Update the alpha value in the echo detector.

webrtc/modules/audio_processing/echo_detector/normalized_covariance_estimator.cc

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/modules/audio_processing/echo_detector/normalized_covariance_estimator.h"
 
 namespace webrtc {
 namespace {
 
 // Parameter controlling the adaptation speed.
-constexpr float kAlpha = 0.01f;
+constexpr float kAlpha = 0.001f;
 
 }  // namespace
 
 void NormalizedCovarianceEstimator::Update(float x,
                                            float x_mean,
                                            float x_sigma,
                                            float y,
                                            float y_mean,
                                            float y_sigma) {
   covariance_ =
       (1.f - kAlpha) * covariance_ + kAlpha * (x - x_mean) * (y - y_mean);
   normalized_cross_correlation_ = covariance_ / (x_sigma * y_sigma + .0001f);
 }
 
 void NormalizedCovarianceEstimator::Clear() {
   covariance_ = 0.f;
   normalized_cross_correlation_ = 0.f;
 }
 
 }  // namespace webrtc