Week 5 (Friday 26th May)
After writing up the new project outline and sending it over to Tony, he made some improvements and additions to give more specific detail. After both Tony and myself were happy with the final draft, we sent it over to Andrew so that any alterations or improvements could be written in. Various alterations were made, but we finally have a project outline from which I can begin writing my literature review.
The new project outline is as follows:
Initially for a single small loudspeaker driver in a cabinet, to develop an adaptive filter for insertion into the signal chain to the speaker input which reduces the effect of the first (floor) reflection on the speaker’s frequency response at the position of a microphone 1 m in front of it. The filter is to operate at low frequency, where existing models tend to be rather inaccurate.
We assume here that our starting point is to make a set of acoustic speaker response measurements (as against, e.g., using an existing model to estimate them based on a measured impedance curve or nearfield response).
In the anechoic chamber, use a log swept sine wave to measure the farfield impulse/frequency response of the speaker, at a distance of up to 1 m directly in front of the speaker, down to the lowest practical frequency (and up to 20kHz) in as nearly as possible freefield conditions. This constitutes our farfield reference response.
Use a similar technique to obtain the nearfield response of the speaker and measurements at incremental distances out to 1m. These measurements will serve as a basis for exploring the chamber anechoic limitations. Additionally, once the nearfield response and the response calculated for the impedance is established, these will be used as a reference response to determine the effects of a gated impulse response measurement.
Again in the anechoic chamber, acoustically measure the frequency/impulse response of the speaker whenan additional microphone is positioned at the point of a single reflection due to a simulated floor (should we make this measurement with or without the floor reflector in place?). The microphone should be placed directly on top of the simulated floor, at the point of intersection of the reflected sound path
With the simulated floor in place, capture the mixture of direct and reflected sound 1 m in front of the speaker.
Identify a suitable model for estimating the reflected signal component arriving in the signal mixture at the farfield position, by adjusting for amplitude and delay knowing the location of the speaker and microphone relative to the floor
Develop an algorithm to shift, scale and then subtract the estimated reflected signal to optimumly remove it from the mixture by minimising the energy of the resulting signal. The process should adapt dynamically so that the estimate remains reasonably accurate even when the farfield measurement point is shifted slightly.
Test the process by comparing to the original floor-free measurement.
Now that I have a much clearer understanding of the scope of the project - I can condense down the reading I have been doing to much more relevant papers. The papers at this stage that are most relevant to the scope of my project, and that I have been studying at length are:
Design and Evaluation of Digital Filters Applied to Loudspeaker/Room Equalisation.
Per Rubak and Lars G. Johansen
Equalization of Loudspeaker and Room Responses Using Kautz Filters: Direct Least Squares Design
Matti Karjalainen and Tuomas Paatero
Low-Frequency Modal Equalization Of Loudspeaker-Room Responses
Aki Mäkivirta, Poju Antsalo1 , Matti Karjalainen
Now that I have a concrete project outline, I am able to start my literature review, which, at the time of writing, I have done so. The abstract for the project is written as well as a general structure for the review. The main sections will be:
1) The existing issues problems with low frequency loudspeaker performance we are trying to improve.
2) Measurement techniques for accurately quanitfying subjective loudspeaker performance
3) Existing techniques attempting to solve the problems of the scope of this project
4) What my project aims to do and how it fits in with similar research
That's it for today, plenty to be getting on with, thanks for reading! As always, comments below.