Q. Why does using a high-pass filter make things seem louder?

Published in SOS December 2010
Bookmark and Share

Hugh Robjohns

Why does applying a high‑pass filter to a sound sometimes result in the output being noticeably higher than it was before? Today I have been working on a sound that peaks at 0dBFS. It has a lot of low‑frequency content. I am applying a high‑pass filter at around 100Hz and the output from the EQ is peaking at around +4dBFS. Why should this happen? Most of the power in this sound is in the low frequencies, and it has little going on above 2kHz, so surely with the high‑pass filter most of the energy from the sound has gone!

Via SOS web site

SOS Technical Editor Hugh Robjohns replies: This is a very common effect and there are several possible reasons for it. Fundamentally, the filtering process changes the shape of the waveform, so although there may be less total energy in the signal, the peak amplitude may well increase.

If you think about a bunch of different‑frequency tones all playing at the same time, their phase relationships vary continuously and add to or cancel each other to create the total waveform. Remove some of those tones and some of those cancellations won't occur. That can result in the waveform becoming bigger. There are various reasons why using a high-pass filter on a signal can make it sound louder. Some equalisers, for example, actually boost the region  just above the turnover frequency, which can produce an increase in peak level.There are various reasons why using a high-pass filter on a signal can make it sound louder. Some equalisers, for example, actually boost the region just above the turnover frequency, which can produce an increase in peak level.

Most equalisers also introduce significant phase shifts and that, again, will change the way different frequencies combine and cancel. It can also happen because some equalisers actually boost the region just above the turnover point below which they are attenuating, potentially increasing peak level.  .


November 2014
On sale now at main newsagents and bookstores (or buy direct from the
SOS Web Shop)
SOS current Print Magazine: click here for FULL Contents list
Click image for November 2014
WIN Great Prizes in SOS Competitions!

 

Home | Search | News | Current Issue | Tablet Mag | Articles | Forum | Subscribe | Shop | Readers Ads

Advertise | Information | Privacy Policy | Support | Login Help

 

Email: Contact SOS

Telephone: +44 (0)1954 789888

Fax: +44 (0)1954 789895

Registered Office: Media House, Trafalgar Way, Bar Hill, Cambridge, CB23 8SQ, United Kingdom.

Sound On Sound Ltd is registered in England and Wales.

Company number: 3015516 VAT number: GB 638 5307 26

         

All contents copyright © SOS Publications Group and/or its licensors, 1985-2014. All rights reserved.
The contents of this article are subject to worldwide copyright protection and reproduction in whole or part, whether mechanical or electronic, is expressly forbidden without the prior written consent of the Publishers. Great care has been taken to ensure accuracy in the preparation of this article but neither Sound On Sound Limited nor the publishers can be held responsible for its contents. The views expressed are those of the contributors and not necessarily those of the publishers.

Web site designed & maintained by PB Associates | SOS | Relative Media