There are several C++ libraries available for signal processing. Here are a few popular ones:

1. The Signal Processing Library (SPL) - SPL is a library developed by the US Naval Research Laboratory. It provides a set of signal processing functions for use in C++ programs. The library includes functions for filtering, signal analysis, and spectral analysis.

2. The GNU Scientific Library (GSL) - GSL is a powerful numerical library that includes a wide range of mathematical functions, including signal processing functions. It provides functions for Fourier transforms, convolution, and signal filtering.

3. The Digital Signal Processing (DSP) library - DSP is a library developed by the Signal Processing Group at the University of California, Berkeley. It provides a range of signal processing functions for use in C++ programs. The library includes functions for filtering, FFTs, and signal analysis.

4. The Audio ToolKit (ATK) - ATK is an open-source library for audio signal processing. It provides functions for filtering, equalization, and time-domain effects. The library is designed for use in music production applications and includes support for real-time processing.

5. The PFFFT library - PFFFT is a C++ library for computing FFTs of real and complex data. It is designed to be fast and efficient, making it suitable for use in real-time applications. The library is also easy to use, with a simple API that makes it easy to integrate into your code.

These are just a few examples of the many C++ libraries available for signal processing. The choice of library will depend on your specific needs and requirements.