Update the alpha value in the echo detector.

webrtc/modules/audio_processing/echo_detector/mean_variance_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/mean_variance_estimator.h"
 
 #include <math.h>
 
 #include "webrtc/base/checks.h"
 
 namespace webrtc {
 namespace {
 
 // Parameter controlling the adaptation speed.
-constexpr float kAlpha = 0.01f;
+constexpr float kAlpha = 0.001f;
 
 }  // namespace
 
 void MeanVarianceEstimator::Update(float value) {
   mean_ = (1.f - kAlpha) * mean_ + kAlpha * value;
   variance_ =
       (1.f - kAlpha) * variance_ + kAlpha * (value - mean_) * (value - mean_);
 }
 
 float MeanVarianceEstimator::std_deviation() const {
   RTC_DCHECK_GE(variance_, 0.f);
   return sqrtf(variance_);
 }
 
 float MeanVarianceEstimator::mean() const {
   return mean_;
 }
 
 void MeanVarianceEstimator::Clear() {
   mean_ = 0.f;
   variance_ = 0.f;
 }
 
 }  // namespace webrtc