonance modes for a regular shaped room (shoe-box model). If the room is not parallelepiped you will have to insert the average dimensions of the room or contact us to study in detail your situation.
The measurement method is the Exponential Sine Sweep. The file played by the browser is a uniform amplitude signal that changes its frequency logarithmically over the entire audible spectrum. This signal, after performing a convolution with its inverse filter, generate the room impulse response. Clicking on "Record Sine Sweep", the process begins.

Automatically, at the end of the registration, a .wav file is downloaded. By clicking on the file selection button, the convolution operation is performed and the frequency response graph is showed up to 400 Hz (the following example shows an untreated room).

Vertical lines represent the theoretical position of resonance modes: in red, length axial modes, in blue, width modes and in yellow the height ones. It should be noted that, in the area between 50 and 200 Hz, the correspondence between theoretical position of axial modes and the frequency response peaks is sufficiently precise. As the frequency increases, the intervention of the tangential and oblique modes makes the correspondence less precise.

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