What is the definition of low frequency when it comes to our playback and recording environments. I have asked that very question in numerous forums and discussion groups over the last few months. Responses range from the classic quoting of frequencies off of known tables and charts to whimsical ones that claim a low frequency is any frequency that rattles your pant legs or if you are older, your teeth fillings. It appears that there is really no set definition and everyone has a varied definition when it comes to low frequency in our listening, home theater, and professional listening environments.
Manufacturers of room acoustic treatments also have their own definition. The literature is full of companies that offer low frequency absorption or bass traps. If you examine multiple companies products especially in the bass trap category, you start to see some common trends. Most companies consider low frequency absorption to begin at 100 Hz. and go from there. The literature is full of companies with performance graphs that start at 100 Hz. or 125 Hz. Most show that at 100 cycles they absorb 20 % of the energy at that frequency. Some show even more with figures around 40% absorption at 125 Hz. and moving up from there. Other companies will claim that absorption coefficients are misleading and we should only focus on sabins which are the real measurement of a product’s ability to absorb low frequency. A sabin is a unit of measurement that measures something? Whether we use absorption coefficients or sabins are we really absorbing low frequency energy at the rates and levels we need to solve our low frequency pressure issues in our rooms when we use product design parameters that start at 100 cycles and go up from there. Does the absorptive rates and levels of the products labeled “bass absorber” really absorb bass energy.
Manufacturers claim that their product is a “bass absorber”. By this nomenclature, one would assume that it really does absorb bass energy and I guess if you use 100 Hz. and above as a bass energy definition, you have achieved your design objective. This is a design objective that does not really address bass or low frequency energy issues in small room acoustics. What this design objective does is allow for manufacturers to attach the label of bass or low frequency absorber to a product that is easy to build, transport, and install rather that achieving any low frequency absorption that will have value and meaning to the end user. It appears to be a marketing slogan instead of an actual performance result. Bass and low frequency energy does not lie above 100 cycles.
Real Low Frequency Room Issues
Low frequency issues in small room acoustics lie below 100 Hz. Measure any room response that has a room volume below 5,000 cu. ft. and you will see the traditional bell curve bump below 100 cycles. It is always there. If you look closely at this bump and look at this bump over many different room measurements, you will see two common patterns occurring. First, there is a pattern that occurs from 20 Hz. – 50 Hz. This frequency region and the over pressurization within the room that occurs within this frequency range is present in the rooms consistently. It is easy to see why. The small room dimensions do not allow for ample space for these frequencies to depressurize, if you will. The pressure these long wavelengths create do not have enough room to run and lose energy with quarter length theory, let alone half wavelength.
Secondly, this pressure issue exists all the way and up through 200 cycles, but it has a more predominant effect up to the 100 Hz. level. From 50 Hz. – 100 Hz., we do not have the 20 – 30 db. bump we see in the region from 20 Hz. – 50 Hz. in small room acoustical environments. We do see that we have 10 db. – 15 db. pressure issues that must be addressed. Therefore, if we look carefully at the frequency responses in small room acoustics, we see two major areas that we should be designing sound absorption products for. We should be focusing on the most predominant frequency range difficulties from 20 cycles through 50 cycles and we should also be looking at a more broadband absorption approach for the areas from 50 Hz. – 200 Hz. Using 100 cycles as a product design criteria and calling it a low frequency absorber is not accurate. When a customer purchases a product called a bass absorber and places it in his or her room and they still have the low frequency problem, no one’s interest is served.
Let All Be Heard
When you absorb this unwanted excess energy below 100 cycles amazing things happen. There is a lot of information and emotion in frequencies from 20 Hz. – 50 Hz. bass notes provide the foundation upon which our middle and high frequencies “ride” upon. With proper rates and levels of low frequency absorption in this area, one is able to hear individual bass notes. Individual bass notes that have an attack and decay that is equal in feeling and emotion as the attack and decay of our middle and high frequency producing instruments. A bass riff or run is equal in importance as a guitar break and can provide the same emotional impact if it can be heard and felt. Without proper rates and levels of absorption in this low frequency area, we miss all of this detail and information and must suffer through a low frequency muddle and blur “soup” that does not have to be if we use low frequency sound absorptive products that were conceived and designed with actual low frequency absorption as a performance mandate. We should be able to play a bass guitar break with the same clarity and enthusiasm as our guitar break without apologizing for the low end performance of our rooms