Equalization system to improve the quality of bass sounds within a listening area

• US 20060147057A1
• Filed: 12/30/2004
• Published: 07/06/2006
• Est. Priority Date: 12/30/2004
• Status: Active Grant

• Current Assignee: Harman International Industries Incorporated (Samsung Electronics Co. Ltd.)
• Original Assignee: Harman International Industries Incorporated (Samsung Electronics Co. Ltd.)
• Inventors: Horbach, Ulrich, Aggarwal, Ashish, Welti, Todd
• Granted Patent: US 9,008,331 B2
• Time in Patent Office: Days
• US Class Current: 381/103
• CPC Class Codes: H04R 3/04 for correcting frequency response
• CPC Class Codes: H04R 3/12 for distributing signals to two or more loudspeakers specially adapted for hearing aids H04R25/407
• CPC Class Codes: H04S 7/301 Automatic calibration of stereophonic sound system, e.g. with test microphone

Abstract

Frequency equalization system substantially equalizes the room frequency responses generated by at least one loudspeaker within a listening area so that the frequency responses in the listening area are substantially constant and flat within a desired frequency range. The frequency equalization system uses multiple microphones to measure the impulse responses of the room and uses the impulse responses to design filters to process the audio signals of one or more subwoofers to achieve an improved bass response that is flat across the relevant frequency range. The system employs an algorithm that is a closed-form, non-iterative, mathematical solution and features very short computation time.

Claims

1. A method for designing one or more filters to substantially equalize frequency responses within a frequency range in a listening area, comprising:

measuring frequency responses in a listening area generated by at least one acoustic transducer;

inverting the frequency responses to determine first stage equalization filter spectra;

smoothing the first stage equalization filter spectra to determine second stage approximate equalization filter spectra;

determining a global frequency response from a combination of frequency responses that result after applying the second stage approximate equalization filters to the measured frequency responses;

inverting the global frequency response to determine a global equalization filter; and

combining the global equalization filter with the second stage approximate equalization filters to determine final equalization filters.

3. The method according to claim 1, the measuring of the frequency responses includes:

receiving impulse responses of the listening area through at least one microphone located within the listening area generated by the at least one acoustic transducer;

removing any common time delay from the impulse responses in the listening area; and

transforming the impulse responses of the listening area into the frequency responses in the listening area.

4. The method according to claim 1, including:

clipping the magnitude responses of the final equalization filters to limit gains within desired frequency bands.

5. The method according to claim 1, the measuring of the frequency responses includes:

receiving impulse responses of the listening area through at least one microphone located within the listening area generated by the at least one acoustic transducer; and

using same number of the at least one microphone in the room to measure the impulse response as the number of the at least one acoustic transducer in the listening area.

6. The method according to claim 5, the inverting of the frequency responses is done through a complex smoothing method.

7. The method according to claim 6, where the complex smoothing is done at two separate frequency bands with two different smoothing index values.

8. The method according to claim 5, where the number of the at least one acoustic transducer is four and the number of the at least one microphone is four.

9. The method according to claim 1, the measuring of the frequency responses includes:

receiving impulse responses of the listening area through at least one microphone located within the listening area generated by the at least one acoustic transducer; and

the inverting of the frequency responses is done through a pseudoinverse method if a number of the at least one microphones used to measure the impulse responses is not equal to a number of the at least one acoustic transducer.

10. The method according to claim 1, including applying a target function to the frequency responses to limit the operating frequency range of each of the at least one acoustic transducer in the listening area.

11. The method according to claim 1, where the at least one acoustic transducer is a subwoofer.

2. A system for designing one or more filters to substantially equalize frequency responses within a frequency range in a listening area within a room, comprising:

at least one acoustic transducer to generate output signals;

at least one microphone to measure frequency responses in the listening area; and

a processor linked to the acoustic transducer and microphone, the processor capable of sending test signals to allow the acoustic transducer to generate the output signals and measure the acoustic response of the room through the microphone, the processor capable of calculating filter coefficients based on the measured acoustic responses so that the filtered output signals to the acoustic transducer generate substantially flat frequency responses within a frequency range within in the listening area of the room.

12. The system according to claim 2, where the number of the at least one acoustic transducer in the room is equal to the at least one microphone in the room.

13. The system according to claim 2, where the number of the at least one acoustic transducer in the room is four and the number of the at least one microphone in the room is four.

14. The system according to claim 2, where the processor is a digital signal processor.

15. The system according to claim 2, where the processor is capable of executing instructions to calculate the filter coefficients, the instructions including:

instructions for measuring frequency responses in a listening area generated by at least one acoustic transducer;

instructions for inverting the frequency responses to determine first stage equalization filter spectra;

instructions for smoothing the first stage equalization filter spectra to determine second stage approximate equalization filter spectra;

instructions for determining a global frequency response from a combination of frequency responses that result after applying the second stage approximate equalization filters to the measured frequency responses;

instructions for inverting the global frequency response to determine a global equalization filter; and

instructions for combining the global equalization filter with the second stage approximate equalization filters to determine final equalization filters.

16. A method for designing a filter to equalize impulse responses across a desired low-frequency range within a room, comprising:

measuring impulse responses of a room from output signals generated by each subwoofer in the room;

transforming the impulse responses of the room into corresponding frequency responses;

inverting the frequency responses to determine an ideal equalization for each of the subwoofers in the room;

smoothing the ideal equalization for each subwoofer in the room to determine an approximate equalization curve where local peaks and dips are minimized;

determining an upper curve from a combination of approximate equalization curves;

smoothing the upper curve across a desired low-frequency range to determine a global equalization curve;

applying the global equalization curve to each of the approximate equalization curves to determine a final equalization curve for each of the subwoofers in the room; and

transforming each of the final equalization curves in the frequency domain into corresponding final impulse responses to determine the corresponding filter coefficients.

17. The method according to claim 16, including:

removing any common time delay from the impulse responses in the room.

18. The method according to claim 16, including:

clipping the final magnitude response to limit the maximum gain outside of the desired low-frequency range.

19. The method according to claim 16, including:

using same number of microphones in the room to measure the impulse response as the number of subwoofers in the room.

20. The method according to claim 19, the inverting is done through a complex smoothing method.

21. The method according to claim 20, where the complex smoothing is done at two separate frequency bands with two different smoothing index values.

22. The method according to claim 19, where the number of subwoofer is four and the number of microphone is four.

23. The method according to claim 16, where the inverting is done through a pseudoinverse method if a number of microphones used to measure the impulse responses is not equal to the number of subwoofers in the room.

24. The method according to claim 16, including applying a target function to the frequency responses to limit the operating frequency range of each of the subwoofers in the room.