Acoustic and Measurements
How do I acoustically treat my room so that I get the best from my studio monitors?
This is a very large subject area covering room geometry, reverberation time, sound reflection and refraction, material properties, etc. We will just give a checklist of the most important features that a listening room should have:
- Ensure that the reverberation time is low and approximately constant with frequency
- Primary sources of reflection should be treated so that reflected levels are at least 10 dB down from the direct sound pressure level at least during the first 15 ms after the arrival of the direct sound at the listening location
- The front wall should be a hard and smooth if monitors are flush-mounted The front wall can be absorptive if monitors are free-standing
Once the room has been acoustically treated the studio monitors can be installed:
- Position the monitors according to the standard orientations (angles) from the listening position
- Position monitors at equal physical distance from the listening position, or use GLM AutoCal to electronically compensate for differences in monitor distances
- Position the monitors so that there are no cancellation effects from the side walls and the wall behind the monitor
- Turn the monitors towards the listening position horizontally and vertically
- Set the room compensation controls as suggested in the Operating Manual/Quick Setup Guide or use GLM AutoCal to compensate for the room acoustics in the case of SAM (Smart Active Monitoring) systems
How do I set the room response calibration controls (I am getting too much bass)?
Genelec monitors are calibrated flat in anechoic free field conditions. When the monitor is placed in a room close to walls or other boundaries, the low frequency output of the monitor increases. To achieve a flat low frequency response an adjustment of typically -4 dB on the bass tilt control is used. Genelec also provides a bass roll-off control to compensate for any remaining excessive LF energy around the low cut-off frequency.
Genelec GLM AutoCal can implement a more precise compensation after measuring the acoustic effects produced by the monitor’s installation location.
Differences in room reverberation time and listening distance can lead to changes being required in the treble region so treble tilt is fitted to most of the models in the Genelec range.
In three-way monitors and large main systems there are additional driver controls for the bass level, mid level and treble level which enable very fine adjustment of the frequency response so that the monitors can be placed in many different listening environments, whilst still achieving a consistent and neutral sound reproduction.
The best way to set the room response controls of a Genelec monitor is by taking an acoustical measurement at the listening location, using a measurement system for those products that offer local controls (DIP switches) on the monitor or subwoofer, or by using GLM AutoCal for the SAM (Smart Active Monitoring) products.
How does a rear reflex port opening work? What are the benefits of such design?
The main benefit of a reflex port in an enclosure is that it enables the loudspeaker to produce low frequencies at a very low distortion level, close to the frequencies where the bass reflex has been tuned. This enables very linear low frequency reproduction systems to be designed.
The bass reflex port allows air to move in and out. The reflex is effective if the energy losses of this air movement remain small. Because of this reason the reflex port is designed so that the speed of the air moving in the reflex port remains fairly small while the port is in resonance.
The direction where the bass reflex port is facing does not greatly influence the work of the bass reflex. While it is in resonance the reflex port generates low frequency sound. This sound sums with the low frequency sound output from the woofer at other frequencies. The distance and location of the bass reflex port is designed so that this summation works correctly.
If the reflex port is not sufficiently large, the air motion in the port can become turbulent (non-linear). This will dramatically increase air flow losses in the reflex port. The higher flow losses can cause the bass reflex to stops working. This will change the low frequency characteristics of the loudspeaker. In Genelec designs we take care to dimension the reflex port to have sufficient capacity to support the system up to the maximum sound level output.
In several Genelec products the bass reflex port opening is behind the enclosure. When you push the loudspeaker close to a wall, a small gap to the wall is needed to enable the bass reflex to work at full capacity. Our experiences show that a distance of twice (2x) the port diameter from the wall to the enclosure back is a very safe distance.
Benefits of a reflex port opening in the rear of the cabinet:
- It allows to have a maximized waveguide (DCW) area on the enclosure front
- It minimizes acoustic diffraction as the front of the enclosure has no holes, vents or slots. This is especially significant near the tweeter.
- The port opening, or flare, can be significantly larger, ensuring laminar flow up to high sound level outputs, high linearity and low system distortion at woofer frequencies.
- It minimizes the audibility of air flow in the port because the port opening is faced away from the listener. When the air flow in the port generates noise, this noise is typically 10…20 times higher in frequency than the port tuning frequency. The flow noise is therefore rather directional, and by having the port opening in the back, this noise is well attenuated compared to having a port in the front and directing such noises towards the listener.
How to flush-mount large studio monitors and how should the wall be constructed?
Ideally the wall for flush-mounting limits the radiation from the monitor to the front hemisphere only.
Flush mounting studio monitors into a wall offers also other important advantages such as eliminating unwanted secondary sound radiation from the monitor cabinet's edges and nearly idealizing the radiation space. The result is minimization of diffraction effects, improved transient response and imaging.
Low frequencies are radiated omni-directionally (equally to all directions). The underlying principle of wall construction is that the larger the wall mass, the less energy transmission there will be through the wall. Therefore the wall should ideally be made of heavy materials, such as bricks or concrete. Any volume behind the wall should be filled with acoustically absorbing material, such as rockwool.
The materials you can use to make the monitor wall are:
This is the best material as it is the heaviest and stiffest. Unfortunately it is not always possible to build concrete walls into existing rooms. No acoustic treatment (rockwool) is needed behind a concrete wall if the wall is air tight. The surface can be finished with wood, soft cloth, etc.
- Bricks (breeze blocks or normal bricks)
This is also a very good material as it is heavy and can produce very stiff walls. A brick wall is easier to build. No acoustic treatment (rockwool) is needed behind a brick wall if the construction is air tight. The surface can be finished with wood, soft cloth, etc.
- Gypsum Board
Two to three layers of gypsum board are needed to increase the wall mass and to lower the wall resonant frequency sufficiently. It is possible to insert other materials, such as sand bags, wood and lead sheets, between the layers to add mass. It is better to put sound absorbing material, such as rockwool, behind the wall as some sound energy may leak through the wall into the enclosed volume due to the relatively low wall mass. These walls are typically constructed using steel frames. Use of wooden frames usually does not result in equally good wall characteristics.
Wooden walls are not recommended as these are typically not lossy and stiff enough and the unit mass of a wooden wall remains rather low. It is good to put some sound absorbing material, such as rockwool, into the cavity volume or the volume behind the wall due to the relatively high chance of sound energy transmission due to the low wall mass. When the wall is used for flush mounting the monitors, they should be mounted on a separate heavy stand built into the wall: a brick foot under the monitor stand is also a good idea to reduce sound energy transmission.
An important issue is to make sure the enclosures are installed exactly flush with the front wall without leaving any gaps or edges between the enclosre and the room wall.
Flush-mounting the monitors into the wall
To reduce structural vibrations due to mechanical conduction of the vibrational energy, the monitors should be mounted on rubber pads so that a resonant frequency of 2–8 Hz can only result. This de-couples the monitors mechanically from the wall and avoids structural vibration transmission.
For some monitors Genelec offers wall mount kits. The kit can be built into the wall and allows the monitor to be installed later as well as removed for servicing. The kits have been designed so that they ensure correct low frequency radiation while providing the acoustical benefits of flush mounting.
I am not getting enough bass, do I have a backwall cancellation?
The wall behind the monitor
A critical factor in the bass response of a monitor in a room is the distance of nearby walls (or boundaries) from the monitor. If a monitor is positioned freely standing in the room, the wall behind the monitor may have a strong effect on the output of low frequencies. The wall will reflect the low frequency energy radiated by the monitor.
This energy will be reflected and sums with the sound radiated by the monitor. See diagram below.
When the reflected sound is out of phase with the original sound, it destructively interferes with the direct sound, causing the sound level to go down, and causing a notch in the bass response of the monitor. This notch can cause a significant reduction in the bass output. For example, if a monitor is placed so that the front of it is 86 cm (34") from the wall behind it, the first cancellation frequency will be at approximately 100 Hz.
Consider the example above. The reflection off the wall behind the monitor causes the notch at 100 Hz. The comb filtering ripple between 1 and 2 kHz is caused by the acoustic reflection from the mixing desk surface. The tolerance of this monitor's anechoic frequency response is ± 2.5 dB. Any deviation larger than that range is an effect of placing the monitor into the room!
The first and best cure for the ‘wall behind the monitor’ cancellation dips is to flush-mount the monitors in a hard wall – also called ‘infinite baffle mounting’ or ‘flush-mounting’ - totally eliminating backwall reflection.
The second best cure is to placing the monitor very close to the wall. The distances below 20 cm (8") from the wall yield the monitor response unaltered (the cancellation dips are at or above 430 Hz). The resulting low frequency boost can be compensated with room compensation adjustments.
Finally, the third cure is to move the monitor very far away from the wall. Then the cancellation frequency goes down. But the direct-to-reflected level ratio also becomes very favourable, and the reflections from walls loose significance as the direct audio from the monitor to the listener dominates.
Placing the free-standing monitor in the room
To avoid cancellation of audio because of the sound reflecting back from the wall behind the monitor, follow the placement guideline below. This reflection happens at relative low woofer frequencies only. Avoiding the cancellation is important because the reflected sound can reduce the woofer output causing the monitor low frequency output to appear to be too low. To avoid the cancellation, push the monitor close enough to the wall. Typically the distance of the monitor front to the wall should be less than 0.6 meters. This ensures that the low frequency output is not reduced. The monitor needs a minimum clearance of 0.05 m to the wall to ensure full output from the rear bass reflex port.
Distances recommendation: from a single wall to the front baffle of free-standing monitors.
Frequency domain notches and distances from the single wall behind a free-standing monitor and its front baffle.
There are some different ways to solve these reflection problems:
- Select a room shape that will direct the reflections away from the listening position.
- Ensure that the back wall behind the listening position (the rear wall) is more than 3 m (9.8 ft) away from the listening position to avoid low frequency cancellation at the listening position. This problem often exists in rooms less than 5 m (16.4 ft) in length.
- Add absorbing to reduce the level of the reflected sound.
Formula for calculating cancellation frequencies
Quarter wavelength cancellation frequency
fc = c / 4dx
fc is the cancellation notch centre frequency c is the speed of sound in air at 20°C at sea level = 344m/s
dx is the distance from the front of monitor to the wall behind it
Minimum distance of the monitor to the wall
dmin =1.4 c / 4 f-3dB
dmin is the minimum distance from the front of the monitor to the wall behind it
c is the speed of sound in air at 20°C at sea level = 344 m/s
f-3dB is the -3 dB low cut-off frequency of the monitor
Half wavelength cancellation frequency
fc = c / 2(dreflect-ddirect)
fc is the cancellation notch centre frequency
c is the speed of sound in air at 20°C at sea level = 344 m/s
dreflect is the distance of monitor to the listening position via the reflecting surface
ddirect is the direct path distance from the monitor to the listening position
Small studio monitors are often placed either horizontally or vertically on the mixing console meter-bridge. Which orientation is better?
- Placing monitors on the meter bridge can cause the mixing desk to vibrate. This can affect the sound quality.
- To reduce mechanical coupling to the mixing desk, several Genelec monitors are equipped with a Genelec Iso-Pod™ rubber foot.
- A better mounting method is to place the monitors on stands behind the mixing desk sufficiently high so that the bass driver is not obscured.
- Mounting monitors vertically increases the distance and angle of the off-axis reflection from the console surface. This reduces sound coloration caused by the desk reflection (usually between 1–2 kHz).
- Below is a real world example. Monitor positioned horizontally on the meter bridge of a large mixing desk (red line) shows comb filtering at 1 kHz and extends up to 7 kHz.
- The monitor was re-measured in vertical orientation (green line). The frequency response is flatter.
- Positioning monitors horizontally causes a notch at the crossover when moving to the side. Positioning the monitor vertically removes this problem.
The imaging on my monitors is poor (room symmetry) - Why?
The room into which you place your studio monitors should always be symmetrical to achieve the best imaging.
- Differences in the direct and reflected sound paths from each monitor will result in different frequency responses at the listening position. This can cause the image to shift slightly to the left or right at different frequencies resulting in poor imaging.
- Symmetry applies also to the equipment in the room (which will affect midrange imaging).
Below is an example where the room was symmetrical but the equipment placed in it was not. The room had almost no absorption so the reflections were strong.
Figure 1: It can be seen that the left and right monitors 'see' different rooms due to the equipment positioning, therefore the left and right frequency responses at the listening position will be different.
This is shown by the red and green lines in the graph below:
Figure 2: The effect of the asymmetrical equipment positioning can be seen on the blue line which represents the difference between the left and right frequency responses. Any deviation from the centre line (0 dB on the right axis scale) means that the phantom image will shift depending on which monitor is louder. The imaging in this room could be considered to be poor.
What measuring techniques do you recommend?
There are a variety of measuring techniques that are available today but some are more suited for measurements in recording studios and other relatively small room environments than others.
Summarized below are most of the commonly available techniques and some tips to ensure that you can make a reliable measurement with the tools that you have available:
1/3rd Octave real time analyser (RTA)
Pink noise is played through the monitor and a typically in 1/3 octave bands, a graphical output waveform is displayed. The sound field in the room should become stable before conclusions are drawn about the measurement. This is a quick measurement technique suitable for subwoofer to monitor level balancing.
Spot frequency using sine wave and a sound level meter
A sine wave signal generator is used to drive the monitor and the sound level is measured using a sound level meter. This method measures the steady state conditions in the room which emphasizes the room resonances. This method is suitable for low frequency room resonance evaluation in highly damped rooms.
Warble tone spot frequency
This techniques is similar to the previous one, but uses a tone that has been slightly modulated to cover a wider frequency range.
Swept sine wave
The traditional swept sine wave measurement measures the instantaneous sound level while the sine frequency is sweeping though the audio band. This method is problematic as there are uncertainties in the settling time of the room response and the effects of possible system non-linearity (distortion) are not detectable.
Time Delay Spectrometry (TDS)
A sine sweep is used as the signal. A tracking filter minimizes noise in the measurement and selects a certain extent of the room response duration for the measurement. This is analogous to time windowing an impulse response. This method can suffer from a loss of accuracy at low frequencies (due to the tight 'time windowing') and the response is smoothed depending on the sweep speed and filter width.
FFT (Fast Fourier Transform) analysis based methods
Using FFT on an impulse measurement
This method measures the impulse response directly and then analyses the frequency response using Fourier analysis on a recorded impulse. Impulses are generated mechanically or electronically. This method suffers from very poor signal to noise ratio in the measurement, and is therefore often impractical or inaccurate.
Using noise (or even music!)
This method uses a wide band statistically stationary random signal (noise) and continuous Fourier analysis. A large number of FFTs are averaged to calculate the frequency response. This method is very low and unreliable at low frequencies. Noise can be replaced with any wideband signal, but this does not improve the performance of the measurement method.
Maximum Length Sequence and FFT (MLS)
A maximum length sequence is a pseudorandom signal having noise-like spectral content, i.e. energy across the whole audio band, but with very high signal level as the method of generating the signal ensures that a high energy level is obtained at most audio frequencies during each measurement cycle. Using an MLS sequence and FFT analysis achieves much better signal-to-noise than using just noise, and taking a reliable measurement becomes fast. Using the MLS method only works for linear systems, and does not produce accurate results when the distortion is high.
Using a variable speed sine sweep and FFT
The sine sweep response can be FFT analysed to calculate estimates of the impulse response and frequency response. The speed of the sinusoidal can be adjusted during the sweep. This can adjust the energy density of the measurement signal, and allows higher energy density to be used where more signal-to-noise ratio is needed, for example at low frequencies. Using the FFT methods can also enable exclusion of the harmonic distortions, improving the quality of the measurement. Using the swept sine with exponentially increasing speed and FFT is considered the method to produce the best signal-to-noise of all methods at the moment. This method is also available widely in general purpose acoustical measurement tools in the Internet.
What should be the target response of a monitoring system at the listening position?
The role of a monitoring system is to reproduce sound without adding or taking anything away from the original input signal. Modern recording systems have a flat electronic frequency response. To accurately monitor what is recorded on the hard drive or tape machine, the monitoring system also has a flat response at the listening position.
Secondly, onto the often quoted "final mix translation" issue, one can observe that domestic and car audio systems are generally improving over time and having better, i.e. flatter, frequency response. As a general note, a good mix should sound good on any system.
One exception to this rule is the X-Curve used in the movie industry. Movie theatre replay systems are installed in very large rooms (e.g. a movie theatre for 200-800 people) and the frequency response across the audience area is never flat. The Dubbing Stage must replicate this response so that the mix translates precisely to the Movie Theatre. Note that the soundtracks for the release of movies on DVD's are re-mixed on flat response monitoring systems for reproduction in domestic environments.
What types of damping materials are used inside Genelec monitor and subwoofer products? Are the materials safe to use?
Genelec products use various damping materials such as glass fiber wool, linen fiber wool and polyester fiber based material (PES). The table below provides detailed listing of our models and the type of damping material used.
During operation, the air moving in and out of the monitor loudspeaker or subwoofer bass reflex openings does not emit significant amounts of fiber particle dust.
The PES wool as material does not emit dust. The linen wool and glass fiber wool can emit a minimum amounts of dust during very high sound level operation. This fiber dust is not hazardous to health.
|SAM Studio Monitors||Damping material type:|
|SAM Studio Subwoofers|
|8000 Series Studio Monitors|
|1000 Series Studio Monitors|
|M Series Studio Monitors|
|7000 Series Studio Subwoofers|
|G Series Active Speakers||Damping material type:|
|F Series Active Subwoofers|
|Home Theater Speaker Series|
|Home Theater Subwoofer Series|
|4000 Series Installation Speakers||Damping material type:|
|Architectural Speaker Series|